EP0172544A2 - Process for heat treating hot rolled steel rod or wire for prestressing concrete - Google Patents

Abstract

For the production of screwable prestressing steels, steels with a C content of 0.50 to 0.80%, preferably approximately 0.75%, an Si content of 0.20, 0.50%, preferably approximately 0.25% and a Mn content of 0.30 to 0.80%, preferably about 0.60% after hot rolling from the rolling heat on the exit side of the finishing stand by means of a cooling liquid, primarily water, subjected to surface quenching such that the material in an edge zone (R1) is immediately and completely converted to martensite, while the heat content remaining in the core zone (K1) during the subsequent cooling does not cause the martensitic peripheral zone to temper beyond the range of the intermediate stage. Prestressing steels of this type have a high ductility and toughness, a high ductility or toughness, are highly corrosion-resistant and have a wear-resistant surface, which makes them particularly suitable for screw-in tensioning rods in which the screwability is either by cold rolled threads or by hot-rolled threads Ribs is generated.

Description

The invention relates to a method for producing rolled steel products, in particular screwable prestressing steels.

Prestressing steels as in the construction industry as tendons for prestressed concrete, but ^g ebrücken as anchor steels for earth and rock anchors as formwork ties, _H ängekabel of Khan, cable-stayed of cable-stayed bridges, bracing, etc. are used generally have a yield strength ranging from 835 to 1080 N / mm ² and a tensile strength that varies from 1030 to 1230 N / mm2. In the manufacturing processes which have hitherto been customary, steels with a C content of 0.65 to 0.75%, an Si content of 0.60 to 1.60% and a Mn content of 0.70 to 1 are used as the starting material. 50% and chrome and / or vanadium and other alloying elements are used.

Prestressing steels of this type are known in various designs, for example as round wires, which are cold-drawn after hardening for strengthening and then tempered again, as flat steels, for example oval steels, which are tempered over the entire cross-section after rolling and as steel rods . Steel bars with diameters between approximately 15 and 50 mm are hot-rolled, then stretched to increase the yield strength and then tempered for relaxation. In addition to the fact that only limited lengths can be produced because of the stretching, this complex production process entails high production costs.

In addition to the static strength values, prestressing steels must have the highest possible elastic limit and good deformability. In the case of screwable prestressing steels, i.e. those in which threaded anchors can be attached, high wear resistance of the surface and corrosion resistance are important. The relaxation properties and a sufficiently high fatigue strength are also important.

In addition to prestressing steel for prestressed concrete components, there are reinforcing steels that are used as slack, not prestressed reinforcement for reinforced concrete. Reinforcing steels of this type are used either naturally hard, whereby the strength is determined by the alloy, or cold worked, for example by drawing or cold rolling, the latter mainly for structural steel mesh. Reinforcing steel of this type must be weldable; their analysis is therefore characterized by a low C content. Such reinforcing steels generally have a yield strength between 420 and 500 N / mm ² and a tensile strength between 500 and 550 N / mm2. Reinforcing steels with higher strength values are usually not manufactured. The analyzes used are: away from those of weldable steels with a C content of less than 0.22%.

Reinforcing bars are manufactured with a smooth surface and as ribbed reinforcing bars. Reinforced ribs usually have crescent-shaped ribs that run diagonally to the longitudinal axis of the bar, which extend in the transverse direction over a large part of the bar circumference and are intended to improve the bond between the bar in question in the concrete.

In order to increase the strength and improve the deformability of hot-rolled reinforcing steels of this type, it is known to subject the bars from the rolling heat on the exit side of the finishing stand to a surface quenching by means of a cooling liquid such that an edge zone made of martensite or Bainite is present, while the heat content remaining in the rod core during the subsequent cooling does not cause the edge zone to be tempered via the bainite stage (DE-AS 23 53 034). This known method is based on the idea of increasing the strength and improving the deformability without increasing the contents of carbon or manganese, which would run counter to the requirement of good weldability. A typical analysis of such heat-treated reinforcing steel shows 0.17 to 0.22% C, 0.05 to 0.30% Si and 0.70 to 1.10% Mn.

Against the background of this prior art, the object of the invention is to provide an accurate and cost-effective manufacturing method for prestressing steels, especially for screwable prestressing steels with the strength properties mentioned at the outset, which makes it possible to use metallurgically easy-to-represent and inexpensive analyzes for the production of prestressing steel which is corrosion-resistant and which is a wear-resistant upper surface that reduces the risk of mechanical damage and is suitable for threading. The prestressing steel should continue to be producible in any bar length, with high yield strength and high strength it should have great ductility or toughness, especially at low temperatures, and with low relaxation it should have high fatigue strength.

