JP3501573B2 - Ferritic stainless steel pipe excellent in secondary work crack resistance and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel pipe having excellent resistance to secondary work cracking produced by welding, and a method for producing the same.

[0002]

2. Description of the Related Art Heat-resistant ferritic stainless steel has a smaller coefficient of thermal expansion than austenitic stainless steel and is advantageous for repeated heating and cooling applications.
Since it is relatively inexpensive, it is beginning to be used in various fields such as automobile exhaust path members and various plant materials.
In particular, since the stainless steel pipe used as the exhaust gas passage member is subjected to severe processing, it is required to have excellent workability and low temperature toughness in addition to heat resistance. However, since strengthening elements such as Nb, Ti, Si and Mo are usually added to these materials in order to improve high temperature strength and oxidation resistance, workability and low temperature toughness tend to be rather lowered. is there.

Stainless steel pipes are generally manufactured by welding both ends in the width direction of a steel plate or steel strip, except for seamless pipes which are expensive to manufacture. Examples of the welding method include TIG welding, high frequency welding, and laser welding. No matter which of these welding methods is used for pipe forming, plastic strain is applied during pipe forming, so that the ductility of the entire pipe is slightly inferior to that of the stainless cold-rolled steel sheet of the same composition. Further, since the welded portion and the HAZ portion have larger crystal grains than the other portions, the workability and the low temperature toughness are inferior in the entire tube.

As a means for improving the workability of stainless steel pipes, a pipe forming method has been proposed in which the plastic strain generated during pipe forming is minimized. However, sufficient workability similar to that of a cold rolled stainless steel sheet cannot always be obtained only by improving the pipe making method. Therefore, in order to obtain even better workability, a method of further annealing the pipe formed is adopted. Since this annealing is intended to soften the material, ferritic stainless steel generally has a temperature of 600-80.
It is performed at a temperature of about 0 ° C. The pipe that has been annealed (hereinafter referred to as an annealed pipe) exhibits better workability than the as-made pipe, and is therefore often used, for example, as a route member for automobile exhaust gas.

[0005]

In recent years, automobile exhaust gas passage members have become more complicated in structure in order to save space and improve exhaust efficiency. Along with this, the shape of the pipe becomes complicated, and strict processing tends to be performed. That is, in addition to the primary processing such as bending, flattening, contraction, and expansion performed on the as-made pipe or the annealed pipe, the pipe subjected to the bending processing as the primary processing is further expanded. So-called secondary processing, such as shrinking, is frequently performed. Since the secondary working is a working in a state close to the ductile fracture limit, it is a working under extremely severe conditions against cracking as compared with the primary working.

When performing such secondary processing, 60
Even after the above-mentioned annealing after the pipe making which is performed at a temperature of about 0 to 800 ° C., the occurrence of cracks cannot always be prevented. The secondary work crack is a crack that occurs when the material is worked within the ductile fracture limit range, and a means for surely preventing this secondary work crack has not been clarified. Under these circumstances, it is difficult to predict the occurrence rate of secondary work cracks, which may lead to unexpected troubles such as sudden frequent occurrence of secondary work cracks during operation.

[0007] The reason why the means for preventing the occurrence of secondary work cracking has not been clarified is that no metallographical knowledge has been obtained on pipes having excellent resistance to secondary work cracking. An object of the present invention is to provide a ferritic stainless steel pipe capable of catching the occurrence of secondary work cracks from a metallographic viewpoint and reliably preventing such secondary work cracks with good reproducibility.

[0008]

[Means for Solving the Problems]
C: 0.03% or less, Si: 2.0% or less, Mn: 2.0
% Or less, Cr: 5.0 to 30.0%, N: 0.03% or less, and Nb: 0.05 to 1.0%, Ti:
0.05-1.0%, Mo: 0.05-3.0%, Cu:
A steel sheet or strip containing one or more of 0.02 to 1.0% and the balance being Fe and inevitable impurities in the production, welded into a pipe, and then annealed pie. The content of the precipitates present in the material steel of the pipe is 0.3 mass% or less, and the grain size of the HAZ part is a grain size number according to JIS-G0552. This is achieved by a ferritic stainless steel annealed pipe that is No. 3 or more and is excellent in secondary work cracking resistance. Also,
This pipe is obtained by a manufacturing method in which it is annealed in the temperature range of 850 to 1000 ° C. after pipe making and then cooled at a cooling rate of 1 ° C./sec or more.

Here, the precipitates are mainly carbides, and include Laves phase when it is formed. The content of the precipitate means the content in the pipe material including all of the welded portion, the HAZ portion and other portions. The HAZ part means a welding heat affected part.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of detailed investigations on the occurrence of secondary work cracks in stainless steel pipes, the present inventors found that the cracks are peculiar to ferritic stainless steel and are likely to occur in winter. It was clarified that the morphology is brittle cleavage fracture, that this crack hardly occurs in the primary working, and that it occurs relatively frequently in the secondary working after bending. The present invention has been completed based on these findings. The reasons for defining the matters that specify the present invention will be described below.

