JPH07268468A - Production of stainless steel tube excellent in formability - Google Patents

Abstract

PURPOSE:To provide a method for producing a stainless steel tube are secured with heat resistance and workability as a steel tube for an automobile exhaust system or the like. CONSTITUTION:A steel having a compsn. contg., by weight, <=0.04% C+N, 0.05 to 2.0% Si, 0.1 to 2.0% Mn and 10 to 25% Cr, furthermore contg. at least one or more kinds of 0.02 to 0.5% Ti and 0.05 to l.0% Nb in the range satisfying C/12+N/14<=Ti/48+Nb/93, and the balance Fe with inevitable impurities is subjected to hot rolling and is thereafter annealed to make a tube, which is subsequently annealed. Thus, the ferritic stainless steel tube excellent in formability can be produced.

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 stainless steel pipe having heat resistance and workability as a steel pipe for an automobile exhaust system or the like.

[0002]

2. Description of the Related Art The exhaust gas temperature has reached 900 ° C. along with the improvement of fuel efficiency and output of automobiles, and it is said that the temperature will further increase in the future. It is also trying to improve fuel efficiency and purify exhaust gas by reducing weight and reducing heat. Against this background, automobile exhaust system materials are also required to have improved heat resistance, compactness and lightweight components, and improved moldability at the same time.

Conventionally, SUS is used for steel pipes for automobile exhaust systems.
Electric resistance welded steel pipes obtained from cold rolled materials of ferritic stainless steels such as 409D, SUS430J1L and SUS436L and austenitic stainless steels such as SUS304 are often used, and especially for ferritic stainless steels, the viewpoint of improving formability Therefore, methods such as performing hot-rolled sheet annealing in the steel sheet manufacturing process and increasing the reduction rate by cold rolling are used. For example, Japanese Patent Laid-Open No. 3-2
In Japanese Patent No. 64652, annealing is performed on the hot rolled sheet. Thus, in order to improve the workability, it is taken as an important condition to anneal with a hot rolled sheet and further increase the cold rolling reduction.

Further, in the case of a member having a complicated shape such as an exhaust manifold, after pipe forming, heat treatment is often performed for the purpose of removing the strain introduced during pipe forming and improving the workability of the steel pipe itself. In this way, when processing a member with a complicated shape with a steel pipe welded structure, a complicated and high-cost manufacturing process such as performing heat treatment after pipe making in addition to high pressure in hot rolling annealing or cold rolling In the present situation, the workability of steel pipe is secured.

On the other hand, the material for automobile exhaust system is relatively thick among the thin plates, and the plate thickness of 1.0 to 2.0 mm is often used. Considering this, it is difficult to obtain a high-pressure reduction rate in cold rolling. Further, regarding heat resistance, particularly high temperature strength, the morphology of precipitates of Ti and Nb has a great influence. Therefore, it is not desirable from the viewpoint of deposit control that the heat history is diverse and complicated.

Further, it is not necessary to strictly control surface characteristics such as surface glossiness and roughness of automobile exhaust system materials, and it suffices if the surface is such that fatigue characteristics are not adversely affected. Conceivable.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a stainless steel pipe which has heat resistance and workability as a steel pipe for automobile exhaust system and the like. INDUSTRIAL APPLICABILITY The present invention simplifies the manufacturing method that has secured the workability of the steel pipe through a number of steps up to now and reduces the cost, and the manufacturing conditions that can secure the pipe-forming property and the forming workability after the pipe-making. Is found.

[0008]

As described above, from the viewpoint of improving workability, steel pipes for automobile exhaust systems are cold-rolled sheets that have been subjected to relatively high pressure by hot-rolled sheet annealing or cold rolling in the manufacturing process. It is made by making a pipe using, and further annealing. But,
It is economically disadvantageous to anneal hot-rolled sheets, anneal under high pressure in cold rolling, or anneal after pipe making. In addition, since automobile exhaust system materials are thick among many thin plates, it is difficult to obtain a high pressure reduction rate in cold rolling, and it is desirable to simplify the thermal history as much as possible from the viewpoint of precipitation control, but the process is simplified. Makes it difficult to secure the formability of steel sheets and pipes.

