Hot dipped steel tube and a method for producing the same

Abstract

A hot dip-metal coated steel tube having a high corrosion resistance and an optionally beautiful appearance, and a method and apparatus for manufacturing a continuous length of the said tube from a steel strip automatically in an in-line process is disclosed. A plurality of uniformly distributed dents are formed in the surface of a continuous length of steel strip. The dented strip is formed into tubing and a metal coating is applied to the tubing. The uniform distribution of the small dents on the surface of the tubing permits a heavy metallic coating to be applied to the tubing and improves the bond between the coating and the surface of the tubing, and the appearance of the tubing is improved. The continuous manufacturing system moves the tubing along its length from a welding apparatus through a temperature controlling device, wherein the heat from the hot welded seam is dispersed over the tube circumference to provide an efficient and uniform pickling in a subsequent pickling step without any local over pickling or under pickling on the tube surface, and the oxide film formed at the welded seam of the tube is removed by means of pickling.

Description

BACKGROUND OF THE INVENTION

Hot dip-metal coated steel tubes are normally manufactured either by dipping precut tubes in a molten metal or by passing a tube of a continuous length through a molten metal bath.

In a continuous hot dip-metal coating process of steel sheet or wire, entry into and exit from the molten metal bath usually are made in a vertical direction because the sheet or wire can be bent in the bath with ease. The excess molten metal adhered to the sheet or wire drips back to the bath as the sheet or wire is drawn in an upward direction from the bath and, consequently, does not cause uneven metallic coating over the surface of the sheet or wire.

In a process in which a steel tube of a continuous length is hot dip-metal coated on its outside surface and then cut to desired length, the steel tube is manufactured from a continuous steel strip and is subsequently hot dip-metal coated in a continuous in-line process. Once the tube is formed, it is not bent and is moved horizontally through the several stages of the manufacturing process. Dripping of the excess molten metal from the tube surface results in an uneven metallic coating over the tube circumference since the tube exits the molten metal bath in a horizontal direction.

In order to prevent uneven coating due to dripping of molten metal, the excess molten metal should be blown off by an air or gas jet immediately after the tube exits from the molten metal bath. Even with this blowing step, it has been difficult to form a heavy metallic coating for higher corrosion resistance as the steel tube surface is too smooth to hold enough molten metal without dripping.

SUMMARY OF THE INVENTION

This invention relates to a hot dip-metal coated tube of beautiful appearance and high corrosion resistance, and a method for manufacturing a continuous length of steel tube from any steel strip by forming a number of small dents with a depth of several microns to a hundred microns and a diameter of several microns to several hundred microns evenly over the surface to receive a heavier metallic coating as well as improve the bonding strength of the coating layer. The resulting product has a uniformly indented surface appearance with a glossy light silver metallic color.

From the above standpoint, an object of this invention is to produce a steel tube having high corrosion resistance and optically beautiful appearance by forming a number of dents with a depth of several to a hundred microns and a diameter of several microns to several hundred microns over the steel tube surface. Uniform distribution of these small dents enables the tube to hold more molten metal on its surface and to form a thick coating layer with a strong bonding.

Another object of this invention is to provide a method for manufacturing the hot dip-metal coated steel tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are schematic illustrations of the manufacturing process of the steel tube according to the present invention, with FIG. 1 illustrating a complete processing line and FIGS. 2 and 3 illustrating variations in the latter portion of the process illustrated in FIG. 1.

FIG. 4 shows dents bored on the steel tube surface.

FIG. 5a is a photograph showing a cross section of the steel tube surface which has been hot dip galvanized without forming any dents on the base metal. This figure is representative of the prior art.

FIG. 5b is a photograph showing a cross section of a hot dip galvanized layer over an indented steel tube surface in accordance with one embodiment of this invention.

