JPH11229034A - Working method for ferritic stainless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for working a ferritic stainless steel pipe less prone to brittle cracks. SOLUTION: A ferritic stainless steel contg., by mass, <=0.03% C, <=2.0% Si, <=2.0% Mn, 10.0 to 35.0% Cr and <=0.03% N, furthermore contg. one or more kinds among <=1.0% Cu, <=3.0% Mo, <=0.8% Nb, <=0.5% Ti and <=0.3% Al, and the balance Fe with inevitable impurities in the production is subjected to TIG welding, laser welding or high frequency welding to make a tube, which is, if circumstances required, furthermore subjected to stress relieving annealing after making the tube and is thereafter subjected to warm working at >=15 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas path member for various internal combustion engines, and more particularly to a method for processing a ferritic stainless steel pipe used for an automobile exhaust gas path member.

[0002]

2. Description of the Related Art In recent years, there has been a demand for an automobile engine capable of meeting strict exhaust gas regulations due to increasing interest in global environmental problems. If measures are taken to satisfy these requirements, the temperature of the combustion gas increases, and the temperature of peripheral members such as the exhaust gas purification system increases. As a result, these members are required to have even better heat resistance and corrosion resistance.

[0003] Austenitic stainless steel has higher high-temperature strength than ferritic stainless steel, but has a large thermal expansion due to large thermal expansion, and there is a concern that thermal fatigue may occur when heating and cooling are repeated. Further, austenitic stainless steel contains a large amount of Cr and Ni, so that the manufacturing cost also increases. For these reasons, ferrite-based stainless steel is often used as the material for the exhaust gas path of automobiles.

The material for the exhaust gas passage portion is a steel plate or a welded steel pipe (refers to a steel pipe manufactured by a method such as high-frequency welding, TIG welding, laser welding, etc .; hereinafter, simply referred to as a steel pipe).
Is processed into a predetermined shape and then welded to obtain a product.
Since the shape of the exhaust pipe is very complicated, a part of the steel plate or the steel pipe undergoes severe processing during forming. In the case of a steel sheet, the elongation of the cold-rolled annealed sheet is about 36%, and the fracture surface transition temperature of the cold-rolled annealed sheet is about −60 ° C. If the processing is performed within the ductility limit range, the transition temperature is lower than the processing temperature. Therefore, brittle cracks hardly occur during processing. However, since a steel strip is formed into a tubular shape and the both ends are welded, the steel pipe includes a strain at the time of forming and a welded portion, and has substantially lower toughness than a steel plate. For this reason, even in the case of a steel pipe using the disclosed steel described above, in the case of pipe expansion after bending, for example, when forming is performed under a low processing temperature in the winter or at a high processing speed, when manufacturing a pipe, Rarely, brittle cracks occur at the joined portion, that is, at the weld.

[0005] For such cracks, it is most effective to perform annealing of the steel pipe to remove strain near the welded portion. The present inventors have disclosed in Japanese Patent Application Laid-Open No. 9-125209 a method in which a steel pipe formed by welding is annealed in a temperature range of 850 to 1000 ° C. and cooled at a cooling rate of 1 ° C./sec or more. According to this method, the workability and toughness of the tube can be improved to the level of a cold-rolled annealed sheet. However, an increase in manufacturing costs due to annealing is inevitable, and in the case of steel pipes that have been made highly alloyed to ensure high levels of heat resistance and corrosion resistance, sufficient toughness cannot be obtained by annealing. There can be. As a method of improving toughness,
A technique for generating a martensite phase in a weld has been proposed. However, this method cannot be applied to a completely ferritic stainless steel that does not generate martensite in a weld. In addition, this publication describes that there is a problem in the prior art because a reduction in the amount of heat input causes welding defects and a warm working causes an increase in cost.

On the other hand, it is also possible to avoid brittle cracking by adjusting alloy components so as not to significantly impair toughness. To improve toughness, C, N, C
Reduction of alloying elements such as r, Si, Mo, Al, Ti, and Nb and addition of trace amounts of Ni, Cu, and the like are performed. But,
The reduction of C and N and the addition of Cu and Ni increase the production cost, and the reduction of Cr, Mo, Al, Ti, Nb and the like deteriorates the heat resistance and the corrosion resistance. Therefore, it has heat resistance and corrosion resistance, without adjusting the alloy elements,
There is a demand for a method of processing a ferritic stainless steel pipe that can be processed into a predetermined shape.

