WO2001027339A1 - Method for producing steel and use thereof in steel construction - Google Patents

Abstract

The invention relates to a method for improving the surface characteristics of low-alloyed heat treatable steel in particular for the production of high strength steel wire comprising a martensitic structural base in order to withstand very high pressure and tensile stress in steel constructions. Said low-alloyed martensitic convertible steel is subjected to one or more processing steps in a thermal treatment procedure of a targetted, evenly distributed edge decarburization process at a depth of 1-15 % of the steel mass located below the surface. An arrangement of several steel wire rods produced according to the inventive method is also disclosed.

Description

METHOD FOR THE PRODUCTION OF STEEL WIRE AND THE USE THEREOF IN STEEL CONSTRUCTION

The invention relates to a method for the treatment of low-alloy steel in order to expand its previous uses while at the same time improving its useful properties and economy. An essential feature of the process is that the low-alloy, mostly tempered steel that is usually used mainly in machine and vehicle construction is made suitable for the first time according to the invention for absorbing very high compressive and tensile stresses in steel construction without the addition of concrete as a wire or rod-shaped main component considerable material savings in construction, construction safety and accuracy can be increased significantly.

According to the preamble of claim 1, it is proposed that the surface of the steel be placed under a uniform carbon depletion (decarburization), this either only moderately and without ferrite formation (i.e. by decarburization) or according to claims 2 and 3 so strongly It should happen that the first surface layer loses its carbon completely (ie through carbonization).

In industrial practice, but also in the entire steel sector, the prevailing opinion is that carbon depletion on the surface of fasteners and machine components is drastic Has an influence on the material properties. There are therefore elaborate processes for avoiding or at least reducing edge decarburization, which often occurs very uncontrolled, very unevenly, for example when heating semi-finished products for hot forming or in the various heat treatment steps for hardening and tempering. However, it is an equally well-known finding that in practice there are no complete decarburization-free steel parts, particularly in the case of mass products made from low-alloy steel, unless the unevenly decarburized surface layer is removed completely by machining. In the manufacturing technology of heat-treated steel parts, we develop and act according to the rule "as little edge decarburization as possible", which does not rule out low edge decarburization or their very uneven C distribution within the zones near the surface. In the screw DIN sheet ISO 898 part 1 "Mechanical properties of fasteners" from April 1979, a "completely decarburized" layer is only permitted to a maximum depth of 0.015 mm even for screw class 12.9, for which a minimum tensile strength of 1220 N / mm ^{2 is} prescribed. Many users only tolerate "Decarburization in places", which certainly does not eliminate the danger of an uneveness of the mechanical properties of the surface areas. This risk increases unequally if, as usual with the intention of reversing edge decarburization, the parts are again heat-treated with great effort in an oven atmosphere with regulated carbon potential, but at best only approximately the same structure as in the core or even not infrequently with enrichments of C. (i.e. carburization) has to be expected, which is a major disadvantage for the durability of such parts.

Since carbon is indispensable for the hardening of low-alloy steel and its martensite formation, it is obvious that its loss on the surface is considered undesirable and is blamed for many defects in the part properties. When closely observing practical relationships However, it turns out that it is not the level of the C content, but its unequal distribution that can be the cause of a number of defects. From many practical cases, as well as fatigue, creep and corrosion stress fracture tests, it is unanimous that - apart from metallurgical, procedural and mechanically caused defects and surface damage - not the missing carbon but mostly the simultaneous presence of "carbide and ferrite" on the surface under certain load conditions leads to material weakening of the steel. In the magazine "Archive for the ironworks" (1972), Issue 12, pp. 913 to 917 it is shown by comparing the theoretical results with test results, for example, that the carbide cracks due to the greater ductility of the ferrite - ie the carburized points - and because of the less deformability of the carbide particles - i.e. not carburized points - with an elongation of approx. 2% lead the ferrite cracks. "The cracking and breaking process" has "its cause in the formation of carbide cracks" is concluded here.

