CA1268957A

CA1268957A - Method for producing rolled steel products, particularly threaded steel tension members

Info

Publication number: CA1268957A
Application number: CA000489259A
Authority: CA
Inventors: Max Aicher; Hans-Wilhelm Klein; Dieter Russwurm; Dieter Jungwirth
Original assignee: Dyckerhoff and Widmann AG
Current assignee: Dywidag Systems International GmbH
Priority date: 1984-08-23
Filing date: 1985-08-22
Publication date: 1990-05-15
Anticipated expiration: 2007-05-15
Also published as: AU4658285A; EP0172544A3; DE3431008A1; EP0172544B2; ES546332A0; JPH0660350B2; BR8504032A; EP0172544B1; JPS6160824A; DE3575577D1; DE3431008C2; AU565805B2; ES8609490A1; US4877463A; ATE49779T1; EP0172544A2; ZA856448B

Abstract

ABSTRACT OF THE DISCLOSURE

To produce threaded steel tension members, steel is used with a C-content of 0.50 to 0.80%, preferably 0.75%, a Si-content of 0.20 to 0.50%, preferably 0.25%, and a Mn content of 0.30 to 0.80%, preferably 0.60%. Exiting from the rolling heat at the outlet side of the finishing stand after hot rolling, the tension member or rod is subjcected to surface quenching by a cooling medium, preferably water, so that the steel in a rim zone R1 is transformed immediately and completely into martensite, while the heat content remaining in the core zone K1 does not effect a tempering of the martensite rim zone during the subsequent cooling beyond the range of the intermediate stage. Steel tension members of this type have a high ductility and toughness at a high yield limit and high strength, they are corrosion-resistant to a great degree and have a wear resistant surface which makes them particularly suitable for threaded tension rods in which the threads are produced either by a cold forming operation or hot rolled ribs.

Description

` 21182-265 The invention is directed to a method of producing rolled steel members, particularly steel threaded tension 1 members or rods.
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Steel tension members such as rods used in building j and heavy construction as tension members for prestressed concrete, but also as steel anchor member for soil and rock l anchors, as formwork anchors, as suspension cables for Il suspension bridges, as diagonal cables for stayed girder ~ briclges, and as braces or stays, generally have a tensile l yield poi.nt varying from 835 to 1080 N/mm2 and a tensile strength in the range of from 103Q to 1230 N/mm2. In the ¦ conventional methods used exclusivèly up to the present tlme, I
steel containing a C-content of 0.65 to 0.75%, a Si-content o ¦
I l¦ 0.60 to 1.60% and an Mn-content of 0.70 to 1.508, as well as chrome and/or vanadium and other alloying elements, have been 1 used as the base material.
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ll~ Steel tension members of this type are known in ¦ various construction operations, such as, round wires which are cold drawn after rolling strength and subsequently are ~ I tempered again, or flat steel members, for example, steel which undergoes a temper-harden;ing across the entire I
cross-section after rolli~ng, or steel rods. STeel rods with , diameters between approximately 15 and 50 mm are hot rolled, then stretched in order to increase the yield point and subsequently tempered for stress relief. Aside from the fact that only limited lengths can be produced because i89S7 of the stretching operation, this costly production method brings about high costs.

Aside from the static strength values, steel tension members must also have the highest possible elastic limit and a good deformability. In threaded tension steels, that is, those on which threaded anchors can be attached, a high wear resistance ~lof the surface and also corrosion resistance are important.
IThe relaxation characteristic, as well as a sufficiently high l~fatigue strength are also important.

