CA2192970A1 - Method of producing a seamless hot-finished tube - Google Patents

Abstract

The invention relates to a process for manufacturing a seamless hot-finished tube of steel with reduced capacity for deformation, especially roller bearing steel, wherein a pretreated and degassed and deoxidized molten steel is continuously cast in the required chemical composition and, after separation from the strand, the continuously cast bar, divided into insertion lengths, is subjected to forming. According to the invention, the insertion length is heated to forming temperature and fed to a tube-machine without any prestraining, i.e., in the cast state and without heat treatment (diffusion annealing).

Description

- -M~n nf ~m~nn AG 2 1 9 2 9 7 0 31 216 M~nr;.~,.,"""-rt:l 2 40213 Dusseldorf Method of Producing a Seamless Hot-Finished ~ube DescriptiQn The invention relates to a process for ll.~lll.t~. (,,.,I,~ a hot-finished tube from high-carbon steel, especially lly~lc uLt~oid steel, according to the generic part of the main claim.
The material designated as DIN 100Cr6 and comparable steels of other standards and regulations, from which, in Europe, it is chiefly roller bearings which are m~nllf~ffllred~ are included because of their high carbon content among the lly~ uLf .~id steels. In order to m~nl1f~f~1re a hot-finished tube as the starting material stock for the production of individual roller bearing rings, the following processes are used.
Starting from pig iron via the LD plant, a ladle furnace and a ladle degassing or, alL~ Li~ly, starting from the E plant, a ladle furnace and a ladle degassing and, in special cases, via a remelting plant, a ingot is cast that is rolled into a tube billet on a cogging train. This tube billet is worked into a hot-finished tube, preferably by means of the Assel process (see the Steel Tube Handbook [Stahlrohr-Handb~ch], 10th ed., Vulkan Verlag, Essen, 1986, pp. 141-143). As its heating unit, the Assel train usually has a rotating hearth furnace, which is followed by a piercing device designed as a piercing mill for the purpose of producing a hollow body. ~his hollow body is fed to an Assel rolling mill that consists of three piercing mills arranged in an evenly distributed manner on the perimeter, which have a shoulder calibration. After the withdrawal of the bar that serves as an internal die, the .

~1 216 2 1 92~70 Int,~rmP~iat~- tube is reheated, and the hot-finished tube is produced via a multi-stand reducing mill and a downstream sizing mill. A disadvantage of this process is that the tube billet used must be close in size to the hot-finished tube; furthermore, in order to meet the delivery program, a large quantity of rolled or forged tube billet material is required.
Although the Assel train is the preferred unit for manl1fartllrin~ roller bearing tubes, other tube making units, such as push bench units or continuous tubular trains, are also used, always with prestrained and hom~-g~ni7~ -1 feed stock.
Instead of a cast ingot, it is also known to produce a continuously-cast irlgot (bloom), primarily in a rectangular format, and then to form this bloom into a tube billet via a rolling or forging process. Alternatively, a rourld continuous c st is produced, rather than a rectangular one, and it, too, is rolled or forged following separation (see L~ Revue de Met~llurg~e CIT April 19r9, pp. 344-350). According to the prior art, the degree of forming is selected in such a way that a forging or rolling degree of )\ = 5 is achieved. The afor~mrnti-~n~d rolling or forging process is always preceded by homr,g~ni7ati~ln, in order to largely remove or reduce the segregations and coarse carbide precipitations stemming from the casting process. All of the afor~m~nti(tn~ processes for ~ r~.lll,;"~ feed stock are pensive, because large capital-intensive machinery is needed for forming the material and because the material is moved a number of times. Since the rods must be repeatedly divided up through stretchings, a considerable quantity of crop material also a~llmlllatl~s Each additional work step and transport step poses the danger of producing additional or more serious defects, the ~iminatil~n of which increases costs.
The object of the invention is to indicate a process for " ,A ~ ", r~ , a hot-finished tube from high-carbon steei, especially hypereutectoid steel, which is more ~ nf~miral than the known processes and with which a better utilization of material as well as a shorter material passage time can be achieved.

