EP0820529B1 - Method of manufacturing hot-worked elongated products, in particular bar or pipe, from high-alloy or hypereutectoid steel - Google Patents

Description

The invention relates to a method for producing a hot-made elongated product, in particular rod or tube made of high alloy or hypereutectoid steel according to the generic concept of the main claim.

High-alloy or hypereutectic steels, especially rolling bearing steel such as B. 100Cr6 form when cooling from high temperatures (1100 to 1250 ° C) Grain boundary carbides and pearlitic structural components. These worsen one mechanical workability and hardenability as well as non-cutting forming. The Suitable structures with spherical cementite can only be processed after long annealing processes (GKZ annealing) of 16 hours and more can be set. There have been a lot of considerations in the past on how to do this Shorten the duration of the soft annealing or even completely replace the soft annealing can.

F. Mladen and E. Hornbogen have examined the influence of a thermomechanical treatment on the mechanical properties of the steel 100Cr6 (Archive Eisenhüttenwesen 49 (1978) No. 2, pages 449 to 453). The austenitizing was carried out at a temperature above the limit temperature for the complete dissolution of Fe ₃ C, which is at a C content of 0.99 parts by mass in percent at a little less than 1100 ° C. Hot rolling was carried out starting at 1100 ° C while cooling down to 720 ° C. The cooling from 720 ° C to room temperature was carried out by water quenching. Details of the forming sequence are not mentioned in this paper. The thermomechanically treated structure shows such a finely dispersed distribution of the carbides that the limits of the resolving power of the light microscope are reached. The more favorable distribution is justified by an increase in the dislocation density and the sub-grain boundaries resulting from the dislocations, which creates new nucleation sites for the carbides.

From DE-PS 2361330 is a process for the production of cylindrical Rolling elements made of steel with 0.7 to 1.2 mass percent in percent carbon known. In this process, the steel wire hot-rolled at 1000 ° C quickly becomes cooled to a temperature corresponding to its lower pearlite level, then isothermally converted and hardness by cold drawing without intermediate annealing brought by 50 HRC. By the rapid cooling of the wire as well as the Subsequent isothermal transformation becomes a structure with fine lamellar pearlite achieved, which makes it possible to close the wire after descaling and phosphating pull without the need for an intermediate annealing.

The object of the present invention is a particularly inexpensive method for the production of a hot-formed elongated product in particular Rod or tube made of high-alloy steel or hypereutectoid steel in particular Rolling steel to specify, in which a structure is created that without a previous one Soft annealing such as B. Annealing on spherical cementite (GKZ) for a chipless Further processing and a final heat treatment is very suitable. Another The task is to create a structure that is without a previous one Soft annealing for subsequent machining with a final one Final heat treatment is also suitable.

This object is with the specified in the characterizing part of claim 1 Features resolved. Advantageous further developments are part of subclaims.

The coordinated process steps make it possible to produce the desired structure, with a Brinell hardness of less than or equal to 280 HB30, preferably less than 250 HB30, being achieved in the case of rolling bearing steel. This structure also makes it possible, for example, to supply hot-worked pipes directly to processing without soft annealing. The optimized manufacturing process is particularly cost-effective because the soft annealing and the associated transport and work steps are eliminated. The processing of the elongated products which are hot-produced according to the invention can be cold drawing or cold pilgrimage or cold rolls or transverse rolls.
The method steps which contribute significantly to the success of the method according to the invention are explained in detail below. After the first transformations and before reheating for the subsequent continuous rolling process, a first process step is the controlled heating or cooling in the sense of temperature compensation over the length and extent of the rolling stock having a different temperature, the controlled compensation temperature being below the predetermined temperature in the reheating furnace. The above-mentioned measure has the purpose, on the one hand, of being able to set the temperature of the rolling stock very precisely, taking into account the control options of the reheating furnace. Furthermore, this measure is intended to ensure that the most exact and reproducible conditions are possible for the temperature-dependent measurement of the wall thickness in the pipe before it enters the reducing mill. The measure to be taken, ie heating or cooling, depends on the thickness of the material to be rolled. In the case of thick-walled tubes, for example in a tube ram bench system, the temperature of the tube after the first forming, punching, elongation and impacting will be above 700 ° C. In such a case, temperature compensation is achieved by controlled cooling to a predetermined compensation temperature in a temperature range between 650 ° and 700 ° C. In the case of thin-walled tubes which lose their temperature very quickly, the temperature is frequently below 650 ° so that the temperature compensation must then take place via controlled heating to a predetermined compensation temperature in the previously mentioned range between 650 and 700 ° C.

