JPH0734124A - Production of steel and steel tube excellent in high temperature property - Google Patents

Abstract

PURPOSE:To produce a steel sheet and steel sheet for a steel tube excellent in high temp. properties by subjecting a high Cr slab having a specified compsn. to hot rolling and cooling under specified temp. conditions, thereafter subjecting it to cold rolling into a cold rolled sheet and executing recrystallization heat treatment. CONSTITUTION:A Cr-contg. slab having a compsn. contg., by weight, 0.3 to 3.0% Si, 0.02 to 2.0%. Mn, 3.5 to 9.9% Cr, 0.01 to 1.0% Nb, 0.01 to l.0% Al, respectively <=0.03% C, P and S and <0.02% N or furthermore contg. V, Ti, Zr, Ta, Hf, Mo, W, rare earth elements and Ca and in which OR value given by [%Cr]+4[%Si]--17[%C]-3[%N] is regulated to >=6.5 is heated to 1150 to 1300 deg.C and is subjected to hot rolling in which the finishing temp. is regulated to >=780 deg.C into a hot rolled steel strip. This is subjected to cold rolling at >40% cumulative draft, and the obtd. cold rolled steel strip is held to >=780 deg.C for >=0.5min and is subjected to recrystallization heat treatment into a cold rolled steel strip excellent in high temp. oxidation resistance and strength, which is formed to form its shape into a cylindrical one, and both edge parts are welded to produce a steel tube excellent in high temp. properties.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing steel and steel pipe having excellent high temperature characteristics, and more specifically, for an exhaust manifold attached to an automobile engine, an exhaust gas purifying catalyst holding substrate, an automobile exhaust system. The present invention relates to a method for producing low cost steel or steel pipe having excellent properties such as oxidation resistance, strength and workability as the steel pipe of the front part of the above.

[0002]

2. Description of the Related Art In recent years, there is an increasing number of cases in which an exhaust manifold (hereinafter referred to as "exhaust manifold") for automobiles is replaced with a conventional cast iron exhaust manifold by using a stainless steel exhaust manifold. This is because the exhaust manifold made of stainless steel can make the wall thickness thinner, which is very useful for reducing the weight of the automobile. JIS (Japanese Industrial Standard) SU
Austenitic stainless steels such as S310 have excellent heat resistance and workability and are therefore widely used for general heat resistance applications. However, this type of steel is generally expensive, and has a large thermal expansion coefficient, and thus has a drawback of being inferior in thermal fatigue characteristics.

On the other hand, ferritic stainless steels generally have a smaller coefficient of thermal expansion than austenitic stainless steels and are therefore excellent in thermal fatigue properties, so that they are suitable for applications such as exhaust manifolds that undergo a temperature cycle of heating and cooling. I can say. Therefore, S specified in JIS standard G4305
US410L, SUS430LX and the like have already been put to practical use as exhaust manifolds and the like. Further, for example, JP-A-64-8
No. 254 and Japanese Patent Laid-Open No. 2-175843, Cr
A ferritic stainless steel containing a large amount of Al and having improved high temperature strength has been proposed. However, these ferritic stainless steels are very expensive, and when used in exhaust manifolds, the cost is significantly higher than in cast iron exhaust manifolds, and the cost for exhaust gas purification catalysts is also high. Moreover, it cannot be said that the workability is good because it contains a large amount of Cr, and in order to form a complicated shape such as an exhaust manifold or an exhaust gas purification catalyst, the manufacturing process becomes complicated and the processing cost is high. There is a drawback that

[0004]

SUMMARY OF THE INVENTION In view of the above situation, the present invention has high strength at high temperatures such as an automobile exhaust system,
And it aims at providing the method of manufacturing the steel and steel pipe which are excellent in oxidation resistance at low cost.