Considering the fact that the unavoidable non-uniformity of the C distribution in the outer layer of a low-alloyed tempering steel, whose martensitic crystal lattice has a high degree of internal lattice tension due to the forced release of the C atoms, increases the surface's sensitivity to hydrogen embrittlement and corrosion, the meaning of the measures according to the invention in claims 1 to 3 for the applications resulting therefrom becomes apparent. Therefore, not only is the formation of a uniform gapless ferrite layer preferred, but it is also required that this should be very uniform as far as possible metallurgically and thermochemically. As a metallurgically effective measure, the content of Si is set to at least 0.35% by weight, since, as tests have shown, compliance with this lower limit in the chemical composition of the low-alloyed tempering steel means that Formation of a coherent ferrite layer under suitable thermochemical and heating conditions. The second limit in claim 2 for the Cr content with a maximum of 0.65% by weight has proven to be necessary in order not to increase the sensitivity of the martensitic steel structure in the high-strength state to the hydrogen-induced stress corrosion cracking in the core area, and that due to the after Claims 1 to 3 ferrite layer constructed according to the invention to effectively achieve permeability of the surface layer for the atomic hydrogen.

To achieve a uniform, targeted decarburization or built-up ferrite layer, it is necessary that the heat treatments - in particular those before the steel is quenched - not only take place thermochemically decarburizing but also that the reacting hot gas flow reaches the entire surface of the steel body as it were. Only in this way and with a determinable and precisely maintainable temperature control of the treatment furnace can the chemical reactions - mainly CO, CO ₂ and descaling - form oxides - reactions with the effusing carbon - but also with Fe and the alloying elements of the steel - as well as the mass transfer due to a fast removal of the reaction products are maintained uniformly in a necessarily turbulent fresh gas flow and the uniform build-up of the ferrite layer according to the invention takes place. Furnace systems and devices for carrying out such precise treatments, in particular for heavy steel wire rings, are described, for example, by DE 44 37 683. With certain restrictions, however, a different type of furnace system according to DE-C 30 35 032 can also be used for the decarburization and ferrite layer formation according to the invention. Test results from the special treatments in this plant have already shown that the measures according to the invention in claims 1 to 3 are surprisingly the essential conditions for the fact that the field of application of low-alloy martensitic convertible tempering steels can be expanded enormously. These can be technically responsible and economically justifiable for use in the strength range from 1000 and up to about 2400 N / mm ² , since according to the invention it is possible to

to make them more corrosion-resistant on the surface than previously

Case has been to facilitate their cold forming at high strength and

Reduce wear costs as well - assembly and disassembly of the steel parts from a modern

To specify mass production in the construction sector and to make it recyclable.

The decarburization according to the invention is explained in more detail below in connection with the drawing. Show:

Fig. 1 example cases (I) and (II) for targeted decarburization before

Hardening the hot-rolled steel wire;

Fig. 2 shows the temperature increase in the martensite structure due to

Transformation of the work into heat;

Fig. 3 shows a cross section through a sleeve package as an example of the

Kink protection of a bundle of rods according to the invention by square sleeves with wedge bolts for 9 rods with the

Steel wire diameter of 23 mm per 200 KN kink-proof permissible load (20cm x 20cm - support for 1.8 MN at 40cm kink protection distance) and Fig. 4 shows an example of the shock training with 9 according to the invention

Bars for a column with a permissible buckling load of 1.8 MN.

The decarburization to be set according to the invention according to claims 3 and 4 leads to changes which have been specified in FIG. 1 to clarify the achievable C depletion profiles in the edge region of a steel wire, for example for one of many possible steel bodies which are generally to be considered as long products. This also indicates the usual C fluctuation of a hot-rolled, low-alloy steel wire, which has both carburized and carburized points that contain carbides and is therefore very uneven. Case (I) in FIG. 1 relates to the usual types of quenched and tempered steels mostly used in the screw industry, the chemical composition of which does not correspond to the inventive restrictions according to the limit values for Si and Cr specified in claim 2. These steels are not suitable for the construction of a coherent ferritic surface layer. Most types of these steels are also not used for the high-strength strength range above 1300 N / mm2, so that the carbides present in the surface edge layer are only very brittle if the carbon content of the layer is not clear and uniform in accordance with that in Fig. 1 for Case (I) drawn profile is reduced. Case (II) in Fig. 1 relates to steel grades according to claims 1 and 2. Here, the surface layer initially contains the contiguous, carburized ferrite layer in accordance with claim 3. The decarburization profile continuing this layer gives the course of decarburization in the intermediate layer from the edge to Core area. For example, if the actual value from the piece analysis of a melt for the C content of a steel wire with a 25 mm rolling diameter in the core area is 0.50 to 0.54% by weight, the following results from FIG. 1: Case (I)

