JPH05269507A - Manufacture of seamless steel tube of iron-based alloy containing chromium - Google Patents

Abstract

PURPOSE:To prevent fouling and stripe-like flaws that are generated at the time of manufacturing a seamless steel tube from a stainless steel by making the thickness of a scale layer which is stuck to the outer surface of a solid billet or hollow tube stock as a material to be rolled to a prescribed thickness. CONSTITUTION:In a method that the seamless steel tube is made from a Fe- based alloy contg. Cr of >=12% by weight by a rolling method, the thickness of the scale layer which is stuck to the outer surface of the billet 10 and hollow shell 20 which is respectively supplied to a piercer 14 and a stretch reducer 26 of the ensuing stages of a heating furnace 12 and a reheating furnace 22 is limited in the range of 10 to 100mum. The thickness of scale which is stuck to that surface is required to be >=10mum to prevent fouling between the material to be rolled and the tool and <=100mum not to make stripe-like flaws remain. Then, the generation of fouling during rolling and naturally stripe-like flaws can be effectively prevented.

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 seamless steel pipe from an Fe-based alloy containing 12 wt% or more of Cr by a rolling method.

[0002]

2. Description of the Related Art Conventionally, Fe-based alloys containing 12 wt% or more Cr, such as JIS-SUS304, SUS310S (austenitic stainless steel), SUS430 (ferritic stainless steel), SU
S420J1 (Martensitic stainless steel), SUS329J2L
In the case of producing a seamless steel pipe from stainless steel such as (duplex stainless steel), the hot extrusion method is generally adopted. The deformation resistance of these stainless steels is high and the tool surface pressure is high.
This is because the (load) becomes high and the hot workability is poor. The hot extrusion method is poor in productivity because the pipe manufacturing efficiency, that is, the number of pipes manufactured per unit time is small, but has an advantage that even a material having relatively poor workability can be processed.

However, recent advances in tool materials and rolling techniques have made it possible to produce seamless pipes of the above-mentioned stainless steel using a rolling method having higher productivity than the hot extrusion method. The rolling method for producing seamless steel pipes is, as typified by the Mannesmann pipe making method, hot rolling rolled round billets in a rolling mill group such as a piercer, a mandrel mill and a stretch reducer in order. Is a method of obtaining a seamless steel pipe as a final product, and has been widely used for the manufacture of seamless steel pipes of carbon steel and low alloy steel.

There is also a method in which a square steel slab (bloom) is used as a raw material, which is perforated by a PPM (press / piercing mill), and then the rolling is continued (except for the piercer) as in the Mannesmann pipe making method. Applied to the department.
Further, during the rolling process, for example, a reheating furnace may be arranged between the mandrel mill and the stretch reducer to stabilize the rolling work.

[0005]

As described above, the rolling method is being applied to produce a seamless steel pipe of stainless steel at a low cost by utilizing the high production efficiency, but it contains 12 wt% or more of Cr. Fe-based alloy (hereinafter collectively referred to as stainless steel)
In the case of producing a seamless steel pipe by the rolling method from the above, it is of course that seizure flaws with tools are likely to occur according to the findings from the past, but in addition to this, it is also observed when producing conventional carbon steel etc. It was discovered that the characteristic streak-like flaws that did not occur occur.

[0006] The streak-like flaw has a sharp-angled or fog-like cross section on the outer surface of the pipe almost in the axial direction, and has a scale bite inside it, so that it can be easily washed by pickling after pipe making. Remains unremoved and greatly impairs the aesthetics of the final product. It is an object of the present invention to provide a pipe manufacturing method in which no seizure flaws or streak flaws are observed when a seamless steel tube is manufactured from stainless steel by a rolling method.

[0007]

[Means for Solving the Problems] As a result of research and development for solving the problems, the present inventors have obtained the following new findings. (1) Although the amount of scale produced in stainless steel is less than that of carbon steel, etc., it is 900 in a heating furnace or reheating furnace.
A considerable amount when exposed to a high temperature atmosphere of ~ 1300 ° C for up to several hours
Generates a scale (for example, several 100 μm or more).

(2) The scale formed on stainless steel is denser than that of carbon steel, etc., and is less likely to fall off during transportation and rolling, so that it is caught between the tool and the material to be rolled during rolling. easy. In particular, a large amount of scale is attached to the surface of the billet or bloom supplied to the piercer or PPM as compared with other rolling processes (because it is exposed to a high temperature atmosphere in the preceding heating furnace for a long time). For example, when sampling and observing stainless steel rolling stop material (rolling material obtained by stopping the rolling mill during rolling) in piercer, when the bite scale is crushed, the outer surface of rolling material is crushed. In some cases, a characteristic morphology (unevenness) that was thought to have been caused by being pressed into the surface or being scratched on the outer surface of the rolled material by a scale crushed in this way was sometimes observed. The occurrence of surface flaws of such a form is consistent with the occurrence frequency of the above-mentioned streak-like flaws.