[0005]

In order to achieve the above object, the present inventors have developed a steel having excellent oxidation resistance and high temperature strength in a high temperature environment including an exhaust system and a method for producing the steel. , Have been examined from various viewpoints. First,
As a result of various studies to achieve the above object, the present inventors have found that Cr is 3.5 to 9.9, which is contrary to the conventional stainless steel.
%, It is possible to greatly improve the high temperature strength and the oxidation resistance at high temperatures by adding Si and Nb in combination to a steel whose content is relatively small, and to strictly control the range of the amount of alloying elements so as to satisfy specific conditions. It has been found that the oxidation resistance at high temperature and the high temperature strength are further improved. Further, the inventors of the present invention further improved the workability by adding V, Ti, Zr, Ta and Hf to such steel, and further improved the high temperature strength by adding Mo and W, while adding the rare earth element or Ca. Then, it has been found that the oxidation resistance at high temperature is also improved. Further, the inventors of the present invention continued to study a method for producing such steel and steel pipe at low cost, and as a result, high C
It has been found that a steel or a steel pipe suitable for such an object can be obtained by precisely selecting the process conditions when the r steel is made into a strip steel from a steel strip and finally subjected to recrystallization annealing or further pipe making.

The present invention has been made on the basis of such findings, and the gist of the first invention is that it is% by weight.
Si: 0.3-3.0%, Mn: 0.02-2.0
%, Cr: 3.5 to 9.9%, Nb: 0.01 to
1.0%, Al: 0.01 to 1.0% contained, and C 0.
03% or less, P 0.03% or less, S 0.0
3% or less, N reduced to 0.02% or less, and an OR value consisting of the balance Fe and unavoidable impurities and given by the following equation: OR value = [% Cr] +4 [% Si] -17 [% C]- Three
A high Cr steel slab having a [% N] of 6.5 or more is sequentially formed into a steel sheet in the following steps, which is a method for producing a steel having excellent high temperature characteristics. After heating the billet to a temperature of 1150-1300 ° C.,
When the rolling completion temperature is 780 ° C or more, the plate thickness is 1.6 mm or more and 6.0
Hot-rolled into hot strips up to mm, at least 6
A step of cooling at an average cooling rate of 10 ° C./sec or more until it is cooled to 00 ° C. or lower and winding it as a strip steel. The above hot-rolled strip steel is subjected to cold rolling with a cumulative rolling reduction of 40% or more. And cold-rolled steel strip, the above cold-rolled steel strip at a temperature of 780 ° C or higher for 0.5 min
The step of recrystallizing by performing the above heat treatment.

The gist of the second invention is that, in the first invention, steel is recrystallized by subjecting it to a heat treatment at a temperature of 760 ° C. or higher for 0.5 min or more before cold rolling the hot-rolled steel. In the manufacturing method. The gist of the third invention is that, in the first invention or the second invention, a high Cr steel slab is further added as an additional component in a weight percentage of V, Ti, Zr, T.
It is a method for producing a steel containing at least one of a and Hf and having a total content of 0.01% or more and less than 1.0%. The gist of the fourth invention is that in the first invention, the second invention or the third invention, a high Cr steel slab is further added as an additional component in a weight percentage of 0 or 1 or 2 in total of Mo and W. It is a method for producing steel containing 0.03% or more and less than 0.5%. A fifth aspect of the invention is that in the first, second, third, or fourth invention, high Cr steel slab is further added as an additional component by weight%.
And rare earth element: 0.005-0.1%, Ca: 0.0
It is a method for producing a steel containing one or two of 01 to 0.03%.

Further, the gist of the sixth invention of the present invention is, in weight%, Si: 0.3 to 3.0%, Mn:
0.02-2.0%, Cr: 3.5-9.9%, N
b: 0.01 to 1.0%, Al: 0.01 to 1.0% are contained, C is 0.03% or less, P is 0.03% or less, S is 0.03% or less, and N is 0.02% or less, consisting of the balance Fe and unavoidable impurities,
OR value given by the following formula OR value = [% Cr] +4 [% Si] -17 [% C] -3
A high Cr steel slab having a [% N] of 7.5 or more is sequentially formed into a steel pipe in the following steps, which is a method for producing a steel pipe having excellent high temperature characteristics. After heating the billet to a temperature of 1150-1300 ° C.,
When the rolling completion temperature is 720 ° C or more, the plate thickness is 1.6 mm or more and 6.0
Hot-rolled into hot strips up to mm, at least 6
A step of cooling at an average cooling rate of 10 ° C./sec or more until it is cooled to 00 ° C. or lower and winding it as a strip steel. The above hot-rolled strip steel is subjected to cold rolling with a cumulative rolling reduction of 40% or more. And cold-rolled steel strip, the above cold-rolled steel strip at a temperature of 780 ° C or higher for 0.5 min
A step of recrystallizing by performing a heat treatment for holding the above, after cutting the above strip steel into a predetermined width, while continuously forming a cylindrical shape by roll forming, welding both ends of the steel strip to form a steel pipe Process.

