GB2085330A - Method of preparing clad steels - Google Patents

Abstract

A method of preparing a clad steel product 34 comprises freely fitting a pipe or bar 32 or a pipe and bar into a pipe 31, cold-drawing the fitted assembly to obtain an intermediate product 33 in which the pipe and bar of the fitted assembly adhere closely to each other, heating the intermediate product, and subjecting the heated intermediate product to hot working for forming into a desired clad steel product such as a clad steel pipe, a clad steel bar 34 or a wire. The confronting faces of the pipe and bar are cleaned prior to the fitting operation, and a metal plating layer or metal foil coating layer is formed on the peripheral face of the pipe or bar located inside for preventing carburization. The intermediate product obtained by cold drawing is a clad billet or clad bloom. The bonded area on the end face of the clad bloom or billet may be sealed by welding prior to the heating operation so as to prevent the occurrence of oxidation in the interior. <IMAGE>

Description

SPECIFICATION Method of preparing clad steels Background of the invention (1) Field ofthe Invention: The present invention relates to a method for the preparation of linear straight steel products such as steel pipes, bars and steel wires, which are composed of at least two laminated metals.

(2) Description of the prior art: A clad steel pipe having a laminated structure comprising an outer layer of carbon steel and an inner layer of stainless steel is known and widely used. In clad steel products represented by such clad steel pipe, by virtue of the presence of a laminated metal (ordinarily, one layer being small in quantity, e.g., stainless steel of the inner layer in the above-mentioned instance), high corrosion resistance, high resistance to hydrogen sulfide cracking and high abrasion resistance are provided and the clad steel products, therefore, can resist severe conditions and be applied to special uses. A cheap metal is normally used as the base metal (ordinarily, a layer large in quantity, e.g., carbon steel of the outerlayer in the above-mentioned instance).

As the method for preparing such clad steel pipes, there has been known a method as described below.

More specifically, as shown in Figure 16 of the drawings, a pipe 2' composed of stainless steel is inserted into a thick pipe 1' composed of carbon steel, and both the end portions of the pipe 2' located in the pipe 1' are continuously welded along the entire peripheries thereof. Nickel is plated in a thickness of 80 to 130 microns on the peripheral face of the pipe 2' so as to prevent diffusion of carbon during the step of metallurgically bonding the two pipes. The outer diameter of the pipe 2' is made smaller by about 1/16 inch than the inner diameter of the pipe 1' so that fitting of both the pipes can be accomplished conveniently. A gas vent hold 1 'a communicating with the cavity between pipe 1' and pipe 2' is formed on the end portion of the pipe 1'.This assembly as a material to be formed into a pipe, that is, a billet, is heated under predetermined conditions, and is passed through a rotary forge mill so that the portion where the gas vent hole 1 'a is formed is located on the bottom side, whereby an intended pipe is formed. This method or similar method is defective in that mechanical processing of the gas vent hold 1 'a and the like is very trouble-some; centering of the inner and outer pipes 2' and 1' is difficult at the welding step; and, since both the pipes are curved, it is difficult to form a long pipe. Therefore, the method involves various factors inhibiting improvements in productivity and manufacturing rates of billets.

As a method of assembling of laminated billets, there is known a press-fitting method. However, since the press stroke for press4itting an inner pipe 2' into an outer pipe 1' is naturally limited, the method is defective in that a long pipe material cannot be obtained.

Also, a shrinkage-fitting method can be mentioned. However, when the shrinkage-fitting method is adopted, a scale is formed on the inner circumferential face of the pipe 1' and the metallurgical bondability during the pipe-forming step is very bad, which results in reduction of the test pass ratio during inspection of the products. There has been adopted a method in which air in the cavity between the pipes 1' and 2' is replaced by an inert gas such as argon in order to prevent formation of scales during the hot rolling step or the like. Even if this method is adopted, in case of, for example, a clad steel pipe having an entire length of 7 m, the top portion along about 2 m and the bottom portion along about 0.5 m must be thrown away because of insufficient metallurgical bonding.

As is seen from the foregoing illustration, even the methods which are regarded as being in practice superior to the explosion welding method, which has been carried out in the past, involve various defects and difficulties.

Objects and brief summary of the invention The present invention proposes means for overcoming the above-mentioned defects and difficulties which are involved in the conventional methods. The present invention provides a method in which a hollow or solid clad billet or clad bloom is obtained as an intermediate material by utilizing a cold drawing technique, and this intermediate material is subjected to hot working to yield a clad steel product having a desired shape.

It is a primary object of the present invention to provide a method of producing steel pipes in which steel pipes can be manufactured in high yields at high manufacturing rates.

Another object of the present invention is to provide a method in which clad, linear, straight steel products such as clad bars, and steel wires, which have not been practically manufactured or marketed, and which are excellent in their metallurgical bondability, can be manufactured at high manufacturing rates in high yields.

Still another object of the present invention is to provide a method of preparing pipes and wires with a high metallurgical bondability between the base metal and laminated metal, in which carburization is prevented, at high manufacturing rates in high yields.

A further object of the present invention is to provide a method of preparing clad steel products, in which large equipment need not be constructed for prevention of carburization.

These and other objects and novel features of the present invention will be more apparent from the following description taken together with the accompanying drawings.

Brief description of the drawings Figure 1 is a diagram illustrating the main steps of the method of the present invention through an embodiment where a clad steel pipe is prepared.

Figure 2 is a diagram illustrating the state of cold drawing in the embodiment shown in Figure 1.

Figure 3 is a metal microscope photograph showing an axial section of the bonded portion of a clad steel pipe prepared according to the embodiment shown in Figure 1.

Figure 4 is a diagram illustrating an embodiment where a metal foil is employed as an intermediate medium material.

Figures 5 and 6 are metal microscope photographs showing an axial section of the bonded portion of a clad steel pipe prepared by using a metal foil as an intermediate medium material.

Figure 7 is a diagram illustrating the main steps of the method of the present invention in an embodiment where a clad steel pipe is prepared through steps inclusive of the step of forming a clad bloom by cold drawing.

Figure 8 is a diagram showing the state of cold drawing in the embodiment shown in Figure 7.

Figure 9 is a metal microscope photograph of an axial section of a clad steel pipe prepared according to the embodiment shown in Figure 7.

Figure 10 is a diagram illustrating the main steps of preparing a clad wire according to the method of the present invention.

Figure 11 is a diagram illustrating the state of cold drawing in the embodiment shown in Figure 10.

Figure 12 is a metal microscope photograph showing an axial section of a clad bar prepared in the embodiment shown in Figure 10.

Figure 13 is a diagram illustrating the main steps of another embodiment of preparing a clad wire according to the method of the present invention.

Figure 14 is a diagram illustrating the state of cold drawing in the embodiment shown in Figure 13.

Figure 15 is a metal microscope photograph showing an axial section of a clad wire prepared according to the embodiment shown in Figure 13.

Figure 16 is a longitudinal sectional view of a pipe material which illustrates the conventional method of preparing a clad steel pipe.

Detailed description of the invention In accordance with a fundamental aspect of the present invention, there is provided a method of preparing clad steel products having a laminated structure comprising at least two metal layers, which comprises freely fitting a core composed of a metal to be formed into an inner layer, into a pipe composed of a metal to be formed into an outer layer, directly or through at least one pipe composed of a metal to be formed into an intermediate layer, simultaneously cold-drawing the pipes and core of the assembly to obtain an intermediate material comprising the core and pipes bonded closely to one another, and heating the intermediate material at a predetermined temperature and subjecting the intermediate material to hot working to form a product having a desired shape.

The present invention will now be described in detail with reference to typical embodiments thereof. In order to clearly define the technical scope of the present invention, the characteristic features of the present invention will be described in brief in advance.