C content directly on the surface: 0.25 to 0.27% by weight

Average C content in the surface layer with a depth of approximately 0.62 mm (corresponding to a 10% area share of the wire cross-section): approximately 0.30 to 0.32% by weight

Case (II)

C content in the carburized edge layer with a depth of the continuous ferrite layer of 0.08 to 0.10 mm: about 0.001 to 0.005% by weight

Average C content in the remaining deeper charred layer of approximately 0.61 mm: approximately 0.25 to 0.27% by weight

In the following, these decarburization values are used, for example, on the basis of the process features according to the invention, to explain the further claims as practical basic values.

In order to be able to fully utilize the strength potential of a low-alloy tempered steel, it is necessary that the hardening process is adapted to the convertibility of the steel body into the martensitic structure, both in terms of process technology and system. The comparison of the mean C content of the two deliberately decarburized layers of case (I) and case (II) makes it clear that both values are in the range from 0.25 to 0.32% by weight and very close to one another. However, the very thin C-free ferrite layer in case (II) is adjacent with a lower-carbon region, so that overall an outer layer with C less than 0.10% by weight and a depth of about 0.3 mm is produced. In view of the high achievable today Quenching effects of liquid hardeners, which allow transition temperatures below 100 ° C, the hardening process contributes both to hardening without a tempering effect, which is conveniently proposed according to claim 4 for case (I), and it leads to the high-grade martensite formation according to claim 5 and also in Case (II), albeit for at least 95% of the total body. Case (I) leads to 100% martensite if the hard technological conditions are met.

As is well known, one of these conditions is compliance with the “critical diameter”. Since the hardenability of the low-alloy steel cannot be increased arbitrarily without having to accept the loss of toughness properties and greater sensitivity to corrosion, the cross-sectional dimensions of the steel body are limited according to the invention to dimensions and long steel products the dimensions less than 45 mm (claim 6).

Therefore, in the following, the hot-rolled steel wire in heavy - e.g. Currently, up to three tons of reelable wire rings are chosen as the main starting material for the description, for example, of the process steps according to the invention for opening up new industrial fields of application and their implementation.

In claim 7 it is proposed according to the invention that the martensitic structure created according to claim 4 without a tempering effect is to be cold formed in the hardened state only.

The fact that a martensitic spring wire, for example according to DE-C2 42 33 462 with initial strengths of over 1300 N / mm ² , can be cold formed and improved by rolling technology, is now known and proven. However, the martensitic structure of such spring wires is - according to the current state of technology - by that for tempering treatment corresponding step of tempering after hardening for the respective application according to the temperature-time relationship that is characteristic of each type of steel. A non-tempered and only - at the hardening agent temperature of at most about 100 ° C, which must necessarily be kept very low - after hardening without a tempering effect in the vicinity of the so-called 1st tempering stage, martensite which is only briefly there because it is considered very hard, brittle and complete unsuitable for cold forming. Because it is evident from the morphology of the formation of martensite that a tempering treatment reduces the so-called "lattice tension" to the extent that the carbon can free itself from its forced position. The C content, the temperature (starting at about 100 ° C.) and time is the determining factor in the starting process.

Surprisingly, however, it has clearly emerged from a very extensive investigation by cold drawing tests with differently tempered martensitic steel wires of the low-alloy steel grades that the higher the strength of the martensite, ie the lower the tempering temperature of the martensite, the lower the "internal friction" in the structure and the higher the "degree of hardening increase" due to the cold working. This will always be the case if it is possible to uniformly set at least 90% lath martensite in the structure of the steel. The basic prerequisites for this are given in the low-alloy martensitic steel according to claims 1 to 6. If the carbon content in this steel is less than 0.50% by weight, its austenite does not convert into a tetragonal as usual, but into a so-called "cubic" martensite. This can mean that this "cubic", possibly even "quasi-spherical" martensite for the microplastic deformations during the transformation processes before and after hardening and especially for the martensite wire to be cold-formed according to the invention directly after hardening without tempering effect or treatment. What is certain, however, is the finding that with excess carbon with well over 0.50% C and the formation of plate martensite - and of course with an insufficient degree of purity or high crystal segregation of the steel melt - the submicroscopic anisotropy increases. For this reason, the term "cubic" is used here for the type of martensite which functions as the structural basis of the method steps according to the invention, even if the proof and the morphology of the exact crystal geometry of this small atomic lattice of the mixed iron crystal have still not been achieved to this day.

