JPH1177115A - Tightly scaled steel tube and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tightly scaled steel tube which is manufactured by stretch-reducing without anxiety about penetrating of scale and work hardening, excellent in adhesive strength of scale in products and eliminates the need of pickling before secondary working and the manufacturing method thereof. SOLUTION: This tightly scaled steel tube has a scale layer 40 on its surface and at least a part just above the base iron 30 of the scale layer 40 is composed of the Fe3 O4 layer (layer C) of Fe dispersed phase 43. The steel tube is manufactured by heating and then stretch-reducing at rolling completing temp. of >=570 deg.C using the stretch reducer of plural stands having grooved rolls and cooling the steel tube after stretch reduction from the rolling completing temp. to 200 deg.C for >=24 hours.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tight-scale steel pipe and a method for producing the same, and more particularly, to a method for producing the same by drawing reduction.
The present invention relates to a tight scale steel pipe excellent in scale adhesion and a method for producing the same.

[0002]

2. Description of the Related Art Steel pipes are classified into seamless steel pipes and welded steel pipes. The seamless steel pipe is heated to a high temperature in a heating furnace as shown in FIG. 6, and then drilled by a piercer, rolled by a mandrel, a plug mill, or the like for extending the drilled pipe, and then reheated in a reheating furnace. After being heated, the drawn tube is cooled through a rolling process using a reducer (stretch reducer) or a sizer for adjusting the outer diameter and the wall thickness of the drawn tube to obtain a product.

[0003] As described above, a seamless steel pipe is produced in a reheating furnace.
Since it is manufactured by hot rolling after being heated to 0 to 900 ° C., it is shipped as a black scale with a scale having a thickness of about 10 μm adhered to the surface. For this reason,
The seamless steel pipe has a problem that the surface roughness is rough, and the scale attached to the surface at the time of secondary processing of the pipe is peeled off, and the surface properties of the pipe are further deteriorated. In addition, heat treatment may be performed for the purpose of removing processing distortion in the secondary processing, and scale is further attached to the surface. Therefore, pickling must be performed in order to improve the surface properties, and there has been a problem that the process is long and complicated.

[0004] Welded steel pipes are formed by forming a steel plate or a steel strip (strip) into a tube and welding the joint thereof, and are manufactured by various manufacturing methods from a small diameter to a large diameter. Examples include electric resistance welding (electric resistance welding), forge welding, and electric arc welding. For small to medium diameter steel pipes, an electric resistance welding method utilizing high frequency induction heating is mainly used (electric resistance welded steel pipes, electric resistance welded pipes). In this method, a steel strip is continuously supplied, formed into a tubular shape with a forming roll to form an open pipe, and then the end faces of both edges of the open pipe are heated by high-frequency induction heating to a temperature equal to or higher than the melting point of the steel. Is a method of manufacturing a steel pipe by butting and welding the end faces of both edges (for example, 3rd Edition Iron and Steel Handbook, Volume III (2) 1056-
1092).

On the other hand, there is a method for producing a forged steel pipe having relatively high productivity for a relatively small diameter steel pipe. This method
After heating the continuously supplied steel strip to about 1300 ° C in a heating furnace, it is formed into a tubular shape with a forming roll to form an open tube, and then oxygen is blown to the end face by a welding horn,
A method in which the end face is heated to about 1400 ° C. by the heat of oxidation, and then the end faces of both edges are abutted with a forging roll to perform solid-phase welding to produce a steel pipe (for example, Vol.
(2) pp. 1056-1092).

However, the method for producing an electric resistance welded pipe and a forged pipe is not suitable for producing a small lot of various kinds of steel pipes because a hole-shaped roll corresponding to the dimensions of a product steel pipe must be used. In order to solve such disadvantages of the steel pipe manufacturing method, Japanese Patent Application Laid-Open Nos.
As disclosed in Japanese Patent Publication No. 14, a method of cold-rolling an ERW pipe in a cold state has been proposed.