The intended product of the present invention is a clad, linear straight steel product such as a pipe, a bar or a wire. The clad steel product prepared according to the present invention has at least a two-layer structure similar to that of the above-mentioned known clad pipe. However, when the clad steel product is used in the field where the clad steel product is used in the field where the clad steel product is exposed to a corrosive atmosphere, it is preferred that the clad steel product should have a three-layer structure including laminated layers of an anti-corrosive metal as the outer peripheral and inner circumferential layers.

Moreover, a linear steel product comprising a base metal as a core and at least two laminated metal layers is often required according to need. Accordingly, the pipe or other linear product according to the present invention comprises at least two metal layers in which the base metal occupies any one of the outer layer, the inner layer and the intermediate layer.

In the case where the pipe or other linear product to be prepared has a two-layer structure, the materials to be freely fitted together are two pipes or one pipe and one bar, and, in the case where the product has a laminated structure including at least three layers, the materials to be freely fitted together are at least three pipes or at least two pipes and one bar. By the term "core" used herein is meant a pipe or bar to be located as the innermost layer.

The materials used in the method of the present invention will now be described.

As the base metal, there are chosen and used carbon steel, alloy steel, stainless steel and nickel-based alloy. As the stainless steel, there can be mentioned martensite steel, ferrite steel and austenite steel, and, furthermore, precipitation hardening stainless steel and chromium-manganese stainless steel can be used.

The nickel-based alloy includes Inconel alloy. As the laminated metal, there can be mentioned abrasion-resistant steel, stainless steel, nickel, nickel alloy, titanium, titanium alloy, copper, copper alloy, chromium and chromium alloy, aluminium and aluminium alloy. The abrasion-resistant steel includes high-carbon, abrasion-resistant steel and abrasion-resistant manganese steel.

The method of the present invention will now be described with reference to an embodiment in which a steel pipe having a two-layer structure is prepared.

When a clad steel pipe having an inner layer of stainless steel and an outer layer of carbon steel is prepared, a large-diameter pipe of carbon steel and a small-diameter pipe or bar of stainless steel are used.

As shown in Figure 1 -(a), a small-diameter pipe 2 or bar 12 is fitted into a large-diameter pipe 1, and the assembly is subjected to cold drawing. The inner diameter of the pipe 1 and the outer diameter of the pipe 2 (or the diameter of the bar 12) are predetermined so that the fitting of both the pipes is performed conveniently. More specifically, both the pipes need not be fitted closely to each other, but the pipes may be loosely fitted together so that the fitting operation can be accomplished very easily. The outer diameter and thickness of the pipe 1 and the inner diameter and thickness of the pipe 2 (or the diameter of the bar 12) may be appropriately determined so far as the cold drawing step of preparing a clad billet is concerned, but they are determined according to working conditions at the tube-making step using the clad billet as the starting material.It is preferred that each of the pipes 1 and 2 be a seamless pipe, but the pipes are not limited to seamless pipes and welded pipes may be used.

Prior to the fitting operation, the inner face of the outer pipe 1 and the outer face of the inner pipe 2 (or the outer face of the bar 12) are subjected to a cleaning treatment. Polishing is preferred as the cleaning treatment, but both of the above-mentioned faces may be subjected to a shot blasting treatment instead of a polishing treatment. Furthermore, acid washing may be adopted as the cleaning treatment. In short, any of various preliminary treatments capable of removing scales will suffice. In order to prevent diffusion of carbon between the outer and inner layers, it is preferred that nickel be plated on one face or the other metal plating attaining a similar effect be formed on one face. A metal foil may be used instead of metal plating.

This embodiment using a metal foil will be described in detail hereinafter.

The fitted pipes 1 and 2 (or pipe 1 and bar 12) are subjected to a reducing treatment and then subjected to a cold drawing treatment using a hydraulic pressure cold drawing bench or chain type cold drawing bench to form a hollow clad billet 3 (or solid clad billet 13), as shown in Figure 1 -(b) as the intermediate material.

Figures 2-(a) to 2-(d) show the step of forming such clad billet 3 or 13. In each of these Figures, the left-hand portion shows the cross-section of the pipes 1 and 2 (or the pipe 1 and the bar 12) in the fitted state, the central portion shows the longitudinal section of the pipe assembly passing through the drawing bench, and the right-hand portion shows the cross-section of the clad billet 3 or 13. Figures 2-(a) and 2-(b) show the preparation of the hollow clad billet 3, Figure 2-(a) showing an embodiment where core drawing is carried out by using a plug 51, and Figure 2-(b) showing an embodiment where blank drawing not using a plug or the like is carried out.Figures 2-(c) and 2-(d) illustrate the preparation of the solid clad billet 13, Figure 2-c) showing an embodiment where a billet suitable for manufacturing a clad steel pipe having an inner layer of a laminated metal is formed and Figure 2-(d) showing a clad billet suitable for manufacturing a clad steel pipe having an outer layer of a laminated metal is formed. A mandrel may be used for core drawing. It is preferred that the drawing operation be carried out once, but such drawing operation may be conducted two or more times.

The degree of working during the drawing step is such that the diameter of the outer pipe 1 is reduced in an appropriate quantity relative to the diameter of the inner pipe 2 or bar 12, and reduction of several % is sufficient. It is desirable that the sectional area be also reduced by the above drawing treatment.

In the resulting clad billet 3 (or 13), the pipe 1 is mechanically bonded strongly to the pipe 2 or bar 12 and no air is left between them.

Then resulting clad billet 3 or 13 is cut into a predetermined length, placed in a heating furnace and heated at a pedetermined temperature.

In the method of the present invention, the pipes 1 and 2 or pipe 1 and bar 12 constituting the clad billet 3 or 13 are strongly bonded to each other. However, in some cases, there is a fear of formation of a minute clearance between the inner and outer layers because of the difference of the thermal expansion coefficient between the two layers. Accordingly, in order to prevent intrusion of air even if such fine clearance is formed, it is preferred that build-up welding be performed along the boundary between the inner and outer layers at the end faces of the clad billet 3 or 13, as shown in Figure 1-(c). If such build-up welding is carried out, even when the above-mentioned fine clearance is formed, scales are not formed at all in this clearance, and, during the subsequent tube-making step, separation of the pipes from each other or the pipe from the bar is completely prevented.However, when the thermal expansion coefficient of the inner metal is larger than that of the outer metal, for example, in the case where the inner pipe 2 or bar 12 is composed of carbon steel and the outer pipe 1 is composed of ferrite type stainless steel, or where the inner pipe 2 or bar 12 is composed of austenite stainless steel and the outer pipe 1 is composed of carbon steel, there is no possibility of formation of a clearance between the inner and outer layers and the above-mentioned build-up welding need not be effected. Furthermore, this build-up welding need not be particularly carried out except in the case where the difference of the thermal expansion coefficient between the inner and outer metals is extremely large, and the length of the clad billet is shorter than a certain value.

After completion of the predetermined heat treatment, the clad billet is transferred to the tube-making step.

Pipe formation may be carried out by using a tube rolling mill or pressing machine. As the tube rolling mill, there may be used a rotary elongator, a plug mill, an assel mill, a mandrel mill, a pilger mill or a reducer. In the case of a hollow clad billet, a pipe is prepared by performing tube rolling with the use of such a rolling mill. In the case of a soled clad billet, the piercing treatment is first carried out by using a piercing mill, for example, a rotary piercer or a press-piercing mill, and the tube rolling treatment is then carried out by using a tube rolling mill such as mentioned above to form a pipe. Through the foregoing steps, an intended clad steel pipe 4 as shown in Figure 1-(d) is obtained. The resulting clad steel pipe may be finished by cold working according to need.When the pressing method is adopted, there may appropriately be chosen an extrusion process such as an Ugine-Sejournet extrusion process or a singer extrusion process or a push bench process such as an Ehrhard push bench process.