FIG. 6 is a magnified photograph of a typical galvanized steel tube surface made by the process of this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIGS. 1 to 3, the apparatus used to perform the process includes an uncoiler 1 for feeding a steel strip 2 from a roll to a shear-end welder 3. A looper system 5 is provided for feeding the strip continuously to the system without any interruption while the trailing end of the exhausted roll of steel strip and the forward end of a new roll of steel strip are connected together by the shear end welder 3. A strip cleaning device 7 receives the strip from looper system 5 and removes oil, stain, water, etc. adhered to the steel strip. The continuous strip 2 is moved along its length from strip cleaning device 7 into shot blast machine 9 which functions to further clean the strip and to form a number of dents evenly about the steel strip surface, said shot blast machine consisting of three compartments. The center compartment 15 is a blast room wherein abrasive blasting wheels 17a, 17b, 17c and 17'a, 17'b and 17'c are respectively arranged above and below the path of the steel strip to throw abrasive particles onto both sides of the strip. The blast wheels 17a, 17c and 17'a, 17'c positioned near the inlet and outlet of blast room 15 are designed to throw abrasive particles towards the center of blast room 15 in order to prevent leakage of the abrasive.

The abrasive flow of each blast wheels 17a, 17b, 17c, and 17'a, 17'b, 17'c is remote-controlled according to strip feeding speed, strip material, and the surface conditions of the strip. During a transition period of starting or stopping the steel strip, the abrasive flow is regulated by an auto-control system for preventing over blasting and under blasting.

Front and rear compartments, 13 and 19, respectively, are seal rooms for preventing abrasive particles from coming out of the shot blast machine 9. The rear compartment 19 incorporates a strip cleaner 21 for removal of scale and iron powder on the steel strip as well as for prevention of the steel strip carrying out abrasive. The strip guides 11a, 11b, 11c and 11d are positioned inside the shot blast machine 9 and the inlet and outlet of shot blast machine 9 for guiding the fast moving steel strip through the shot blast machine, enabling the strip to have a uniform and efficient shot blasting over its surfaces.

A cold-roll-forming machine 23 receives the strip 2 from shot blast machine 9 to roll-form the continuous length of steel strip into a tubular shape. A seam welder 25 welds along the length of the tubing to close the tube and form a seam, and temperature controller 27 cools the welded tube to a proper pickling temperature and distributes the heat from the welded seam about the circumference of the tube.

The steel tube is extremely hot at the welded seam after welding and the seam of the tube would carry excessive heat into the subsequent pickling device unless it is cooled in the temperature controller 27 prior to pickling. The heat distribution over the tube circumference is also corrected in temperature controller 27 in order to obtain uniform pickling over the surface. The controller applies water to the tube to wash off the iron powder and scale adhered to the tube surface in the shot blasting, roll-forming or welding steps and to prevent pollution and consumption of acid. At the outlet of temperature controller 27, an air blower 29 is incorporated for removal of water from the steel tube surface, preventing the pickling solution from becoming diluted with water.

The continuous tubing is moved along its horizontal path progressively to pickling device 31 which removes oxide film from the steel surface, to a water rinse device 33, to a prefluxing device 35 for applying flux solution to prevent oxidization of the surface in the subsequent steps, to a drying or preheating device 37, to a hot dip metal coating device 39, to a blow-off device 41 which blows air or inactive gas against the tubing to remove excess molten metal adhered to the tube surface and to prevent dripping of the molten metal, to a water quench device 43, to a sizing machine 45 for cold-roll-forming the tube to a desired shape or cross sectional dimensions, to a straightener 47 for correcting any bend of the tube, to a surface treatment device 49, and to a tube cut-off machine 51.

OPERATION

The steel strip 2 is fed from uncoiler 1 through looper 5 to strip cleaner 7 wherein oil, stain and moisture are eliminated. The strip which passes through cleaner 7 is fed into shot blast machine 9 and subjected to bombardment of abrasive. The particle sizes and the amount of abrasive mixture thrown by blast wheels 17a, 17b, 17c and 17'a, 17'b, 17'c are determined in accordance with the surface conditions and quality of the steel strip and subsequent coating weight of the metal to be applied to the tubing. The shot blast machine removes the iron oxide and other contaminants from the continuous steel strip and forms a plurality of dents in the steel strip with a range of depths from a few microns to a hundred microns and a range of diameters from several microns to several hundred microns, and the dents are formed uniformly over the steel strip surface. The abrasive, scale, iron powder, etc. adhered to the strip surface are removed in seal room 19 by surface cleaner 21.