[0007]

SUMMARY OF THE INVENTION In order to cope with such a current situation, the present invention secures the heat resistance and corrosion resistance required for a vehicle exhaust gas path member, without adding or reducing a third element. It is still another object of the present invention to provide a method of processing a ferritic stainless steel pipe that can be processed into a predetermined shape without increasing the manufacturing cost.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for producing
And C: 0.03% or less, Si: 2.0% or less, Mn:
2.0% or less, Cr: 10.0 to 35.0%, N: 0.
Cu: 1.0% or less, M
o: 3.0% or less, Nb: 0.8% or less, Ti: 0.5
% Or less, Al: 0.3% or less, and after forming a ferritic stainless steel composed of Fe and unavoidable impurities in production by TIG welding, laser welding or high frequency welding, Further, in some cases, it is attained by performing strain relief annealing after pipe forming, and then performing warm working at a temperature of 15 ° C. or more.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied the workability of a pipe under various conditions in order to improve the method of processing the pipe. It has been clarified that it is possible to suppress brittle cracking by adjusting to, and the present invention has been achieved.

FIG. 1 shows Fe-14Cr-1.2Mn-
Outer diameter 38.1.2Si-0.6Nb as a basic component;
FIG. 4 shows the effect of the flat test temperature on the crack occurrence rate in a flat test of a steel pipe having a thickness of 1 mm and a thickness of 2.0 mm. The flatness test was performed at a low temperature according to the flatness test method described in “JIS G3449” for stainless steel pipes for piping. That is, a test piece having a length of 1250 mm was immersed in a cooling or heating medium, and after holding for 15 minutes, the welded portion was placed so as to be perpendicular to the compression direction, and the rolling speed was reduced by 250 mm.
/ Min flattened to close contact. At each temperature, a total of eight cracks (total length 10 m) were tested, and the number of cracks generated was divided by the number of tests to obtain a crack occurrence rate. In addition, the test was stopped when cracks occurred, and the height (flat height) of the tube after the test was measured. The flat height after the test of the cracked material was averaged (the parameter was the number of cracks) to obtain the average flat height of the cracked material, which was indicated by a number in the figure.

FIG. 1 shows that the crack generation rate strongly depends on the processing temperature, and the lower the processing temperature, the higher the crack generation rate. However, in the processing at 15 ° C. or higher, the rate of occurrence of cracks is very small, about 10%, and cracks occur only in flatness up to close contact. Further, when the temperature was 30 ° C. or higher, it was clarified that brittle destruction did not occur even when a very severe deformation was applied to the welded portion, which is referred to as an adhesion flatness test. In addition, since all fracture surfaces of the material in which cracks occurred exhibit brittle fracture surfaces, it can be said that these fractures were processed in a brittle fracture region equal to or lower than the ductile brittle transition temperature of the material. As described above, the fracture surface transition temperature in the Charpy impact test of the cold-rolled annealed sheet is about −60 ° C., but under the test conditions, the crack is −10 ° C.
From the above results, it has been clarified that a large amount is generated in the temperature range from to 10 ° C. Then, next, in order to compare the toughness of the pipe with that of the cold-rolled annealed sheet, a Charpy impact test was performed in which the pipe was pushed open and a V-notch was formed in the welded portion and the base material other than the welded portion.

FIG. 2 is a graph showing the relationship between the Charpy impact value and the test temperature when a Charpy impact test was performed on a pipe in the same component system as in FIG. In the Charpy impact test, a V-notch test piece having a thickness of 2.0 mm according to “JISZ2202” was prepared, and a metal material test method (Charpy impact test) specified in “JISZ2242” was performed. I asked.

According to FIG. 2, the ductile brittle transition temperature of the pipe base material is around 0 ° C., which is inferior to the toughness of the cold-rolled annealed sheet. The cause of this is that the pipe base material is subjected to strain during pipe making and strain during processing of a test piece. From this fact, in the secondary processing after the primary processing, if the pipe is subjected to impact processing at a low temperature of 0 ° C. or less, if there is a notch, the pipe base material may be broken. . On the other hand, the ductile brittle transition temperature of the pipe weld is
The temperature is around 0 to 15 ° C, and the toughness is further inferior to that of the base material. In the processing of a general pipe, when brittle fracture occurs, it is considered that the fact that the brittle fracture often occurs from the welded portion is due to the low toughness of the welded portion relative to the base material.