However, when the steel pipe is cold drawn and rolled, the rolling load is large, so that it is necessary to install a lubricating rolling device for preventing seizure with rolls and a large mill capable of withstanding a large rolling load. In addition, the work strain due to cold drawing rolling is superimposed on the forming strain when forming a steel strip into a raw tube, and the work hardening of the material is remarkable. There is.
In addition, if the strip used is still hot-rolled, pickling is indispensable to remove the adhered scale.
If heat treatment is added after pipe production, scale will be generated and pickling will be required.

[0008] Japanese Patent Publication No. 2-24606,
As disclosed in -15082, a method of hot-rolling an ERW pipe has been proposed. However, when the ERW pipe is hot drawn and rolled, the main pipe is heated to 800 ° C or more in a reheating furnace, which causes new scale adhesion and induces biting of the scale during drawing and rolling. There's a problem.

[0009]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, can be manufactured by drawing and rolling without concerns about scale entrapment and work hardening, and has excellent scale adhesion in products. To provide a tight-scale steel pipe that does not require pickling before secondary processing and a method for producing the same.

[0010]

First, the results of experiments and studies on which the present invention is based will be described. 31.8mm φ × 3.
After heating a 2mmt base steel pipe to a temperature of 900-600 ° C,
The rolling end temperature and the cooling history after rolling were adjusted by a two-stand three-roll reducing mill (stretch reducer) to obtain a product tube of 25.4 mmφ to 15.1 mmφ. The peeling property of the scale formed on the surface of this product tube was measured by a three-point bending tape peeling test (tape was attached between test specimen marks, and then peeled, and then peeled off. The scale peeling amount was evaluated based on the difference in tape weight before and after bending). FIG. 3 shows the results of the investigation. In the figure, ○
Indicates a small amount of peeling, and X indicates a large amount of peeling. From Fig. 3, the rolling end temperature is 570 ° C or higher, and the cooling time to 200 ° C after rolling is
It was found that setting the time to 24 hours or more is important for improving the adhesion of the scale.

As a result of microscopic observation of the surface layer portion of these product tubes and examination of the scale composition, as shown schematically in FIG.
In FIG. 3, in the case of the evaluation “O”, the Fe ₃ O ₄ layer 43 (provisionally referred to as “C layer”) in which Fe is dispersed (hereinafter referred to as “dispersion phase”) is remarkably present immediately above the base iron 30 in the scale layer 40. An FeO layer 41 (tentatively referred to as a B layer) and an Fe ₃ O ₄ layer 42 (tentatively referred to as an A layer) exist in this order (FIG. 4).
(A)) Those with an evaluation of “×” indicate that the B layer and A
It was found that only the layers were present in order (some layers were only the A layer) and the C layer was not present at all (FIG. 4B). In other words, in order for the surface scale layer of the steel pipe to have good adhesion, it is important that at least immediately above the ground iron of the scale layer is composed of an Fe ₃ O ₄ layer in which Fe is dispersed, It is a mandatory requirement.

On the other hand, considering the chemical reaction of the scale formation, it is considered that the layers A to C are formed according to the following formulas (A) to (C), respectively. 2Fe + O ₂ → 2FeO …… (A) 6FeO + O ₂ → 2Fe ₃ O ₄ …… (B) 4FeO → Fe ₃ O ₄ + Fe …… (C) Therefore, in the drawing reduction process of steel pipe, Fe
O 2 is generated, and mainly the Fe
If the decomposition reaction of FeO according to the formula (C), which mainly occurs on the interface side with the base iron, is promoted while suppressing the oxidation reaction of O 2, the C layer can be effectively formed.

