JP3232857B2 - Manufacturing method of stainless steel seamless pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a stainless steel seamless steel pipe,
In particular, the present invention relates to a method for manufacturing a ferritic stainless steel seamless steel pipe, a duplex stainless steel seamless steel pipe, and an austenitic stainless steel seamless steel pipe by a Mannesmann-mandrel mill method.

[0002]

2. Description of the Related Art A seamless steel pipe is generally subjected to a Mannermann rolling method in which piercer piercing and rolling is followed by elongation rolling by a mandrel mill method, a plug mill method, an Assel mill method, or a hot extrusion method such as a Eugene Sejournel method or an Erhard push bench method. The mandrel mill rolling method, which is excellent in dimensional accuracy and productivity, is widely used for pipes having relatively small diameters.

In the mandrel mill method, for example, as shown in FIG. 1 to be described later, a material billet 1 is heated to a predetermined temperature (usually 1100 ° C. to 1300 ° C.) by a rotary hearth type heating furnace 2 and then perforated by a mannes man piercer 3. After rolling to produce the hollow shell 4A, it is stretched and rolled by the mandrel mill 5. In the mandrel mill 5 which is an elongation rolling machine, a lubricant for hot rolling is applied to the surface of the mandrel bar 6 and the mandrel bar 6 is inserted into the hollow shell 4A, and the mandrel bar 6 and the grooved roll 7 are inserted. And elongation rolling.
The rolling temperature in the mandrel mill is in the range of 1050 ° C to 1200 ° C on the inlet side of the roll, and 800 ° C to 10 ° C on the outlet side of the rolling.
It is generally in the range of 00 ° C. The hollow shell subjected to wall thinning and elongation rolling by a mandrel mill is usually referred to as a finish rolling base tube 4B.

[0004] The finishing tube 4B is reheated to a predetermined temperature (850 ° C to 1100 ° C) by a reheating furnace 11, and then the outer diameter of the tube is reduced to a predetermined size by a finishing rolling machine such as a stretch reducer 12. It is diametered and becomes the finish pipe 13. After that, although not shown in FIG. 1, if necessary, the product is subjected to a heat treatment for solution treatment, an inner surface grinding for removing a carburized portion on the inner surface of the pipe, or an acid washing for removing scale, and the like. Become.

[0005] In elongation rolling using a mandrel mill, wear and seizure of the mandrel bar 6 may occur as the rolling load increases, or scratches due to seizure may occur on the inner surface of the hollow shell 4A to be rolled. is there. For this reason, it is necessary to apply a lubricant for hot rolling excellent in hot lubricity to the surface of the mandrel bar and perform mandrel mill rolling. As such a lubricant, a lubricant containing graphite as a main component is widely used because it is inexpensive and exhibits excellent lubrication performance.

[0006]

[Problems to be Solved by the Invention] Ferrite (especially 13
When a mandrel bar coated with a lubricant containing graphite as a main component is inserted into a mandrel mill using a relatively low-carbon material such as a Cr-based, two-phase, or austenitic stainless steel, and mandrel mill rolling is performed. Then, graphite is pressed against the inner surface of the tube, and when the raw tube for finish rolling is reheated thereafter, it reacts with oxygen in the atmosphere in the furnace to cause carburization on the inner surface of the raw tube for finish rolling. As described above, the corrosion resistance of a stainless steel pipe in which carburization has occurred on the inner surface of the pipe is significantly deteriorated.

[0007] In order to prevent the occurrence of carburization on the inner surface and ensure the corrosion resistance of the steel pipe, an austenitic stainless steel pipe which is subjected to finish rolling after removing about 20 to 50 µm of the thickness of the inner surface of the finish rolling pipe immediately after the mandrel mill rolling. (See Japanese Patent Application Laid-Open No. 4-111907), this method requires a new step of polishing or cutting in order to remove the carburized portion on the inner surface of the raw tube, which significantly increases the cost.