The fact that the conversion to such a cubic martensite has been experimentally established alone makes it possible to choose this obvious name for cold forming only hardened and non-tempered steel with a high ductility and high strength to an extent not known to date.

The hardening condition of any tempering steel that results without a tempering effect has the highest possible strength value. This phenomenon of maximum hardness results from the fact that further approximation of the atoms is not readily possible due to the lattice reduction that has already taken place. From a morphological point of view, the formation of cubic martensite creates a crystal structure with packets of parallel slats, which are arranged without any order within the former austenite grains. So the slats are inside the grains. There is a high dislocation density within the slats, which is indicated by the large number 10 ^{1 1} to 10 ¹² per cm ² . It is several orders of magnitude larger than, for example, ferrite. These many dislocations within the slats are the result of the microplastic deformation of the slats; they increase disorder. There is a state of extreme disorder within and between former austenite grains this in the special way that the large number of small atomic grids of cubic martensite can create a new order of higher quality, in which predominantly quasi-isotropic stress fields dominate.

Regardless of this interpretation or other possible ideas, the sole cause for the occurrence of the mechanical properties determined experimentally according to the invention in any case is the residual stress state of the material, which can be adjusted to the so-called "cubic" martensite by means of a conversion that is as uniform as possible free of austenite.

The elasto-plastic behavior of the "cubic" martensite is somewhat well understood in terms of engineering mechanics if one assumes that the above-mentioned quasi-isotropic stress fields consist of elementary internal stress cells that attract each other very much in accordance with their maximum proximity. The elastic component arises when the fields are pulled apart, the plastic when they experience twists, it is clear that it is impossible to achieve a purely elastic or only plastic behavior, since this is not a structureless continuum. It is also understandable that the proximity of the elementary internal stress cells, the high hardness and the elasticity of the cubic martensite structure are different manifestations of the same marten seat composition or in other words the same stress field configuration. If the hardness of the steel is very high, then both the elasticity and the plasticity can height Show values if it is possible to force martensite formation very evenly with a quasi-cubic slatted structure. This ensures that the hardened steel has the following special features at the same time:

- Over 40 Rockwell hardness Over 35% fracture constriction in the tensile test Over 1300 N / mm ² compressive strength and Over 1500 N / mm ² tensile strength

It is easy for a person skilled in the art to notice that the value for the constriction of fracture of> 35% for the high strength values above 1300 N / mm ^{2 characterizes} an unusually high property potential of a material - in particular a structural steel of the type "low-alloy tempered steel" - in "reinforced concrete columns with high strength Stahl St 90 / DafStb, Issue 222/1972 ", the following values are given for this reinforcing steel, which is particularly emphasized as" high strength ":

Tensile strength Rm: 1040 N / mm ² break constriction Z: 29%

Considering the fact that this pair of values for the reinforcing steels BSt 420 S or BSt 500 with the tensile strength values of 500 or 550 N / mm2 - according to DIN 488 part 1 or DIN 1045 ("Concrete and reinforced concrete" edition July 1988) - despite the strength level being half as high - can hardly be achieved, is rarely determined and specified, then the importance of the process technology according to the invention for a new high-quality steel application in building construction becomes more apparent. On the basis of the combination of values described above for hardness (strength) and toughness (constriction of fracture) ) according to the invention, a resistance of the steel to a very high level for absorbing all the stresses occurring in building construction is ensured.