According to the results of this test, if the temperature is 25 ° C. or more, both the base material and the welded part exhibit ductile fracture, and the Charpy impact value becomes 50 J / cm 2 or more, indicating ductile fracture. become. That is, it is understood that brittle fracture hardly occurs even when subjected to an impact deformation such as the Charpy test. The temperature at which the brittle fracture hardly occurs is close to the temperature (15 ° C.) described in FIG. 1, and it is apparent that brittle fracture can be avoided by heating the material in any of the deformation modes. became. Further, as described in the examples, similar tests were performed on various ferrite stainless steels for exhaust gas path members. As a result, if the processing was performed at a temperature of 15 ° C. or more, the incidence of brittle fracture was reduced. It has been clarified that it can be suppressed to a low level, leading to the present invention.

Hereinafter, the action of each component in the present invention and the reason for limiting the range of the content thereof will be described.

C and N are generally effective as elements for improving the creep characteristics, but it has long been known that ferritic stainless steels have a bad influence on intergranular corrosion susceptibility and toughness. C and N are also preferably as low as possible with respect to the toughness of the stainless steel pipe. For this reason, the contents of C and N are each set to 0.03% by mass or less. In order to ensure toughness at a higher level, it is preferable to set each to 0.02% by mass or less.

Si is an element effective for improving high-temperature oxidation characteristics, high-temperature salt damage characteristics, and high-temperature fatigue characteristics. However, an excessive addition causes a decrease in toughness of the steel pipe. For this reason, the content of Si is set to 2.0% by mass or less.

Addition of an appropriate amount of Mn significantly improves the high-temperature oxidation characteristics, particularly the adhesion of the surface oxide. However, if it is contained excessively, it becomes hard and causes a decrease in toughness and workability. Therefore, the content of Mn is set to 2.0% by mass or less.

Cr is a very effective element for securing corrosion resistance and high-temperature oxidation resistance, and it is necessary to add 10% by mass or more to maintain each characteristic. On the other hand, if it is added excessively, an embrittlement phase such as a σ phase is generated, resulting in embrittlement of steel. In addition to becoming hard and deteriorating workability,
Raw material prices increase. Therefore, the range of Cr is 10.
0 mass% to 35.0 mass%.

Cu is an element effective for improving corrosion resistance, toughness and workability when added in an appropriate amount. However, when Cu is added excessively, workability is impaired. Therefore, the content of Cu is set to 1.0% by mass or less.

Mo is a very effective element for improving corrosion resistance and high-temperature strength, and is an element appropriately added to satisfy these characteristics at a higher level. However, when Mo is added excessively, toughness and workability are reduced. Then, M
The content of o was 3.0% by mass or less.

Nb is one of the most effective alloying elements for improving the high-temperature strength of ferritic stainless steel. Further, by binding to C and N in steel, the formation of Cr carbonitride is suppressed, and the intergranular corrosion resistance is improved. On the other hand, N
Excessive addition of b leads to embrittlement of steel and deterioration of workability.
Nb should not affect the workability and toughness so much.
Was set to 0.8% by mass.

Ti, like Nb, combines with C and N in steel to suppress the formation of Cr carbonitride and improve intergranular corrosion resistance. However, if Ti is added excessively, it is oxidized at the time of pipe making, so that an oxide is apt to remain in the welded portion, and the toughness of the welded portion is reduced. Therefore,
The upper limit of Ti was set to 0.5% by mass.

Al is effective as a deoxidizing agent for removing residual oxygen during smelting of steel. In other words, if oxygen remains in the steel, the weldability deteriorates, so Al deoxidation is effective,
If Al remains excessively, an oxide is generated at the time of pipe-forming similarly to Ti, and the toughness of the weld is deteriorated. Therefore, A
The content of 1 was 0.3% by mass or less.

The type of ferritic stainless steel pipe containing these alloy components mainly refers to a welded pipe formed by TIG welding, laser welding, or high-frequency welding. Further, the present invention can be applied to a welded pipe that has been subjected to processing after manufacturing the welded pipe, such as straightening, strain relief annealing, pickling, polishing, and plating. Since the seamless pipe does not include a welded portion, it is not particularly included in the claims of the present invention.

In order to process the ferritic stainless steel pipe so as to reduce the defective rate due to brittle cracking,
As described above, processing may be performed at a temperature of 15 ° C. or higher.
More preferably, the processing temperature is 30 ° C. or higher. Further, the processing form is not particularly limited, and can be applied to any processing such as bending, flattening, contraction, expansion, and a combination thereof. The ductile-brittle transition temperature of a ferritic stainless steel pipe changes depending on the alloy composition of the steel, the pipe-forming method, the presence or absence of annealing, and the processing form.
From the viewpoint of reducing the defective rate, the above-mentioned processing at 15 ° C. or more is sufficient, but in order to completely suppress cracking, a transition temperature is determined for each pipe, and processing is performed in a complete ductile fracture temperature range. Is preferred.