1 In addition to steel tension members for prestressed Iconcrete structural parts, steel reinforcing bars are used as untensioned or nonpretensioned reinforcing for steel-reinforced :
concrete. Reinforcing rods of this type are either used with natural hardness, wherein the strength is determined by means of ~the alloying, or are cold formed, for example, by means of drawing or cold rolling, the latter being employed especially for steel reinforcing mats~ Steel reinforcing members of this type must be weldable, accordingly, their analysis or make-up is distinguished by a low C-content. As a rule, such lreinforcing members have a yield point between 420 and 500 N/mm and a tensile strength between 500 and 550 N/mm2. Steel re-inforcing members w1th higher strength values are generally not produced. The steel make-up content used deals exclusively with ;`those steels, suitable for welding, having a C content of less ;than 0.22%.
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~895~7 ,, Steel relnforcing bars or rods are produced with a smooth surface and as ribbed reinforcing members. Ribbed reinforcing rods usually have sickle-shaped ribs extending diagonally relatively to the long axis of the rod, and these ribs extend ,~ in the transverse direction over a large part of the circum-.l ferential surfaces of the rod and are intended to improve the bonding action of the rei.nforcing rod in the concrete.

To increase the strength and improve the deformability of ! hot rolled steel reinforcing bars or rods of this type, it is ~ known to suhject the rods exiting out of the rolling heat at the I outlet side of the finishing stand to a surface quenching by means of a coollng medium so. that there is a rim zone of martensite or bainite in the rod cross-section immediately after quenching, while the heat content remaining in the rod core i does not cause a tempering of the rim zone beyond the bainite stage during the subsequent cooling (DE-AS 23 53 034). This known method is based on the concept of increasing the strength and improving the deformability without increasing the carbon or manganese content which runs contrary to the requirement for ~ I ~a good welding ability. A typical analysls or make-up content : of a reinforcing steel heat-treated in this manner is 0~17 to : ll 0.22% C, o.ns to 0.30% Si, as well as 0.70 to 1.10% Mn.

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~8957 ~UMMARY OF THE INV~TION
With this prior art sexving as background, the invention has the object of providing an accurate and econo~ical production method for steel tension members, partiGularly for threaded steel tenslon members with the ~trength characteristics named in the beginning, which permitæ
~he use of the s~eel cons~i~uent ~ake-up, which i~ easily and economically reproducible metallurgically, for forming steel tension membars which are corrosion-resistant, have a wear-resistant surface for reducing the danger of mechaniaal damageand can be suitably threaded. In addi~lon, steel tension members can be producecl in desired rod lengths with a greater ductility and touyhne s and a hiyher tensile yield point and higher strength, particuIarly at low temperatures, and with a high ~atigue strength and a reduced relaxation e~fect.
The inventlon provides a method of producing an ~axially extendlng rolled steel member ~or use as prestressing steel member and having a generally cireular transverse section with an axially extending core zone (K) enclosed by an axially extending annular rim zone (R) and comprising the steps o:E
forming the s~eel member with a C-conten~ in the range of 0.50 to 0.80%, a Si-content in the range of 0.20 ~o 0.50~, a Mn-: content in the range of 0.30 ~o 0.80%, hot rolling ~he steel member ln ~ rolling mill with a fini~hing stand having an outlet~ removing the rolled mé=ber from the outlet of ~he flnishing stand at tbe final rolling temperature and selecting the ~inal rolling temperature in the range of 860 to 1060C, ~urface quenching in a. single operation the rolled member at the rolling temperature from the finishl~g s~and using a cooling medium so that the rim zone (R) is trans~ormedimmediately in~o mar~ensite whil2 malntaining the heat contellt . ", ,.. ~, .

, - ... -~L~6895~7 . 211~2-265 in ~he core zone (K) so that the heat content does not temper the mar~ensite rim zone during subsequent eooling beyond the ranye of the intermediat,e staye, and surface quenching and tempering the steel product so that the surface temperature of the rim zone, depending on the diameter of the steel product, i~ in the range of 400 and 500C in the tima interval between the second and sixth second following the commencement of surface quenching.
The invention also provides a hot rolled steel rod comprising a generally circular transverse section ~ormed from a ~teel wlth a C-content in the range 0.50 to 0.80%, a Si-content ln the ranye of 0.20 to 0.50%, and a Mn-conten~ ln the range of 0.30 to 0.80%, said rod havlng an annular rim zone o~
fine-grained martensite, said annular rim zone having been formed by ~urface quenching of the hot rolled steel rod uæing a cooling medium while maintaining the heat conten~ in the core zone of the rod such that during subse~uent cooling the marten~ite rim zone ls not tempered beyond the inter~ediate stage.
The steel has a C-con~ent of preferably approximately 0.75%, a Si-content of preferably appro~imately 0.25%, and a Mn-content of preferably approximately 0.60%. The steel exitln~ from the rolling heat at the outlet ~ide of the finlshlng ~and o~ a hot rolling mill i5 subjected to a surface quenching preferably~by water.