A hollow clad billet 3 finished to have a predetermined diameter may be subjected to a lateral pressing step by an extrusion press of the Ugine-Sejournettype after the heat treatment. However, a solid clad billet 13 or a hollow clad billet 3 having an inner diameter corresponding to a guide hole is first subjected to the piercing treatment using a press, and is then subjected to the lateral pressing step. In the case of the solid clad billet 3, of course, a treatment for the information of a guide hole and a conical cone treatment are carried out before the solid clad billet 3 is placed in the heating furnace. Holes for lateral pressing may be formed by a mechanical treatment before the heat treatment. In the case of the Ehrhard push bench process, the solid clad billet 13 is used.After the heat treatment, the solid clad billet 13 is processed into a bottomed hollow pipe by using an Ehrhard press, and, then, the bottomed hollow pipe is subjected to the reducing treatment using a tandem die or single die.

When the pipe-forming step of the method of the present invention is carried out according to the Ehrhard push bench process, the solid billet is not limited to a circular billet, but an angular billet may also be used.

The clad steel pipe thus obtained by the pressing method [see Figure 4-(d)l may be finished by cold working according to need.

Embodiments where clad steel pipes are prepared by using austenite stainless steel as the laminated metal of the inner layer will now be described.

Example 1 Referring to Figure 1, an outer pipe 1 composed of carbon steel having a carbon content of 0.20%, which has an outer diameter of 214mm, an inner diameter of 151 mm and a thickness of 31.5mm, and an inner pipe 2 composed of austenite stainless steel having a carbon content of 0.07%, a nickel content of 8.5% and a chromium content of 18.0%, which has an outer diameter of 148mm, an inner diameter of 122mm and a thickness of 12.5mm, are used.The inner face of the outer pipe 1 and the outer face of the inner pipe 2 are polished, and both the pipes are fitted together and subjected to a reducing treatment and then to a cold drawing treatment using a hydraulic pressure drawing bench of 200 tons, in such manner as can be seen in Figure 2-(d) to obtain a hollow clad billet 3 having an outer diameter of 205mm, an inner diameter of 121 mm and a thickness od 42mm [30mm (outer layer) + 12mm (inner layer)j. The clad billet is cut into a predetermined length, the boundary area between the inner and outer layers is welded on both the end faces and the billet is heated at 1170"C, for 100 minutes by a rotary hearth furnace.Then, the hollow billet is subjected to tube rolling using a rotary elongator so that the outer diameter if 223mm, the inner diameter is 198mm and the thickness is 12.5mm. The hollow shell is then subjected to tube rolling using a plug mill so that the outer diameter is 217mm, the inner diameter is 196mm and the thickness is 10.5mm. The tube is then treated by a reeler so that the outer diameter is 230mm, the inner diameter is 209.5mm and the thickness is 10.25mm, and, finally, the tube is subjected to a sizing treatment using a 6-stand sizing mill to obtain a clad steel pipe having an outer diameter of 219mm, an inner diameter of 198mm and a thickness of 10.5mm [ 8mm (outer layer) + 2.5mm (inner layer)j.

When the resulting clad steel pipe is subjected to a supersonic flaw detecting test along the entire length and periphery thereof, it is found that the base metal of the outer layer and the laminated metal of the inner layer are metallurgically bonded completely along the entire pipe length of about 7.7m except 20cm of the top portion and 5cm of the bottom portion. The reason why metallurgical bonding is insufficient in small top and bottom portions is that during the tube-making step, both the end faces of the hollow billet are tucked in, and the resulting insufficient metallurgical bonding is irrelevant to the characteristic features of the present invention.

Figure 3 is a metal microscope photograph of 100 magnifications which shows one section of the clad steel pipe obtained according to the above procedures in a portion close to the pipe end. From Figure 3, it will readily be understood that the carbon steel of the base metal (the pearlite micro structure in the upper portion of the photograph) and the stainless steel of the laminated metal (the austenite micro structure in the lower portion of the photograph) are metallurgically integrated and bonded.

The following effects can be attained by the above-mentioned embodiment of the method of the present invention.

(1) Since the clad billet used as the material during the tube-making step is obtained by drawing in combination at least two pipes or at least one pipe and one bar by cold drawing, air is not left at all in a clearance in the bonded interface. Therefore, no scale is formed in the interior of the billet during the heating step, and the inner and outer layers are metallurgically bonded to each other completely by the tube-making operation conducted at high temperatures. Moreover, since discharge of air need not be considered during the hot working step, mechanical processing conducted for the formation of a gas vent hole or the like in the conventional method becomes unnecessary. Furthermore, there is no risk of separation of the inner and outer layers during the hot working operation.

(2) A long clad billet can be prepared and the formed billet may be cut into any desired length according to need. Therefore, the billet-manufacturing efficiency can be enhanced.

(3) Since the base metal and the laminated metal are mechanically completely bonded to each other already at the stage of the billet, both the layers are completely bonded metallurgically throughout the billet from the top to the bottom by hot tube rolling or hot pressing, and the examination pass ratio can be increased.

(4) Priorto formation of a clad billet, polishing need not always be effected on both the inner and outer pipes, and only a preliminary treatment for removing scales is sufficient. For example, if only an acid washing treatment is carried out, good results can be obtained. Accordingly, the preliminary treatment is remarkably simplified according to the method of the present invention, and this advantage is enhanced by virtue of the feature that mechanical processing can be omitted as pointed out hereinbefore.

As will be apparent from the foregoing illustration, according to the present invention, cold drawing is carried out to obtain a hollow or solid billet as the material to be used during the tube-making step, that is, cold drawing is conducted prior to hot working contrary to the technically accepted views held in the art, and, by virtue of this characteristic feature, excellent effects such as mentioned above can be attained in the method of the present invention.

When a clad steel pipe is prepared using carbon steel and stainless steel, in order to prevent diffusion of carbon and carburization, a nickel plating layer having a thickness of 30 to 50 microns is formed, for example, on the peripheral face of the inner pipe 2 (or bar 12). However, costs of plating equipment and accessory means are very large and much labour is necessary for material handling during the plating step. Moreover, since plating is performed by electrolysis, the running cost is increased, and, since the size of the plating tank is limited, the size of length of the clad billet is inevitably limited. Therefore, increases in the length of the clad billet are inhibited and the pipe-manufacturing cost per unit weight is inevitably increased.

These problems can be solved effectively according to the present invention. Namely, the present invention proposes a method in which a metal foil is used instead of the above-mentioned plating layer. This embodiment will now be described.

As shown in Figure 4-(a) or 4-(b), an appropriate metal foil 5, for example, a nickel foil, is spirally wound on the peripheral face of pipe 2 or bar 12, constituting the inner layer, and the foil-wound pipe or bar is fitted into the pipe 1 constituting the outer layer. Then, the pipe assembly is subjected to cold drawing in the same manner as described above to yield a hollow or solid clad billet and then, the resulting billet is subjected to hot working as described above.

In the case of a clad steel pipe having at least three layers, a metal foil is wound also on the peripheral face of the pipe constituting the intermediate layer.

The portion on which a metal foil is wound or the confronting portion, that is, the peripheral face of the pipe 2 or bar 12, or the inner circumferential face of the outer pipe 1 in Figure 4, is subjected to a preliminary treatment such as polishing, acid washing or shot blasting. For winding of the metal foil, it is preferred to adopt a method in which a metal foil tape is spirally wound on the peripheral face of the pipe 2 or bar 12 so that both the end-portions of the metal foil tape are lapped together along a short length, and the entire peripheral face of the pipe 2 or bar 12 is covered with the metal foil tape.

A metal foil tape having a thickness of 20 to 80 microns is preferably used, and, when relaxation during the subsequent step is large, a tape having a larger thickness is used. In view of the adaptability to the winding operation, a metal foil tape having a thickness larger than 30 microns is preferred, and, from an economical viewpoint, a tape having a thickness smaller than 40 microns is preferred.