On the other hand, since the automobile exhaust system material and the like are mounted on the lower portion of the vehicle body, it is not necessary to strictly control the surface glossiness, ridging and roping.
Japanese Patent Publication, Japanese Patent Publication No. 4-9852, Japanese Patent Publication No. 4-985.
It is not necessary to take into consideration the ridging resistance as in the technique described in Japanese Patent No. 3 publication. For the above reasons, the present invention aims to simplify the heat history, reduce the cost, and secure the workability by forming a hot-rolled steel sheet into a pipe and subsequently annealing it.

First, as a ferrite component system, solid solution C
In addition, Ti and N
The workability was improved by adding b alone or in combination and fixing C and N. Here, the lower limits of the amounts of addition of Ti and Nb were limited so as to be excessive with respect to the amount of C + N. Further, Ti and Nb also contribute to improvement of high temperature strength and oxidation resistance.

For high temperature strengthening, it is common to add Nb or Mo. When improving the high temperature strength and the thermal fatigue resistance, the existing state of Nb and Mo becomes a particular problem. That is, it is effective for high temperature strengthening to have Nb and Mo dissolved in solid solution in "Materials and Processes" VOL. 4 (199
1) p. 1796 etc. Therefore, in order to improve the high temperature strength and the heat fatigue resistance, in order to secure the amount of solid solution Nb and the amount of solid solution Mo in the state of the final product plate, the precipitation of these elements in each heat treatment step should be suppressed as much as possible. It is necessary to set various process conditions. For this reason, it is desirable to simplify the heat history, and it is important to secure the amount of solid solution in the product plate.

On the other hand, in the austenite component system,
Basically, it has higher workability and heat resistance than the ferrite type, but the cost of the material itself is high because Ni is added,
It is necessary to reduce costs by simplifying the manufacturing process. For the above reasons, as shown in FIG. 1, the inventors of the present invention have performed a process in which a cold-rolled annealed plate is conventionally used for annealing after pipe forming,
A method of manufacturing a stainless steel pipe that secures workability at low cost by annealing after pipe making using a hot-rolled sheet or hot-rolled annealed plate that specifies hot-rolling conditions and by specifying an annealing temperature after pipe-making. I found it.

With respect to the hot rolling conditions, the conditions were found to promote recovery in the as-hot-rolled state and to secure toughness and ductility capable of forming pipes. It is well known that Nb and Mo significantly delay the recovery. Therefore, steel containing a large amount of Nb or Mo may not be sufficiently recovered even when the finishing temperature for hot rolling is increased, and the toughness and workability may be insufficient. In such a case, hot-rolled sheet annealing is performed at (700
The toughness and workability can be secured by adding a heat treatment step of + 200 × Nb + 10 × Mo) ° C. or higher and (900 + 200 × Nb + 10 × Mo) ° C. or higher for an austenite type.

Further, in the annealing after pipe forming, in order to sufficiently recrystallize in order to secure the workability and to sufficiently dissolve the high temperature strengthening elements Nb and Mo into a solid solution, (750 + 200 × Nb + 10 × Mo) in the ferrite system. ) ° C. or higher, (950 + 200 × Nb + 10 × Mo) in austenite type
The workability of the steel pipe was secured by setting the temperature to ℃ or higher.

[0015]

[Action]

1. Ferrite-based component: Shown below in claims 2 to 4. C + N: C and N deteriorate the moldability and have a strong affinity with Ti and Nb to lower the high temperature strength. Further, since C and N are austenite formers, it is necessary to keep them low from the viewpoint of suppressing martensite formation, and the upper limit of the amount of C + N is set to 0.04%.
Further, when the effect of further improving the workability and the high temperature strength by the solid solution Nb is utilized, the amount of C + N is 0.02%.
The following is desirable.

Si: Si is a deoxidizing element, and is added in an amount of 0.05% or more in order to improve oxidation resistance. Also,
Addition of Si in excess of 2.0% lowers ductility and hardens the steel (increases yield stress), so 2.0% was made the upper limit. Mn: Mn is a deoxidizing element, so 0.1% or more was added. On the other hand, Mn deteriorates the oxidation resistance and is also an austenite former, so the upper limit was made 2.0% from the viewpoint of suppressing martensite formation.