For this purpose, it is most important that the rolling completion temperature of the reduction rolling and the cooling history after the completion of the rolling are controlled within the above ranges. In general, the thinner the scale layer, the better the adhesion. Therefore, it is preferable to make the scale of the product tube as thin as possible. Thus, the effects of the drawing rolling conditions on the scale thickness of the product tube in the above experiment were arranged, and the results shown in FIG. 5 were obtained. From FIG. 5, it is assumed that the scale is 3 μm thick, for example.
In order to reduce the thickness to about m or less, the temperature at the end of drawing rolling is 700 ° C.
Hereinafter, the outer surface magnification ratio is preferably set to 1.3 or more (see FIG. 5).
(A)) Further, the heating temperature before the reduction rolling is preferably set to less than 800 ° C. (FIG. 5 (b)). The outer surface enlargement ratio is the ratio of the outer surface area of the steel pipe after drawing and the outer surface area before drawing. That is, the outer diameters before and after reduction rolling are D ₀ ,
D, assuming that the lengths before and after the drawing rolling are L ₀ and L, the outer surface enlargement ratio can be expressed by (DL / D ₀ L ₀ ).

Further, when the base steel pipe is drawn and rolled under specific conditions, the average ferrite grain size becomes finer, the grain boundary oxidation becomes denser, and the adhesion between the scale and the base material further increases. The present invention has been made based on these findings. That is, the present invention is a tight-scale steel pipe having a scale layer on a surface, at least immediately above the ground iron, comprising a Fe ₃ O ₄ layer in which Fe is dispersed.

The present invention also relates to a method for producing a steel pipe by reduction rolling using a multiple-stand reduction rolling mill having a grooved roll, wherein the reduction rolling temperature is set to 570 ° C. or higher after heating the base steel pipe. Is a method for producing a tight-scale steel pipe, characterized in that the steel pipe after rolling is cooled from the rolling end temperature to 200 ° C. for 24 hours or more, preferably at least one of the following requirements I to V: It is a method that includes. I The outer surface expansion ratio of the steel pipe before and after reduction rolling is 1.3 or more, and the temperature at the end of reduction rolling is 700 ° C or less. II Heating temperature before drawing rolling of steel pipe is 500 ℃ or more, 800 ℃
Less than III Descale the steel pipe after heating and before rolling. IV Surface roughness at least at the stand of the final rolling
Ra: Draw rolling is performed using a bright roll of 0.5 μm or less. V The average ferrite grain size in the section perpendicular to the tube axis is d
After heating the base steel tube of _F (μm), reduction rolling is performed at an average rolling temperature θ _m (° C.) and a total diameter reduction ratio R _G (%) satisfying the following equation (1).

[0016]

(Equation 2)

Here, θ _m = (θ ₀ + θ _N ) / 2, R _G
= (D ₀ -D _N ) × 100 / D ₀ , θ: temperature (° C.), D:
Outer diameter (mm), suffix 0, N: entrance side of the first stand of the rolling mill, exit side of the last stand.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of an equipment line suitable for implementing the present invention. In the present invention, a multi-stand drawing rolling device 21 having a grooved roll is used. The number of stands of the rolling mill can be appropriately selected according to the combination of the mother pipe diameter and the product pipe diameter. The hole type roll of each stand is a general-purpose 2 roll,
In addition to three rolls, four or more rolls may be used.

The heating temperature of the steel pipe before the reduction rolling is preferably 500 ° C. or more and less than 800 ° C. If the heating temperature exceeds 800 ° C., the amount of scale generation increases rapidly, and the surface of the steel pipe surface deteriorates, and the scale thickness of the product pipe increases as shown in FIG. 5B. Heating temperature is 500 ℃
The rolling end temperature due to the heat generated during
The temperature can be increased to 570 ° C or higher, but in this case, the rolling load increases, the rolling mill needs to be increased in size, seizure occurs on the rolls, the surface flaw of the product tube increases, Work hardening increases.