Embodiments using a metal foil will now be described in detail.

Example 2 A pipe 1 composed of killed steel having a carbon content of 0.18%, which has an outer diameter of 214 mm, an inner diameter of 151 mm and a thickness of 31.5 mm is used as the base metal constituting the outer layer, and a pipe 2 composed of austenite stainless steel having a carbon content of 0.06%, a nickel content of 8.2% and a chromium content of 18.2%, which has an outer diameter of 148 mm, an inner diameter of 122 mm and a thickness of 13 mm, is used as the laminated metal constituting the inner layer.

The inner face of the pipe 1 and the outer face of the pipe 2 are polished, and a nickel foil having a thickness of 50 microns is spirally wound on the outer face of the pipe 2 so that both the end-portions are lapped together along a small length and the outer face of the pipe 2 is entirely covered with the nickel foil. Then, the pipes 1 and 2 are fitted together and the assembly is subjected to a reducing treatment and then to cold drawing using a hydraulic pressure drawing bench of 200 tons to form a hollow clad billet 3 having an outer diameter of 205 mm, an inner diameter of 121 mm and a thickness of 42 mm [30 mm of the outer layer and 12 mm of the inner layer]. The clad billet 3 is cut into a predetermined length, and circumferential welding is performed in the boundary portion between the inner and outer layers on both the end faces of the cut billet.

Then, the billet is hated at 1 170"C. for 100 minutes in a rotary hearth furnace, and the billet cut in the predetermined length is drawn and rolled by a rotary elongator so that the outer diameter is 223 mm, the inner diameter is 198 mm and the thickness is 12.5 mm. The hollow shell is further rolled by a plug mill so thatthe outer diameter is 217 mm, the inner diameter is 196 mm and the thickness is 10.5 mm, and the tube is passed through a reeler so that the outer diameter is 230 mm, the inner diameter is 209.5 mm and the thickness is 10.25 mm.Finally, the tube is subjected to draw-size rolling using a 6-stand sizing mill to form a clad steel pipe having an outer diameter of 219mm, an inner diameter of 198mm and a thickness of 10.5 mm (8mm of the outer layer and 2.5mm of the inner layer).

Figure 5 is a metal microscope photograph of 100 magnifications of a section of the resulting clad steel pipe in the vicinity of the pipe end. From this photograph, it will readily be understood that carbon steel as the base metal (the pearlite micro structure in the upper portion of the photograph) and stainless steel as the laminated metal (the austenite micro structure in the lower portion of the photograph) are metallurgically integrated and bonded completely through the nickel layer as the intermediate medium (the intermediate thin portion of the photograph).

Example 3 An outer pipe composed of low alloy steel having a carbon content of 0.11%, a silicon content of 0.38%, a manganese content of 0.46%, a chromium content of 4.78% and a molybdenum content of 0.04%, which has an outer diameter of 252mm, an inner diameter of 149mm and a thickness of 51.5mm, and an inner pipe 2 composed of austenite stainless steel having a carbon content of 0.04%, a nickel content of 12.5%, a chromium content of 17.7%, a molybdenum content of 2.6% and a titanium content of 0.41%, which has an outer diameter of 145mm, an inner diameter of 98mm and a thickness of 23.5mm, are used.The inner face of the outer pipe 1 and the outer face of the inner pipe 2 are polished, and a nickel foil having a thickness of 80 microns as the intermediate medium is spirally wound entirely on the peripheral face of the pipe 2, so that the ends of the nickel foil are lapped together along a short length. The inner pipe 2 is fitted into the outer pipe 1 and the assembly is subjected to the reducing treatment. In this state, the pipe assembly is subjected to cold drawing using a hydraulic pressure drawing bench of 200 tons to yield a hollow clad billet 4 having an outer diameter of 248.5mm, an inner diameter of 97.9mm and a thickness of 75.3mm (51.75mm of the outer layer and 23.55mm of the inner layer). The clad billet 4 is cut into a predetermined length and circular welding is performed in the boundary area between the inner and outer layers on both the end faces of the billet 4.Then, the billet is heated at 1 1300C. for 30 minutes in an induction heating furnace and the billet is processed by an Ugine-Sejournet extrusion press to form a clad steel pipe having an outer diameter of 1 14.3mm, an inner diameter of 92.6mm and a thickness of 10.85mm (8.55mm of the outer layer and 2.30mm of the inner layer).

Figure 6 is a metal microscope photograph (100 magnifications) of a section of the resulting clad steel pipe in the vicinity of the pipe end. As is seen from this photograph, the low alloy steel as the base metal (the martensite microstructure in the upper portion of the photograph) and the austenite stainless steel as the laminated metal (the austenite micro-structure in the lower portion of the photograph) are metallurgically integrated completely through the nickel layer as the intermediate medium (the intermediate portion in the photograph).

The following effects can be attained according to the above-mentioned embodiment of the present invention, in which a metal foil is wound as an intermediate medium for prevention of carburization: (1) Since a metal foil acting as the intermediate medium is spirally wound on the outer face of a pipe or bar constituting the inner layer, and the pipe or bar is fitted in a pipe constituting the outer layer and subjected to cold drawing, this embodiment is advantageous over the method in which the intermediate medium is formed on the outer face of the inner layer by plating, because special equipment, for example, plating equipment, is not necessary and, hence, the initial cost can be reduced to zero.

(2) Since the intermediate medium is only wound, the length or size of the pipe or bar is not limited and a long billet can be prepared conveniently. In contrast, in the plating method, the size and length of the pipe or bar are limited according to the size of the plating tank. Therefore, the running cost per unit weight is much lower than in the plating method, and the yield can be increased during the pipe-forming step.

(3) The cost of the metal foil to be wound is much lower than the plating cost, and even in view of expenses for labor necessary for the winding operation, the cost is only several % of the cost required in the plating method.

Another embodiment of preparing clad steel pipes, which is different in some points from the foregoing embodiments, will now be described. This embodiment is in agreement with the foregoing embodiments at the point where an intermediate material is formed by cold drawing, and, therefore, this embodiment is included in the scope of the present invention. However, the present embodiment is different from the foregoing embodiment at the point where a bloom (bloom material described hereinafter) is used as the bar to be formed into the inner layer. Accordingly, the intermediate material formed by cold drawing is a clad bloom, and the step of forming a clad billet from this clad bloom by heating and billetting is additionally conducted. This embodiment will now be described in detail.

A pipe and a bloom material are appropriately chosen according to the intended structure of a clad billet to be formed. For example, when it is intended to prepare a clad steel pipe having an outer layer of stainless steel and an inner layer of carbon steel, a bloom material composed of carbon steel and a pipe composed of stainless steel are used. For example, a bloom prepared according to continuous casting may be used as the bloom material, and a seamless pipe or welded pipe may be used as the pipe. As shown in Figure 7-(a), for example, an angular bloom material 22 is freely fitted in an angular pipe 21. The sectional shape of each of the pipe and bloom is not limited to an angular shape. For example, a combination of a circular pipe and an angular or circular bloom material may be adopted. The relation of the interior size of the pipe 21 to the sectional size of the bloom material 22 may be determined so that fitting of the pipe and bloom can be accomplished conveniently. The exterior size and thickness of the pipe 21 and the sectional size of the bloom material 22 may be determined appropriately, so far as the cold drawing step is concerned. In the actual operation, however, these factors are determined in view of the conditions adopted at the subsequent billetting and tube-making steps.

Prior to the fitting step, the inner face of the pipe 21 and the surface of the bloom material 22 are polished.

