DE19520833C2 - Process for the production of a seamless hot-worked pipe - Google Patents

Description

The invention relates to a method for producing a hot-formed tube high-carbon especially hypereutectoid steel according to the generic term of Main claim.

The material with the DIN designation 100Cr6 and corresponding steels of others Standards and regulations, from which mainly roller bearings in Europe produced, is one of the high carbon content hypereutectoid steels. For the production of a hot-worked pipe as Starting material for the production of individual rolling bearing rings are as follows Procedure applied.

Starting from the pig iron via the LD steel mill and a ladle furnace as well as one Ladle degassing or, alternatively, starting from the E steel mill, a ladle furnace and ladle degassing and in special cases via a remelting steel plant a block is cast, which turns into a tube round on a roughing mill is rolled. This round tube is preferably closed using the Assel method formed into a hot-finished pipe (see steel pipe manual 10th edition, Vulkan-Verlag, Essen, 1986, pages 141-143). Assel Street usually points a rotary hearth furnace as a heating system, one as a cross rolling mill trained perforator for producing a hollow body follows. This hollow body an Assel mill consisting of three is evenly distributed over the circumference arranged cross rollers supplied with a shoulder calibration are. After pulling out the rod that serves as the inner tool, this becomes  Intermediate pipe reheated and over a multi-stand reducing mill and over a downstream sizing mill produces the hot-finished pipe. A disadvantage of this method is that the tube round used close to the dimension must be hot-made pipe and to cover the delivery program A large number of rolled or forged tubular round material is required.

Assel Street is the preferred facility for the production of roller bearing tubes, but also other tube production systems, such as push bench systems or Pipe continuous lines are always used using preformed and diffusion annealed feed.

It is also known, instead of a ingot (ingot), a continuous casting block (bloom) - predominantly in rectangular format - to generate - and this over a rolling or To transform the forging process into a tube round. Alternatively, instead of one Rectangular format produced a continuous casting, with this continuous casting block rolled or forged after separation (see La Revue de Metallurgie CIT April 1989, pages 344-350). According to the state of the art, the degree of deformation chosen such that a forging or rolling degree of λ = 5 is achieved. The The mentioned rolling or forging process is always a diffusion annealing upstream to the segregations resulting from the casting process and coarse To largely reduce or reduce carbide deposits.

From US 4 809 423 a method for producing seamless metal tubes is large Known diameter. Here, thick-walled hollow bodies are first Continuous casting made and cut to certain lengths. Then there is a Rolling to a weight per unit length that corresponds to that of the finished product. This Pipes are then expanded without axial extension and while maintaining the Weight per unit length to the dimensions of the finished pipe.

Another method for producing seamless tubes is from DE 20 44 815 A1 known.

Here, rods are first produced by continuous casting, which are then formed into blocks predetermined weight can be separated. These blocks are then pressed and Punching creates a pipe blank. The blank becomes a tube by extrusion deformed long length, the wall thickness corresponds to that of the desired tube. The final dimensions are then set by rolling.

All mentioned Manufacturing processes for the primary material are costly, since large capital-intensive Plants for the forming are needed and the material is moved several times. Since the bars have to be divided again and again by the stretching, falls also a corresponding amount of cropping material. Any additional work and Transport step means a risk of generating more or intensifying Errors whose elimination increases costs.

The object of the invention is a method for producing a hot-finished Specify tube from high-carbon, in particular hypereutectoid steel, which in Compared to the known method is cheaper and with a higher Material utilization and a shorter material throughput time can be achieved.  

This task is carried out in the characterizing part of the main claim specified features solved. Advantageous further training is part of Subclaims.

The key point of the invention is the use of undeformed continuous casting from steel, in particular from the material group of rolling bearing steels of any cross-section on a tube production plant, eliminating the previously required upstream rolling or forging process and without the diffusion annealing required by the prior art. The elimination of these steps saves considerable costs and time. In addition, the material is used better because it does not have to be divided and scooped as often. When using non-deformed and non-diffusion-annealed continuous casting, care must be taken that during the perforation process as part of the overall process of the tube production system, a stress state is built up in the length of use to be reformed, which has the highest possible negative mean stress value σ _m while minimizing shear stresses.

The method according to the invention can be used for all types of continuous casting, such as vertical continuous casting or curved continuous casting, and regardless of the cross section, ie whether square, octagonal, polygonal or round. The continuous casting block produced is divided into insert lengths without pre-deformation, ie in the as-cast state, heated to the forming temperature and then fed to the tube production system. The push bench and the drawing press method have proven to be particularly advantageous. In all of the methods mentioned, the heated insert length for the punching process is placed in a round die and, as already mentioned, a negative mean stress value σ _{m is} built up during the punching process in the workpiece to be formed. This prevents the material from tearing open. However, other punching units with an unclosed work space, for example a conical cross-rolling mill, connected upstream in pipe continuous or plug rolling processes, can also be used for this. However, it must be ensured through the choice of the roller geometry, the position and the type of guides that a tension state σ _{m also} arises here, in which the proportion of tensile stress is minimized and the risk of the workpiece tearing during the punching process is no longer present is.