2i6 21 92970 This object is attained with the features specified in the rhAI Al ~rl ;~;11~ part of the main claim.
Advantageous further dcvcloL)~ are contained in subclaims.
The core of the invention is the use on a tube making machine of rr,ntinllrl1cly cast steel that has not been deformed, cspecially steel from the material group of the anti-friction bearing steels, in any desired cross-section, while omitting the previously usual upstream rolling or forging processcs as well as the homogenization necessary according to the prior art. The omission of these work steps leads to significant savings in costs and time. Furthermore, the material is better utilized, because it docs not need to be divided up and cropped so often.
When such continuously cast stecl which has not been deformed or hr~mng~ni7~ is used, it must be noted that during the piercing proccss (as part of the overall process in the tube making unit) a strcssed state builds up in the inserts to be shaped; this strcssed state has, while shear stresses are minimi7~ the greatcst possible negative stress average m.
The process according to the invention can be used for all typcs of continuous casting, such as vertical rr,ntintlmlc casting and bow-type continuous casting, and can be used regardless of cross-section, i.e., regardless of whether the cross-section is square, octagonal, polygonal or round. After being divided into inserts, the rrlntinllollcly-cast bloom is heated without prestraining i.e., in its cast state, to the forming t~:lllp.ldlUI~:, and is then fed to the tube making unit. The punch-bench process and the drawing-press process have proved to be cspecially advantageous. In all of the afor~m~ntir,nr~l processes, the heated insert is placed into a round tool for the piercing process and, as mentioned earlier, a negative stress average m builds up during the piercing sequence in the workpiece bcing shaped. In this way, cracking of the material is prevented. However, it is also possible to use other piercing devices having non-closed work spaces; for example, an upstrcam conc-type piercing mill can be used in the rr ntinllr,~l~ tubular process or the plug mill proccss. It is still necessary, however, to select the roller geometry, the A~ and the guide type in such a way as to ensure that a stressed state m is created, in which the share of tensile stresscs is ~ mized and -the danger that the workpiece will crack during the piercing process thus no longer exists.
Cornpared to the conventional use of pre-deformed and h~".,..~,. .;,~.1 material, the direct use of continuously cast steel requires rontinl~o~ y cast material which has the finest dispersion and most homogeneous distribution of primary carbides possible, along with the finest possible grain and a high degree of purity. These ~h~r~tPricri~c are provided by the melting and casting process described below. A high degree of purity is necessary because the non-metallic intrusions inevitably created during steel production are retained in their original particle size and number per mass unit up to the point of use as tube starting material; they are not stretched or changed by deformation. This is also true of the medium grain siæ. The reduction the coarse carbide prPririt~tir~n C is connected to the omission of the previously usual h~ullu~~ dLion, which in the conventional process ensures a concentration balance of the carbon and the homogenization of the microstructure.
In order to produce a continuously cast material with these ~h~ra~tPricti~c as the feed stock for a tube making unit, the measures called for according to the invention include a modified metaliurgy as well as special measures to be taken during ~r~ntin1lr~lc casting. These include an adjusted casting speed in the range of, preferably, 1.8 -1.2 m/min for the size range of 170 -240 mm [illegible]. These casting speeds, in ~r~njllnr~ir~n with careful ladle metallurgy, an advduL..~ )u~ design of the distributor in respect to air exclusion, refractory-grade lining and steel flow, a special immPrcion nozzle and a steel composition adjusted, especially [in respect to] the elements Al, S and O~, for the sake of castability and the degree of purity ensure the desired degree of purity. The ranges of casting speed indicated above make it easier to achieve the ascent of blisters and inclusions as related to a high degree of purity, but ne~nPthPIPcc are so selected that they do not interfere with the desired grain fineness. Rapid solidification in the relatively small cross-sections results in a I~ Ll u~Lu~ which has high grain fineness even in the cast state, and also prevents the precipitation of coarse carbides. In respect to the evenness of cooling across the cross-section and the distribution of segregations, the round format must be considered preferable. Basically, however, it is equally possible to cast a comparable square, octagon or polygon. The special immersion nozzle is characterized by a - ~1216 21 92970 diameter of c 2s mm at a . . " ,.,.,. ,~, dle structural height. ~his has an ddvd~ Ju~> effect on the ferrostatic pressure and the flow behavior during casting.
For the special case of lly~tltuLe~toid steel from the material group of anti-friction bearing steels, the ~ ,..lr~ tl~ of roller bearings prescribe a minimum content of sulphur, in order to ensure that the hot-finished tubes or the separated rings can be machined flawlessly and without difficulty. ~o achieve a good degree of purity, there should be as little sulphur as possible in the steel; however, for the purpose of m~rhinin~, the steel should have a certain minimum sulphur content. It has proved advantageous for the sulphur to be in the range of 0.008 - 0.015 percent by mass. In contrast, when the subsequent working of the steel is to be largely chipless, a maximum sulphur content of 0.005 percent by mass is considered alvdllldg~ ~u~. For both types of further processing, i.e., machining or chipless, a chrome-carbon ratio in the range of 1.35 to 1.52, preferably 1.45, is advdll~&g.~u~. It is possible to positively influence the undesired carbide banding via this ratio.