C and N are generally effective elements for improving high temperature strength such as creep strength and creep rupture strength. However, if the contents of C and N increase, the oxidation resistance, workability and toughness decrease. Further, if a large amount of C and N is added, it is necessary to increase the addition amount of carbonitrides such as Nb and Ti, resulting in high cost. Therefore, in the present invention, the content of C and the content of N are both 0.03 mass% or less.

Si is an element that improves high temperature oxidation resistance. However, if added excessively, the hardness increases and the workability and toughness decrease. Therefore, the Si content is 2.0% by mass.
Below.

Mn is an element that remarkably improves the high temperature oxidation characteristics, particularly the adhesion of the surface oxide. However, if it is contained excessively, it becomes hard and the low temperature toughness and workability are deteriorated.
Therefore, the Mn content is set to 2.0% by mass or less.

Cr stabilizes the ferrite phase, and also
It is an essential element for improving corrosion resistance and high temperature oxidation resistance. Cr for improving high temperature oxidation resistance
The higher the content of, the more preferable. However, if it is contained excessively, it causes embrittlement of the steel, becomes hard, deteriorates the workability, and raises the raw material price. Therefore, the content of Cr is set to 5.0 to 30.0 mass%. In order to achieve both high levels of corrosion resistance and high temperature oxidation resistance, and workability,
The Cr content is preferably in the range of 10.0 to 20.0 mass%.

Nb is an element that effectively acts to increase the high temperature strength. In order to increase the high temperature strength, it is necessary to contain at least 0.05% or more. On the other hand, if Nb is excessively contained, low temperature toughness and workability are deteriorated. The Nb content is 0.0 in order to maintain the high temperature strength and not to affect the low temperature toughness and workability deterioration.
It was set to 5 to 1.0 mass%.

Ti is an element that improves the r value of the steel sheet and is effective for deep drawability. However, if Ti is added, TiN
Is likely to occur, resulting in a decrease in yield due to the occurrence of bald defects in the steel sheet and a decrease in weldability. Therefore, the content of Ti is set to 0.05 to 1.0 mass% or less.

Mo is an element effective for improving corrosion resistance, oxidation resistance and high temperature strength. However, if contained in a large amount, the steel becomes brittle. Therefore, the Mo content is 0.05 to 3.0.
It was defined as mass%.

Cu is an element effective for improving both low temperature toughness and workability, and its effect becomes remarkable when its content is 0.02%. However, if Cu is contained excessively, workability is impaired. Therefore, the Cu content is 0.02 to 1.
It was set to 0 mass%.

The content of precipitates present in the steel material of the pipe and the grain size of the HAZ part are the most important matters in specifying the present invention. The present inventors have established the content of precipitates and the grain size of the HAZ part by investigating the relationship between the metal structure and the secondary workability of steels having various chemical compositions. The investigation methods and results are shown below, Fe-14% Cr-1.0% Si-1.1% Mn.
An example of a test conducted using −0.5% Nb steel will be illustrated.

An annealed pipe obtained by annealing a high-frequency pipe having a diameter of 42.7 mm × 2.0 mm at various temperatures was used as a test material. Secondary workability was investigated by performing the low temperature bending flatness test shown below. That is, after bending at 90 ° with a radius of the central axis of 42.7 mm so that the high-frequency weld is on the outside of the bend (primary processing), JIS-G0202 is applied at each temperature
-Flatness according to No. 2021 was performed up to a height of 1/3 of the diameter (secondary processing), and the presence or absence of cracks was investigated. And
The temperature at which cracking does not occur in the low-temperature bending flatness test (hereinafter referred to as the cracking limit temperature) was used as an index for evaluating the secondary workability. For the content of precipitates, a sample was cut out from the pipe so that the welded part was included, and the sample was cut by 10% AA.
Electrolysis was performed using a liquid (10% acetylacetone + 1% tetramethylammonium chloride + methanol) to extract precipitates, and the weight of the precipitates was divided by the amount of electrolysis to obtain the value. H
The crystal grain size of the AZ portion was obtained by a comparison method based on JIS-G0552.

FIG. 1 shows the test results. The numbers in the figure represent the cracking limit temperature. In order to prevent cracks in actual pipe processing, this cracking limit temperature is -50 ° C.
The present inventors have confirmed separately by experiments that the following is acceptable. For example, if the cracking limit temperature of the pipe after the primary working is -50 ° C or less, cracking can be prevented when the pipe is subjected to the secondary working. From FIG. 1, the content of the precipitate is set to 0.3% by mass or less, and the HAZ
It can be seen that when the grain size of the part is No. 3 or more, the cracking limit temperature becomes −50 ° C. or less. Based on such test results, the content of precipitates and the grain size of the HAZ part were determined within the ranges specified in the present invention.

FIG. 2 shows a summary of the influence of the annealing temperature of the pipe on the cracking limit temperature in the low-temperature bending flatness test, based on the test results of FIG. From the results of FIG. 2, it became clear that a pipe having good resistance to secondary work cracking can be obtained by strictly defining the annealing temperature of the pipe in the range of 850 to 1000 ° C. The reason for this is that, as can be seen by comparing FIGS. 1 and 2, when the annealing temperature is low, a large amount of precipitates are generated, which becomes the starting point of brittle fracture, and when the annealing temperature is high. It is considered that this is because the toughness itself deteriorates because the crystal grains of the HAZ portion become coarse.

