SK159796A3 - Method of producing a seamless hot-finished tube - Google Patents

Abstract

During the production of a seamless hot-finished tube from s teel with limited possibilities of changing the shape, particular ly of the steel for roller bearing, in which it is pre-processed and degassed, as well as deoxidized liquid steel of the required chemical composition molten in the zonal cast with an arbitrary c ross-cut and after separating from the zone a rode of the zonal c ast is divided into the used length, submitted to the transformat ion. The used length without pre-shaping, i. e. in the state afte r casting and without thermal processing (diffuse annealing), is heated up to the temperature of transformation and introduced to the device for production of pipes.

Description

METHOD OF MANUFACTURING SEAMLESS-FREE TUBE

Technical field

The invention relates to a process for the production of a hot-formed steel tube of high carbon content, in particular of a super-ovoid steel according to the preamble of the main claim.

BACKGROUND OF THE INVENTION

Due to the high carbon content, the material with the designation DIN 100Cr6 and suitable steel of other standards and rules, from which the rolling bearings are predominantly made in the European region, belongs to the superaduttoid steels. The following methods are used as starting materials for the production of a single roller bearing ring for the production of a hot-formed tube.

Starting from pig iron through an LD steel mill and ladle furnace as well as ladle degassing or alternatively starting from an E steel mill, ladle furnace and furnace degassing and in particular cases through a remelting steel mill, an ingot is rolled into an ingot rolling mill ingot. ingot. This circular ingot is primarily an Assel - shaped tube into a hot-formed tube (see Steel Tube - Manual, 10th Edition, Vulcan Publishing, Essen, 1986, pp. 141-143). The Assel line usually has a rotary hearth furnace as a heating device, followed by a hollow body perforator formed as a punching machine for tube rolling. This hollow body is fed to an Assel rolling mill consisting of three circumferentially spaced cylindrical cylinders which are fitted with calibration. After pulling out the rod, which serves as an internal tool, the tube blank is additionally heated and a hot rolling tube is produced by a reduction mill with a plurality of rolling stands and a calibrating machine connected in series. A disadvantage of this method is that the inserted circular ingot must be close to the dimension of the hot-formed tube and a larger number of rolled or forged material in the form of circular ingots is required to cover the delivery program.

Assel line is a preferred device for the production of tubes for rolling bearings, but also other equipment for the production of tubes, such as extrusion mill equipment or continuous tube production lines, are always used using pre-formed and diffused annealed used material.

It is also known to produce a continuously cast block (mostly in a rectangular shape) instead of an ingot and to convert it into a round ingot by rolling or forging. Alternatively, instead of a rectangular format, a spherical zone casting is produced, and the zone casting is also rolled or forged after separation (see Revue de Metallurgie CIT, April 1989, pages 344 350). According to the state of the art, the degree of deformation is chosen such that a degree of overlap or a degree of shaping of l = 5 is achieved. The selected rolling or forging process is always preceded by diffusion annealing so that the segregates and coarse carbide compounds resulting from the casting process are largely removed or possibly reduced. All of the above-mentioned production methods for the starting material are costly, since large capital-intensive equipment is required for transformation and the material moves multiple times. Since the bars must always be redistributed by stretching, an appropriate amount also falls on the end material. Each additional work and transport step entails the danger of creating a further or increasing error, the elimination of which increases costs.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for the production of hot-formed high carbon steel tubes, in particular of nadadectoid steel, which is more cost-effective compared to the known method and which can achieve better material utilization and shorter material flow.

This task is solved by the features given in the characterizing portion of the main claim. Advantageous further improvements are part of the subclaims.

DETAILED DESCRIPTION OF THE INVENTION

The core of the invention is the use of non-reshaped steel strip castings, in particular of the cross-sectional material group of steel of any cross-section, for the tube making plant, avoiding the previously existing rolling and forging processes and the diffusion annealing required by the prior art. Omitting this step saves considerable costs and time. In addition, the material is better utilized because it does not have to be divided and collected as often. When using such a non-deformed and diffused annealed zone casting, it is necessary to take into account that during the punching process a state of stress is created as part of the entire process of the tube manufacturing plant in the deformed used length, which has the highest negative average value. voltage with _m .

The process according to the invention can be used for all types of zone castings as perpendicular zone castings or arc zone castings and irrespective of the cross-section, ie whether they are square, octagonal, polygonal or spherical. The produced zone casting is, after being split at the lengths used without preforming, i.e. in the post-cast condition, heated to the deformation temperature and then fed to the tube making apparatus. The method of extrusion mill and extruder has proved to be particularly advantageous. In all of the above methods, the heated punching process length used is embedded in a spherical die and, as punched, an average stress value of _{m is} formed in the reshaped workpiece as mentioned _above . This prevents the material from tearing. However, other punching units with an unoccupied working space, for example a conical punching machine for tube rolling, can be used for this in the continuous tube manufacturing or automation process. However, it must be ensured by the selection of the roller geometry, the adjustment and the type of guides that a stress condition with _m is also created, where the fraction of the tensile stresses is minimized, so that the risk of tearing the workpiece in the punching process is no longer possible.