Further, the gist of the seventh invention is that the total strain amount ε added to the steel from the stage of the strip steel recrystallized by the process of the sixth invention to the time when it is manufactured as a steel pipe, Strip steel plate thickness t (mm), final outer diameter of steel pipe D (mm)
On the other hand, in the steel pipe manufacturing method, ε ≦ t / D + 0.03. The gist of the eighth invention is that in the sixth invention or the seventh invention, before cold rolling the hot-rolled steel,
It is a method for manufacturing a steel pipe in which a heat treatment of holding at a temperature of 760 ° C. or higher for 0.5 min or longer is performed to recrystallize. The gist of the ninth invention is that it is the sixth invention, the seventh invention or the eighth invention.
In the present invention, there is a method for producing a steel pipe which is produced as a steel pipe and is recrystallized by subjecting a steel pipe having a final outer diameter to a heat treatment at a temperature of 780 ° C. or higher for 1 min or longer. The gist of the tenth invention is that in the sixth invention, the seventh invention, the eighth invention or the ninth invention, a high Cr steel piece is further added as an additional component in a weight percentage of V, Ti, Zr, Ta, Hf. Of 1
A method for producing a steel pipe, which comprises one kind or two or more kinds, and whose total content is 0.01% or more and less than 1.0%.
The gist of the eleventh invention is that in the sixth invention, the seventh invention, the eighth invention, the ninth invention or the tenth invention,
A high Cr steel billet is added as an additional component in a weight percentage of Mo,
0.03% or more as a total of one or two types of W 0.5
In the method for producing a steel pipe containing less than%. The gist of the twelfth invention is that in the sixth invention, the seventh invention, the eighth invention, the ninth invention, the tenth invention or the eleventh invention, a high C
r Steel slab is an additional component, and further by weight%, rare earth element: 0.005 to 0.1%, Ca: 0.001 to 0.0
It is a method for producing a steel pipe containing 1% or 2% of 3%.

Further, the gist of the thirteenth invention is that the sixth invention, the seventh invention, the eighth invention, the ninth invention, and the tenth invention.
In the invention, the eleventh invention or the twelfth invention, there is a method for manufacturing a steel pipe in which both ends of a steel strip are welded by electric resistance welding to produce an electric resistance welded steel pipe, and a gist of the fourteenth invention is the sixth invention, In a seventh invention, an eighth invention, a ninth invention, a tenth invention, an eleventh invention or a twelfth invention, there is a method for manufacturing a steel pipe in which both ends of a steel strip are welded by laser welding to produce a steel pipe, and a fifteenth invention. Is the sixth invention, the seventh invention, the eighth invention, the ninth invention, the first invention.
In the 0th invention, the 11th invention, or the 12th invention, there is a method for manufacturing a steel pipe, wherein the means for welding both ends of the steel strip is combined welding of electric resistance welding and laser welding.

[0011]

First, the reason why the range of each component is limited in the high Cr steel targeted by the method of the present invention will be described. Si: When Si is added in combination with steel containing 3.5% or more of Cr, the effect of improving oxidation resistance at high temperatures is remarkable,
It is also an element necessary to secure a ferrite structure, and if its content is less than 0.3%, its effect is not sufficient, and if it exceeds 3.0%, toughness and workability at room temperature deteriorate. Therefore, the content range is limited to 0.3 to 3.0%. Mn: Mn is necessary as a deoxidizing agent for steel and it is necessary to contain 0.02% or more, but even if it is contained in an amount of more than 2.0%, the effect is not saturated but Mn is excessively added.
If it is contained, it becomes difficult to secure a ferrite single phase structure, so the upper limit content is made 2.0%.