After annealing in the above temperature range, precipitates should not be formed in the cooling process to exceed 0.3% by mass. Therefore, the cooling rate is 1 ℃
Regulate to more than / sec.

In the method of the present invention, the heating rate and soaking time in annealing are not particularly specified, but if the heating rate is too slow, precipitates are produced during heating, and if the soaking time is too short, they are produced during heating. The formed precipitate may not be solid-resolved again. Therefore, it is preferable that the heating rate is 1 ° C./sec or more and the soaking time is 10 min or less. Further, generally, after annealing, the shape is corrected and the annealing scale is removed by pickling, but these methods are not particularly limited.

[0025]

[Examples] Ferritic stainless steels having chemical compositions A to H shown in Table 1 were melted, hot rolled, annealed, and cold rolled into a 2.0 mm plate, and annealed at a temperature of 800 to 1050 ° C. After that, it was pickled to obtain a steel strip for pipe making. These steel strips were formed into pipes having an outer diameter of 42.7 mm by high frequency welding, annealed at various temperatures, and cut into lengths of 500 mm to obtain pipes for working test.

[0026]

[Table 1]

The secondary workability was evaluated by obtaining the crack limit temperature in the above-described low temperature bending flatness test. For reference, as-annealed pipe (not bent)
The direct low-temperature flatness test was also conducted for the above, and the primary workability was also investigated. The content of the precipitate and the grain size of the HAZ part were also investigated by the above-mentioned method. Table 2 shows the results of these tests.

[0028]

[Table 2]

No. 1 to No. 17 are according to the present invention. Since the content of the precipitates is 0.3 mass% or less and the grain size number is 3 or more in any of the steel types, the cracking limit temperature in the low temperature bending flatness test (secondary workability test) is -50. It is below ℃. Since all of these pipes were manufactured with an annealing temperature of 850 ° C. to 1000 ° C. and a cooling rate after annealing of 1 ° C./sec or more, the above-mentioned precipitate ratio and grain size number were secured.

No. 18 to No. 22 show comparative examples. No. 18 and 22 have low annealing temperature,
Since No. 20 had a low cooling rate after annealing, the amount of precipitates was out of the range specified by the present invention, and as a result, it cracked at a temperature higher than -50 ° C in the low temperature bending flatness test (secondary workability test). There has occurred. Since Nos. 19 and 21 have high annealing temperatures, the grain size number of the HAZ part is out of the range specified by the present invention, and as a result, cracking occurs at a temperature higher than -50 ° C in the low temperature bending flatness test (secondary workability test). There has occurred. In both of the present invention and the comparative example, no crack was generated even at −50 ° C. in the low temperature flatness test (primary workability test) of the raw pipe which was not subjected to the primary working, and therefore, for mild working. It can be said that all of these pipes have sufficient workability.

[0031]

According to the present invention, means for preventing secondary work cracking of ferritic stainless steel welded pipes, which was conventionally unclear, has been clarified from the viewpoint of the metallographic structure, so that reproducibility is assured. It is now possible to provide ferritic stainless steel pipes that can prevent secondary work cracking.
For this reason, it becomes possible to stably manufacture a pipe having a complicated shape using ferritic stainless steel, and ferritic stainless steel can be widely spread particularly in applications such as automobile exhaust path members.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the secondary working crack limit temperature of an annealed pipe, the content of precipitates, and the grain size number of the HAZ part.

FIG. 2 is a graph showing the influence of the annealing temperature of the pipe on the secondary work cracking temperature of the annealed pipe.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshiyuki Fujimura 4976 Nomuraminami-cho, Shinnanyo-shi, Yamaguchi Nisshin Steel Co., Ltd. Technical Research Institute (72) Inventor Yoshihiro Uematsu 7th Takatani-machi, Ichikawa-shi, Chiba Nisshin (56) References JP-A-7-268468 (JP, A) JP-A-6-279948 (JP, A) JP-A-7-286239 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 B21C 37/08 C21D 9/08

Claims (2)

(57) [Claims] 1. In mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 5.0 to 30.0%, N: 0.03%. Contains the following, and Nb: 0.05 to 1.0%, Ti: 0.05 to 1.0%, Mo: 0.05 to 3.0%, Cu: 0.02 to 1.0% A steel sheet containing at least one of the above, the balance being Fe and a steel strip or steel strip consisting of unavoidable impurities in manufacturing, welded into a pipe, and then annealed. The content of the precipitates present therein is 0.3% by mass or less, and the grain size of the HAZ part is JIS-G.
A ferritic stainless steel annealed pipe having a grain size number of 3 or more in accordance with 0552 and excellent in secondary work cracking resistance. 2. A pipe made by pipe forming is 850 to 1000 ° C.
The method for producing a ferritic stainless steel annealed pipe excellent in secondary work cracking resistance according to claim 1, which is annealed in the temperature range of 1 and cooled at a cooling rate of 1 ° C./sec or more.