The actual reheating takes place at a temperature either below Ac ₁ but above 650 ° C. or above Ac ₁ but below Ac _ma (a = beginning of the area of carbide dissolution). It should be borne in mind that, as is known, the Ac ₁ or Ac _ma temperature is primarily dependent on the carbon content of the material quality used and on the forming history. The former temperature range corresponds to the two-phase region α + Fe ₃ C in the continuous time-temperature conversion diagram (ZTU), the latter to the two-phase region γ + Fe ₃ C.

Another measure for the proposed combination of coordinated process steps relates to the final continuous rolling process, preferably in the stretch-reducing mill. In contrast to other rolling processes, the intervention possibilities in this fast-running continuous rolling process are low. Nevertheless, it is important for the proposed method that, on the one hand, a minimum partial deformation per extension in the reducing mill is expressed as the elongation λ _RW ≥ 1.03 and, on the other hand, a minimum degree of elongation for the total deformation λ ≥ 1.5. In special cases, the total stretch can even be somewhat lower, ie λ ≥ 1.4. In addition, the temperature increase resulting during the rolling due to the dissipation work or a temperature drop resulting due to excessive cooling should be as small as possible. In any case, it must be ensured that the rolling takes place in the respective two-phase area and that the rolling stock has a temperature corresponding to the respective area even when it leaves the last stand. With the preferred rolling in the γ + Fe ₃ C region, this means that the temperature of the rolling stock must not exceed A _cma . This narrow temperature range can be maintained by means of a controlled coolant control in special cases, heat supply by means of an external heating device, and by varying the roller geometry, the roller speed and the stitch take-off. With the roller geometry, the pressed length is of particular importance.

The method according to the invention is generally known for all Pipe production processes applicable, which end up with a reduction mill or without train or sizing mill. For example, this can be a procedure a Rohrkontistraße, Stopfenstraße or an Asselwalzwerk. All it is particularly suitable for the push bench process for the production of seamless tubes Rolling steel suitable. As a feedstock for the process of the invention block casting (forged or rolled) or continuous casting (square or round) come in Question, the continuous casting material in a known manner before rolling use is deformed and annealed. Experiments have shown that the method is special is to be used advantageously if the chemical analysis of the known Rolling steel is modified. This affects the sulfur and Phosphorus content and on the other hand the ratio of chromium to carbon. The The sulfur and phosphorus content should not exceed 0.005 percent by mass amount to possible melting at the grain boundaries with increasing Forming speed taking into account the ratio of manganese to Avoid sulfur by suppressing FeS. This is a risk of melting through the required high forming temperature in the first forming steps given when the strain rates are such that they become one lead to a corresponding temperature increase. For this reason, the The rate of deformation in the first forming stages was chosen so that the Temperature inside the rolling stock, d. H. not at the worst point 1170 ° C exceeds. In addition, low levels of S and P may have a beneficial effect following chipless forming processes.

The lowered S and P levels are also beneficial in secondary metallurgy to set a low oxygen content in the melt, which leads to a Improvement in the oxide purity leads.

The chromium to carbon ratio is said to be in the range between 1.35 and 1.52 preferably 1.45. The carbon content is then, for example, 0.94 Mass fractions in percent and the chromium content about 1.36 mass fractions in percent. This ratio can have a positive effect on the undesired carbide rate become.

The cost advantage resulting from the elimination of the otherwise required Soft annealing results can be increased if one in the case of Rolling steel as a feedstock a continuous casting rod without any pre-deformation, d. H. used in the as-cast state and without prior heat treatment (diffusion).

Another improvement is the cooling process after the last one Forming process. After leaving the rolling mill, the rolling stock is in still air or cooled to a temperature by means of an air shower, which is shown in the ZTU diagram corresponds to a structure above the martensite point and below the nose of the bainite. The formed material is isothermal for several hours in this area held. This practice has been aimed at reducing the Internal stresses found to be favorable. This can be done in the way take place that the cooling bed at a suitable location, for example, insulating covered or the rolling stock is fed to a temperature compensation or tempering furnace becomes.