The use of a straight zone casting, in contrast to the commonly used preformed and diffused annealed material, determines a zone casting having as homogeneous and fine a dispersion distribution of the primary carbides as possible and a high degree of purity as much as possible as finely grained as possible. and a casting process. A high degree of purity is desirable, since non-metallic inclusions, necessarily arising in the manufacture of steel, remain in their original particle size and count per unit weight until use as a starting material for the tubes and are not stretched and altered by deformation. This relationship also applies in the same way to the mean grain size. The reduction of the coarse carbide deposition is associated with the omission of the hitherto conventional diffusion annealing which, in the conventional process, takes care of equalizing the carbon concentration and homogenizing the structure.

In order to produce a continuously cast material thus characterized as a material used for tube production equipment, the invention provides for a modified metallurgy as well as special precautions for the casting of the zone casting. This also includes an adapted pouring speed in the range of, in particular, 1.8-1.2 m / min. for dimensional range 170 - 240 mm □. These casting rates in conjunction with the conscientious metallurgy of the ladle and the favorable adjustment of the screens with respect to air lock, non-flammable equipment and steel flow, as well as the special submerged effluent and steel composition adapted to castability and degree of purity, especially Al, S and O2 , guarantee the required degree of purity. The abovementioned casting rate ranges facilitate the rise of blowing and inclusions in terms of a high degree of purity, but are selected such that the desired fine grain size is not limited. The rapid solidification of relatively small cross-sections results in a structure with high fine grain size even after casting, and at the same time precludes coarse carbide precipitation. In view of the cooling seen in cross-section and the distribution of scoops, a rounded format is preferred. In principle, however, it is equally possible to cast a comparable quadrilateral, octagon or polygon. A particularly submerged outflow is characterized in that the diameter is < 25 mm at a suitable height. This has a favorable effect on the ferostatic pressure and on the flow behavior during the casting.

For the special case of nadadectoid steel of the rolling bearing group, a minimum sulfur content is prescribed by the rolling bearing manufacturers so that the hot-formed tube or the separate rings can be machined without any problems by cutting tools. In the sense of a good degree of purity, there should be as little sulfur in the steel as possible on the one hand, but for machining by cutting tools, the steel should have a minimum sulfur content. A range of 0.008-0.015% by weight of sulfur has proven expedient in this respect. If, on the other hand, further processing is largely flakless, then the sulfur content of max. 0.005% by weight is preferred. For both types of further processing, i. particle or chrome-free, a chromium-carbon ratio of between 1.35 to 1.52, preferably 1.45, is preferred. The undesirable rows of carbide can be positively influenced by this ratio.

Furthermore, due to the low oxygen content and fine grain size of the material, a minimum aluminum content is often required. Since generally conventional calcium treatment in terms of the formation of liquid calcium aluminate in order to improve the degree of purity of the steel for rolling bearings is not permitted, the aluminum content must be selected so that the amount of Al 2 O 3 forming in the cast zinc cast is small. Therefore, in the process according to the invention, the addition of aluminum is omitted without the oxygen content reaching undesirable values. The abandonment of the addition of Al does not mean that there is no trace of Al in the melt if it is in spite of the low Al content of the cast iron or lining components.