According to the invention, this object is achieved in that steels with a C content of 0.50 to 0.80%, preferably about 0.75%, an Si content of 0.20 to 0.50%, preferably about 0 , 25% and a Mn content of 0.30 to 0.80%, preferably about 0.60% after hot rolling from the rolling heat on the outlet side of the finishing stand by means of a cooling liquid, primarily water, are subjected to surface quenching in such a way that the material in an edge zone is immediately and completely converted to martensite, while the heat content remaining in the core zone during the subsequent cooling does not cause the martensitic edge zone to temper beyond the range of the intermediate stage.

The final rolling temperature on the finishing stand is expediently chosen so that it is just above the transformation point A ₃ at the lower limit of the hot formability of the steel. The final rolling temperature is advantageously between 860 and 1060 ° C, preferably between 880 and 940 ° C.

Tempering is advantageously carried out according to the invention in such a way that the surface temperature of the edge zone in the period between the second and sixth seconds of the heat treatment is not more than approximately 500 ° C., preferably between 400 ° and 500 ° C., depending on the rod diameter.

The invention is based on the knowledge that it Combinatorial interaction of certain factors is required to produce a prestressing steel with the specified properties in an economical manner.

What is particularly important here is the analysis, the relatively high C content of which results in high strength, which is further increased by the subsequent heat treatment. While the austenitization takes place during tempering as a special homogenization anneal, the prestressing steel production according to the invention does this by heating in the rolling mill furnace and the rolling process itself. Decisive for the product are the degree of homogeneity of the analysis, the size of the austenite grain and the temperature of the solution annealing.

The size of the austenite grain is determined, among other things, by the recrystallization that occurs during hot rolling after each pass. In absolute terms, the more frequently and more intensively it is deformed, the smaller it is. The final grain size is only achieved in the last rolling pass; Here again, the deformation and the temperature as well as the dwell time at this temperature until the cooling process starts are decisive.

In the manufacture of prestressing steels according to the invention, a very fine-grained structure, at least in the area of the strong deformation in the edge zone of the rod, must be present before cooling begins. This reduces the risk of the formation of very resistant austenite grains, which adversely affect the corrosion resistance of the steel.

It is particularly advantageous if the temperature at the last roll pass, i.e. on the finishing stand, is at the lower limit of hot formability, i.e. just above the transformation point A ₃ lies. This creates a very fine-grained structure and recrystallization is largely prevented. In connection with this, the cooling must start so quickly and so intensively that the cooling curve of the edge zone reaches the martensite area without reaching the areas of ferrite, pearlite and the intermediate stage. This is particularly important in the case of a relatively high-carbon steel in which the martensite start temperature M _{S is} relatively low. At the same time, however, the core zone of the rod must still have such a large heat content in order to cause the martensite present in the peripheral zone to temper.

The conditions under which this process can proceed can be explained with reference to FIG. 1, which shows a time-temperature conversion diagram for a steel which roughly has the analysis preferred for the invention, namely 0.76% C, 0 , 23% Si and 0.63% Mn.

Curve R _{1 shows} the profile of the surface temperature of a steel rod with a relatively small diameter, for example 15.1 mm, and curve K _{1 shows} the profile of the temperature of the core zone of the rod in question. R ₂ is the corresponding curve of the surface temperature of a rod with a larger diameter.

The course of curve R ₁ in the area of tempering is essential for the heat treatment. In order to achieve a structural structure that meets the requirements for prestressing steel, the curve R _{1 of} the surface temperature must run in the temperature range between 400 and 500 ° C. in the time between the second and the sixth second of the heat treatment recorded in FIG. 1; under no circumstances should it extend into the pearlite area.

The strong cooling of the edge zone also accelerates the heat transfer from the core zone. Depending on the chemical composition, conversion takes place directly in the intermediate stage or pearlite pre-precipitation takes place. It is considered particularly advantageous if the core zone of the rod converts in the upper intermediate stage area, which is characterized by a finely dispersible distribution of the carbides.

The intensity of the cooling of the peripheral zone essentially depends on the cooling capacity of the system available. The cooling capacity depends on several factors. For a tried and tested cooling system, a water quantity of 10 to 20 l / sec kg is considered to be particularly advantageous. Inertia supporting this process can also be achieved through the chemical composition of the steel. For example, the increase in carbon content, but also the usual alloying elements of steel, such as Mn, Si, Cr, Ni, Mo, work in this sense.

By adding further alloying elements, certain properties of the prestressing steel can be improved. The addition of chromium (up to approx.0.8%) and copper (up to approx.0.5%) increases the corrosion resistance, the addition of vanadium (up to approx.0.15%) and niobium ( up to approx. 0.06%) and the microalloying elements titanium and boron in traces an increase in toughness and fatigue strength. By choosing the appropriate alloying elements, the C content can also be reduced to the lower limit.