Shot blasting may be conducted instead of polishing. Furthermore, acid washing may be carried out instead of polishing or shot blasting. In order to prevent diffusion of carbon between the inner and outer layers, it is preferred that the inner face of the pipe 21 and the surface of the bloom material 22 be plated with nickel or that a nickel foil be wound on the bloom material 22. These preliminary treatments are carried out in the same manner as in the foregoing embodiments.

The thus fitted pipe 21 and bloom material 22 are subjected to the reducing treatment on the pipe 21 and to the cold drawing treatment, using a hydraulic pressure cold drawing machine or chain type cold drawing machine, to obtain a clad bloom 23 in which the pipe 21 and bloom material 22 are mechanically bonded closely to each other as shown in Figure 7-(b).

Figures 8-(a), 8-(b) and 8-(c) show the steps of preparing the clad bloom 23. In each Figure, the left-hand portion shows the cross-section of the pipe 21 and bloom material 22 in the fitted state, the central portion shows the longitudinal section of the assembly passing through the cold drawing machine, and the right-hand portion shows the cross-section of the clad bloom 23. Figure 8-(a) shows an embodiment where both the bloom material and pipe are angular; Figure 8-(b) shows an embodiment where an angular bloom material and a circular pipe are used; and Figure 8-(c) shows an embodiment where both the bloom material and pipe are circular. In these Figures, reference numeral 52 represents a die. The degree of working may be such that the pipe 21 is reduced in an appropriate quantity relative to the bloom material 22, and reduction of several % will suffice.It is preferred that cold drawing be carried out only once, but the cold drawing operation may be conducted two or more times. In the thus prepared clad bloom 23, the pipe 21 and bloom material 22 are mechanically bonded together very strongly and tightly, and no air is left between them.

The clad bloom 23 is cut into a predetermined size, placed in a soaking pit and heated at a predetermined temperature.

Also, in this embodiment, in order to prevent formation of scales owing to formation of a clearance because of the difference of the thermal expansion coefficient between the inner and outer metal layers, build-up welding is performed along the boundary between the inner and outer layers on the cut end-face of the clad bloom 23 as shown in Figure 7-(c). This build-up welding need not be particularly carried out except in the case where the difference of the thermal expansion coefficient between the inner and outer layers is extremely large, or the length of the cut clad bloom 23 is extremely short.

After completion of this predetermined heat treatment, the clad bloom is hot-rolled by a blooming mill such as a reversing blooming mill or a continuous billet mill to obtain a rod-like clad billet 24 as shown in Figure 7-(d). Since this clad billet 24 has passed through hot working, that is, blooming, and no air is left between the inner and outer metal layers, both the layers can be bonded completely to each other metallurgically.

The thus obtained clad billet 24 is cut to a predetermined length to form a material to be fed into the tube-making step and the cut billet is then fed to this tube-making step. For the tube-making step, a so-called skew rolling process such as a Mannesmann plug mill process, a Mannesmann assel mill process, a Mannesmann mandrel process of a Mannesmann pilger mill process, an extrusion process such as an Ugine-Sejournet extrusion process or a Singer extrusion process, or a press process such as an Ehrhard push bench process may appropriately be chosen and adopted according to the inteneded use of the product and the material and equipment conditions.For example when the Mannesmann plug mill process is adopted, the clad billet 24 is cut into a predetermined length, heated in a rotary hearth furnace and treated by a rotary piercer, a rotary elongator, a plug mill, a reeler and a sizer in sequence to form a pipe. Thus, a desired clad steel pipe 25 as shown in Figure 7-(e) is obtained. This clad steel pipe 25 may be subjected to cold working according to need.

In the case of the press piercing mill process or Ehrhard push bench process, there may be adopted a method in which an angular billet is formed by blooming and this angular billet is used as the tube-making material.

An example of preparing a clad steel pipe by using austenite stainless steel as the laminated metal for the outer layer according to the above-mentioned embodiment will now be described.

Example 4 An angular pipe composed of austenite stainless steel having a carbon content of 0.07%, a nickel content of 8.5% and a chromium content of 18.0%, which has an outer side length of 500 mm, an inner side length of 408 mm and a thickness of 46 mm, is used as the outer pipe 21, and a continuously cast bloom composed of carbon steel having a carbon content of 0.20%, which has a side length of 400 mm, is used as the inner bloom material 22. The inner face of the pipe 21 and the surface of the bloom material 22 are polished and the pipe 21 and bloom material 22 are fitted together.After the reducing treatment of the pipe 21, the assembly is subjected to cold drawing by using a hydraulic pressure drawing bench of 500 tons in such manner as shown in Figure 8-(a) to obtain a clad bloom 23 having a side length of 492 mm (the side length of the portion of the bloom material 22 is still 499 mm). Then, the clad bloom 23 is cut into a predetermined length, and the boundary area between the inner and outer layer is welded o both the end faces. The clad bloom 23 is placed in a soaking pit and heated at 12500C. for 180 minutes.

The clad bloom 23 is then hot-rolled by a reversing blooming mill to reduce the side length to 285 mm (the thickness of the outer layer is 27 mm and the sectional side length of the inner layer is 231 mm), and the bloom 23 is treated by a 6-stand continuous billet mill to obtain a circular clad billet 24 having a diameter of 197 mm (the thickness of the outer layer is 18.5 mm and the diameter of the inner layer is 160 mm).

The clad billet 24 is heated at 11 700C. for 100 minutes in a rotary hearth furnace and the tube-making operation is carried out according to the Mannesmann plug mill process. The dimensions of the pipe at the outlets of respective mills used at the tube-making step are as follows: Thickeness (mm) Outer Diameter Inner Diameter (inner layer + (mm) (mm) outer layer) Rotarypiercer 205 121 42 (12 + 30) Rotary elongator 223 198 12.5 Plug mill 217 196 10.5 Reeler 230 209.5 10.25 Sizer (6 stands) 219 198 10.5 (3.5+7) The clad steel pipe coming from the sizer is subjected to a supersonic flaw detecting test along the entire length and periphery thereof.It is found that the base matal as the inner layer and the laminated metal as the outer layer are metallurgically bonded completely along the entire length except 20cm of the top portion and 5cm of the bottom portion. The reason why incomplete metallurgical bonding is caused in limited areas of the bottom and top portions is that both the end faces of the hollow billet are tucked in itself. Accordingly, this defect is irrelevant to the characteristic features of the method of the present invention.

Figure 9 is a metal microscope photograph of 100 magnifications of a section of the thus obtained clad steel pipe in the vicinity of the pipe end. From this photograph, it will readily be understood that the carbon steel as the base metal (the pearlite micro-structure in the lower portion of the photograph) and the stainless steel as the laminated metal (the austenite micro-structure in the upper portion of the photograph) are metallurgically integrated and bonded completely.

The following effects can be attained according to the above-mentioned embodiment adopting a characteristic technique of forming a clad bloom as the intermediate material by cold drawing.

(1) Since the clad bloom is obtained by drawing the pipe and bloom material in combination, no air is left in the bonded interface between the pipe and bloom material. Therefore, even if the clad bloom is heated for slabbing, no scale is formed in the interior of the clad bloom, and a completely metallurgically bonded clad billet and, in turn, a completely metallurgically bonded clad steel pipe can be obtained. Further, there is no risk of separation of the inner and outer layers during hot working. Moreover, since both the inner and outer layers are caused toadhere closely to each other by cold drawing, discharge of residual air during hot working need not be taken into account and mechanical processing for forming a gas vent hole or the like need not be performed, as in the foregoing embodiments where a clad billet is once formed.

(2) Since clad processing is carried out during the step of forming a bloom to be used for formation of a billet to be subjected to the tube-making operation, the efficiency of manufacture of billets can be enhanced.

Furthermore, a long billet and, in turn, a long clad pipe can easily be obtained.