Although the upper limit of the processing temperature is not particularly specified, the processing is preferably performed at 80 ° C. or lower in consideration of the processing operation and the operation cost. Should be avoided. Regarding the heating method of the steel pipe, any method such as immersion in water or oil, atmosphere heating by a batch furnace or the like, direct heating by energization, or heating of a work place in winter may be used.
Not specified. The heating time and the cooling method after processing are not particularly specified.

[0028]

Embodiments of the present invention will be described below. Table 1 shows the chemical components of the test materials. No. in the table. 1 to No. Steel No. 11 is included in the scope of the method of the present invention. In addition, No. 1
2 is a comparative steel not included in the method of the present invention. These are melted in a weight of 400 kg or 70 tons, and then annealed after hot rolling to form a hot-rolled steel strip, then cold-rolled to 2.0 mm, annealed, and then pipe-formed to obtain φ38.1 × 2.0 mm Steel pipe.

[0029]

[Table 1]

Table 2 shows the results of the adhesion flattening test. The flattening test was performed by the test method shown in FIG. 1 described above. In addition, the pipe making is performed by any of a high-frequency pipe making, a TIG pipe making, and a laser pipe making, and some of the samples are subjected to strain relief annealing. Table 2 also shows the pipe forming method and whether or not annealing was performed.

[0031]

[Table 2]

When the hot working of 15 ° C. or more, which is the method of the present invention, is performed, 1 to No. Steel No. 11 did not undergo brittle cracking in any of the pipe forming methods. On the other hand, when the adhesion flatness test was performed in a temperature range lower than 15 ° C., more precisely, lower than the ductile fracture range of the welded portion of each steel, one or more cracks occurred. This is because at temperatures below the ductile brittle transition temperature range of the weld, severe processing and processing with impact are performed.
It suggests that brittle fracture can occur. In addition, the comparative steel No. In No. 12, since the ductility was low and the ductile-brittle transition temperature was 15 ° C. or higher, brittle cracking occurred even in the flat test at 15 ° C.

Table 3 shows the processing test results assuming actual processing. As the processing method, 90 ° bending with the inside radius of the pipe being the outside diameter of the pipe (φ38.1 mm) was performed as primary processing for 5 seconds.
And then cut along the maximum thickness reduction,
The tube end was expanded by 3% as the next processing. The processing temperatures of the primary processing and the secondary processing were set to be the same, and the crack occurrence rate (number of cracks / number of processing) was evaluated.

[0034]

[Table 3]

Although the cracking rate is smaller than that of the above-mentioned flattening test, the influence of the processing temperature clearly appears. According to the method of the present invention, the crack generation rate is 1% or less, but the crack generation rate reaches several% in the processing under 15 ° C. which is the comparative method.

[0036]

According to the method of the present invention, it is possible to reduce the frequency of occurrence of brittle cracks which are likely to occur during the processing of ferritic stainless steel pipes. For this reason, the method of the present invention is suitable for machining of automobile exhaust path members that perform severe machining, particularly in winter in which the machining temperature is low.

[Brief description of the drawings]

FIG. 1 shows the effect of test temperature on the crack generation rate in the adhesion flatness test.

FIG. 2 shows the effect of test temperature on Charpy impact test values of a base material and a weld.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C21D 1/26 C21D 1/26 A // C22C 38/00 302 C22C 38/00 302Z 38/38 38/38 (72) Inventor Naoto Hiramatsu 4976 Nomura Minamicho, Shinnanyo City, Yamaguchi Prefecture Inside Nisshin Steel Co., Ltd.

Claims (2)

[Claims] (1) In mass%, C: 0.03% or less, Si:
2.0% or less, Mn: 2.0% or less, Cr: 10.0 to
35.0%, N: 0.03% or less.
u: 1.0% or less, Mo: 3.0% or less, Nb: 0.8
% Or less, Ti: 0.5% or less, Al: 0.3% or less, and a balance of ferrite stainless steel composed of Fe and inevitable impurities in production by TIG welding, laser welding or A method for processing a ferritic stainless steel pipe, comprising forming a pipe by high-frequency welding and then warm-working the pipe at a temperature of 15 ° C. or higher. 2. The ferritic stainless steel according to claim 1 is pipe-formed by TIG welding, laser welding, or high-frequency welding, and is further subjected to strain relief annealing.
A method for working a ferritic stainless steel pipe, comprising warm working at the above temperature.