To harden the individual finished product after machining it is also proposed to save the rolled stock after cooling to 600 heat to 700 ° C, cool and then at 180 to 210 ° C to start. After that, the rolling stock has a corresponding hardness that the the required final hardness of the finished product.

a) Einsparung der Investitionsmittel für einen speziellen Glühofen und der Betriebskosten für die langzeitige GKZ-Glühung.

b) Einsparung von Transport- und Arbeitsgängen (Glühen, Richten) und die damit verringerten Fehlermöglichkeiten führen bei kürzeren Betriebsdurchlaufzeiten zu einem kostengünstigeren warmgefertigten Erzeugnis bzw. zu einem preiswerteren Vormaterial für weitere Umformvorgänge.

c) Bessere Materialausnutzung durch Verkürzung der Arbeitsfolgen und durch geringe Entkohlungstiefen aufgrund des Wegfalls der oxidierenden Glühung. Dadurch ergeben sich geringe Aufmaße und damit kleinere Zerspanungsvolumina sowie für den Kunden die Möglichkeit, seine Spannzangenmaße beibehalten zu können.

d) Das aus dem Walzwerk auslaufende Walzgut hat wegen der abgesenkten Umformtemperatur eine höhere Steifigkeit und wird auf dem Kühlbett ausreichend gerade. Ein Richten kann deshalb im Regelfall entfallen.

e) Das erzeugte Gefüge ist ausgesprochen feinkörnig. Dies führt bei der Vergütung zu einer höheren und homogeneren Härte sowie zu einer besseren Zähigkeit. Das wirkt sich positiv auf die spätere Lebensdauer des Fertigerzeugnisses, z. B. Wälzlager, aus.

f) Das durch die neue Verfahrenstechnologie erzielte Gefüge kann ohne zusätzliche Wärmebehandlung einem Kaltumformvorgang wie z. B. Kaltziehen, Kaltpilgern, Kaltrollen bzw. Querwalzen unterzogen werden. Kaltgezogene Rohre weisen nach einem Spannungsarmglühen die gleichen Eigenschaftsmerkmale wie kaltgepilgerte Rohre auf.

g) Die abgesenkten S- und P-Gehalte sowie die auf die Untergrenze gelegten Cr- und C-Gehalte wirken sich bei der Schmelzenerstellung kosteneinsparend aus. Die Minimierung der Karbidzeiligkeit und die Verbesserung des oxidischen Reinheitsgrades wirken sich steigernd auf die Gebrauchseigenschaften des Fertigerzeugnisses aus.

The proposed new process technology for the production of hot-worked elongated products, in particular rods or tubes made of bearing steel, has the following advantages:

a) Saving the investment funds for a special annealing furnace and the operating costs for long-term GKZ annealing.

b) Saving on transport and work steps (annealing, straightening) and the resulting reduced possibility of errors lead to a cheaper, hot-made product or a cheaper raw material for further forming processes with shorter operating times.

c) Better material utilization by shortening the work sequence and by lower decarburization depths due to the elimination of the oxidizing annealing. This results in small oversizes and thus smaller cutting volumes, as well as the possibility for the customer to maintain his collet dimensions.

d) The rolling stock running out of the rolling mill has a higher rigidity due to the reduced forming temperature and becomes sufficiently straight on the cooling bed. Straightening can therefore generally be omitted.

e) The structure produced is extremely fine-grained. This leads to a higher and more homogeneous hardness as well as better toughness. This has a positive effect on the later life of the finished product, e.g. B. rolling bearings.

f) The structure achieved by the new process technology can be a cold forming process such as. B. Cold drawing, cold pilgrimage, cold rolling or cross rolling. Cold drawn tubes have the same properties after stress relief annealing as cold piled tubes.

g) The reduced S and P contents as well as the Cr and C contents placed on the lower limit have a cost-saving effect when producing the melt. The minimization of the carbide line and the improvement of the oxide purity increase the usage properties of the finished product.