(3) In the stage of preparation of the clad bloom, the base metal and laminated metal of the inner and outer layers are mechanically bonded completely, and, therefore, by subsequent hot rolling, complete metallurgical bonding can be obtained throughout the clad pipe from the top to the bottom. Accordingly, the examination pass ratio of the products can be remarkably increased.

(4) As the preliminary treatment to be conducted prior to preparation of a clad bloom, polishing is not always necessary, but a simple scale-removing treatment such as acid washing will suffice. For this reason and, because mechanical processing is unnecessary as pointed out above, the preliminary treatment step can be simplified and the efficiency of the preliminary treatment can be enhanced.

Preparation of clad steel bars and clad wires will now be described.

The principle of preparation of clad steel bars and clad wires is not significantly different from that of the above-mentioned method of preparing clad steel pipes. That is, the characteristic feature resides in formation of an intermediate material by cold drawing.

A bar (inclusive of a bloom material as described hereibefore) is used as the core constituting the inner layer. The process is roughly divided into two embodiments, that is, an embodiment in which a clad billet is formed by cold drawing and an embodiment in which a clad bloom is first formed by cold drawing and, then, a clad billet is formed by heating'and blooming.

The former embodiment is first described. At first, a small-diameter bar 32 is freely fitted in a larger-diameter pipe 31 as shown in Figure 10-(a). The inner diameter of the pipe 31 and the diameter of the bar 32 are appropriately determined so that fitting of the bar and pipe can be accomplished conveniently and so that the frequency of the cold drawing operation is not increased (it is preferred that the cold drawing operation be conducted only once). Furthermore, as in the case of preparation of clad steel pipes, the inner face of the pipe 31 and the outer face of the bar 32 are cleaned prior to the fitting operation. According to need, the surface of the bar 32 may be plated with nickel or a nickel foil may be wound on the bar 32.

After the reducing treatment of the portion of the pipe 31, the assembly of the fitted pipe 31 and bar 32 is cold-drawn by using, for example, a hydraulic drawing machine or chain type drawing maching to obtain a solid clad billet 33 in which the inner face of the pipe 31 adheres closely to the outer face of the bar 32 as shown in Figure 10-(b).

Figures 1 -(a) to 1 -(c) show the states of preparation of various clad billets. In each Figure, the left-hand portion shows the longuitudinal section of the fitted pipe and bar, the central portion shows the longitudinal section of the pipe and bar passing through a die 52 and the right-hand portion shows the section of a clad billet formed by cold drawing.Figure 1 -(a) shows an embodiment in which a solid clad billet 33 having a circular section is formed from a pipe 31 having a circular section and a bar 32 having a circular section; Figure 11-(b) shows an embodiment in which a solid cald billet 33 having a rectangular section is formed from a pipe 31 having a circular section and a bar 32 having a rectangular section; and Figure 11-(c) shows an embodiment in which a solid clad billet 33 having a rectangular section is formed from a pipe 31 having a rectangular section and a bar 32 having a rectangular section. It is preferred that the cold drawing operation be conducted only once, but the cold drawing operation may be conducted two or more times.The degree of working during the cold drawing step is such that the outer pipe 31 is reduced relative to the inner bar 32 and reduction of several % is sufficient. In order to improve close adherence between the pipe and bar, it is preferred that the bar 32 be also reduced in its sectional area and be caused in this state to adhere closely to the pipe 31. In the thus prepared billet 33, the pipe 31 and bar 32 are mechanically bonded to each other strongly and tightly and no air is left between them.

The clad billet 33 is fed into a heating furnace and heated at a predetermined temperature If necessary, the boundary region between the inner and outer layer is welded and sealed on the end face to prevent intrusion of air into a clearnace which may be formed between the inner and outer layers in some cases.

After completion of the predetermined heat treatment, the billet 33 is transferred to the rolling step which is conducted at high temperatures. At this draw-rolling step, the clad billet 33 is rolled by a continuous bar mill according to a pass schedule selected depending on the kind of the intended product.

The hot-rolled clad steel or bar is cooled and cut into an appropriate length, and is subjected to a straightening or heat treatment according to need. In the case of the wire 34, after cooling, the wire is wound in the form of a coil as shown in Figure 10-(c), and is finished by a necessary heat treatment.

An example of preparing a clad bar according to the above embodiment will now be described.

Example 5 A rolled solid billet composed of killed steel having a carbon content of 0.16%, which has a diameter of 191 mm, is used as the bar 32 i.e. the base metal and a pipe 31 composed of austenite stainless steel having a carbon content of 0.06%, a nickel content of 9.5% and a chromium content of 19.0%, which has an outer diameter of 216 mm, an iner diameter of 206 mm and a thickness of 5 mm, is used as the laminated metal.

The peripheral face of the bar 32 and the inner circumferential face of the pipe 31 are polished, and a nickel foil having a thickness of 80 microns is wound as an intermediate medium on the peripheral face of the bar 32 so that both the ends of the nickel foil are lapped together along a short length. Then, the bar 32 is fitted and inserted into the pipe 31, and both the end portions are subjected to the reducing treatment. The assembly is cold-drawn by a hydraulic pressure drawing bench of 200 tons to botain a clad billet 33 in which the outer diameter of the pipe 31 is 200 mm, the diameter of the bar is 190.5 mm and the thickness of the outer layer is 4.75 mm. The clad billet 33 is cut into a predetermined length, and the entire periphery of the bonded area between the inner and outer layers is welded and sealed on each end face.The billet is then heated at 1160 C. for 90 minutes in a heating furnace and passed through a continuous bar mill including roughening stands 1 to 8, intermediate stands 9 to 12 and finishing stands 13 to 16, to obtain a bar in which the outer diameter of the outer layer is 22 mm, the inner diameter of the outer layer is 20.95 mm and the thickness of the outer layer is 0.525 mm. The bar is subjected to a scale-removing treatment using nitric acid-hydrofluoric acid, and is then subjected toa surface-lubricating treatment using a mixed liquid of Ferbond and Bondalube, and to a reducing treatment. The resulting bar is cold-drawn to yield a clad bar in which the outer diameter of the outer layer is 19 mm, the inner diameter of the outer layer is 18.1 mm and the thickness of the outer layer is 0.45mm. Figure 12 is a metal microscope photogrph of 100 magnifications showing a section of the resulting clad wire in the vicinity of the bar end. From this photograph, it will readily be understood that the carbon steel of the inner layer as the base metal (the pearlite micro-structure in the lower portion of the photograph) and the austenite stainless steel of the outer layer as the laminated metal (the austenite micro-structure in the upper portion of the photograph) are metallurgically integrated and bonded to each other completely through the intervening nickel layer.

According to the present embodiment, the effects of obtaining a high-quality clad bar or other linear product metallurgically bonded completely, improving the manufacturing efficiency because of possibility to form a long billet, increasing the yield and simplifying the preliminary treatment can be attained as in the foregoing embodiments directed to the manufacture of clad steel pipes. Furthermore, according to the present embodiment, clad bars and clad wires, which have hardly been manufactured according to the conventional techniques, can be prepared at a high efficiency on an industrial scale. This is a characteristic effect attained according to the present embodiment.

An embodiment where a clad bloom is first prepared by cold drawing, the cold bloom is subjected to blooming to obtain a solid clad billet and the billet is drawn and rolled to form a clad wire will now be described.