The steel strip with a number of dents bored over its surface by shot blast machine 9 is hauled into forming machine 23 and cold-rolled to a tubular form, and the seam is welded along the length of the tubing by seam welder 25. Due to weld heat, the seam and the tubing circumference are covered with an oxide film with a thickness proportional to the surface temperature. The thickness of the oxide film is not even over the circumference of the tubing.

The steel tube is then fed to temperature controller 27 where heat distribution over the tube circumference is controlled by cooling water in such a manner that the pickling of the metal will be accomplished evenly over the surface of the tubing within the same period without causing a local over-pickling or under-pickling. The steel tube with a controlled surface temperature is fed into pickling device 31 after removal of water by air blow-off device 29. As pickling speed is proportional to the surface temperature of the steel tube, the tube should be, for an efficient operation, warm enough to shorten the pickling time when it enters pickling device 31. On the other hand, if the tube is too warm, it carries excessive heat into the pickling solution and causes an excessive consumption of acid. The heat distribution over the tube circumference is also controlled in such a manner that the thicker oxide film portion is warmer than the thinner oxide film portion, enabling the tube to be pickled evenly over the circumference within a predetermined time.

The tube is then rinsed by water rinse device 33 and fed to prefluxing device 35. Due to the number of dents formed by the shot blast, the steel tube can hold a heavier flux coating on the surface which will provide greater resistance to higher temperature conditions for a longer time in the subsequent steps.

The flux-coated tube is then fed to drying and preheating device 37 and subsequently to the hot dip metal coating device 39, wherein molten metal is applied to the surface of the steel tube. The excess molten metal on the steel tube is removed by blow-off device 41 to avoid dripping of the molten metal. Since the steel tube has a larger surface because of the number of dents, it can hold more molten metal on the surface without dripping. Accordingly, a heavier metallic coating is provided without growth of the alloy layer. The coating layer is superior not only in corrosion resistance but also in mechanical and physical properties, since growth of the alloy layer of poor mechanical properties is kept small while bonding of the coating layer is strengthened by the number of dents on the base metal.

In hot dip galvanizing, for example, the base metal, i.e., the surface of the tube, has an alloy layer of iron-zinc, on which a pure zinc layer is formed. According to this invention, a heavier zinc coating can be accomplished without growth of iron-zinc alloy since the galvanized layer of this invention consists mostly of a pure zinc layer over a relatively thin alloy layer. This will help shorten the dipping time and permit a higher rate of production by a smaller equipment.

The tube passes through blow-off device 41 where the coating weight is controlled and enters water quench device 43. Then it is cold-rolled by sizing machine 45 to a desired shape or cross-sectional dimensions. In this process, the coating layer which is softer than the base metal is subjected to plastic deformation and bonded more firmly to the base metal. Thus, the finished coating layer is of a strong and fine structure. Furthermore, the finished surface is a smooth, lustrous, and beautiful silver-colored aventurine face.

The steel tube is fed to straightener 47 for correction of any bend after the sizing and fed to cut-off machine 51 and cut to a specified length.

The method of this invention is summarized as follows:

1. The shot blast machine 9 does not incorporate any steel strip drive unit. The steel strip is hauled by forming machine 23 and subjected to a proper and constant tension when it passes through shot blast machine 9, wherein the abrasive particles are thrown to the strip for forming a number of dents evenly over the surface. Because of the tension applied to the strip and the strip guiding system incorporated in the shot blast machine, vibration and/or twist of the strip is minimized in spite of a high speed feeding of the strip. The steel strip processed at the shot blast machine has a uniform distribution of the dents with a depth of several to a hundred microns and a diameter of several to several hundred microns over its surface.

As illustrated in FIG. 4, the dents formed on the strip differ from each other from a microscopic viewpoint while the surface roughness of the steel strip is uniform from a macroscopic viewpoint. In FIG. 4, a and b show the dents bored by the first particles thrown and c shows the dent bored by the second particle thrown against the dent previously bored by the first particles. The second particle is not likely to deepen the dent but to bore up the dent since the surface is hardened by the first particles. Consequently, dents of nearly uniform depth are formed all over the surface as shown in FIG. 6.