Furthermore, for the sake of low o~ygen content and grain fineness in the material, a minimum aluminum content is often required. Because no calcium treatments involving the formation of liquid calcium ~ min~trs, such as are commonly used to improve the degree of purity, are permitted in the case of anti-friction bearing steel, the aluminum content must be selected in such a way that the quantity of Al~03 which forms in the rnntin~lr,llc cast is low.
For this reason, the addition of aluminum is dispensed with in the process according to the invention, without the o~ygen content assuming undesirable values. Omitting to add Al does not mean that no traces of Al are present in the melt, whether they originate from the slight Al content of the alloy ~ u~orl.~ or from the masonry.
I~he process according to the invention is described in greater detail in reference to an arbitrarily chosen e~rample, which illustrates the use of a non-deformed and non-homr,g~ni7rd continuous cast of preferably lly~..tu~e~oid steel in the m~nllf~rt~lre of seamless roller bearing tubes in a tube making unit having a piercing press, wherein a state of compression stress is created during piercing by the selection of the forming rnn~litir,n~ A hot-finished tube of roller bearing steel 100Cr6 having a si e of 60.3 [illegible] and an 8.0 mm wall is to be ~12i6 2l9297~
produced, for example. Starting from pig iron produced in a blast furnace, the pig iron is desulphurized in a first step, specifically, to a range lower than 0.010 percent by mass. The pig iron is converted into steel via a LD converter with a holding capacity of up to 250 t.
During slag-free tapping into the casting ladle, alloying is carried out, as is a combined i~o~ ri~n with carbon, silicon and manganese only. ~he otherwise common step of adding aluminum for the sake of pre-killing is omitted here, for the reasons already discussed. In the casting ladle, the steel is further processed, i.e., fine alloying is carried out in order to exactly meet the pre-established analysis limits; degassing takes place, including carbon~ rion in a vacuum; and there is also purity-degree rinsing with an inert gas. In contrast to the prior art, this process dispenses with the use of a ladle furnace. A~ ctm~nt of the t~ CId~UlC
budget in the sense of m~int~inin~ the pre-established casting tClll~.ld~Ult: is ensured by means of a high charge weight with a corresponding heat content. Casting in a round format to a diameter of 220 mm is then carried out on a multi~core bow-type continuous casting machine.
The casting speed is approximately 1.4 m/min. The distributor is equipped with ipartitions for better flow control, and the immersion nozzle preferably has a diameter of ~
25 rnm with a longer pouring funnel, which always remains full. The entire casting area is sealed with argon, and the melt is not rinsed with argon during casting, so that no [permanent]
stopper is needed. In order to control the flow behavior of Ly~clcu~e~uid steels, special stopping and withdrawing heads are used for casting. After the separation into long bars is carried out, the bars are identified and visually inspected for defects and then ~ldul~t~ul~ed to the punch-bench machine. The long bars are divided up into inserts of pre-established length on a cold saw and heated to 1140 - 1180C in a rotating hearth furnace. The first forming of the heated bloom into a hollow body with a size of 223.0 x 51.0 mm takes place in the piercing press. This is equal to a stretching of 1.39. In the elongator, the hollow body is further stretched to asize of 192.0 x 40.0 mm. This corresponds to a stretching of 1.43. After this, the stretched hollow body, which has a bottom, is placed into the punch bench. By means of an inserted rod, the hoLow body is pushed through several stands and thereby attains a size of 127.5 x 8.25 mm. The stretching is equal to 6.18. The rod is withdrawn and the ~2i6 21 92~70 bottom area of the hollow body is separated. Up to this work step, the previously described forming occurs under heat. In order to permit sllhs~ T.~nt forming to be carried out in a stretch reducing mill, the cooled ;"~ tube must again be heated to the forming t. .~ The final finished size of 60.3 [illegible] x an 8.0 mm wall with a stretching of 2.35 is produced in the stretch reducing mill without an internal die. Cooling in the air takes places on a standard cooling bed. The hot-finished tube produced in this manner tllen serves, after soft annealing, as feed stock for downstream cold working, such as cold pilgering or cold drawing. Rings are [run off~ from these cold~finished tubes in automatic rotary devices, and the final contour of the roller bearing is formed.
The essential advantage of the above-discussed process according to the invention is that the upstream rolling or forging process and the homogeni~ation previously necessary for tube starting materials (semi-finished products) of high-carbon steels, especially l~y~ u~ oid steels, can be omitted and that the continuously cast ingot has, by virtue of the Al-free metallurgy as well as the described casting conditions, a mi~ LI u.l u. ~l formation of a type which allows it to be directly used on a tube making machine, which preferably has a piercing press in its unit deslgn.