Cr: Cr must be added in combination with Si to contain 3.5% or more in order to secure the corrosion resistance and oxidation resistance required for the exhaust manifold, but 9.9.
If it is contained in excess of%, not only the cost is unnecessarily increased, but also the workability is deteriorated, so the upper limit content is made 9.9%. Nb: Nb is an element useful for stabilizing the ferrite structure, and is added to a steel containing 3.5% or more of Cr in order to secure the high temperature strength required as a steel for automobile exhaust system. It must contain more than 01%,
Even if added in excess of 1.0%, the effect will no longer be saturated, but the cost will be unnecessarily increased and coarse precipitates will be formed to reduce the workability, so the upper limit content is 1. 0% Al: Al needs to be added as 0.01% or more as a deoxidizing agent for steel, and is a useful element for securing a ferrite structure, but if added in excess of 1.0%, coarse oxidation will occur. The content is limited to 0.01 to 1.0% because it forms a material-based inclusion and reduces workability.

C, N: C and N form a compound with Nb to precipitate Nb and lower the high temperature strength, and C also forms a carbide with Cr to lower the oxidation resistance. Therefore, it is desirable that the amounts of C and N are small, and the upper limit contents are 0.03% for C and 0.02% for N. P: If P is present in a large amount, it lowers the toughness, so it is preferable that the amount be small, and the upper limit content is made 0.03%. S: If S is also present in a large amount, the hot workability and ductility are deteriorated, so it is preferable that the S content is small, and the upper limit content is 0.03.
%.

Further, in the present invention, it is necessary that the OR value defined by the following formula is 6.5 or more as a combination of the contents of the respective elements. OR value = [% Cr] +4 [% Si] -17 [% C] -3
[% N] This is a finding based on the results of the present inventors' detailed examination of the optimum component composition and combination for obtaining the characteristics as the steel for automobile exhaust system, and the OR value is 6.5.
If the temperature is less than 100%, the oxidation resistance at high temperature, which is the most important property for exhaust system steel, is insufficient. For example, assuming an operating temperature in an automobile exhaust system of 800 ° C or higher,
If the OR value is less than 6.5, abnormal oxidation occurs and the product cannot be used. To withstand use as an automobile exhaust system, O
It is necessary to secure an R value of 6.5 or more. Here, according to the study by the present inventors, in a steel containing Cr in an amount of 3.5% or more, Si has an effect of four times that of Cr with respect to oxidation resistance, and C and N are both It is based on the finding that it forms a precipitate with Cr and impairs the oxidation resistance.

The above is the basic composition and conditions of the steel for automobile exhaust system which is the subject of the present invention. In the present invention, however, the steel is further improved in the properties by further adding the following elements as required. Intended. Needless to say, the above-mentioned condition that the OR value is 6.5 or more must be satisfied even when the following components are added.
V, Ti, Zr, Ta, Hf: V, Ti, Zr, Ta,
Hf is effective in increasing workability when it is added in combination with steel containing 3.5% or more Cr and 0.01% or more Nb. If the total content of one or more of V, Ti, Zr, Ta and Hf is less than 0.01%, the effect is not sufficiently exhibited. On the other hand, even if added in excess of 1.0%, the effect will no longer be saturated, but it will only unnecessarily increase the cost, and coarse precipitates may be formed to reduce toughness. So V, Ti, Z
The total content of one or more of r, Ta, and Hf is 0.01% or more and 1.0% or less.

Mo, W: Mo and W contain 3.5% of Cr
When it is added to the steel containing the above and containing 0.01% or more of Nb, it is effective in further improving the high temperature strength.
The total content of one or two of Mo and W is 0.0
If it is less than 3%, the effect is not sufficiently exhibited. On the other hand, 0.
Even if 5% or more is added, the effect is no longer saturated, whereas it only unnecessarily increases the cost.
The total content of one or two of o and W is 0.03% or more and less than 0.5%.

Rare earth elements (REM), Ca: Rare earth elements and Ca are elements effective for improving hot workability and oxidation resistance at high temperatures, but rare earth elements are 0.00
If 1% or more and 0.0005% or more of Ca are not added, the effect of addition is not sufficient, while if it exceeds 0.1% for rare earth elements and exceeds 0.03% for Ca, coarse addition occurs. Since such non-metallic inclusions are generated to conversely deteriorate the hot workability and the oxidation resistance, the upper limit contents are 0.1% for rare earth elements and 0.03% for Ca. In the present invention, the rare earth element has an atomic number of 57 to 7.
It refers to elements Nos. 1 and 89 to 103 and Y.