(3) The occurrence of streak defects was also concerned with the effect of internal oxidation (grain boundary oxidation) accompanying the oxidation of the material to be rolled. For example, the microstructure near the surface of SUS304 stainless steel was investigated in detail. As a result, it was confirmed that even the billet surface after heating only had a maximum depth of about 0.1 mm, and that the flaw in question could not progress.

(4) In the case of carbon steel or the like, scale is not completely bitten, but secondary oxidation (generation of secondary scale) of the rolled material after biting is significant. In the end, it is judged that the scale biting part itself falls off and cannot remain as a streak. The surface morphology (irregularities) that was very similar to that observed in the above-mentioned SUS304 stainless steel was actually observed on the carbon steel half-rolled material in the piercer, but finally no streaky flaw remained.

(5) Therefore, the scale layer densely and in large quantity adhered to the outer surface of the material to be rolled is bitten by the outer surface tool such as a roll and pressed into the outer surface of the material to be rolled while being crushed, so that the material to be rolled is deformed. At the same time, the scale grains stretched or pulverized generate sliding flaws on the outer surface of the material to be rolled due to the relative slip between the outer surface tool and the material to be rolled, so that an uneven shape is generated on the outer surface of the material to be rolled. Then, the concavo-convex shape in which the scale is bitten remains in the final product while being crushed and stretched in the subsequent rolling step, and remains as streak defects without being removed even by pickling or the like.

(6) The above-mentioned streaky flaws are not observed on the inner surface of the tube. Regarding this reason, considering the pipe manufacturing process in order, first, when the inner surface is formed as a new surface in the piercer or PPM, the scale formation is almost negligible, and then the elongation rolling of the elongator, mandrel mill, etc. Alternatively, the amount of secondary scale generated on the inner surface during transportation up to this point is small, and when constant-temperature rolling such as a stretch reducer is equipped with a reheating furnace immediately before that, a large amount of scale is formed on the inner surface of the pipe. It is easily generated, but due to the fact that the inner surface tool is not used during rolling,
This is because no harmful scale bite is generated on the inner surface of the pipe in any of the steps.

(7) However, in order to prevent seizure between the tool and the material to be rolled, it is essential to secure the minimum required scale layer thickness. Therefore, it has been revealed from the above that the following means should be taken to prevent the occurrence of seizure and streak defects, and the present invention has been completed.

The thickness of the scale layer on the outer surface of the material to be rolled used for rolling should be within an appropriate range of 10 to 100 μm. Since the influence of the secondary scale generated during rolling and transportation can be ignored, it is necessary and sufficient if the above is satisfied on the inlet side of the rolling process arranged in the next process of the heating furnace or the reheating furnace. Since there is no effect of grain boundary oxidation, it is sufficient to adjust the scale formation thickness in the heating furnace or reheating furnace, or to adjust the scale thickness between the furnace exit side and the next rolling mill entrance side. ..

The gist of the present invention is as follows.
In a method for producing a seamless steel pipe from a Fe-based alloy containing wt% or more of Cr by a rolling method, at each inlet side of the rolling process following the heating furnace or the rolling process following the heating furnace and the rolling process following the reheating furnace. , The thickness of the scale layer that adheres to the outer surface of the solid steel slab or hollow shell that is the material to be rolled is 10
This is a method for seamless pipes, which is characterized in that the thickness is set to -100 μm.

[0016]

Next, the present invention will be described in detail along with the actual pipe manufacturing process. Here, of the seamless pipe manufacturing process by the rolling method, a Mannesmann mandrel mill line will be described as an example. As shown in FIG. 1, in the seamless pipe manufacturing step, processing is performed in the order shown by white arrows in the figure.
Rotating hearth type heating furnace 12
After heating to about 1200 to 1300 ° C. (hereinafter, simply referred to as a heating furnace), it is perforated by a piercer 14 to obtain a thick hollow piece 15.

Then, the mandrel 16 with the mandrel 16 inserted therein is stretch-rolled by a mandrel mill 18 to form a thin hollow shell 20. Then, the hollow shell 20 is heated in a reheating furnace 22 to about 1000 ° C., and then a stretch reducer 26 is used. The final product is obtained by carrying out outer diameter drawing rolling according to.

In the above steps, the thickest scale layer formed on the surface of the material to be rolled is the heating furnace 12 or the reheating furnace.
Therefore, if the material to be rolled is limited to stainless steel, the amount of scale produced during rolling and transportation is small. In the present invention, the thickness of the scale layer formed on the outer surface of the rolled material is 10 μm or more in order to prevent seizure of the rolled material and the tool as much as possible, and 100 μm in order not to leave streaky defects. As described below, pipe rolling after heating and after reheating is performed.