Cr: Cr secures corrosion resistance, which is a basic performance of stainless steel, and at the same time, assures oxidation resistance capable of responding to a high exhaust gas temperature of an automobile, that is, at least 600 ° C. To achieve this, 10% or more is added. On the other hand, when the amount of Cr exceeds 25%, the effect of further improving the corrosion resistance and the oxidation resistance becomes small and the workability also deteriorates, so the upper limit was made 25%.
Further, if considering both the oxidation resistance up to 1000 ° C. and the processability, 10 to 20% is sufficient, and this range is economically desirable.

Ti: Ti fixes C and N, and is 0.02% or more and C / 12 in order to improve molding processability.
The range was + N / 14 ≦ Ti / 48 + Nb / 93. Further, Ti contributes to high temperature strengthening by causing fine precipitation during hot rolling, but addition of more than 0.5% causes coarsening of precipitates and reduces high temperature strength.
Was set as the upper limit. From the viewpoint of improving workability, 0.1%
The above Ti addition is desirable.

Nb: Nb, like Ti, fixes C and N, and is 0.05% or more in order to improve moldability and C / 12 + N / 14≤Ti / 48 + Nb / 93.
The range was set to satisfy. Further, Nb contributes to high temperature strengthening by solid solution strengthening, but addition of more than 1.0% deteriorates toughness, so 1.0% was made the upper limit. Further, Nb is preferably added in an amount of 0.2% or more from the viewpoint of improving workability and high temperature strength, and is preferably 0.6% or less from the viewpoint of improving toughness.

Mo: Mo is an element that enhances high temperature strength and high temperature salt damage resistance, but since it deteriorates ductility and weldability,
The range was 0.1 to 2.0%. Further, from the viewpoint of improving the high temperature strength and the high temperature salt damage resistance, Mo is preferably added in an amount of 0.4% or more. 2. Austenitic components; Claims 5 to 7 will be described below. C: C deteriorates not only moldability but also intergranular corrosion resistance, so the additive amount was made 0.1% or less.

N: N deteriorates the molding processability, so the addition amount was made 0.1% or less. Si: Si is a deoxidizing element, and is added in an amount of 0.05% or more in order to improve oxidation resistance and high temperature salt damage resistance.
Further, addition of Si in excess of 5.0% lowers ductility and hardens the steel (increases yield stress), so 5.0% was made the upper limit. Mn: Mn is a deoxidizing element, so 0.1% or more was added. On the other hand, Mn deteriorates the oxidation resistance, so the upper limit was made 2.0%.

Cr: Cr is added in an amount of 13% or more in order to secure the corrosion resistance which is the basic performance of stainless steel. on the other hand,
Since Cr is a ferrite former and deteriorates workability, Cr is set to 25% as the upper limit. Ni: Ni is an austenite former and is added in an amount of 7.0% or more in order to improve heat resistance, workability and toughness. On the other hand, the excessive addition of Ni is economically disadvantageous, and the hot deformation resistance is likely to increase, which may deteriorate the productivity. Therefore, the upper limit was set to 15.0%.

Ti: Ti adheres C and N, and is 0.1% or more in order to improve molding workability and intergranular corrosion resistance, and C / 12 + N / 14≤Ti / 48 + Nb / 93.
The range was set to satisfy. Further, addition of more than 0.5% Ti deteriorates toughness, so 0.5% was made the upper limit. Nb: Nb, like Ti, fixes C and N, and is 0.2% or more and C / 12 + N / 14 ≦ Ti / 48 + Nb / 93 in order to improve molding processability and intergranular corrosion resistance.
The range was set to satisfy. Further, Nb contributes to high temperature strengthening by solid solution strengthening, but addition of more than 0.6% deteriorates toughness, so 0.6% was made the upper limit.

Mo: Mo is an element that enhances high temperature strength and high temperature salt damage resistance, but since it deteriorates ductility and weldability,
The range is 0.1 to 3.0%. Next, the manufacturing conditions of the present invention specified the following conditions. Hot rolling finishing temperature: The hot rolling finishing temperature was 750 to 950 ° C. If the hot rolling finishing temperature is lower than 750 ° C, the ductility of the hot rolled sheet decreases, so 750 ° C is set as the lower limit, and if it is higher than 950 ° C, the toughness of the hot rolled sheet deteriorates, so 950 ° C is set as the upper limit.