Further, it has been disclosed in Japanese Unexamined Patent Publication No. H10 (1999) that the use of austenitic stainless steel which has been improved in carburization resistance by adding Sb and Sn to the steel and which is used as a material, ensures the corrosion resistance.
No. 63-274741. However, in this method, in order to provide complete carburization resistance, Sb, Sn
It is necessary to add a large amount of steel to steel, and as a result, the hot workability of the raw material may be reduced, and rolling defects may occur frequently.

[0009] Further, in Japanese Patent Application Laid-Open No. 4-294803, when a mandrel bar coated with a lubricant containing graphite is inserted into a base tube and subjected to mandrel mill rolling, a gap between the mandrel bar and the rolled base tube is reduced by CO _2. A rolling method for a mandrel mill has been proposed in which rolling is performed in a state of being filled with oxidizing gas, which is a mixture of O ₂ , O ₂ , steam, or a mixture thereof. However, the amount of graphite that reacts with the oxidizing gas and burns during this rolling is extremely small, and the rolled raw tube is sent to the next step with most of the graphite adhered to the inner surface of the tube. Carburization is caused by the action of oxygen in the atmosphere. Also, due to the reaction with the oxidizing gas used in the mandrel mill rolling, oxygen in the gas and C in the lubricant react to generate CO gas, which inevitably causes carburization. Become. Therefore, the proposed rolling method cannot be a measure for preventing carburization of the rolled tube.

In view of the above-mentioned problems of the prior art, the present invention relates to a case in which a graphite-based lubricant is employed in mandrel mill rolling in producing a ferritic, two-phase, or austenitic stainless steel seamless pipe. Even if there is a problem, an object of the present invention is to establish a method capable of preventing carburization generated on the inner surface of a raw tube for finish rolling and inexpensively producing the steel tube without deteriorating the corrosion resistance of the steel tube.

[0011]

SUMMARY OF THE INVENTION The gist of the present invention is the following manufacturing methods (1) and (2).

(1) In a method of manufacturing a stainless steel seamless tube by mandrel mill rolling using a lubricant containing graphite as a main component, an oxide scale having a thickness of 0.05 to 5 μm is formed on the inner surface of the hollow shell before the mandrel mill rolling. A method for producing a seamless stainless steel pipe, characterized in that a mandrel mill is rolled after forming the stainless steel pipe.

(2) In a method of manufacturing a stainless steel seamless tube by mandrel mill rolling using a lubricant containing graphite as a main component, an inert gas having a volume not less than twice the inner volume is passed through the inner surface of the tube after the mandrel mill rolling. And then re-heating the stainless steel seamless pipe.

[0014]

[Function] Lubricant adhering to the inner surface of the finish rolling element tube during mandrel mill rolling, specifically, O ₂ in the atmosphere during rolling
The graphite in the lubricant remaining on the inner surface of the finish rolling tube without reacting and burning is activated by the subsequent heating in the reheating furnace and the reaction with oxygen in the atmosphere, and at the same time, the CO gas is activated. Occurs, causing diffusion to the inner surface of the pipe and gas carburization.

On the premise of the above factors, the following findings were obtained as a result of a detailed investigation of the carburizing phenomenon in order to establish measures for preventing the inner surface of the finishing roll tube from being carburized.

Investigation of the relationship between the state of the inner surface of the hollow shell before the mandrel mill rolling and the state of occurrence of carburization after the rolling revealed that the inner surface of the hollow shell was covered with a uniform oxide scale having a thickness of 0.05 to 5 μm. No carburization occurs after rolling. This oxide scale acts as a protective film against diffusion to the inner surface of the raw tube and gas carburization, thereby preventing carburization occurring on the inner surface of the finished rolling raw tube after rolling.

According to the result of a detailed study of the relationship between the inner surface atmosphere of the finish rolling element tube after the mandrel mill rolling and the state of occurrence of carburization, the inner surface of the finish rolling element pipe was purged with an inert gas (purged). If it is charged into the reheating furnace, no carburization occurs on the inner surface of the raw tube.

[0018] By the ventilation of inert gas,
The generation of CO gas can be suppressed and the activation of graphite can be prevented, so that the occurrence of carburization of the finish rolling element tube after reheating can be prevented. Inert gases that can be used for ventilation include Ar,
N ₂ and He are mentioned.