Based on an arbitrary example, the method of the invention is explained in more detail - the use of a non-deformed and diffusion-annealed zone casting of preferably nadutectoid steel for the production of seamless rolling bearing tubes on a punch press tube machine in which punching conditions result in punching conditions . For example, a 60.3 tepla tube with a 8.0 mm wall of steel for rolling bearings of 100 Cr6 should be produced. Starting from pig iron produced in the blast furnace, in the first step, the iron is sulfur-free, to a extent less than 0.010% by weight. The LD converter with a capacity of up to 250 t is pig iron forged to steel. The slag-free discharge of melting into the ladle is followed by alloying and combined deoxidation exclusively by carbon, silicon and manganese. Otherwise, the usual aluminum additions in the sense of preliminary sedation are not used here for the reasons explained above. In the ladle, the steel is further processed, i. fine alloying occurs in the sense of precisely meeting the specified limits of analysis, degassing including carbon deoxidation in vacuum and an additional purge with inert gas purge. In contrast to the prior art, the use of a ladle furnace has been omitted in this method. The setting of the temperature endurance in terms of adhering to a predetermined casting temperature is guaranteed by a high batch weight with a suitable volume of heat. Thus, on a multi-core arc band casting machine, the casting of a circular format reaches a diameter of 220 mm. The casting speed is about 1.4 m / min. The manifold is provided with baffles for better flow control and the submerged outflow is preferably <25 mm in diameter with another pouring funnel, which always remains filled. The entire casting space is sealed with argon and during casting the melt is not flushed with argon so that no permeation agent is required. Special creep heads are used to control the creep behavior of NAD. After being separated into long rods, they are marked and visually inspected for caries, and then transported to the extrusion mill. For cold saws, the long bars are subdivided at the specified lengths and heated to 1,140-1,180 ° C in a rotary hearth furnace. In the punching press, the first heated zone casting is converted into a hollow body having dimensions of 223.0 x 51.0 mm. This corresponds to an overstretch of 1.39. In the elongator, the hollow body is further drawn to a dimension of 192.0 x 40.0 mm. This corresponds to an elongation of 1.43. Then, the hollow body is drawn, which has a bottom inserted into the extrusion mill. By means of a retracted rod, the hollow body is impacted by a plurality of stools while being extended to a dimension of 127.5 x 8.25 mm. The stretching is 6.18. The rod is pulled out and the bottom area is separated. Up to this working step, the first described heat treatment takes place. In order to be able to carry out the final deformation in the extension reduction mill, the cooled tube blank must be reheated to the deformation temperature. In the draw-through mill, the final dimension of the 60.3 □ x 8.0 mm wall is made without an internal tool, with a draw-out of 2.35. Cooling in air occurs on a conventional cooling bed. Thus, the hot-formed tube is, after soft annealing, a material used for downstream cold forming such as cold rolling, pilgrimage or cold drawing. The rings are cut from these cold-made tubes in automatic lathes and the final contour of the rolling bearing is worked out.

An essential advantage of the above-described method according to the invention is that the hitherto required rolling or forging process, preceded by the starting material for tubes (semi-finished products) of high carbon steels, in particular of overutectoid steels, and diffusion annealing can be omitted and the zinc casting has as well as the described casting conditions, such a structural composition that it can be inserted directly onto the pipe manufacturing device, which in the plant concept preferably has a punching press.

Claims (12)

PATENT CLAIMS 1. A process for the production of a seamless, hot-formed high carbon steel tube, in particular of a nadutectoid steel, in which preconditioned and degassed as well as deoxidized liquid steel in a strip casting in the desired chemical composition is cast with any cross-section and The casting, divided into the length used, undergoes re-shaping, characterized in that the high-purity zone cast material is produced with fine carbide precursors and high grain fineness and that the lengths used are pre-formed, i.e., after casting and without heat treatment (diffusion). annealing) are heated to the deformation temperature and fed to a tube manufacturing plant, preferably having a punching press in the design of its apparatus, wherein during the punching process, as part of the entire process of the tube making plant, Tosti, who is in minimizing shear stresses high negative voltage with an average value of _m. Method according to claim 1, characterized in that in the punching process a tool is used which receives the length in use and which has a closed sheath surface. Method according to claim 1 or 2, characterized in that the length used optionally has a quadrangular or other polygonal, preferably round cross-section. Method according to claim 1, characterized in that in the punching process an aggregate with an open working space is used, in particular a conical punching machine for tube rolling, the length used preferably having a round cross-section. PTF Y / Method according to Claims 1 to 4, characterized in that the steel in the spheroidal casting is cast with a format in the range of 170 mm to 450 mm□ and a casting speed in the range of 1.8 - 0.6 m / min, preferably in the range 1.8-1.2 m / min. for dimension range 170 - 240 mm □. Method according to one of Claims 1 to 5, characterized in that, for the non-deutectoid steel of the rolling stock bearing material group, the liquid steel is initially partially deoxidized exclusively by carbon, silicon and manganese without the addition of aluminum and subsequently completely deoxidized by carbon under vacuum. Method according to claim 6, characterized in that, in the further heat treatment of the tube by cutting tools, the sulfur content is in the range of 0.008 to 0.015 percent by weight and in the further largely chipless treatment the sulfur content is at most 0.005 percent by weight. percentage. Method according to claims 6 and 7, characterized in that a range between 1.35 and 1.52 is selected for the chromium-carbon ratio. Method according to claim 8, characterized in that the Cr / C ratio is preferably 1.45. Method according to any one of the preceding claims 5 to 9, characterized in that, during casting, a submerged outlet having a diameter of <25 mm at a suitable construction height as well as a divider with partitions are inserted into the zone casting device. Method according to any one of the preceding claims 1 to 3 and 5 to 10, characterized in that the pipe manufacturing device is an extrusion mill with a press as a punching unit. Method according to one of the preceding claims 1 to 11, characterized in that the steel production process has the following steps: pig iron production, pig iron desulphurisation, blowing process (LD - converter), ladle tapping (alloying, combined deoxidation with C, Si and Mn without aluminum addition), v - treatment in a ladle without electric arc heating (degassing and 'deoxidation by carbon under vacuum, fine alloying, rinsing), casting in a circular format on an arc-zone casting apparatus while abandoning argon flushing during casting.