The method according to the invention is explained in more detail using an exemplary embodiment. A hot tube of 40.9 mm outer diameter x 4.8 mm wall thickness made of 100Cr6 is to be produced on a pipe ram bench system. Insert blocks with a length of approx. 850 mm are cut from a continuous casting rod with a diameter of 220 mm and a length of 11000 mm. The 100Cr6 insert blocks are in the as-cast state, ie they are neither heat-treated nor pre-deformed. The cut blocks are placed in a rotary hearth furnace and heated to approx. 1140 ° C. After a total heating time of 150 minutes, the blocks are removed individually from the furnace and, after descaling the press water, fed to the punch press. The first forming into a perforated piece takes place in the punch press. For this embodiment, the hole piece has the following dimensions
Outside diameter 223 mm
Inner diameter 121 mm
Wall thickness 51 mm.

This deformation corresponds to a cross-sectional decrease of 29.4% or an elongation of λ = 1.42. The strain rate in this example is 0.45 s ^-1 and affects the optimal temperature window. Another punching process follows the punch press, namely elongation in a shoulder mill. This deformation creates a sleeve with an outer diameter of 192 mm, an inner diameter of 112 mm and a wall thickness of 40 mm. The decrease in cross-section is 30.7% or the elongation λ = 1.44. In this forming stage, high temperatures arise on the inner surface during rolling. For this reason, special care must be taken at this point to ensure that the temperature on the inside surface of the sleeve does not exceed 1170 ° C, since otherwise internal surface defects can be expected due to melting of the grain boundaries. Changes in the roller speed and in the transport angle can be used as control variables. The third forming step is followed by bumping on the push bench. For the selected final dimension, a bench bench blank is manufactured with an outside diameter of 122.8 mm, an inside diameter of 112 mm and a wall thickness of 5.4 mm. After pushing through a number of stands, the blanks are released from the rod as an internal tool in a release roller mill. The temperature of the slug drops further until the pull-out duo and in the aforementioned case reaches a level in the range of 650 to 700 ° C. After pulling out the bumper, the slug bottom is scooped. According to the invention, prior to the entry of the slug into the reheating device, it is subjected to controlled cooling in order to achieve a uniform temperature distribution in the range between 650 ° C. and 700 ° C. In this case, a temperature level of approx. 670 ° C is aimed for. The slugs are held for a certain time by means of a heat-insulated buffer, so that heat can flow from the areas of the slug with a higher temperature level to the areas with a low temperature level. The thermal insulation ensures that the overall level of the bobbin temperature does not drop below the specified target value. In this example, the temperature of the reheating furnace is set so that the temperature of the material to be formed is approximately 740 ° C. At this temperature, the billet enters a stretch-reducing mill. This consists of a large number of three-roll stands, which are each offset by 120 ° in a rolling line. 19 frames are used for the selected example with the final dimensions of 40.9 x 4.8 mm. The partial forming in the basic scaffolding is estimated to be between 7.1 and 8.1% decrease in cross-section. The total deformation is 72.7% corresponding to an elongation λ of 3.66. The forming conditions are chosen such. B. by the choice of calibration and rolling speed and the setting of the cooling that a slight temperature increase up to 760 ° C is allowed. This ensures that the forming in the stretch-reducing mill takes place completely in the two-phase region γ + Fe ₃ C. The tube made of 100Cr6 rolled in this way has, after cooling, a structure that approximates the GKZ structure. The finely dispersed structure consists of molded cementite with minor pearlite residues. The Brinell hardness of the tube thus produced is less than 250 HB30. The spread of hardness values is low. The structure is finer than after conventional GKZ annealing as the comparison of Figure 1 with Figure 2 shows.

• gezielte Temperaturführung vor dem Eintritt in den Wiedererwärmungsofen
• abgesenkte Temperatur des Wiedererwärmungsofens im Vergleich zur üblichen Fahrweise
• Walzen im Zweiphasengebiet
• Entfall der GKZ-Glühung von mehr als 16 Stunden

erzielt man gegenüber dem bekannten Stand der Technik eine viel dünnere entkohlte Schicht. Die für die spanende Bearbeitung erforderlichen Rohraufmaße können deshalb verringert werden. Trotz eines Spannungsarmglühens nach dem Richten weisen kaltgezogene Rohre, die ein entsprechend der erfindungsgemäßen Verfahrensweise erzielbares Gefüge aufweisen, die gleichen Eigenschaftsmerkmale wie kaltgepilgerte Rohre auf.The tube produced according to the procedure according to the invention can be further processed without cutting or without additional heat treatment. For example, this can be cold drawing. By the chosen procedure

• targeted temperature control before entering the reheating furnace
• Lower temperature of the reheating furnace compared to the normal driving style
• Rolling in the two-phase area
• No more than 16 hours of GKZ annealing

one achieves a much thinner decarburized layer compared to the known prior art. The pipe allowances required for machining can therefore be reduced. Despite stress relief annealing after straightening, cold-drawn tubes which have a structure which can be achieved in accordance with the procedure according to the invention have the same properties as cold-piled tubes.