The use of direct continuous casting compared to that used conventionally preformed and diffusion annealed material a continuous casting, the one as possible homogeneous and finely dispersed distribution of the primary carbides and a high Has purity and is as fine-grained as possible, characteristics that by the melting and casting process explained below can be guaranteed. Of the high degree of purity is required as that is inevitable in steel making resulting non-metallic inclusions in their original particle size and The number per unit of mass is preserved until use as a tube material and not be stretched and changed by deformation. This connection applies in in the same way for the average grain size. Reducing Coarse Carbide Eliminations are associated with the elimination of the usual ones Diffusion annealing used in the conventional process for one Compensation of the concentration of carbon and for a homogenization of the structure worries.

In order to produce a continuous casting material identified in this way as a feedstock for a pipe production plant, modified metallurgy and special measures for casting the continuous casting are provided as measures according to the invention. This also includes an adapted casting speed in the range of preferably 1.8-1.2 m / min. for the dimension range 170-240 mm ∅. These pouring speeds in connection with careful ladle metallurgy and the favorable design of the distributor with regard to air exclusion, refractory lining and steel flow as well as a special immersion nozzle and a steel composition adapted in terms of pourability and degree of purity, in particular of the elements Al, S and O _2, ensure the desired degree of purity. The casting speed ranges given above make it easier for the bubbles and inclusions to rise in the sense of a high degree of purity, but are selected so that the desired fine grain size is not impaired. The rapid solidification of the relatively small cross sections leads to a structure with a high degree of fine grain even in the as-cast state and at the same time prevents the precipitation of coarse carbides. With regard to the uniformity of the cooling across the cross-section and with regard to the distribution of the segregations, preference is given to the round format. In principle, however, it is also possible to cast a comparable four-octagon or polygon. The special immersion spout is characterized in that the diameter is ≦ 25 mm with a corresponding overall height. This has a favorable effect on the ferrostatic pressure and the flow behavior during casting.

For the special case of hypereutectoid steel from the material group of Rolling bearing steel is a minimum sulfur content by rolling bearing manufacturers prescribed to the hot-finished pipe or the separated rings without Difficulty machining perfectly. In the sense of a good one Purity levels should on the one hand be as little sulfur as possible in the steel be, but have a minimum sulfur content for machining the steel. In this sense, a range of 0.008-0.015 mass fraction has been found to be expedient exposed in percent sulfur. In contrast to this is further processing largely without chips, then a sulfur content of max. 0.005 mass fraction in Percent considered advantageous. For both types of processing, i.e. H. A chromium-carbon ratio in the range between 1.35 is cutting or non-cutting to 1.52, preferably from 1.45 is advantageous. It cannot do that desired carbide line can be influenced positively.

Furthermore, in the sense of a low oxygen content and a fine grain of the material, a minimum content of aluminum is often required. Since a generally customary calcium treatment in the sense of the formation of liquid calcium aluminates to improve the degree of purity for the rolling bearing steel is not permitted, the aluminum content must be chosen so that the amount of Al ₂ O _{3 formed} in the cast continuous casting is small. For this reason, no aluminum is added in the process according to the invention without the oxygen content assuming undesirable values. Dispensing with the addition of Al does not mean that no traces of Al are present in the melt, either because of the low Al content of the alloy components or because of the lining.