When a clad wire comprising, for example, an inner layer of carbon steel as the base metal and an outer layer of stainless teel as the laminated metal is prepared, a bloom material of carbon steel having, for example, a rectangular section and a pipe of stainless steel having, for example, a rectangular section, which is slightly larger in size than the bloom material, are used. A bloom formed by continuous casting may be used as the bloom material as it is or after it has been cut into a predetermined length. The pipe is formed appropriately in conformity with the bloom material by welding, or a seamless pipe or welded pipe may be used as it is. Of course, the sectional shapes of the pipe and bloom material are not limited to rectangular shapes, but they may have a circular or other section.The bloom material 42 is fitted into the pipe 41 as shown in Figure 13-(a) and the assembly is then subjected to cold drawing. The sectional areas of the pipe 41 and bloom material 42 are determined so that the fitting operation can be accomplished conveniently. From the practical view point, it is preferred that the difference of the sectional area between the pipe and bloom material be as small as possible as long as the fitting operation can be performed smoothly, because the frequency of repetition of the cold drawing operation is increased if the difference of the sectional area between the pipe and bloom material is relatively large.Prior to fitting of the bloom material 42 into the pipe 41, the inner face of the pipe 41 and the outer face of the bloom material 42, which are to be bonded together, are polished so as to improve the metallurgical bondability between the pipe 41 and the bloom material 42 and to remove scales. Surface cleaning may be accomplished by shot blasting or acid washing instead of polishing. For example, when the bloom material 42 is composed of carbon steel and the pipe 41 is composed of stainless steel or the like, in order to prevent diffusion of carbon from the bloom material 42 into the pipe 41, a nickel plating layer having a predetermined thickness (a thickness of 30 to 50 microns is ordinarily peferred) is formed on the inner face of the pipe 41 or the outer face of the bloom material 42. Also, of course, a nickel foil may be wound for attaining the same effect.The fitted pipe 41 and bloom material 42 are subjected to the reducing treatment on the pipe 41, and they are passed through a die by using a hydraulic pressure drawing machine or chain type drawing machine and simultaneously cold-drawn to form a solid clad bloom 43 in which the inner face of the pipe 41 adheres closely to the surface of the bloom material 42, as shown in Figure 13-(b).

Figures 14-(a) to 14-(c) show steps of preparing clad blooms from various pipes and bloom materials differing in the sectional shape. In each Figure, the left-hand portion shows the section of the pipe and bloom material in the fitted state, the central portion shows the section of the pipe and bloom material being cold-drawn by the die 52, and the right-hand portion shows the section of the clad bloom obtained by cold drawing.

figure 14-(a) shows an embodiment where a clad bloom 43 having a rectangular section is formed from a pipe 41 having a rectangular section and a solid bloom material 42 having a rectangular section; Figure 14-(b) shows an embodiment where a clad bloom 43 having a rectangular section is prepared from a pipe 41 having a circular section and a solid bloom material 42 having a rectangular section; and Figure 14-(c) shows an accomplished by shot blasting or acid washing instead of polishing.For example, when the bloom material 42 is composed of carbon steel and the pipe 41 is composed of stainless steel or the like, in order to prevent diffusion of carbon from the bloom material 42 into the pipe 41, a nickel plating layer having a predetermined thickness (a thickness of 30 to 50 microns is ordinarily preferred) is formed on the inner face of the pipe 41 or the outer face of the bloom material 42. Also, of course, a nickel foil may be wound for attaining the same effect. The fitted pipe 41 and bloom material 42 are subjected to the reducing treatment on the pipe 41, and they are passed through a die by using a hydraulic pressure drawing machine or chain type drawing machine and simultaneously cold-drawn to form a solid clad bloom 43 in which the inner face of the pipe 41 adheres closely to the surface of the bloom material 42, as shown in Figure 13-(b).

Figures 14-(a) to 14-(c) show steps of preparing clad blooms from various pipes and bloom materials differing in the sectional shape. In each Figure, the left-hand portion shows the section of the pipe and bloom material in the fitted state, the central portion shows the section of the pipe and bloom material being cold-drawn by the die 52, and the right-hand portion shows the section of the clad bloom obtained by cold drawing.Figure 14-(a) shows an embodiment where a clad bloom 43 having a rectangular section is formed from a pipe 41 having a rectangular section and a solid bloom material 42 having a rectangular section; Figure 14-(b) shows an embodiment where a clad bloom 43 having a rectangular section is prepared from a pipe 41 having a circular section and a solid bloom material 42 having a rectangular section; and Figure 14-(c) shows an embodiment in which a clad bloom 43 having a circular section is formed from a pipe 41 having a circular section and a solid bloom material 42 having a circular section. It is preferred that the cold drawing operation be conducted only once, but the cold drawing operation may be conducted two or more times. The degree of working during the cold drawing step is such that the sectional area of the pipe 41 is reduced and the inner face of the pipe 41 is caused to adhere closely to the outer face of the bloom material 42, and relative reduction of several % will suffice. However, it is preferred that the degree of working during the cold drawing step by such that, also, the sectional area of the bloom material is reduced to some extent.

In the thus prepared clad bloom 43, the pipe 41 and bloom material 42 are mechanically bonded to each other strongly and tightly, and no air is left between them. The clad bloom 43 is placed in a heating furnace and heated at a predetermined temperature. Prior to this heat treatment, if necessary, the boundary area between the inner and outer layers is welded and sealed on the end face of the clad bloom so as to prevent intrusion of air when a clearance such as mentioned in the foregoing embodiment is formed.

The clad bloom 43 heated at a predetermined temperature is hot-rolled by a blooming mill such as a reversing blooming mill or a continuous billet mill to effect complete metallurgical bonding and obtain a clad billet 44 as shown, for example, in Figure 13-(c). Since this clad billet 44 has passed through the hot working and rolling steps and no air is present between the metals of the inner and outer layers, oxides such as scales are not formed at all and both the metals can be metallurgically bonded completely. The thus obtained clad billet 44 is fed as the starting material to the step of preparing an intended linear steel product, such as a die steel, a bar or a wire. In the process for preparing a linear steel product, the clad billet 44 is rolled by a continuous wire mill according to a pass schedule corresponding to the sectional configuration of the intended bar or wire.For example, the clad billet 44 is heated at a predetermined temperature in a heating furnace, and it is then hot-rolled by the continuous wire mill to obtain a clad linear steel product having configurations shown in Figure 13-(d).

An example of preparing a clad wire by using austenite stainless steel as the outer layer according to the above embodiment will now be described.

Example 6 A circular pipe composed of austenite stainless steel having a carbon content of 0.08%, a nickel content of 7.35% and a chromium content of 17.9%, which has an outer diameter of 646 mm, an inner diameter of 566 mm and a thickness of 46 mm, is used as the outer pipe 41, and a continuously cast bloom, which has a side length of 400 mm and is composed of carbon steel having a carbon content of 0.17% is used as the inner bloom material 42 i.e. the base metal. The inner face of the pipe 41 and the surface of the bloom material 42 are polished, and the bloom material 42 is fitted into the pipe 41.After the reducing treatment of the pipe 41, the fitted assembly is cold-drawn by using a hydraulic pressure drawing bench of 500 tons in such manner as shown in Figure 14-(a) to obtain a clad bloom having a side length of 480 mm (the side length of the portion of the bloom material is still 400 mm).

Then, the clad bloom is cut into a predetermined length and the boundary area between the inner and outer layers is welded and sealed on each end face. The clad bloom is heated at 125000. for 180 minutes in a soaking pit and hot-rolled by a reversing blooming mill to reduce the side length to 240 mm (the thickness of the outer layer is 20 mm and the side length of the side length of the section of the inner layer is 200 mm).

Then, the bloom is passed through a 6-stand continuous billet mill to form an angular clad billet having a side length of 115 mm (the thickness of the outer layer is 9.75 mm and the sectional side length of the inner layer is 95.5 mm).