In claim 7, as already mentioned above, it is therefore proposed that the steel wire hardened according to the invention martensitically without a tempering effect, which has high hardness and strength, but at the same time has high ductility - expressed by high constriction values - without - as with conventional hardening and tempering Steel grade common - cold form. The cold formability of the non-tempered cubic martensite according to the definition used for the present invention is based on the fact that the potential of the cold formability becomes better with decreasing tempering action, ie with increasing strength, which was very surprising at first, but was experimentally ascertainable and also becomes theoretically plausible. if you abandon the traditional assumptions that come from conventional steel treatments. With the new knowledge, the classic idea that the increasing starting effect and thus the increased cementite formation leads at the same time to a change of the "cubic" martensite to the "α-iron" is compatible. This increases the displaceability of the unit cells against each other at the expense of their elasticity. The more carbides in the only hardened martensite structure, the less the mutual attraction of the elementary cells, the lower the hardness or strength and the higher the displaceability. In a word: Compared to the state of the "cubic" martensite, every tempered state has a lower cold forming potential.

The invention is based on the conclusion that the martensite is cold formed from the austenite as soon as possible.

The technical use of this new knowledge, however, stands in practical execution against the material problem of the tools and their service life. In order to achieve a significant reduction in the wear of the cold forming tools, the proposal according to claims 1, 3 and 4 comes to the aid. This increases the difference in hardness between the tool and the wearing edge layer of the martensitic hard steel wire to be formed. If this boundary layer is ferritic, as is proposed according to claim 3, the wear is least. In the still effective measure according to claim 8 the wear-reducing effect will be less, since in this case the higher tempered surface layer is relatively richer in cementite in the "α-iron" than in the core area. According to claim 8, only a surface layer lower in strength than that without this measure on the Strength level of the core area would be.

Multi-dimensional cold forming - e.g. by cold rolling according to DE-C2 42 33 462 - at the same time causes a thermal lattice activation in the martensite structure that has not yet been tempered, which leads to a novel "thermo-mechanical" low-temperature tempering treatment. As was also found experimentally, this treatment of the martensite, which without Tempering effect obtained by hardening, advantages over a conventional tempering treatment In particular, the elastic limit of the steel wire increases compared to that of a tempered and separately cold-formed steel wire condition.

The entire forming volume records macromechanical changes in the martensite - new dislocations are created in the grid

Change in micro-expansions in the structure

Thermal action as a result of the thermal energy generated simultaneously and comprehensively from the cold forming work in the whole

Forming volume - blocking of the newly created dislocations in the structure

Despite the increase in elastic values and tensile strength, the

Ductility - measured by determining the constriction of the fracture in the tensile test - is retained, since the structure base with the "cubic" martensite with the temperature increases of around 100 to occurring due to cold forming 200 grd still prevails. This temperature increase has been shown in FIG. 2 as a function of the tensile strength for various degrees of shape change - expressed in logarithmic shape change φ = In (Ao / A) with Ao as the cross-sectional area before and A after the deformation - from theoretical calculations. In the lower temperature range, tests for φ = 0.20 could be confirmed and the changes in properties determined. 2 shows the course of the temperature increase for φ = 0.25 in the shaded part that it will be possible to temper a non-tempered "cubic" martensite during and by cold working in the first tempering stage, φ = 0.25 corresponds to a related one Shape change ε = (1 -Ao / A). 100 [%] of about 20%.

According to the invention, the wear-reducing effect of the ferritic surface layer of a tempered steel wire with a high tensile and compressive strength is of great importance, since otherwise the mass production of releasably connectable high-strength steel rods is not economically possible. The measure proposed according to this claim requires approximately 20% cold forming in order to e.g. Apply suitable grooves, ribs or threads to the rod by pressing or rolling.

The mechanical values of the ferrite layer - with 20% cold forming - are:

BEFORE u. LATER

Tensile strength Rm [N / mm ² ] 250 +/- 50 500 +/- 200

Elongation at break A10 [%] 40 +/- 3 33 +/- 2

As can be seen, the 20% cold forming increases the tensile strength of the ferrite layer on average by two, while the elongation at break decreases remains relatively small and is less than 20%. This means that there is still a very high degree of extensibility on the steel rod surface in order to counteract material separation and cracking under bending stress. A comparison with the corresponding values in the core area of a steel rod manufactured according to FIG. 1, for example Case II (decarburization with decarburization), makes the importance of the measures according to the invention in the preamble of the invention even clearer:

The mechanical values in the core area - with 20% cold forming - are:

BEFORE u. LATER

Tensile strength Rm [N / mm ² ] 1750 +/- 50 2100 +/- 25

Elongation at break A 10% 15 +/- 2 10 +/- 1

Without the decarburization according to the invention, the elongation on the surface is therefore only 10% and would greatly increase the sensitivity to cracking and corrosion of the steel rod under load. In contrast, the ductility in the case of a carburized surface layer according to claim 3 is 33% more than 3 times and in the case of only decarburization without a ferrite layer, depending on the degree of decarburization, it is higher than 10% and less than 33%.