2. Mechanical removal by means of a rotary brush or the like has been one means for removing the oxide film formed at the welded bead and on the tube circumference. In this existing method, however, a uniform and reliable operation in removal of the oxide film cannot be assured because of wear of the brush or the like. According to the method of this invention, the removal of the oxide film is accomplished by pickling while the weld heat remains on the tube.

The temperature of the steel tube surface, which has risen because of welding, is controlled by temperature controller 29 to provide the tube with a proper temperature distribution corresponding to the thickness of the oxide film. As pickling speed is proportional to temperature, the tube is uniformly pickled by pickling device 31 in the predetermined time without causing local over pickling or under pickling if the tube has a proper heat distribution.

3. In shot blast machine 9, an abrasive mixture is thrown over the surface of the steel strip to form a number of dents with a depth of several to a hundred microns and a diameter of several to several hundred microns evenly on the surface thereof. These small dents play an important role in the following sequence.

i. In prefluxing device 35, the steel tube can hold a heavier and more uniform flux coating without dripping on the surface, enabling the tube to be protected from oxidation for a longer time at higher temperature conditions in the subsequent steps.

ii. In the hot dip metal coating process, more molten metal is held on the steel tube surface, enabling the tube to have a heavier metallic coating without growth of the alloy layer. This coating layer is not only alloy-bonded to the steel tube surface but also is bonded mechanically. Further, the coating layer provides an optically beautiful sheen surface of the aforementioned dents formed over the steel tube surface.

One embodiment of the steel tube produced in the above described process will be shown hereinbelow.

(Shot blasting conditions)

*Quality of steel strip: Hot-rolled steel sheet--- SPHT 3

*surface condition of the steel strip: Strong hot roll scale is formed but no rust is seen over the surface.

*Abrasive Particle: Shot S-60

*abrasive throwing velocity: 88 m/sec

*Amount of abrasive thrown: 80 kg/m.sup.2

(Galvanizing condition)

*Temperature of molten zinc bath: 450.degree.C

According to the above process, dents of approximately 20 microns in depth were formed on the steel strip surface as shown in the photograph of FIG. 5 b and the coating of the steel tube was 33 microns thick on an average. The comparative photograph of FIG. 5 a shows the surface of the steel tube manufactured from the cold rolled steel sheets with no rust on its surface under the same conditions but without shot blasting. The comparative steel tube has a coating layer of approximately 15 microns in thickness, which is much thinner than the coating of the steel tube of this invention.

In the production line configuration shown in FIG. 1, if sizing machine 45 is moved to the position between water rinse device 33 and prefluxing device 35 as shown in FIG. 2, a rough surface finished hot-dip metal-coated tube suitable for painting can be manufactured.

Another version of production line configuration is the replacement of surface treatment device 49 in FIG. 2 with the painting or plastic coating device 53. This arrangement enables paint or plastic film to be overcoated on the rough surface of the hot dip metal coated steel tube in a continuous in-line process.

Claims

I claim:

1. A method of producing a hot galvinized steel tube in a continuous in-line process comprising:

continuously pulling a length of steel strip from a supply and moving the strip along its length through a predetermined path and maintaining tension in the strip as it is pulled from the supply;

continuously shot blasting at least one surface of the strip as the strip is moved along its path at a position along the path where the strip is in tension;

continuously cold-forming the strip into tubing after the strip has been shot blasted with the surface of the strip which has been shot blasted forming the outside surface of the tubing;

continuously hot welding the tubing closed along its length as the tubing continues to move along its length;

continuously cooling the welded tubing and controlling the heat distribution from the weld seam about the circumference of the tubing to maintain the welded seam warmer than the remaining portions of the tubing;

continuously acid pickling the exterior surface of the tubing to remove the oxide film from the surface of the tubing;

continuously applying a hot metal coating to the exterior surface of the tubing;

continuously quenching the tubing; and

continually cutting the tubing into lengths of tubing.

2. The method of manufacturing a metal coated steel tube according to claim 1 wherein the step of shot blasting at least one surface of the strip comprises forming dents in the strip with a range of depths of several microns to several hundred microns and a range of diameters of several microns to several hundred microns, with the dents being uniformly dispersed about the surface of the strip.

3. The method of manufacturing a metal coated steel tube according to claim 1 and further including the step of progressively applying flux material to the exterior surface of the tubing after the acid pickling step and prior to the metal coating step.