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395~7 The final rolling temperature at the finishing s~and is advisably selected in such a way that it lies ~ust under the transformation point A3 at the lower limit of the hot deformability of the steel. The final rolling temperature is preferably be~ween 880 and 940C.
The invention is based on the discovery that the co-operation of a combination of determined factors is required in order to produce a steel capable of use as tension members, with the indicated characteristics, in an economical manner.
Of particular importance is that the steel has a relatively hiyh C-content producing a high strength which is increased even further by means of the subsequent heat treatment. Whlle khe ~ustenitizatlon taking place during the temper-hardenlng as a special homogenization treatment, the latter ls performed by the heatlny in the rolllng mlll ~urnace, as well as by the rolling process itself, during the produetion of steel tension members aacording to the invention. The degree o~ homogeneity of the constituents making up the steel, the size of the austenite grain and the temperature of the solution heat treatment are dealsive for the product.

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~i89~7 The size of the austenite grain is determined, amon~ other fa~tors, by the recrystallization which occurs duriny the hot rolling after each pass. In absolute terms, the grain size is smaller the more frequently and intensively the steel member is deformed. However, the final grain size achieved appears only in the last rolling pass; the deformation and the ~¦ temperature, as well as the dwell time at this temperature until the start of the cooling process, are decisive here.

~ In the production of steel tension members according to ¦¦ the invention, there must be, beEore the coolin~ commences, a very fine-grained structure which forms at least in the area of the strong deformation in the rim zone of the steel rod j being rolled. The risk of the formation of very resistant austenite grains, which unfavorably influence the corrosion ¦resistance of the steel, is accordingly reduced.

, It is particularly advantageous if the temperature at the ¦ last rolling pass, that is at the finishing stand, lies at the lower limit of the hot forming property or hot deformability, that is, just under the transformation point A3. A very fine-grained structure results and recrystallization is prevented to a great degree. In connection with this, a cooling must take place rapidly and intensively so that the cooling curve of the rim zone reaches the martensite range without reaching the range of the ferrite, pearllte, and the intermediate stage.
This is particularly important in a relatively high-carbon steel in which the martensite starting temperature MS is ~7--3~i~
relatively low. However, a core zone within the rod must, at the same time, have a sufficient heat content to bring about a tempering of the martensite present in the rim zone.
BRIEF DESCRIPTIO~ OF T~IE DRAWI~G

_ In the drawing:
Fig. 1 is a graph illustrating a time-temperature transformation curve for a steel tension member produced according to the present invention;
Fig. 2 is a partial side view of a tension rod with a smooth sur-face and a thread rolled on one end of the rod;
Fig. 3 is a cross-sectional view along line III-III
in Fig. 2;
Fig. 4 is a partial axially extending sectional view of the thread displayed on an enlarged scale;
Fig. 5 is a partial side view of a tension rod with hot rolled thread ribs extending along a hel~cal line;
Fig. 6 is a cross-sectional view along the line VI-VI
in Fig. 5 in normal projection;
Fig. 7 is a partial side view of a tension rod with hot rolled, transversely extending thread ribs; and Fiy. 8 is a cross-section taken along line VIII-VIII
in Fig. 7 in normal projection.
The conditions unler which the process according to the invention takes place can be explained with the aid of Fig. 1 showing a time temperature transformation curve for a steel which approximately has the preferred constituent content for the invention, that is, 0.76~C, 0.23~ Si, and 0.63% Mn.
The curve Rl ehows the curve of the surface tempera-ture of a steel rod with relatively small diameter, for exam-ple, 15.1 mm and the curve Kl shows the curve of the tempera-~ 3'~7 ture of the core zone for the same rod. R2 is the correspond-ing curve of the surface temperature for a rod with a greater diameter.
Significan-t for the heat tre~tment is the course of the curve Rl in the tempering range. To achieve the grain structure satisfying the demands for a steel tension member, the curve Rl of the surface temperature must be within the temperature range between 400 and 500C in the time interval between the second and the sixth second of the heat treatment shown in Fig. l; under no circumstances may it reach the pear-lite range.
The heat transfer from the core zone is also acceler-ated by the strong cooling of the rim zone. In accordance with the steel constituent composition, transformation is effected directly in the intermediate stage area or prior to the .