The clad billet is heated at 1150 C. for 90 minutes in a heating furnace, and the clad billet heated at 1150 C.

is taken out and passed through a continuous wire mill including roughening stands 1 to 7 and intermediate stands 8 to 15, to effect rolling, and to obtain a rough wire having a diameter of 17.5 mm. Then, the rough wire is passed through a continuous block mill including finishing stands 16 to 25 to obtain a finished wire in which the outer diameter of the outer layer is 5.5 mm, the inner diameter of the outer layer is 4.6 mm and the thickness of the outer layer is 0.45 mm. The finished wire is subjected to a scale-removing treatment using nitric acid-hydrofluoric acid and to a surface lubricating treatment using a mixed liquid of Ferbond and Bondalube.The wire is subjected to a reducing treatment and is cold-drawn to obtain a wire in which the outer diameter of the outer layer is 3.0 mm, the inner diameter of the outer layer is 2.5 mm and the thickness of the outer layer is 0.25 mm. Figure 15 is a metal microscope photograph of 100 magnifications showing a section of the resulting clad wire in the vicinity of the wire end. From this photograph, it will readily be understood that the carbon steel of the inner layer as the base metal (the pearlite micro-structure in the lower portion of the photograph) and the ferrite stainless steel of the outer layer as the laminated metal (the austenite micro-structure in the upper portion of the photograph) are metallurgically integrated and bonded completely.

The following effects can be attained by the present embodiment of the method of the present invention.

(1) Since the clad bloom is obtained by cold-drawing the pipe and bloom material in combination to cause them to adhere closely to each other, no air is present in the boundary area between them, and, even if the clad bloom is heated for blooming, no scale is formed in the interior of the clad bloom at all. Therefore, a clad billet metallurgically bonded completely and, in turn, a completely metallurgically bonded linear steel product can be obtained. Furthermore, there is no risk of separation of the inner and outer layers. Moreover, since complete close adhesion is attained between the inner and outer layer, discharge of residual air during the hot working operation need not be taken into account and the cost of formation of the bloom can be remarkably reduced.

(2) Since the cladding treatment is carried out during the step of preparing a bloom to be used for formation of a billiet to be used as the starting material during the step of forming a linear steel product, the efficiency of manufacture of billets can be enhanced, and a long billet and, in turn, a long linear steel product can be manufactured very easily.

(3) The base metal and laminated metal of the inner and outer layers are mechanically bonded completely in the stage of the billet and complete metallurgical bonding is obtained along the entire length from the top to the bottom by hot-rolling. Therefore, the examination pass ratio of products can be remarkably increased.

(4) Polishing is not always necessary as a preliminary treatment conducted prior to preparation of a clad bloom, and such a simple treatment as acid washing is sufficient if only scales are removed. For this reason and because mechanical processing is unnecessary as pointed out hereinbefore, the preliminary treatment can be simplified.

These effects can be attained in the present embodiments as well as in the above-mentioned embodiments of preparing clad steel pipes and clad linear steel products.

As will be apparent from the foregoing detailed illustration, according to the present invention, by adopting a technique quite contrary to the technically accepted views held in the art, that is, the technique of preparing a material to be subjected to hot working by cold drawing, clad steel pipes, clad steel bars and other clad linear steel products having excellent metallurgical bondability and high quality can be manufactured at a high efficiency in a high yield. Furthermore, novel bars and steel wires which have not previously been marketed, can be manufactured on an industrial scale according to the method of the present invention, and is is expected that these novel products will be applied to various fields where conventional products have not been used. Moreover, when a metal foil is used as an intermediate medium for this prevention of carburization, large equipment such as plating equipment need not be used and clad steel products where carburization is prevented can be manufacture very simply and conveniently. Thus, the present invention makes valuable contributions to the art.

Claims (24)

1. A method of preparing a clad steel product having a laminated structure comprising at least two metal layers, which comprises freely fitting a core composed of a metal to be formed into an inner layer, into a pipe composed of a metal to be formed into an outer layer, directly or through at least one additional pipe composed of a metal to be formed into an intermediate layer; simultaneously cold-drawing the pipes and core of the assembly, to obtain an intermediate material comprising the core and pipes bonded closely to one another; heating the resulting intermediate material at a predetermined temperature; and subjecting the intermediate material to hot working to form a product having a desired shape.

2. A method of preparing a clad steel product according to claim 1, wherein among the metals constituting the inner layer, the outer layer and the intermediate layer, a metal acting as the base metal is selected from the group consisting of carbon steel, alloy steel, stainless steel and nickel-based alloy, and the laminated metal is selected from the group consisting of abrasion-resistant steel, stainless steel, nickel, nickel alloy, titanium, titanium alloy, copper, copper alloy, chromium, chromium alloy, aluminium and aluminium alloy.

3. A method of preparing a clad steel product according to claim 2, wherein the stainless steel is selected from the group condisting of martensite stainless steel, ferrite stainless steel, austenite stainless steel, precipitation hardening stainless steel and chromium-manganese stainless steel.

4. A method of preparing a clad steel product according to claim 2, wherein the nickel-based alloy is Inconel alloy.

5. A method of preparing a clad steel product according to claim 2, wherein the abrasion-resistant steel is selected from the group consisting of high carbon abrasion resitant steel and abrasion-resistant manganese steel.

6. A method of preparing a clad steel product according to claim 2, wherein the core is a pipe.

7. A method of preparing a clad steel product accordinging to claim 1, wherein the core is a bar.

8. A method of preparing a clad steel product according to claim 1 or 7, wherein the intermediate material is a clad bloom.

9. A method of preparing a clad steel product according to claim 1, 6 or 7, wherein the intermediate material is a clad billet.

10. A method of preparing a clad steel product according to claim 1, 8 or 9, wherein the final clad steel product is in the form of a pipe.

11. A method of preparing a clad steel product according to claim 1,8 or 9, wherein the final clad steel product is in the form of a linear product selected from the group consisting of a bar and a wire.

12. A method of preparing a clad steel product according to claim 1, wherein the confronting faces of the pipe and core are subjected to a surface cleaning treatment before the fitting operation.

13. A method of preparing a clad steel product according to claim 1 or 12, wherein a metal plated layer having an appropriate thickness is formed on the peripheral face of the core of the metal to be formed into the inner layer and/or the peripheral face of the pipe of the metal to be formed into the intermediate layer before the fitting operation.

14. A method of preparing a clad steel product according to claim 13, wherein the plated metal is nickel.

15. A method of preparing a clad steel product according to claim 1 or 12, wherein a metal foil having an approrpriate thickness is wound on the peripheral face of the core of the metal to be formed into the inner layer and/or the peripheral face of the pipe of the metal to be formed into the intermediate layer prior to the fitting operation.

16. A method of preparing a clad steel product according to claim 15, wherein the metal foil is a nickel foil.

17. A method of preparing a clad steel product according to claim 8, wherein the clad bloom obtained by cold drawing is heated and subjected to blooming to form a clad billet in which the respective layers are metallurgically bonded and the so formed clad billet is fed to a tube-making step.

18. A method of preparing a clad steel product according to claim 8, wherein the clad bloom obtained by cold drawing is heated and subjected to blooming to form a clad billet in which the respective layers are metallurgically bonded and the so formed clad billet is fed to a linear product-forming step.

19. A method of preparing a clad steel product according to claim 17 or 18, wherein the bonded area between the pipe and core is welded and sealed on the end face of the clad bloom prior to the heating operation.

20. A method of preparing a clad steel product according to claim 9, 17 or 18, wherein the clad bloom obtained by cold drawing, or the clad billet obtained by the blooming treatment, is cut into a predetermined length prior to the heating operation and the bonded area of the respective layers is welded and sealed on the cut end face.

21. A method of peparing a clad steel product according to claim 10, wherein an elongator is used as the tube-making means.

22. A method of preparing a clad steel product according to claim 7, wheren the clad billet which is heated or not heated is subjected to a piercing treatment and is then subjected to the tube-making operation according to a press process.

23. A method of preparing a clad steel product according to claim 10 or 21, wherein the clad billet is heated, subjected to a piercing treatment and the rolled.

24. A clad steel product prepared in accordance with the method of claim 1.