The method for heating the base steel pipe is not particularly limited.
Heating with a heating furnace or an induction heating coil is preferred. Above all, the induction heating method is more preferable in terms of production efficiency. Desirably, the scale steel adhered to the surface of the mother steel pipe 8 heated by the reheating device 25 is removed by the descaling device 23 before the drawing rolling. Fe ₃ O ₄ and FeO of about _{4 to} 10 μm adhere to the base steel pipe as hot rolled.
It is preferable to remove such a scale as much as possible before rolling. Descaling sprays high pressure water,
Alternatively, removal is preferably performed by brush rolling, shot blasting, or light reduction rolling using a rough roll having a Ra of 3 μm or more. Among them, it is preferable to use mechanical descaling such as brush roll or shot blast.

With the primary scale on the surface removed or not removed, the base steel tube 8 is subjected to reduction rolling. The reduction rolling is preferably performed so that the outer surface enlargement ratio is 1.3 or more (FIG. 5A). As a result, the peeling of the scale attached before the reduction rolling and the compression of the scale generated during the rolling are sufficiently promoted, and a product pipe having a thin scale layer of about 3 μm or less can be obtained. The more preferable range of the outer surface enlargement ratio is 1.5 or more and 10 or less. If the outer surface expansion ratio exceeds 10, the tube temperature will be too low, so that the target rolling end temperature (570 ° C. or higher) may not be secured. The outer surface magnification ratio for each stand is 1.01 to 1.18
It is preferred that

At the final pass stand of the rolling mill 21,
Ra: Rolling is preferably performed using a bright roll of 0.5 μm or less. By rolling using a bright roll, the surface roughness of the product pipe can be reduced after rolling. Of course, a stand other than the final pass stand may be rolled using a bright roll. If a bright roll is used in the final pass stand, the surface roughness of the product tube can be reduced to Ra: 1.0 μm or less. If a bridle roll is used in a stand other than the final pass stand, Ra: 0.7 μm
In the following, the tube surface properties are improved. When a bright roll is not used for the final pass stand, the surface roughness of the product tube is Ra: 3.0 μm or less.
70 ° C or higher is mandatory. If the end temperature of the reduction rolling is lower than 570 ° C., the reaction of the formula (3) is delayed, the formation of the C layer shown in FIG. 4A becomes insufficient, and the adhesion deteriorates as shown in FIG. . From the viewpoint of reducing the thickness, it is preferable that the drawing rolling temperature is set to 700 ° C. or lower (FIG. 5B).

[0023] The steel pipe (product pipe 16) after the reduction rolling is 20
It must be cooled to 0 ° C over 24 hours. If the cooling time is shorter than 24 hours, the time required for the reaction of the formula (3) is insufficient, the formation of the C layer shown in FIG. 4A becomes insufficient, and the adhesion deteriorates as shown in FIG. I do. In normal air cooling, the product pipe temperature drops to 200 ° C or less in at most about 0.5 hours.Therefore, the product pipe 16 is cut by the cutting machine 18 and charged into the slow cooling box 19. It is necessary to slow down the cooling rate.

In the present invention, a temperature adjustment device 26 may be provided on the entrance side of the reduction rolling device 21 or in the middle of the reduction rolling device 21 to adjust the temperature of the steel pipe during rolling. The temperature adjusting device 26 includes an induction coil (for heating), a spray nozzle (for cooling), and the like. Scale composition, heating temperature, determined by the rolling conditions and cooling conditions after rolling, according to the above production conditions, the surface of the steel pipe, Fe ₃ O ₄ layer was dispersed in Fe and at least the base steel right above exists The tight-scale steel pipe of the present invention having a scale layer can be obtained.

Further, when the average ferrite grain size of the section perpendicular to the tube axis (referred to as C section) of the base steel pipe is d _F (μm), the average rolling temperature θ _m (° C.) If reduction rolling is performed in the range of the diameter reduction ratio R _G (%), the crystal grain size of the product tube is reduced to 1 μm or less, and as shown in FIG.
The grain boundary oxide layer (inner oxide layer), which is the deepest phase of the scale, contains metal precipitates and becomes denser, and the interface between the base material (base iron) and the scale becomes intricate. Adhesion with the material is further enhanced.