If you consider the fact that the DIN 488 specification for the elongation at break A10 is not greater than 10% for any of the reinforcing steel types in use, although the tensile strength has to be a maximum of only 550 N / mm ² , the magnitude of the effect according to the invention becomes decarburized surface layer with - but also without - ferrite clearly. The St 90 steel referred to above as "high strength", which was tested for use in building construction in the highly reinforced reinforced concrete columns, also provided tensile strength of 1040 N / mm ² only A10 = 1 1%, which led to this steel, which has a relatively high Cr content of approx. 3% was no longer a low-alloy grade and therefore, among other things, had relatively low decarburization and was highly susceptible to corrosion in concrete. In the years around 1970, the direction of development "away from weak types of reinforcing steel and towards those with higher strengths" was a reaction to "difficulties with concrete" itself to be sought in the fundamental problem of concrete properties. This was discussed in the test report from 1972 (DafStb , Issue 222 in Part 2, page 63):

“Since the load in highly reinforced reinforced concrete columns is almost entirely shifted to the steel anyway due to the concrete crawling and depending on the degree of reinforcement, this is stressed to the point of squeezing, so it made sense to examine whether such columns were not better suited to participate the concrete is not subjected to pressure "It is also said on the same page:

"The concrete then only has the task of securing the bars from kinking and protecting against corrosion and fire; the best way to do this is to tie them with a spiral reinforcement."

The aim of the invention was therefore to produce a steel wire that significantly improves the current state of the art in building construction when using concrete and reinforced concrete with major disadvantages. Such a steel made of a low-alloy martensitic steel wire according to claims 1 to 10 has a tensile strength of up to 2400 N / mm ² in the presence of high ductility and low sensitivity to corrosion. The crushing limit of the steel rod according to the invention can be described as particularly high with values of up to 1600 N / mm ² , so that support structures with permissible propellant loads of, for example, 10 MN at 0.5 x 0.5 m ^{2 are} made possible for the first time in a completely new type of lightweight steel construction becomes. In order to utilize the high steel strength potential according to the method according to the invention, the high-quality role of the steel wire is also freed from the concrete-specific restrictions in building construction. In the inventive arrangement of steel wire rods according to claims 1 1 to 20 it is achieved that the steel wire according to the invention

"great buckling protection" - without the help of concrete - and only through precise and secure steel elements with a releasable non-detachable welding non-positive and / or positive connection,

"With regard to the risk of corrosion" is better without concrete than with it, according to the invention by deliberately decarburizing the surface layer, the chemical composition and the structure of the steel wire, the crack formation both in the edge and in the core area and the sensitivity to hydrogen crack corrosion is minimized and its effective Permanent monitoring as a load-bearing element in the entire building complex is given in that all elements are easily accessible and can be replaced if necessary and finally

"Protection against fire" can be obtained very effectively by the fact that the accessibility also allows the installation of an automatic monitoring and fire extinguishing system also inside the supports and ceilings.

From what has been said so far, it is evident that the use of the steel wire according to the invention as the sole element for load-bearing in building construction is to be preferred in all respects, although here the use of concrete as a "filling and covering material" compared to its use according to the current state of the art Technology certainly also has clear advantages for the classic construction in reinforced concrete construction. In particular, wherever one is forced to extend reinforcement bars by "pressure surges" of any kind, ie by "pressure overlaps", "contact butts" or "press sleeve butts" but also by "welding binding wire" in reinforced concrete columns, this need is eliminated , because the steel wire according to the invention is elastically coiled in heavy wire rings after cold forming and profiling, it can be uncoiled and cut from the ring in full length as a straight rod for supports of any height. These weaknesses of the current construction would certainly be eliminated, but the fact remains exist that a not insignificant part of the total potential of the anyway not high-strength reinforcing steel is used to hold the concrete together under load. This also applies to prestressed concrete construction and bridge construction. Especially in buildings with many floors, the weight of the concrete causes depending the higher the load capacities and additional bending stress, and thus more concrete and reinforcing steel. These do not apply if the present invention is used.