Specifically, the thickness of the scale layer attached to the outer surfaces of the billet 10 and the hollow shell 20 supplied to the piercer 14 and the stretch reducer 26, which are the next steps of the heating furnace 12 and the reheating furnace 22, respectively, is set to 10 to 10. Limit within 100 μm. The reason why the scale layer thickness of the hollow piece 15 supplied to the mandrel mill 18 is not a problem is that if the appropriate scale layer thickness is satisfied in the piercer 14, the mandrel mill 18 will eventually have an appropriate scale layer thickness. This is because rolling is realized.

In order to set the scale thickness attached to the billet surface supplied to the piercer 14 to 10 to 100 μm, the scale generation thickness is adjusted to 10 by adjusting the atmosphere and temperature in the heating furnace 12.
~ 100 μm or allow the scale formation thickness in the heating furnace 12 to exceed 100 μm, and then perform high-pressure water injection or various surface treatments on the heating furnace outlet side to the piercer inlet side and water surface. Or by performing an appropriate descale (scale removal work) by combining with air injection, etc.
It should be ~ 100 μm. Thus, according to the present invention,
By setting the scale thickness to 10 to 100 μm, it is possible to effectively prevent not only seizure during rolling but also generation of streak defects.

The scale formed on the surface of the material to be rolled in the heating furnace is determined by the temperature at which it is produced, the temperature settings in each zone of the furnace, the atmosphere (concentration of oxygen, CO, H ₂ O, etc.), the material to be rolled. Composition (material) of the rolled material before heating (as-rolled,
Although it may vary depending on the machining finish etc.), regarding the streak defect which is a problem in the present invention, as will be confirmed in Examples described later, there is no significant difference in the range of scales produced in stainless steel. However, it can be prevented if the thickness of the scale layer is proper.

In other words, the scale thickness of 10 to 100 μm may be realized mainly by adjusting the temperature rising pattern of the steel material or the atmosphere in the furnace in consideration of them.
Trouble may occur in the actual pipe manufacturing work, the line may be temporarily stopped, and it may be difficult to achieve an appropriate scale thickness in the heating furnace. In this case, since an excessive scale layer is formed on the billet surface, it is necessary to perform descaling on the billet extracted from the heating furnace before supplying it to the piercer.

Generally, the scale formed on the surface of stainless steel is dense and has strong adhesion, and the peelability is lower than that of carbon steel and low alloy steel. Therefore, the scale of 150 to 150 that is usually attached to a Mannesmann pipe line is used. It is not possible to adjust the scale layer thickness, that is, scale removal work with a high-pressure water descaler of about 200 kg / cm ² , and it is necessary to use a high-pressure water descaler of at least 300 kg / cm ² . At this time, the scale removal thickness by the descaler is the type of scale generated on the billet surface according to the descaler nozzle pressure, high-pressure water jet area, billet moving speed with respect to the nozzle, descaling conditions such as jet flow rate, and billet material. Therefore, the required amount of scale removal and the descaling conditions associated therewith may be determined in consideration of the initial scale thickness immediately before descaling expected from the heating history of the billet to be descaled.

Descaling with high-pressure water may be carried out by spraying the jetted water onto the billet surface only once, or by spraying the jetted water a plurality of times. It is also effective to facilitate descaling by lightly reducing the billet surface by rolling, forging, die drawing, spinning, shot processing, or the like prior to high-pressure water jetting to partially crush the billet surface scale. However, if the amount of processing is too large, the scale will be pushed in rather, and the next descaling may become difficult, or streak defects may be caused. Therefore, a reduction of about 3% is sufficient at most. The processing step immediately before the descaler illustrated here is the "rolling step following the heating furnace or reheating furnace" in the claims of the present invention from the viewpoint of its purpose and processing degree.
Is not included in. In short, descaling is performed to reduce the scale layer thickness from 10 to
It should be 100 μm.

The above method can be applied to the reheating furnace and the subsequent rolling step, ie, the outer diameter drawing step by the stretch reducer, in the completely same background. However, since the heating temperature in the reheating furnace is lower than that in the heating furnace and the in-house time is short, the stainless steel with a Cr content of more than 18% has a scale layer thickness of 100% in the reheating furnace during operation.
It rarely exceeds μm, and the application range of the present invention regarding the steps after the reheating furnace may be considered only for the Fe-based alloy having a Cr content of 12% or more and 18% or less. Next, the operation of the present invention will be described more specifically with reference to Examples.

[0026]

[Example] A heating billet was prepared under various conditions.
Rolling was carried out on the Mannesmann mandrel mill line shown in, and the occurrence of streak defects was compared.