The tight-scale steel pipe of the present invention can be manufactured regardless of whether the base steel pipe before the reduction rolling is any of a seamless steel pipe, an electric resistance welded steel pipe, and a solid-state pressure welded steel pipe. The rolling device can be made continuous with the production line for these steel pipes. FIG. 2 shows an example in which a drawing rolling device is connected to a solid-state pressure welded steel pipe production line. The method of manufacturing the solid-state pressure welded steel pipe by this manufacturing line will be described. The strip 1 discharged from the uncoiler 14 is connected to the strip 1 by the joining device 15.
After being preheated in the preheating furnace 2 via the looper 17,
An open pipe 7 is formed by a forming apparatus 3 composed of a group of forming rolls, and the open pipe is heated to a temperature range lower than a melting point by an edge preheating induction heating apparatus 4 and an edge heating induction heating apparatus 5. At the same time, the base steel pipe 8 is brought into contact with and pressed against the squeeze roll 6.

Next, the mother steel pipe 8 is heated and soaked to a predetermined temperature in a soaking furnace 22, and after the surface scale is removed by a descaling device 23, the drawn steel is rolled by a drawing rolling device 21 composed of a plurality of stands of hole type rolls. After rolling, the product is cut by a cutting machine 18 and gradually cooled in a slow cooling box 19 to produce a product tube.
It becomes 16. The temperature of the steel pipe is measured by a thermometer 20.

[0028]

【Example】

(Example 1) JIS STKM13B with a pipe size of 42.7mmφ × 2.6mmt
The steel pipe (base steel pipe) was heated to the heating temperature shown in Table 1 using the apparatus row shown in FIG. 1 and then subjected to drawing rolling shown in Table 1. Some of the steel pipes were rolled and then rolled in the annealing box 19. The product tube was gradually cooled.

Some of the steel pipes are descaled by the descaling device 23 before the reduction rolling, and some of the steel tubes are the final pass stand (the final stand = the twelfth stand) of the reduction rolling device 21 (three-roll type). ) With stand: Ra: 0.3 μm
It was rolled by the following bridle roll. Regarding the product pipe, the layer composition immediately above the ground iron in the scale layer, the scale adhesion by a three-point bending tape peel test ("◎" without peeling, "○" with little peeling, "、" with slight peeling, considerable Table 1 shows the results obtained by examining the four grades of "X" to peel off, the scale thickness, and the surface roughness.

[0030]

[Table 1]

From Table 1, in all of the "invention examples" whose production conditions are within the scope of the present invention, the C layer (Fe
Fe ₃ O ₄ layer containing a dispersed phase), and the scale adhesion is evaluated as “○” or more. B layer (Fe
O layer) or A layer (Fe ₃ O ₄ layer), and the scale adhesion is below “△”. In the comparative example, some scale biting and work hardening occurred, but in the invention example, none occurred.

In the invention examples, the above-mentioned requirements I to III
(Re-listed below)
In the case where the scale was further thinned and the adhesion was "◎", and those satisfying the requirement IV (represented below), the surface roughness was further reduced and the surface skin was improved. Note I The outer surface expansion ratio of the steel pipe before and after the reduction rolling is set to 1.3 or more, and the end temperature of the reduction rolling is set to 700 ° C or less. II Heating temperature before drawing rolling of steel pipe is 500 ℃ or more, 800 ℃
Less than III Descale the steel pipe after heating and before rolling. IV Surface roughness at least at the stand of the final rolling
Ra: Draw rolling is performed using a bright roll of 0.5 μm or less. (Embodiment 2) Table 3 shows the results of implementing the present invention on the base steel pipes shown in Table 2 under the conditions shown in Table 2 that satisfy the above requirement V using the apparatus row shown in FIG. As a result of the average ferrite grain size of the cross section of the product C being 1 μm or less and the grain boundary oxide layer being densified, the interface between the inner oxide layer of the scale (corresponding to the C layer) and the base material was complicated as shown in FIG. As shown in Table 3, the scale adhesion was “◎”.