To use the high strength potential of the steel wire according to the invention as a compression rod, it is proposed in claim 13 that the load-bearing capacity of a support consisting of several rods is made possible by the fact that interconnectable sleeves protect the compression rods against thrust and bending according to the invention. If one chooses a permissible kink length of the single bar as far as possible from the crushing limit of the steel wire according to the invention as the maximum distance between the sleeves of each bar of a bundle, then very high load capacities for unusually slim loads can be achieved with such simple but very effective buckling protection with sleeve packages Columns are realized according to the invention, wherein pressure resistance potentials of the column are still preserved, which can be used at any time. 3 shows the cross section through such a sleeve package, for example and schematically. The square sleeves and the selected form of this example for the wedge bolts in FIG. 3 reduce any relative movement of the supporting rods to one another to an extremely small extent and bring about a high degree of kink protection. According to this design principle, a 40 cm x 40 cm support consisting of 36 23 mm rods according to the invention would have a load capacity of 36 x 0.20 MN = 7.20 MN, ie approx. Resist 700 tons, which corresponds to a floor area of more than 1000 m ² . The specific steel mass of this column per meter of column height - without the mass fraction of the socket packages - is approx. 150 kg / m; with the anti-kink elements max. 160 kg / m. This results in the specific total steel mass of approx. 0.23 kg / mt

The following comparison of this value according to the invention with the highest currently. Specific values that can also be achieved in steel construction are very revealing:

a) IP steel (according to DIN 1025. BI.2; calculated according to DIN 4114):

I P 40 - shaped steel with 164 kg / m brings the specific value of 0.62 kg / m.t with a free buckling length of 2.0 m and the specific value of 0.77 kg / m.t with a free buckling length of 5.0 m. This is three times the value of 0.23 kg / m.t.

b) Double I - steel column (according to DIN 1025, BI.1; calculated according to DIN 41 14):

Two I 40 - shaped steel buckling bars with a support spacing of 33 cm and standard-compliant binding plates bring the specific value of 0.56 kg / mt with a free buckling length of 2.5 m and the specific value with a free buckling length of 5.0 m 0.61 kg / mt These values are negligible smaller than with IP steel column under a). However, they are significantly less favorable than the value according to the invention at 0.23 kg / mt

If lower loads are to be resisted, the number of compression rods according to the invention can be correspondingly smaller and the spacing between them can be chosen larger by the sleeves on the bars according to the invention, according to claim 15, in order to create larger radii of inertia by means of further — here longer — coupling sleeves be placed further away. The lowest statically determined number of bars with three results in the inventive values of the above example with a bar diameter of 23 mm and a pressure carrying capacity of 60 tons.

According to the invention, the unused potential of the bundle of rods in the pressure resistor is partially used in accordance with the invention by subsequently adding new packages or partial connections to the compression rods of the support, which are already under load, at intervals between the existing socket packages by means of split sockets. The split sleeves can be closed positively or positively after the application via the thread according to claim 17. The forces occurring at the level of the interconnected or pressed together sleeves are very small in the buckling protection; a fact that arises from knowledge of the concrete cover regulations for reinforced concrete columns.

According to claim 18, the ball offers a shape base according to the invention for the design of nodes of the truss structure but also support sleeves.

If, according to claim 19, it is made from the same low-alloy tempering steel according to the invention and if it was produced economically as a grinding ball, then they receive a threaded through-hole for the main rod and several threaded holes, in which only short rods are screwed in, with which the secondary rods can then be connected as tension or compression rods by means of long sleeves.

According to claim 20 and as shown schematically in Fig. 4, the impact formation at high loads by means of solid steel cores and plates for the transmission of the loads according to the invention should be carried out uniformly by screwing assembly in order to reduce the usual eccentricities to the low level of precise mechanical engineering to minimize.