9~;7 `, formation of pearlite. This is viewed as particularly advantageous if the core zone of the rod transforms in the upper ,ntermediate sta~e area which is distinguished by a fine dis-persion of the carbidesO

The intensity of the cooling in the rim zone depends sub~
stantially upon the cooling output of the available installation.
The cooling output is dependent upon a plurality of the factors.
For a known cooling installation a water quantity of 10 to 20 l/sec kg is viewed as particularly advantageous. A transforma-tion inertia supporting this process can also be achieved by i means of the constituent composition of the steel. Thus, forexample, not only the increase of the carbon content, but also l of the rest of the alloying elements of the steel such as Mn, ,I si r Cr, Ni, Mo, act in this manner.
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sy adding additional alloying elements certain character-istics of the steel tension members are successfully improved.
Thus, the addition of chrome (up to approximately 0.8%) and ', copper (up to approximately 0.5%) cause an increase of the i corrosion resistance, the addition of vanadium (up to 1l1 approximately 0.15%) and niobium (up to approximately OD06%) as well as traces of the micro-alloying elements titanium and boron, cause an increase of the toughness and fatigue strength.
The C content can also be successfully lowered to the lower ; limit by a corresponding selection of the alloying elements. I

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As tests have shown, the steel tension membersproduced according to the invention meet the demands imposed on them to a high degree. Compiled in Table l, see end of the specifica-tion, are the constituent compositions of some melts of the ~ steel types 835/1030 (ratio: yield limit/tensile strength) .1 1 and 885/1080 for steel rods with diameters of 26.5 mm and 15.1 mm, respectively. Table 2 provides the static strength values calculated as mean values of some steel rods produced, I¦ according to the invention, ~ith diameters 36.0 mm, 26.5 mm and 15.1 mm. Re designates the yield limit, Rm designates the tensile strength, Alo designates the rupture elongation over a measured length corresponding to ten times the diameter of the I rod, and AG designates the elonyation before a reduction in ',I area.

The high corrosion resistance of the steel produced according to the invention is chiefly the result o~ great j uniformity of structure, the low temperature during rolling, and the rapid cooling preventing dlsturbance factors from developing. Moreover, relaxation tests for determining the I inelastic extension or elongation at lO00 hours show that the relaxation losses are very low. Bending tests show excellent ductility characteristics of the specimens examined. ~ ¦

Since the~steel tension~ members produced according to the invention have a very fine grain structure in the rim zone ; and a correspondingly high surface strength, they are particularly suitable for the production of threaded steel tension members.

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t39frl7 To transfer the tension forces to a structural part, threaded anchors are often used on tension rods. In this I context, it is known to roll threads onto the ends of the rods I rolled with smooth surfaces in cold forming operation. In contrast to a cut thread, such a thread formation has the advantage that a stengthening of the steel structure, particu-larly in the region of the thread grooves, is achieved with il reduced core cross-section so that the steel rod can also be j~ utilized in the threaded region with the full force correspond-`~ ing to its cross-section while taking lnto account the allowable tension. It is also known to construct the thread so that the rounded portlon of the thread grooves has a substantially ~¦ I greater radius of curvature than the rounded portion at the outer tips of the thread teeth or ribs (DE-PS lO 68~454). A thread with such rounded grooves allows considerably greater tolerances relative to the thread of the nut and accordinqly provides the ;
conditions for the abllity to absorb Qr adjust to tolerances during the installation of the anchor member without damage.
i In addition, it is known to provide a tension rod by hot rolling, with helically extending ribs arranged an two : ~ . l opposite sides of the circumferential surface of the rod forming parts of a thread OA: which an anchor member provided with a corresponding counter-thread (DE-PS 17 84 630j can be placed.
The partlal thread achieved in this manner has very coarse tolerances relative to a metric thread so that it effectlvely ' ;