In the steel of the present invention, in addition to the above-mentioned components, Ni, Cu, B and other components are contained as impurities mixed from scraps or raw materials or for the purpose of adjusting toughness, workability and the like. However, the addition of Ni, Cu, B, etc. does not depart from the scope of the present invention.

Next, the process of the present invention and the reason for limiting the process will be described. Billet heating temperature: Uniformly heat the billet to its center,
It is necessary to secure hot workability in hot rolling. However, heating above 1300 ° C. is not preferable because the slab will be burned down and the material loss due to the formation of oxide scale will be significant and the yield will be reduced. on the other hand,
When the heating temperature is lower than 1150 ° C., the deformation resistance in hot rolling becomes too large, and in addition, the solid solution of Nb is insufficient, so that the high temperature characteristics cannot be secured. Therefore, the billet heating temperature is set to 1150 to 1300 ° C.

Hot rolling: For hot rolling, a conventional plate rolling process can be used. In order to satisfy the properties as a plate while ensuring the productivity in the subsequent cold rolling, it is appropriate that the plate thickness after hot rolling is 1.6 mm or more and 6.0 mm or less. Further, from the viewpoint of productivity in the subsequent cold rolling, the shape of the plate is a hot coil. Rolling completion temperature: The end temperature of hot rolling is 780 ° C. for the purpose of ensuring ductility after hot rolling and suppressing precipitation of Nb.
It is necessary to be above.

Cooling rate after hot rolling: If the cooling rate after hot rolling is low, the workability of the hot-rolled sheet deteriorates even if Nb or the like precipitates during cooling. , The average cooling rate is 10 ℃ / at least below 600 ℃
Must cool in more than a second. After the temperature of the steel sheet becomes 600 ° C. or lower, there is no problem even if the cooling rate is made smaller.

Cold rolling: When manufacturing parts and the like using the steel sheet according to the method of the present invention, in order to ensure good workability, it is necessary to set the cumulative rolling reduction to 40% or more in cold rolling. is there. If this cumulative reduction ratio is satisfied, a normal rolling process can be applied as cold rolling, and it is possible to select the plate thickness before cold rolling according to the plate thickness of the final product.

Heat treatment: In order to secure good workability when manufacturing parts and the like using the steel sheet according to the method of the present invention, in order to ensure good workability, when heat treating the steel sheet after cold rolling, 78
It is necessary to hold at a temperature of 0 ° C. or higher for 0.5 min or longer. This is because when the holding temperature is lower than 780 ° C., it takes a very long time to recrystallize the steel targeted by the present invention. However, if the holding time is less than 0.5 min, the required heating temperature will be too high.
This is because the steel sheet is oxidized and the surface quality is deteriorated. As long as the above conditions are satisfied, either a continuous heat treatment equipment for steel sheets or a batch (box) heat treatment equipment may be used as the heat treatment.

Forming and Welding to Steel Pipe: A normal steel pipe manufacturing process can be applied to forming and welding to a steel pipe, and a steel strip is cut into a predetermined width according to the outer diameter required for a steel pipe for automobile exhaust system. From this, it can be roll-formed and welded to produce a steel pipe. As the welding, ordinary electric resistance welding, ordinary laser welding, or combined welding of electric resistance welding and laser welding can be used.

In the present invention, in addition to the above steps, additional steps may be added or the conditions thereof are limited, if necessary, for the following reasons. Pre-cold rolling heat treatment: According to the study by the present inventors, for example, when particularly high workability is required for an automobile exhaust system, the hot-rolled steel strip should be recrystallized before cold rolling. Is more effective. For this purpose, the holding temperature must be 760 ° C or higher, and the holding time is 0.5 min.
It is necessary to be above. As long as the above conditions are satisfied, either a continuous heat treatment equipment for steel sheets or a batch (box) heat treatment equipment may be used as the heat treatment.