• Streckgrenze/Zugfestigkeit) bzw. 885/1080 zusammengestellt, die an Spannstäben mit den Durchmessern 26,5 mm bzw. 15,1 mm genommen wurden. Tabelle 2 gibt die als Mittelwerte errechneten statischen Festigkeitswerte einiger nach der Erfindung hergestellter Spannstäbe der Durchmesser 36,0 mm, 26,5 mm und 15,1 mm an. Hierin bedeuten die Abkürzungen Re die Streckgrenze, Rm die Zugfestigkeit, A₁₀ die Bruchdehnung über eine Meßlänge, die dem 10-fachen Durchmesser des Stabes entspricht und A_G die Gleichmaßdehnung.

As tests have shown, the prestressing steels produced according to the invention largely meet the requirements placed on them. Table 1 shows the analytical values of some melts of steel grades 835/1030 (ratio:

• Yield strength / tensile strength) or 885/1080, which were taken on tension rods with diameters of 26.5 mm and 15.1 mm. Table 2 shows the static strength values, calculated as mean values, of some of the tensioning rods of the diameters 36.0 mm, 26.5 mm and 15.1 mm produced according to the invention. The abbreviations Re here mean the yield strength, Rm the tensile strength, A ₁₀ the elongation at break over a measuring length which corresponds to 10 times the diameter of the rod and A _G the uniform elongation.

The high corrosion resistance of steels produced according to the invention is primarily a result of the great uniformity of the structure; due to the low temperature during rolling and the rapid cooling, disruptive factors are prevented from occurring. In addition, relaxation tests to determine the inelastic elongation after 1000 hours of service have shown that the relaxation losses are very low. Bending tests have shown excellent ductility properties of the samples examined.

Since the prestressing steels produced by the process according to the invention have a high grain size in the edge zone and a correspondingly high surface strength, they are particularly suitable for the production of screwable prestressing steels.

Threaded anchorages are often used in tension rods to transmit the clamping forces to the component in question. In this context, it is known to cold-roll threads on rods rolled with smooth surfaces at the ends. In contrast to a cut thread, such a non-cutting deformation has the advantage that in the thread area with a reduced core cross section, the steel structure is strengthened, in particular in the area of the thread fillets, so that the steel rod also can be used in the thread area with the full force corresponding to its cross-section, taking into account the permissible stresses. It is also known to design this thread so that the fillet in the thread grooves has a much larger radius of curvature than the fillet on the outer thread tips (DE-PS 10 68 454). A thread with such rounded fillets allows considerably greater tolerances compared to the thread of the nut and thus creates the prerequisite for inaccuracies in the installation of the anchoring bodies to be safely absorbed.

It is also known to provide a tensioning rod already by hot rolling with helically running ribs which are arranged on two opposite sides of the rod circumference and form parts of a thread onto which an anchoring body provided with a corresponding counter thread can be screwed (DE- PS 17 84 630). The partial thread achieved in this way has very rough tolerances compared to a metric thread, so that it meets the requirements of rough construction operations very well. In addition, the screw thread is available over the entire length of the rod in question without additional expenditure.

Finally, it is also known to form the ribs produced by hot rolling as transverse ribs which extend approximately over half the circumference of the cylindrical tension rod and decrease in width and height towards their ends (DE-PS 20 43 274). Only partial areas of these ribs lie on a helical line, but this opens up the possibility that both anchoring bodies with a right-hand thread and those with a left-hand thread can be screwed onto such a partial thread.

The invention accordingly also relates to the application of the method to the production of hot-rolled steel rods or wires with a smooth surface, which at least at the ends are provided with cold-rolled threads suitable for screwing on a connecting or anchoring body, in which the fillet in the Thread fillets have a much larger radius of curvature than at the thread tips and on the production of steel rods or wires, which are provided with ribs by hot rolling, which at least in some areas run along a helix, which are arranged on two opposite sides of the rod circumference and Form parts of a thread onto which a connection or anchoring body provided with a corresponding counter thread can be screwed.

In the case of steel bars with hot-rolled ribs, it is important that the quenching effect is not impaired by the surface shape of the bars or that the surface profile of the bars is designed so that the bars have a uniform coating layer in the area of the ribs.

It has been shown that if the height of the ribs, their mean width and their spacing are in a certain ratio to one another, during the heat treatment of this rod, the ribs behave relative to the cooling medium like cooling ribs, that is to say that the heat output in the region of the Ribs is comparatively larger than in the area of the smooth rod surface, so that the border zone between the core zone and the peripheral zone of the rod runs in a straight line. This ratio is expediently 0.5 to 1 to 4.