Annealing of hot-rolled sheet: When the amount of addition of Mo, Nb, etc. is high, the toughness and ductility required in the pipe-making process may not be ensured in the as-hot-rolled state even if the finishing temperature is 750 to 950 ° C. Occurs. Therefore, (700+
The temperature is set to 200 × Nb + 10 × Mo) ° C. or higher, and the annealing process at (900 + 200 × Nb + 10 × Mo) ° C. or higher is added to the austenite type. This is to secure the toughness and workability capable of forming a pipe by further promoting the recovery.

Annealing temperature after pipe making: The annealing temperature after pipe making is
In order to ensure sufficient workability of the steel pipe after pipe forming and to sufficiently dissolve Nb and Mo, which are high-temperature strengthening elements, in the ferrite system, (750 + 200 × Nb
+ 10 × Mo) ° C or higher, (950 for austenite type)
+ 200 × Nb + 10 × Mo) ° C. or higher. Below this temperature, the bending workability of the steel pipe will be insufficient.

[0027]

[Examples] Table 1 shows the chemical composition of the test steel. The sample steel is
A steel ingot of 50 kg was cut out from an actual slab, hot-rolled under each condition, and subsequently annealed to produce a 2 mmt hot-rolled sheet, which was used as a 38 mmφ electric resistance welded tube. Table 2 shows the expansion and contact flatness (crushing) test results for each ERW pipe.
Shown in.

The steel pipes produced according to the present invention all have good working characteristics.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

According to the present invention, as a steel pipe for an automobile exhaust system or the like, a stainless steel pipe having heat resistance and workability can be manufactured at a low cost by simplifying the process.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional manufacturing method and a manufacturing method of the present invention in comparison.

Claims (7)

[Claims] 1. A method for producing a stainless steel pipe having excellent formability, which comprises hot-rolling stainless steel as it is or after annealed a hot-rolled sheet, followed by pipe forming and subsequent annealing. 2. By weight%, C + N: 0.04% or less Si: 0.05 to 2.0% Mn: 0.1 to 2.0% Cr: 10 to 25%, and Ti: 0. 0.02 to 0.5% Nb: 0.05 to 1.0% of at least one kind is contained in a range satisfying C / 12 + N / 14 ≦ Ti / 48 + Nb / 93, and the balance Fe and unavoidable impurities are contained in the steel. A method for producing a ferritic stainless steel pipe having excellent formability, which comprises performing hot rolling, annealing, pipe forming, and subsequent annealing. 3. By weight%, C + N: 0.02% or less Si: 0.05 to 2.0% Mn: 0.1 to 2.0% Cr: 10 to 20%, and Ti: 0. 1 to 0.5% Nb: 0.2 to 0.6% of at least one kind is contained in a range satisfying C / 12 + N / 14 ≦ Ti / 48 + Nb / 93, and the balance is Fe and inevitable impurities. A method for producing a ferritic stainless steel pipe having excellent formability, which comprises hot rolling, pipe forming, and subsequent annealing. 4. The method for producing a ferritic stainless steel pipe having excellent formability according to claim 2, wherein a steel containing Mo: 0.1 to 2.0% is used. 5. By weight%, C: 0.1% or less N: 0.1% or less Si: 0.05 to 5.0% Mn: 0.1 to 2.0% Cr: 13 to 25% Ni Of an austenitic stainless steel pipe having excellent formability, characterized in that the steel containing 7.0 to 15.0% and the balance of Fe and unavoidable impurities is formed into a tube after hot rolling and subsequently annealed. Production method. 6. The method for producing an austenitic stainless steel pipe having excellent formability according to claim 5, wherein a steel containing Mo: 0.1 to 3.0% is used. 7. At least one of Ti: 0.1 to 0.5% Nb: 0.2 to 0.6% is contained in a range satisfying C / 12 + N / 14 ≦ Ti / 48 + Nb / 93. Claim 5 characterized by the above-mentioned.
Alternatively, the method for producing an austenitic stainless steel pipe having excellent moldability according to Item 6.