Total strain amount: When particularly high workability is required for bending, expanding, etc., as a steel pipe, from the stage of re-crystallized strip steel in the heat treatment process after cold rolling, a pipe is formed as a steel pipe. It is effective to reduce the total strain amount ε added to the steel as much as possible. High Cr for the method of the present invention
For the plate thickness t (mm) of steel and the final outer diameter D (mm) of the steel pipe,
If the steel is manufactured so that ε is t / D + 0.03 or less, a steel pipe having particularly excellent workability can be obtained.

Post pipe heat treatment: When workability as an electric resistance welded steel pipe is extremely severely required, pipe forming as a steel pipe and annealing of a steel pipe having a final outer diameter are performed until a steel pipe having a final outer diameter is obtained. It is effective to remove the introduced strain, and a holding temperature of 780 ° C or higher is necessary for this purpose.
Hold time must be 1 min or more.

In the manufacturing process of the steel sheet or steel pipe produced by the method of the present invention, descaling treatment or pickling treatment of the steel sheet or steel pipe is carried out for the purpose of obtaining the surface texture of the cold-rolled sheet and the predetermined characteristics of the steel pipe. The application does not deviate from the essence of the method of the present invention, and all are included in the scope of the present invention, and if necessary, a normal descaling step or a pickling step may be performed. Can be added.

The steel produced according to the method proposed in the present invention may be used, for example, as an exhaust manifold.
After being manufactured as a steel plate, the steel plate may be press-molded and further processed / welded to be manufactured as an exhaust manifold, or after the steel plate is formed into a steel pipe shape, for example, as an electric resistance welded steel pipe,
Any steel that may be processed into an exhaust manifold by secondary processing, welding, etc. and that has a composition and a combination of elements that are limited in the present invention, including other processes, is a target of the method of the present invention, and is cost-effective. It is possible to select the optimal exhaust manifold manufacturing process due to restrictions on existing manufacturing equipment, etc., and whichever exhaust manifold manufacturing process is selected,
It does not depart from the scope of the invention. When it is used as an exhaust gas purification catalyst supporting substrate, it is generally used as a steel plate, and when it is used as a front pipe, it is generally used as a steel pipe. Therefore, the object of the steel or the steel pipe targeted by the method of the present invention is, of course, not limited to these product shapes.

[0030]

EXAMPLES Examples of the present invention will be described below. (Example 1) Steels having the components shown in Table 1 were melted and hot-rolled into a strip steel having a thickness of 4.0 mm, and then cold-rolled and heat-treated under the conditions shown in Table 2 to obtain steel strips. Was manufactured. In the hot rolling, the billet heating temperature was 1250 ° C. Inventive Example No. In Nos. 6 and 12, heat treatment was performed under the conditions shown in Table 2 before cold rolling. Next, test pieces were taken from these steel plates and were tested in accordance with JIS G0567 to obtain 80
A high temperature tensile test at 0 ° C and a continuous high temperature oxidation test at 800 ° C were carried out. Further, a thermal fatigue test in which the maximum heating temperature was 800 ° C was also carried out.

The test results are also shown in Table 2. In the high temperature tensile test results of Table 2, ⊚ is 0.2% at 800 ° C.
It shows that the proof stress was 22 N / mm ² or more, and ○ means 17 N / mm ^2.
mm ² or more and less than 22 N / mm ² , and x is 17 N
It was less than / mm ² . In addition, in the high temperature oxidation test result, ○ indicates that the increase in oxidation at 800 ° C is 0.1 g /
was less than or equal to m ² / hr, and x indicates that the increase in oxidation amount is 0.
It was shown to be larger than 1 g / m ² / hr. Furthermore, in the thermal fatigue test results, ◯ indicates that the number of repetitions up to fracture was 500 or more, and x indicates that the number of repetitions was less than 500.

As is clear from Table 2, the steel of the present invention.
No. Nos. 1 to 12 are excellent in high temperature strength, oxidation resistance and thermal fatigue resistance, while No. 13-14
No cracks were generated during cold rolling and it could not be made into a product. It is understood that 15 to 18 are inferior in high temperature strength, high temperature oxidation resistance or heat fatigue resistance.