The present invention has been completed on the basis of the above findings, and includes ferrites such as SUS405, and ferrites such as SUS329.
It can be applied to all methods of manufacturing austenitic stainless seamless steel pipes such as SUS304, SUS310, SUS316, SUS321, and SUS347.

FIG. 1 is a process diagram showing the procedure for manufacturing a seamless steel pipe by the mandrel mill method of the present invention, including the step of venting an inert gas after removing the mandrel bar. Hereinafter, the contents of the present invention will be described with reference to FIG.

In the first invention of the present invention (based on the above findings), a material blank 1 is heated, for example, in a rotary hearth heating furnace 2 and then a hollow shell 4 A is manufactured by piercing and rolling a Mannesmann piercer 3. And then on the inner surface of the hollow shell
The method is characterized in that an oxide scale having a thickness of 0.05 to 5 μm is generated and the mandrel mill 5 is rolled.

In the first invention, it is important to control the thickness of the oxide scale formed on the inner surface of the hollow shell 4A to 0.05 to 5 μm. If the thickness of the oxide scale is less than 0.05 μm, the oxide scale is too thin, and carburization occurs due to the subsequent temperature increase in the reheating furnace and the reaction with oxygen.

On the other hand, when the thickness of the oxide scale on the inner surface of the hollow shell 4A exceeds 5 μm, the oxide scale itself peels off during the mandrel mill rolling, and the metal surface on the surface of the hollow shell 4A is exposed. When the graphite in the lubricant is pressed, carburization occurs in the subsequent reheating treatment. Also,
The oxide scale pieces that peeled off during rolling were removed from the mandrel bar 6.
In addition to the occurrence of seizure on the surface of the hollow shell 4A, a surface flaw that cannot be repaired even when it becomes a final product is generated on the inner surface of the hollow shell 4A. Therefore, the thickness of the oxide scale must be 5 μm or less.

As a method of forming the above-mentioned oxide scale on the surface of the hollow shell 4A, a means of piercing and rolling by the Mannesmann piercer 3 and then allowing it to cool for a predetermined time or to heat it for a predetermined time in a heating furnace is employed. In the case of cooling, the hollow shell 4A is kept in the air for 2 minutes or more and 6 minutes or less after rolling by the Mannesmann piercer 3 to secure the thickness of the oxide scale to 0.05 to 5 μm. I do. On the other hand, when heating in a heating furnace for a predetermined time, the hollow shell 4A
It is kept for a predetermined time in a heating furnace containing an oxidizing gas of not less than ° C. The holding time is appropriately selected depending on the heating temperature and the type of steel.

Second invention (based on the above findings)
After the material billet 1 is heated in the heating furnace 2 and the piercing-rolling of the mannes man piercer 3 is performed, immediately after the finish rolling raw tube 4B is manufactured by the mandrel mill 5 rolling, the mandrel bar 6 is extracted and the finishing rolling raw material is removed. An inert gas of Ar, N ₂ or He is passed through the ventilation nozzle 10 to the inner surface of the tube 4B to replace the atmosphere on the inner surface of the tube, and then charged into the reheating furnace 11.

In the present invention, "aeration with an inert gas"
This means that since the inside of the finish rolling element tube 4B after the mandrel mill rolling is in an air atmosphere, this is replaced with an inert atmosphere. This ventilation time may be selected according to the volume of the inner surface of the raw tube and the gas flow rate. In practicing the present invention, a flow rate twice or more the internal volume of the raw tube is secured. If the ventilation rate is less than twice the inner volume of the raw tube, a sufficient effect cannot be exhibited. In addition, it is better to vent the inert gas immediately before insertion into the reheating furnace. If it is not possible to insert the gas immediately after venting, seal both pipe ends after venting to maintain the effect of suppressing carburization. Can be.

[0027]

The effects of the present invention will be described in detail based on the first invention (Example 1) and the second invention based on (Example 2) to (Example 3).