[0027]

[Example 1] Billet: Diameter 143 mm x length 1800 mm (surface machining finish) was heated in an electric heating furnace in the atmosphere, and then rolled with a piercer and a mandrel mill, diameter 110 mm x wall thickness 3.5. mm
C. After forming a hollow shell having a length of 25 m, it was pickled and the outer surface was visually inspected.

The billet material used and the heating temperature are as follows, and the thickness of the generated scale layer in the heating furnace is changed to 10, 50, 10 as an average value by changing the time in the furnace.
It was set to 0, 150, and 200 μm. In addition, since a test material having a scale layer thickness of approximately 0 μm prepared by heating in an Ar gas atmosphere was prepared, the scale layer thickness became 6 levels.

[0029]

[Table 1]

In the visual inspection, the degree of occurrence of seizure flaws and streak flaws was evaluated. The results are shown in Table 2.

[0031]

[Table 2]

As shown in Table 2, the scale layer thickness is 10 to
By setting the thickness to 100 μm, it was possible to perform pipe manufacturing work without seizure flaws or streak-like flaws.

[0033]

[Example 2] As in Example 1, billets of the five materials shown in Table 1 were heated in the atmosphere to the heating temperature shown in Table 1 in an electric heating furnace to obtain a scale layer of about 200 to 500 µm. Immediately after the formation, the surface scale was partially removed by a high-pressure water descaler with a nozzle pressure of 500 kg / cm ² , and the residual scale thickness was adjusted to about 20 to 70 μm. After that, the same rolling, pickling and visual inspection as in Example 1 were performed. The results are all good, and it was confirmed that even if an excessive scale layer is formed, if the scale layer is adjusted to an appropriate scale layer thickness before rolling by descaling, pipe manufacturing work without seizure flaws and streak flaws is possible. did.

[0034]

Example 3 In a heavy oil combustion furnace, the same experiment as in Example 1 was conducted by adjusting the scale layer thickness in the heating furnace according to the in-furnace time and the air-fuel ratio (fuel / air mixing ratio). However,
It was not carried out when the scale thickness was 0 μm. The results are almost the same as in Table 2, and it was confirmed that the pipe manufacturing work without seizure flaws and streak flaws is possible by setting the thickness of the scale layer to an appropriate range even if the combustion method is different.

[0035]

[Example 4] In Example 3, for SUS420J1 and SUS430, pipes were piped up to the mandrel mill under the condition that a proper hollow shell was obtained on the piercer entrance side with a proper scale thickness of 10 to 100 µm, and then the hollow shell was immediately reused. Heating furnace (10
Temperature (50 ° C, heating with heavy oil) and adjusting the scale thickness between 50 and 200 μm by changing the in-furnace time and descaling conditions on the stretch reducer inlet side, and then using a stretch reducer, a diameter of 34 mm × a thickness of 3 mm × Outer diameter reduction rolling was performed to a length of 96 m.

When the obtained hollow shells were pickled and visually inspected, no streaky flaws were observed in the hollow shells having a scale thickness of 100 μm or less, but those having a scale thickness of more than 100 μm differed to some extent. However, the occurrence of streak defects was confirmed. That is, it was confirmed that the present invention is effective because there is an appropriate range necessary for preventing streak defects even when the rolling method is different from tilt rolling (piercer) to hole rolling (stretch reducer).

In all of the above examples, the Mannesmann mandrel mill line was used as an example of the pipe making method. However, the piercer is a PPM (press piercing mill), or the piercer, elongator, and plug mill. Needless to say, the present invention is effective even in the case of other rolling methods such as a Mannesmann plug mill line mainly having a reel, a reeler, and a sizer.

[0038]

As described above, according to the present invention, 12 wt%
It becomes possible to prevent seizure flaws and streak flaws that occur when producing a seamless steel pipe from the Fe-based alloy containing Cr by the rolling method, and a product having good surface properties can be obtained.

[Brief description of drawings]

[Figure 1] Seamless pipe manufacturing process by rolling method (Mannesmann
FIG. 3 is a schematic view showing a mandrel mill line).

[Explanation of symbols]

 12: Rotary hearth type heating furnace 14: Piercer 18: Mandrel mill 22: Reheating furnace 26: Stretch reducer

Claims (1)

[Claims] 1. A method for producing a seamless steel pipe from a Fe-based alloy containing 12 wt% or more of Cr by a rolling method, which comprises a rolling step following a heating furnace or a rolling step following a heating furnace and a rolling step following a reheating furnace. The method for producing a seamless steel pipe, wherein the thickness of the scale layer adhering to the outer surface of the solid steel slab or the hollow shell, which is the material to be rolled, is 10 to 100 μm on each inlet side.