[0033]

[Table 1]

[0034]

[Table 2]

(Example 2) Steels having the components shown in Table 3 were melted, and hot-rolled under the conditions shown in Table 4 to obtain strip steels having a thickness of 4.0 mm, which were then cooled under the conditions shown in Table 4. After being subjected to hot rolling and heat treatment, it was formed into a steel pipe having the dimensions shown in Table 4 by forming and welding. All pipe welding was ERW welding. In the hot rolling, the billet heating temperature is 12
The temperature was 40 ° C. Inventive Example No. In No. 10, heat treatment was performed before cold rolling under the conditions shown in Table 4, and No. 10 was used. In No. 9, the steel pipe after pipe making was heat-treated under the conditions shown in Table 4. Next, test pieces were taken from these steel sheets, and a high temperature tensile test at 800 ° C. and a continuous high temperature oxidation test at 800 ° C. were carried out in accordance with JIS standard G0567. Also, the maximum heating temperature is 800
A thermal fatigue test at ℃ was also performed.

The test results are also shown in Table 4. In the high temperature tensile test of Table 4, ⊚ indicates that the 0.2% proof stress at 800 ° C was 22 N / mm ² or more, and ∘ indicates 17 N / mm.
² or more and less than 22 N / mm ² , where x is 17 N /
It was less than mm ² . In addition, in the high temperature oxidation test result, ○ indicates that the increase in oxidation at 800 ° C is 0.1 g / m 2.
² / hr or less, and x indicates an increase in oxidation of 0.1
It is shown that it was larger than g / m ² / hr. further,
In the thermal fatigue test results, ∘ indicates that the number of repetitions up to fracture was 500 or more, and x indicates that the number of repetitions was less than 500.

As is clear from Table 4, the steel of the present invention.
No. Nos. 1 to 12 are excellent in high temperature strength, oxidation resistance and thermal fatigue resistance, while No. 13-14
Was unable to manufacture a steel pipe due to cracking during cold rolling. It is understood that 15 to 18 are inferior in high temperature strength, high temperature oxidation resistance or heat fatigue resistance.

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

Industrial Applicability As described above, the present invention provides a method for producing a steel and a steel pipe having excellent high temperature characteristics, and is extremely large in contributing to the development of industry.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location C22C 38/00 302 Z 38/12 38/18

Claims (15)