Example 1 A material billet was a ferrite-based material having a chemical composition shown in Table 1 (indicated by 13Cr-based in the table), 2
Ten grades of phase and austenitic stainless steels were used.

[0029]

[Table 1]

The conditions of heating the material billet (outer diameter 192 mm), piercing and rolling, forming oxide scale, rolling and reheating were as follows.

1. Billet heating Heated in a rotary hearth heating furnace in the temperature range of 1100 ° C to 1200 ° C.

2. Perforated rolling 192 mm outer diameter, 16 m wall thickness by Mannesmann Piercer
Manufactures hollow shell with a length of 6650 mm.

3. Oxidation scale formation on the inner surface of the hollow shell Part of the air is allowed to cool to the atmosphere (hold for 3 to 5 minutes), and the other part is placed in a heating furnace in an oxidizing atmosphere and heated to 1100 to 1150 ° C for a predetermined time ( 10 minutes to 5 hours) hold.

(Detailed production conditions and oxide scale thicknesses are shown in Tables 2 and 3). Mandrel mill rolling Lubricant: A graphite-based lubricant (graphite 20)
%, Organic binder 10%, water 70%).

Rolling dimensions: 151 mm outside diameter, 6.5 mm thickness, length
Manufactures 20m finish rolling stock.

5. Reheating Heat at 1100 ° C for 20 minutes in a reheating furnace.

6. Reducer finish The stretch reducer was used as a finish tube for finish rolling with an outer diameter of 63.5 mm, a wall thickness of 5.5 mm, and a length of 56 m.

7. Heat treatment after reducer finish Heat treatment was performed for each steel type to obtain the final product.

(The heat treatment conditions are shown in Tables 2 and 3.) A sample was taken from the central portion in the axial direction of the seamless steel pipe manufactured under the above conditions, and was subjected to sulfuric acid-copper nitrate prescribed in JIS G 0575. The intergranular corrosion test was performed to check the occurrence of carburization based on the presence or absence of cracks on the inner surface of the steel pipe. Table 2 shows the results.
And Table 3 collectively.

[0040]

[Table 2]

[0041]

[Table 3]

As is clear from Tables 2 and 3, in the examples of the present invention in which the oxide scale was formed under the conditions specified in the present invention, no cracks were observed on the surface even in the intergranular corrosion test.
It can be seen that carburization can be prevented. On the other hand, in the comparative example in which the oxide scale was not generated more than the specified thickness, cracks occurred due to carburization, and in the comparative example in which the oxide scale was generated beyond the specified thickness, there was no carburization, but due to seizure. Surface scratches occurred.

(Example 2) As the material billet, a steel type having a chemical composition shown in Table 1 was used in the same manner as in Example 1.

The adopted pipe making process is basically the same as that of Example 1.
In the same manner as described above, tube forming conditions and inert gas ventilation shown below were performed.

1. Billet heating Heating from 1100 ° C to 1200 ° C in a rotary hearth heating furnace.

2. Outer diameter 140mm, wall thickness 10m by Mannesmann Pierce
Manufactures hollow shells with a length of 7300 mm.

3. Mandrel mill rolling Lubricant: A graphite-based lubricant (graphite 20)
%, Organic binder 10%, water 70%).

Rolling dimensions: outer diameter 100 mm, wall thickness 5.0 mm, length
Manufactures 20m finish rolling stock.

4. Inert gas through finish rolling pipe for (internal volume: about 130 liters) of the tube end from the inert gas so (Ar, N ₂ and He) becomes a predetermined aeration (blank tube per 100 to 600 liters) Ventilation.

(Detailed ventilation conditions are shown in Tables 4 and 5). Reheating Heat at 1100 ° C for 20 minutes in a reheating furnace.

6. Reducer finish The stretch reducer was used as a finish tube for finish rolling with an outer diameter of 63.5 mm, a wall thickness of 5.5 mm, and a length of 56 m.

7. Heat treatment after reducer finish Heat treatment was performed for each steel type to obtain the final product.

(The heat treatment conditions are shown in Tables 4 and 5.) As in Example 1, a sample was taken from the axial center of the seamless steel pipe, and subjected to a grain boundary corrosion test (specified in JIS G 0575). The survey was conducted to check the occurrence of carburization. Table 4 shows the results of the present invention, and Table 5 shows the results of the comparative example.