To distinguish the new process technology from the known state of the art To make technology clear, the same final dimension is 40.9 mm outer Diameter x 4.8 mm wall thickness made of 100Cr6 also rolled in the usual way been. The hardness determined on these tubes is 328 HB30 with one adjustment of the reheating oven to 1000 ° C. This hardness is so high that before a Further processing a GKZ annealing is required.

When manufacturing thick-walled hot pipe dimensions, for example 60.3 x 8.0 mm, it is advantageous to control the cooling according to the ZTU diagram so that above the martensite point but below the bainite nose isothermal Hold time is introduced. The temperature range is preferably between 240 and 300 ° C. After holding in this temperature area for more than 3.5 hours can be cooled to room temperature.

Claims (10)

1. A process for the production of a hot-manufactured elongate product, in particular a rod or tube of high-alloy or hypereutectoid steel, in which the feed length of the selected feedstock is heated to deformation temperature, after one or more shaping steps is re-heated to a temperature below the original deformation temperature, and is shaped into the final dimension by means of a multi-stand reducing mill by continuous rolling and is then cooled in still air,
   characterised in that
   after the first shaping steps and before the reheating, a temperature distribution which is uniform when viewed over the length and thickness of the intermediate product is produced by controlled heating or cooling to a predetermined temperature within a certain temperature range and the subsequent reheating to a temperature either below Ac₁ but higher than 650°C or above Ac₁ but below A_cma takes place in the two-phase region and the shaping and cooling and also the optionally additional separate heating in the reducing mill are adjusted such that the temperature increase in the rolled stock relative to the initial temperature remains low and the rolled stock during the shaping in the reducing mill and on leaving the rolling mill is in the two-phase region in terms of temperature, the minimum deformation as elongation expressed for the total deformation λ being ≥ 1.5 and for the minimum partial deformation in the individual stand of the reducing mill λ_RM being ≥ 1.03.
2. A process according to Claim 1, characterised in that the predetermined temperature for the temperature equalisation before reheating lies in a range between 650°C and 700°C.
3. A process according to Claims 1 and 2, characterised in that the reheating of the rolled stock takes place to a temperature in a range of more than 650°C but less than 710°C or in a temperature range of more than 710°C but less than 880°C.
4. A process according to Claims 1 to 3, characterised in that the temperature increase or the temperature drop of the rolled stock in the reducing mill by means of controlled adjustment of coolant in exceptional cases supply of heat by means of an external heat means and also by means of the variation of the roll geometry, the rolling speed and the reduction per pass is kept within narrow limits [sic].
5. A process according to Claims 1 to 4, characterised in that when using a hypereutectoid steel, in particular anti-friction bearing steel, the sulphur and phosphorus contents are at most each 0.005 mass percent and a range between 1.35 and 1.52 is selected for the chromium/carbon ratio.
6. A process according to Claim 5, characterised in that the Cr/C ratio is preferably 1.45.
7. A process according to Claims 1 to 6, characterised in that a continuously-cast rod without any deformation, i.e. in the as-cast condition and without prior heat treatment (diffusion annealing) is used as feedstock.
8. A process according to Claims 1 to 7, characterised in that the deformation rate in the first shaping steps before the temperature equalisation is selected such that the maximum temperature in the interior of the rolled stock does not exceed 1170°C.
9. A process according to Claims 1 to 8, characterised in that the rolled stock after rolling to a temperature above the martensite temperature and beneath the bainite "nose" (continuous TTT diagram) is cooled and held for a relatively long time and then cooled to room temperature in known manner.
10. A process according to Claims 1 to 9, characterised in that the finish-rolled product after cooling to a temperature between 650 and 700°C is heated and held for a predetermined time and then cooled again, and then tempering is effected at a temperature between 180 and 210°C with subsequent cooling in still air.

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