[Claims] 1. By weight%, Si: 0.3 to 3.0%, Mn: 0.02 to 2.0%, Cr: 3.5 to 9.9%, Nb: 0.01 to 1.%. 0%, Al: 0.01 to 1.0%, C is 0.03% or less, P is 0.03% or less, S is 0.03% or less, and N is 0.02% or less. However, the OR value consisting of the balance Fe and unavoidable impurities and given by the following equation: OR value = [% Cr] +4 [% Si] -17 [% C] -3
A method for producing a steel having excellent high-temperature properties, characterized in that high Cr steel billets having a [% N] of 6.5 or more are sequentially formed into steel sheets in the following steps. After heating the billet to a temperature of 1150-1300 ° C.,
When the rolling completion temperature is 780 ° C or more, the plate thickness is 1.6 mm or more and 6.0
Hot-rolled into hot strips up to mm, at least 6
A step of cooling at an average cooling rate of 10 ° C./sec or more until it is cooled to 00 ° C. or lower and winding it as a strip steel. The above hot-rolled strip steel is subjected to cold rolling with a cumulative rolling reduction of 40% or more. And cold-rolled steel strip, the above cold-rolled steel strip at a temperature of 780 ° C or higher for 0.5 min
The step of recrystallizing by performing the above heat treatment. 2. The excellent high temperature property according to claim 1, characterized in that, before cold rolling the hot rolled steel strip, it is recrystallized by performing a heat treatment of holding it at a temperature of 760 ° C. or higher for 0.5 min or longer. Steel manufacturing method. 3. The high Cr steel slab as an additional component according to claim 1, further comprising 0.01% or more by weight of one or more of V, Ti, Zr, Ta and Hf. A method for producing steel having excellent high-temperature characteristics, characterized by containing less than 1.0%. 4. The high Cr according to claim 1, 2, or 3.
A method for producing a steel having excellent high-temperature characteristics, characterized in that a steel slab further contains, as an additional component, by weight, one or two kinds of Mo and W in a total amount of 0.03% or more and less than 0.5%. . 5. The high Cr steel slab as an additional component according to claim 1, 2, 3 or 4, further comprising: by weight, rare earth elements: 0.005 to 0.1%, Ca: 0.001 to 0. 03% of 1 type or 2 types is contained, The manufacturing method of the steel excellent in the high temperature characteristic characterized by the above-mentioned. 6. By weight%, Si: 0.3 to 3.0%, Mn: 0.02 to 2.0%, Cr: 3.5 to 9.9%, Nb: 0.01 to 1.%. 0%, Al: 0.01 to 1.0%, C is 0.03% or less, P is 0.03% or less, S is 0.03% or less, and N is 0.02% or less. However, the OR value consisting of the balance Fe and unavoidable impurities and given by the following equation: OR value = [% Cr] +4 [% Si] -17 [% C] -3
A method for producing a steel pipe having excellent high-temperature characteristics, characterized in that high Cr steel slabs having a [% N] of 6.5 or more are sequentially formed into steel pipes in the following steps. After heating the billet to a temperature of 1150-1300 ° C.,
When the rolling completion temperature is 780 ° C or more, the plate thickness is 1.6 mm or more and 6.0
Hot-rolled into hot strips up to mm, at least 6
A step of cooling at an average cooling rate of 10 ° C./sec or more until it is cooled to 00 ° C. or lower and winding it as a strip steel. The above hot-rolled strip steel is subjected to cold rolling with a cumulative rolling reduction of 40% or more. And cold-rolled steel strip, the above cold-rolled steel strip at a temperature of 780 ° C or higher for 0.5 min
A step of recrystallizing by performing a heat treatment for holding the above, after cutting the above strip steel into a predetermined width, while continuously forming a cylindrical shape by roll forming, welding both ends of the steel strip to form a steel pipe Process. 7. The total strain amount ε added to the steel from the stage of the steel strip recrystallized by the process of to the step of forming a steel pipe according to claim 6, is defined by the plate thickness t (mm) of the steel strip. A method for producing a steel pipe having excellent high temperature characteristics, characterized in that ε ≦ t / D + 0.03 with respect to the final outer diameter D (mm) of the steel pipe. 8. The high temperature characteristic according to claim 6 or 7, characterized in that, before cold rolling the hot rolled steel strip, it is recrystallized by being heat-treated at a temperature of 760 ° C. or higher for 0.5 min or longer. Excellent steel pipe manufacturing method. 9. The high temperature process according to claim 6, 7 or 8, wherein a steel pipe formed as a steel pipe and having a final outer diameter is subjected to a heat treatment of holding it at a temperature of 780 ° C. or higher for 1 min or longer to recrystallize it. A method of manufacturing steel pipes with excellent characteristics. 10. The method according to claim 6, 7, 8 or 9,
It is characterized in that the high Cr steel slab further contains 0.01% or more and less than 1.0% in total of one or two or more of V, Ti, Zr, Ta, and Hf as an additional component by weight. A method for manufacturing a steel pipe having excellent high temperature characteristics. 11. The high Cr steel billet as an additional component according to claim 6, 7, 8, 9 or 10, wherein the total content of one or two of Mo and W is 0.03% or more 0. A method for producing a steel pipe having excellent high temperature characteristics, characterized by containing less than 0.5%. 12. The method according to claim 6, 7, 8, 9, 10 or 1.
1, the high-Cr steel slab further contains, as an additional component, one or two kinds by weight%, rare earth element: 0.005 to 0.1%, Ca: 0.001 to 0.03%. And a method for producing a steel pipe having excellent high temperature characteristics. 13. The excellent high temperature characteristics according to claim 6, wherein both ends of the steel strip are welded by electric resistance welding to form an electric resistance welded steel pipe. Steel pipe manufacturing method. 14. A steel pipe is produced by welding both ends of a steel strip by laser welding.
A method for manufacturing a steel pipe having excellent high temperature characteristics according to 7, 8, 9, 10, 11 or 12. 15. The high temperature characteristic of claim 6, 7, 8, 9, 10, 11 or 12, wherein the means for welding both ends of the steel strip is a composite welding of electric resistance welding and laser welding. Excellent steel pipe manufacturing method.