[0054]

[Table 4]

[0055]

[Table 5]

As is clear from Tables 4 and 5, in the present invention example in which the inert gas was passed through twice or more the inner volume of the raw tube for finish rolling before reheating, the inside of the steel tube was subjected to the intergranular corrosion test. No cracks were observed on the surface, indicating that no carburization occurred.
However, when the inert gas is not ventilated, or when the ventilated gas does not reach the specified ventilation rate (1.
(Less than 4 times), cracks due to carburization were observed on the inner surface.

(Example 3) The effect of the present invention was confirmed while comparing it with a conventionally proposed method. As a conventional example,
A method in which the gap between the mandrel bar and the rolling element tube is filled with an oxidizing gas such as CO ₂ when rolling the mandrel mill (proposed in Japanese Patent Application Laid-Open No. 4-294803) was taken up.

Each of the material billets was a steel type SUS 304 among the austenitic stainless steels shown in Table 1. The adopted pipe making process (billet heating-piercing rolling-mandrel mill rolling-reheating-reducer finish) was the same as in Example 2 described above, and the product heat treatment after the reducer finish was performed under the conditions of heating at 1080 ° C. and water cooling. .

The prevention of carburization between the present invention and the conventional example was performed under the following conditions.

1. Ar gas was applied from the end of the base tube for carburization prevention of the tube of the present invention to the finish tube for 280 times.
It was ventilated to liters. 2. Rolling was performed while the gap between the mandrel bar and the rolling tube was filled with CO ₂ gas at the inlet side of the conventional case of carburization prevention mandrel mill rolling.
(Continuously ventilates CO ₂ gas from the insertion of the mandrel bar to the start of rolling.) A sample is taken from the axial center of the seamless steel pipe manufactured under the above conditions, and the thickness of the inner surface layer of each steel pipe is measured. The distribution of carbon concentration was investigated. Table 6 shows the results.
Shown in

[0061]

[Table 6]

From Table 6, it can be seen that the carbon concentration in the inner surface layer of the present invention is lower and shallower than that of the conventional example.
It is clear that the carburizing effect is excellent.

[0063]

According to the present invention, a seamless steel pipe free of carburization on the inner surface can be manufactured at low cost even when a graphite-based lubricant is used in the mandrel mill rolling of stainless steel.

[Brief description of the drawings]

FIG. 1 is a process diagram showing a manufacturing procedure of a seamless steel pipe by a mandrel mill method of the present invention, including a process of extracting an inert gas and then ventilating an inert gas.

[Description of Signs] 1 ... Mill billet, 2 ... Rotating hearth heating furnace, 3 ... Mannel man piercer 4A ... Hollow blank tube, 4B ... Finishing roll tube 5 ... Mandrel mill, 6 ... Mandrel bar, 7 ... Hole Mold roll 8 ... Mandrel bar removal process, 9 ... Inert gas ventilation process, 10 ... Vent nozzle 11 ... Reheating furnace, 12 ... Stretch reducer, 13 ... Finishing tube

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI // C10N 40:24 C10N 40:24 Z (56) References JP-A-48-102744 (JP, A) JP-A-4- 344804 (JP, A) JP-A-4-168221 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 17/02 B21B 25/04

Claims (2)

(57) [Claims] 1. A method for manufacturing a stainless steel seamless tube by mandrel mill rolling using a lubricant containing graphite as a main component, wherein an oxide scale having a thickness of 0.05 to 5 μm is formed on the inner surface of the hollow shell before the mandrel mill rolling. And then performing mandrel mill rolling. 2. A method for producing a seamless stainless steel pipe by mandrel mill rolling using a lubricant containing graphite as a main component, wherein an inert gas having a volume equal to or more than twice the internal volume is passed through the inner surface of the pipe after the mandrel mill rolling. And a method of manufacturing a stainless steel seamless steel pipe by reheating.