Using an arbitrarily chosen example, the method according to the invention is Use of undeformed and non-diffusion annealed continuous casting preferably hypereutectoid steel for the production of seamless roller bearing tubes a tube production plant with a punch press, in which the choice of Forming conditions a compressive stress state arises during punching - closer explained. For example, a hot-rolled pipe is to be manufactured Dimension 60.3 ∅ with 8.0 mm wall made of bearing steel 100Cr6. Starting from  A pig iron produced in the blast furnace is in a first step desulfurized, to a range less than 0.010 percent by mass. over The pig iron is turned into an LD converter with a capacity of up to 250 t Blown steel. Alloying takes place in the slag-free tapping into the ladle as well as a combined deoxidation exclusively with carbon, silicon and Manganese. The usual addition of aluminum in the sense of a Calming is not used here for the reasons explained above. In the The ladle is treated further, i.e. H. fine alloying takes place in the sense a precise fulfillment of the specified analysis limits, a degassing including carbon deoxidation in vacuum and an additional one Purity purge with an inert gas. In contrast to the prior art this method does not use a pan oven. The setting of the temperature balance in terms of compliance with the specified casting temperature is guaranteed by a high batch weight with appropriate heat content. The casting is then carried out in a round format on a multi-core continuous casting machine to a diameter of 220 mm. The casting speed is approx. 1.4 m / min. The distributor is provided with partitions for better flow control and the immersion nozzle preferably has a diameter ≦ 25 mm with a longer one Pouring funnel that always remains filled. The entire casting room is made with argon sealed and when pouring the melt is not flushed with argon, so that no perm plug is required. To master the flow behavior of hypereutectoid steels, special starting heads are used for casting. After When they are cut into long rods, they are marked and visually for defects checked and then transported to the push bench system. They are on a cold saw long bars divided into given lengths and in a rotary hearth furnace heated to 1140-1180 ° C. The first forming of the takes place in the punch press heated continuous casting block to a hollow body with the dimensions 223.0 × 51.0 mm instead of. This corresponds to an extension of 1.39. The hollow body is in the elongator stretched further to a dimension of 192.0 × 40.0 mm. This corresponds to one Elongation of 1.43. Then the elongated hollow body having a bottom becomes inserted into the push bench. The hollow body is inserted by means of an inserted rod pushed through several scaffolds and thereby a size of 127.5 × 8.25 mm lengthened. The aspect ratio is 6.18. The rod is pulled out and the Separated floor area. Up to this step, the previously described is done Forming in a heat. To the final forming in  The cooled intermediate pipe must be able to carry out the stretch-reducing mill heated to the forming temperature again. In the stretch-reducing mill, without an internal tool the final finished size of 60.3 ∅ × 8.0 mm wall with a Elongation of 2.35 generated. The cooling in air takes place on a usual Cooling bed. The hot-produced pipe thus produced is then that after soft annealing Material for the downstream cold forming such as cold pilgrims or Cold drawing. These cold-machined pipes are turned into rings in automatic lathes tapped and the final contour of the rolling bearing machined.

The main advantage of the method according to the invention explained above is therein to see that for tube stock (semi-finished) from high-carbon in particular hypereutectoid steels previously required upstream rolling or The forging process and the diffusion annealing can be omitted and the continuous casting block due to the Al-free metallurgy and the described casting conditions Microstructure formation in such a way that it directly on a tube production plant preferably used a punch press in their system concept, can cry.

Claims (12)

1. A process for the production of a seamlessly hot-formed tube made of high-carbon, in particular hypereutectoidal steel, in which a pretreated and degassed and deoxidized liquid steel is cast in continuous casting in the required chemical composition with any cross-section and, after being separated from the strand, the casting is divided into insert lengths and subjected to forming , characterized in that a continuous casting material with a high degree of purity, with fine carbide precipitates and a high degree of granularity is produced and that the insert length is heated to the forming temperature without any pre-deformation, ie in the as-cast state and without heat treatment (diffusion annealing) and a tube production system, which preferably has a hole press in its system concept has, is supplied, with the hole process as part of the overall process of the tube production plant in the insert length to be reshaped, a stress state is built up which minimizes vo n shear stresses have the highest possible mean stress value σ _m . 2. The method according to claim 1, characterized, that a tool that takes up the insert length during the punching process is used, which has a closed lateral surface. 3. The method according to claim 1 or 2, characterized, that the insert length is either a square or other polygonal preferably has a round cross section. 4. The method according to claim 1, characterized, that during the punching process, an aggregate with an unclosed work area, in particular bevel cross rolling mill is used, the length of use preferably has a round cross section.   5. The method according to claims 1 to 4, characterized, that the steel in the round casting 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 of 1.8-1.2 m / min for the dimensional range 170-240 mm ∅ is shed. 6. The method according to claims 1 to 5, characterized, that for hypereutectoid steel the material group of rolling bearing steels liquid steel initially only over carbon, silicon and manganese Partially deoxidized without the addition of aluminum and then in a vacuum Carbon is fully deoxidized. 7. The method according to claim 6, characterized, that during machining of the hot-finished pipe Sulfur content is in the range 0.008 to 0.015 percent by mass and with largely non-cutting further processing the maximum sulfur content 0.005 percent by mass. 8. The method according to claims 6 and 7, characterized, that for the chromium-carbon ratio a range between 1.35 and 1.52 is chosen. 9. The method according to claim 8, characterized, that the Cr / C ratio is preferably 1.45.   10. The method according to any one of the preceding claims 5 to 9, characterized, that when pouring into the continuous casting system, a dip spout with a Diameter ≦ 25 mm with a corresponding overall height and a distributor is used with partitions. 11. The method according to any one of the preceding claims 1 to 3 and 5 to 10, characterized, that the tube production plant as a push bench with a press Punching unit is. 12. The method according to any one of the preceding claims 1 to 11, characterized in that the steel manufacturing process comprises the following steps:

• - Pig iron production
• - hot metal desulfurization
• - blowing process (LD converter)
• - ladle racking (alloying, combined deoxidation with C, Si and Mn without adding aluminum)
• - ladle treatment without arc heating (degassing and carbon deoxidation under vacuum, fine alloy, purging)
• - Casting in round format on a continuous sheet caster without an argon purge during casting.