Claims

Claims: 1.Procedure for improving the surface properties of low-alloy tempered steel, in particular for producing a high-strength steel wire with a martensitic microstructure base for absorbing very high compressive and tensile stresses in steel construction, the low-alloy martensitic steel being subjected to targeted, uniformly distributed edge decarburization in one or more processing stages of its heat treatment. characterized in that the edge decarburization takes place with a depth corresponding to 1 to 15% of the steel mass located below the surface and that the edge decarburized layer becomes martensitic during the transformation during the quenching process without self-starting effect. 2. The method according to claim 1, characterized in that the low-alloy martensitic convertible steel has a basic content of Si of more than 0.35 wt .-% and Cr of less than 0.65 wt .-%. 3. Process according to claims 1 and 2, characterized in that the edge decarburization of the steel surface begins with a closed carbon-free ferritic carburization layer of up to 20% of the depth of the total decarburization. 4. Process according to claims 1 and 2, characterized in that the decarburized layer is without decarburization and without coherent ferritic points and that its degree of decarburization on the surface is between 20 and 50%. 5. The method according to claims 1 to 4, characterized in that the low-alloy steel contains at least 90% martensite after hardening and at least 95% bainite of the lower intermediate structure in the structural residue. 6. The method according to claims 1 to 5, characterized in that the low-alloy martensitic hardenable steel has a diameter of 4 to 45 mm as hot-rolled steel wire. 7. The method according to one or more of claims 1 to 6, characterized in that the steel wire decarburized coherently after hardening in the edge layer after the hardening process without a tempering effect undergoes a first tempering treatment by cold forming and thus by the heat of deformation resulting from this structural deformation and after the forming is cooled at a rate of less than 10 K / s. 8. The method according to one or more of claims 1 to 6, characterized in that the hardened in the edge layer coherent edge decarburized martensitic steel wire after hardening in the heat treatment for the first tempering of the martensite undergoes a specifically different heating from the edge layer to the wire core and thereby get a softer edge layer. 9. The method according to one or more of claims 1 to 8, characterized in that the surface of the hardened and tempered steel wire undergoes a change in shape appropriate to the particular use due to the single continuous or subsequently discontinuous cold forming. 10. The method according to any one of claims 1 to 9, characterized in that the total tensile strength Rm resulting from martensite formation, tempering treatment and strain hardening is between 1000 and 2400 N / mm ² . 1 1. Arrangement of several steel wire rods produced by a method according to one of claims 1 to 10, characterized in that the straight steel rods cut to length by the high-strength steel wire after the cold forming for the purpose of absorbing tensile, shear and compressive stresses in a non-positive and / or positive manner geometrically arbitrary spatial arrangements are connected to one another in a manner that is detachable at any time. 12. The arrangement according to claim 1 1, characterized in that the high-strength martensitic steel rods are kink-resistant and assembled as units of e.g. Support, ceiling or bridge constructions can be used in steel construction with or without the addition of concrete. 13. Arrangement according to claim 1 1 or 12, characterized in that the kink protection of a pressure-loaded rod bundle is ensured by interconnectable, acting against thrust and bending sleeves. 14. Arrangement according to claim 13, characterized in that the connection of the sleeves is positively by means of wedge bolts and any number of bars as kink-proof bundle of bars is converted by sleeve packages at equal intervals of the permissible buckling length to a heavy-duty support. 15. The arrangement according to claim 13, characterized in that the sleeve of each rod is connected by screwing to the other remotely located sleeves to create larger radii of inertia. 16. The arrangement according to claim 13, characterized in that the kink-proof connection is subsequently expanded by divided sleeves in order to increase the load capacity. 17. The arrangement according to claim 16, characterized in that the split sleeves can be mounted on the rod by spring washers and are connected to each other by a bandage with steel tape to form sleeve packs. 18. Arrangement according to claims 1 1 to 15, characterized in that the sleeves have a spherical shape and act as a node of a truss structure. 19. The arrangement according to claim 18, characterized in that the ball sleeves produced by the technology of the production of grinding balls, consist of the same low-alloy steel according to the invention, martensitic hardened, long-term tempered and are prefabricated with threaded holes for the tension and compression rods. 20. Arrangement according to one of claims 1 1 to 19, characterized in that the forwarding of the loads to the supports takes place exclusively via the socket connections and compression rods to the solid steel cores and plates on the head and / or foot of the support supports.