-" . ~., i7 It is important, particularly in steel rods with hot rolled ribs that the quenching action is not impaired due to the surEace shape of the rods and that the surface profilling of the rods is selected in such a way that the rods have a uniform heat-treated layer in the region of the ribs.
It has been shown that when the height, the average width, and spacing of the ribs, are in a detlermined ratio relative to one another, with the ribs acting as cooling ribs during the heat treatment of the rod, the heat removal in tha region of the ribs is comparatively greater than in the area of the smooth rod surface so that the boundary zone between the core zone and the rim zone of the rod extends linearly. This ratio is advisably -0.5 to l to 4.

meets the demands of heavy duty construction practices.
Moreover, the thread is present along the entire length of the rod without any additional expenditure.

Finally, it is also known to form the ribs produced by hot rolling as transverse ribs which extend approximately along ll half the circumference of the cylindrically shaped tension rod ; 1 and whieh decrease in width and height toward their ends (DE-PS 20 43 274). Only portions of these ribs lie on a helical line whieh, however, provides the possibility that anehor members 1 with right-hand as well as left-hand threads ean be serewed onto I
sueh a partial thread.

ccordingly, the subject matter of the invention is still the application of the method to the productlon of hot rolled steel rods or wires with smooth surfaees whieh arè provided, ~!
~ at least at the end, with threads formed by cold rolling and .
suitable for serewing on a eonneeting or anehoring member, having ;
threads with the rounded portion of the thread grooves having a substantially greater radius of eurvature than at the thread tips; as well as to the produetion of steel rods or wires ~ ~ 20 ~ provided with ribs by hot rolling, with such ribs extending at ;~ l least partially along a helical line, arranged at two opposite 1 sides of the eireumferential surfaee of the rod and form parts , of a thread on whieh a eonnecting or anchoring member provided ~ with a eorrespondlng eounterthread ean be applied.
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~:68~57 DETAILED DESCRIPTION OF THE INVENTION

Steel rod 1, shown in Fig. 2, rolled as a smooth rod, was subjected to the heat treatment according to the invention.
A thread has been rolled on the rod end in a cold forming operatlon and the thread 2 is indicated in Fig. 4~ in an axially extending section on a greatly enlarged scale. The thread is a so-called asymmetric partial thread, that is, the radius of the rounded portion in the region of the thread 'I I
i grooves 3 is substantially greater than the radius of the tip 4 I of the thread tooth or rib.
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The steel tension rod 11 set Eorth in Figs. 5 and 6 is a so-called threaded rod provided with thread ribs 12 in a hot rolling procedure. The ribs 12 have a height h, an average width s and are spaced apart from one another a~ a dlstance A
,l with the ratio of h : B : A being approximately 0.5 to l to 4.
The ribs 12, in each instance extend approximately over a third ! j ~
of the circumference of the rod in full height with the transi-tion from the full height into the circumferential surface 14 I of the rod core 15 being at the rib ends 13.

`I In Figs~ 5 and 6, it is indicated that the boundary zone l6 between the rim zone R, where the material of the steel rod hds been transformed lnto martensite by surface quenching, and the core zone K,~where heat content remaining after surface "
quenching causes tne subsequent tempering of the martenslte rim zone R, to extend approximatel~ linearly. This is the result of the enhanced cooling effect of the ribs during ;; ~
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surface quenching and has the advantage of a continuously high surface strength of the steel and a very favorable corrosion resistance.

Similar ratios also prevail in the threaded rod 21, shown in Figs. 7 and 8, where the ribs 12 are formed as transverse ribs. In this embodiment, the boundary zone extends linearly between the core zone K and the rim zone R of the rod without beiny influenced by the ribs.