Claims (12)

Claims

1. Process for manufacturing a seamless hot-finished tube from high-carbon steel, especially hypereutectoid steel, in which a pretreated and degassed and deoxidized molten steel is continuously cast in any desired cross-section in the required chemical composition and, after separation from the strand, the continuously cast bar divided into inserts is subjected to forming, characterized by the fact that a continuously cast material having a high degree of purity and fine carbide precipitations and high grain fineness is produced and that the insert is, without any pre-deformation, i.e., in the cast state and without heat treatment (homogenization), heated to the forming temperature and fed to a tube making unit, which preferably has a piercing press in its unit design, whereby during the piercing process, as part of the overall process of the tube making unit, a stressed state is built up in the insert to be formed, which has, as shear stresses are minimized, the highest possible negative stress average m.

2. Process as in Claim 1, characterized by the fact that during the piercing process, a tool holding the insert is used that has a closed mantle surface.

3. Process as in Claim 1 or 2, characterized by the fact that the insert optionally has a rectangular cross-section or another polygonal cross-section, preferably a round one.

4. Process as in Claim 1, characterized by the fact that during the piercing process, a device is used that has a work space that is not closed, especially a cone-type piercing mill, whereby the insert preferably has a round cross-section.

5. Process as in Claims 1 to 4, characterized by the fact that the steel in the round continuous cast is cast with a format in the range of 170 mm to 450 mm [illegible] and at 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 size range of 170 - 240 mm [illegible].

6. Process as in Claims 1 to 5, characterized by the fact that for hypereutectoid steel from the material group of the anti-friction bearing steels, the molten steel first is partially deoxidized via carbon, silicon and manganeseexclusively, without the addition of aluminum, and then is fully deoxidized in avacuum via carbon.

7. Process as in Claim 6, characterized by the fact that during subsequent machining of the hot-finished tube, the sulphur content is in the range of 0.008 to 0.015 percent by mass, and during subsequent processing which is largely chipless, the maximum sulphur content is 0.005 percent by mass.

8. Process as in Claims 6 and 7, characterized by the fact that a range between 1.35 and 1.52 is selected for the chrome-carbon ratio.

9. Process as in Claim 8, characterized by the fact that the Cr/C ratio preferably is 1.45.

10. Process as in one of the above Claims 5 to 9, characterized by the fact that during casting into the continuous casting unit, an immersion nozzle with adiameter 25 mm at a commensurate structural height as well as a distributor having partitioning pieces is used.

11. Process as in one of the above Claims 1 to 3 and 5 to 10, characterized by the fact that the tube making unit is a punch bench having a press as the piercing unit.

12. Process as in one of the above Claims 1 to 11, characterized by the fact that the steel production process has the following steps:
- pig iron production - pig iron desulphurization - conversion process (LD converter) - ladle tap (alloying, combined deoxidation with C, Si and Mn without addition of aluminum) - ladle treatment without electric arc heating (degassing and carbon deoxidation in a vacuum, fine alloying, flushing) - casting in round format on a bow-type continuous casting machine, omitting argon flushing during casting.