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

According to the present invention, it is possible to manufacture by drawing rolling without concern for scale biting and work hardening, and it has excellent scale adhesion to a product and requires pickling before secondary processing. This has a remarkable effect that a tight-scale steel pipe that does not have a large diameter can be obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an example of an equipment row suitable for implementing the present invention.

FIG. 2 is a conceptual diagram of a solid-state pressure welded steel pipe production line to which the equipment row in FIG. 1 is connected.

FIG. 3 is a graph showing a range of a rolling end temperature and a cooling history after rolling at which the scale adhesion of a drawn rolled steel pipe is improved.

FIG. 4 is a schematic view of a surface scale structure of a drawn rolled steel pipe of the present invention (a) and the conventional (b).

FIG. 5 is a graph showing the effects of (a) the end-of-rolling temperature and the outer surface enlargement ratio and (b) the heating temperature on the scale thickness.

FIG. 6 is a conceptual diagram of a conventional seamless steel pipe production line.

FIG. 7 is a schematic view showing one example of a surface scale structure of a product pipe of Example 2.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Strip steel 2 Preheating furnace 3 Forming apparatus 4 Induction heating apparatus for edge preheating 5 Induction heating apparatus for edge heating 6 Squeeze roll 7 Open pipe 8 Base steel pipe 14 Uncoiler 15 Joining apparatus 16 Product pipe 17 Looper 18 Cutting machine 19 Slow cooling box 20 thermometer 21 reducing rolling device 22 Hitoshinetsuro 23 descaler device 25 reheating device 26 temperature controller 30 base iron 40 scale layer 41 FeO layer (B layer) 42 Fe ₃ O ₄ layer (A layer) 43 Fe dispersed phase Fe ₃ O ₄ layer containing C (C layer)

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Motoaki Itaya 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Inside the Chita Works, Kawasaki Steel (72) Inventor Yuji Hashimoto 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Nobuki Tanaka 1-1-1, Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Masanori Nishimori 1-1-1, Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Taro Kanayama 1-1-1 Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Nochika Okabe 1-1-1, Kawasaki-cho, Handa-city, Aichi Prefecture Address Kawasaki Steel Corporation Chita Works

Claims (7)

[Claims] 1. A tight-scale steel pipe having a scale layer on its surface and at least a portion of the scale layer immediately above the ground iron comprises a Fe ₃ O ₄ layer in which Fe is dispersed. 2. A method for producing a steel pipe by reduction rolling using a multi-stand reduction rolling machine having a grooved roll, wherein after the base steel pipe is heated, reduction rolling is performed at a rolling end temperature of 570 ° C. or higher, and reduction is performed. A method for producing a tight-scale steel pipe, comprising cooling the rolled steel pipe from the rolling end temperature to 200 ° C. for 24 hours or more. 3. The expansion ratio of the outer surface of the steel pipe before and after reduction rolling is determined.
3. The method according to claim 2, wherein the temperature at which the rolling is completed is 1.3 ° C. or more and 700 ° C. or less. 4. The heating temperature of the mother steel pipe is 500 ° C. or more, 800 ° C.
The method according to claim 2 or 3, wherein 5. The method according to claim 2, wherein descaling is performed on the steel pipe after heating and before rolling. 6. The method according to claim 2, wherein at least at the stand of the final pass of the reduction rolling, the reduction rolling is performed using a bright roll having a surface roughness Ra of 0.5 μm or less. 7. After heating a base steel pipe having an average ferrite grain size of d _F (μm) in a cross section perpendicular to the pipe axis, an average rolling temperature θ _m (° C.) satisfying the following expression (1) and a total reduction ratio R
The method according to any one of claims 2 to 6, wherein reduction rolling is performed at _G (%). (Equation 1) Where θ _m = (θ ₀ + θ _N ) / 2 and R _G = (D ₀ −D
_N ) × 100 / D ₀ , θ: temperature (° C.), D: outer diameter (mm),
Subscripts 0, N: entrance side of the first stand of the rolling mill, exit side of the last stand.