Il Due to the high surface strength of the heat-treated rim I zone Rl comprising the hot rolled ribs, it is possible to I construct the anchoring and connecting members, such as nuts, sleeves or the like, shorter than in known tension rods with a homogeneous rod cross section~. With shorter anchoring and 11 1 i connecting members, however, the force transmisslon in the threaded region between the rod and nut or sleeve will be improved.

While specific embodiments of the invention have been shown and described ln detail to illustrate the appllcation of the inventive prlnciples, lt will be understood that the 20~ ~ I invention may be embodied ot~herwise wIthout departing from such principles.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producing an axially extending rolled steel member for use as prestressing steel member and having a generally circular transverse section with an axially extending core zone (K) enclosed by an axially extending annular rim zone (R) and comprising the steps of forming the steel member with a C-content in the range of 0.50 to 0.80%, a Si-content in the range of 0.20 to 0.50%, a Mn-content in the range of 0.30 to 0.80%, hot rolling the steel member in a rolling mill with a finishing stand having an outlet, removing the rolled member from the outlet of the finishing stand at the final rolling temperature and selecting the final rolling temperature in the range of 860° to 1060°C, surface quenching in a single operation the rolled member at the rolling temperature from the finishing stand using a cooling medium so that the rim zone (R) is transformed immediately into martensite while maintaining the heat content in the core zone (K) so that the heat content does not temper the martensite rim zone during subsequent cooling beyond the range of the intermediate stage, and surface quenching and tempering the steel product so that the surface temperature of the rim zone, depending on the diameter of the steel product, is in the range of 400° and 500°C in the time interval between the second and sixth second following the commencement of surface quenching.

2. A method, as set forth in Claim 1, including forming the steel product with a C-content of approximately 0.75%, a Si-content of approximately 0.25%, and a Mn-content of approximately 0.06%.

3. A method, as set forth in Claim 1, including selecting the final rolling temperature at the finishing stand so that the temperature is at the lower limit of heat deformability of the steel just below the transformation point A3.

4. A method, as set forth in Claim 1, including selecting the final rolling temperature to be in the range of 880°C and 940°C.

5. A method, as set forth in Claim 1, 2, 3 or 4, including in the constituent composition of the steel product at least one of the following: up to approximately 0.8%
chrome, up to approximately 0.5% copper, up to approximately 0.15% vanadium, up to approximately 0.06% of niobium, and traces of titanium and boron.

6. A hot rolled steel rod comprising a generally circular transverse section formed from a steel with a C-content in the range 0.50 to 0.80%, a Si-content in the range of 0.20 to 0.50% and a Mn-content in the range of 0.30 to 0.80%, said rod having an annular rim zone of fine-grained martensite, said annular rim zone having been formed by surface quenching of the hot rolled steel rod using a cooling medium while maintaining the heat content in the core zone of the rod such that during subsequent cooling the martensite rim zone is not tempered beyond the intermediate stage.

7. A hot rolled steel rod as set forth in Claim 6 having threads formed in a cold forming operation in said rim zone, said threads being suitable for the threaded engagement with connecting or anchoring members and defining alternating thread tips and grooves with the grooves having a radius of curvature substantially greater than the radius of curvature of the tips.

8. A hot rolled steel tension rod as set forth in Claim 6 ribs formed on the outer circumferential surface thereof in a hot rolling operation, said ribs extending along a helical line at least along portions of said rods located at the opposite ends of said rods, said ribs form a part of a thread arranged for threaded engagement with a connecting or anchoring member having a corresponding counterthread.

9. A hot rolled steel tension rod as set forth in Claim 8, wherein said ribs have a ratio of height from the outer circumferential surface to the average width in the direction between the opposite ends, to the spacing between adjacent ribs of approximately 0.5 to 1 to 4.

10. A hot rolled steel tension rod as set forth in Claim 6 having a C-content of approximately 0.75%, a Si-content of approximately 0.25%, and a Mn-content of approximately 0.60%.

11. A hot rolled steel product as set forth in Claim 6, 7, 8, 9 or 10 including in the constituent composition of the steel product at least one of the following: up to approximately 0.8% chrome, up to approximately 0.5% copper, up to approximately 0.15% vanadium, up to approximately 0.06% of niobium, and traces of titanium and boron.