• Fig. 1 ein Zeit-Temperatur-Umwandlungs-Schaubild für einen erfindungsgemäß herzustellenden Spannstahl,
• Fig. 2 einen Spannstab mit glatter Oberfläche und am Stabende aufgerollten Gewinde,
• Fig. 3 einen Querschnitt entlang der Linie III-III in Fig. 2,
• Fig. 4 einen Ausschnitt aus dem Gewinde in größerem Maßstab,
• Fig. 5 eine Seitenansicht eines Spannstabes mit warmgewalzten, auf einer Schraubenlinie liegenden Gewinderippen,
• Fig. 6 einen Querschnitt entlang der Linie VI-VI in Fig. 5 in Normalprojektion,
• Fig. 7 einen Spannstab mit warmgewalzten, quer verlaufenden Gewinderippen und
• Fig. 8 einen Schnitt entlang der Linie VIII-VIII in Fig. 7 in der Normalprojektion.

In the drawing are still some types of prestressing steels that can be produced according to the invention, Darge poses. It shows

• 1 is a time-temperature conversion diagram for a prestressing steel to be produced according to the invention,
• 2 a tension rod with a smooth surface and thread rolled up at the end of the rod,
• 3 shows a cross section along the line III-III in FIG. 2,
• 4 shows a section of the thread on a larger scale,
• 5 is a side view of a tie rod with hot-rolled threaded ribs lying on a helix,
• 6 shows a cross section along the line VI-VI in FIG. 5 in normal projection,
• Fig. 7 is a tie rod with hot-rolled, transverse thread ribs and
• Fig. 8 is a section along the line VIII-VIII in Fig. 7 in the normal projection.

The tension rod 1 shown in Fig. 2 is rolled as a smooth rod and has been subjected to the heat treatment according to the invention. In a cold way, a thread 2 was rolled onto the rod end, which is indicated in FIG. 4 in longitudinal section in large enlargement. This thread is a so-called asymmetrical partial thread, that is to say the rounding radius in the area of the thread fillets 3 is significantly larger than in the area of the thread tips 4.

The tension rod 11 shown in FIGS. 5 and 6 is a so-called threaded rod, which has already been provided with threaded ribs 12 in the course of the hot rolling process. The ribs 12 have a height h, an average width B and are arranged at a distance A from one another, which are approximately in a ratio of 0.5 to 1 to 4. The ribs 12 each extend over a third of the rod circumference to their full height and merge with their end faces 13 into the surface 14 of the rod core 15.

It is indicated here that the boundary zone 16 between the edge zone R, in which the material of the steel has been converted into martensite by the surface quenching, and the core zone K, the heat content of which remains after the surface quenching causes the subsequent tempering of the martensitic edge zone R approximately linear . This is the result of the increased cooling effect of the ribs during surface quenching and has the advantage of a consistently high surface strength of the steel and very good corrosion resistance.

Corresponding conditions also prevail in the threaded rod 21 shown in FIGS. 7 and 8, in which the ribs 12 are designed as transverse ribs. Here, too, the border zone between the core zone K and the edge zone R of the rod runs in a straight line without being influenced by the ribs.

As a result of the high surface strength of the tempered edge zone R, which also includes the hot-rolled ribs, it is possible to make the anchoring and connecting elements, such as nuts, sleeves or the like, shorter than in the case of known tension rods with a homogeneous rod cross section. However, the shorter these elements are, the better the force transmission in the thread area between the rod and nut or sleeve.

Claims (8)

1. A method for producing rolled steel products, in particular screwable prestressing steels or the like, characterized in that steels with a C content of 0.50 to 0.80%, preferably about 0.75%, an Si content of 0.20 to 0.50%, preferably about 0.25% and an Mn content of 0.30 to 0.80%, preferably about 0.60% after hot rolling out of the rolling heat on the exit side of the finishing stand by means of a cooling liquid, primarily Water, surface quenching such that the material in an edge zone (R) is immediately and completely converted to martensite, while the heat content remaining in the core zone (K) during the subsequent cooling does not temper the martensitic edge zone beyond the range of the intermediate stage beyond. 2. The method according to claim 1, characterized in that the final rolling temperature on the finishing stand is selected so that it is just above the transformation point A ₃ at the lower limit of the hot formability of the steel. 3. The method according to claim 2, characterized in that the final rolling temperature is between 860 and 1060 ° C, preferably between 880 and 940 ° C. 4. The method according to any one of claims 1 to 3, characterized in that the tempering is carried out such that the surface temperature of the edge zone in the period between the second and sixth second of the heat treatment depending on the rod diameter not more than about 500 ° C, preferably is between 400 ° and 500 ° C. 5. The method according to any one of claims 1 to 4, characterized ge indicates that the alloy contains up to approx. 0.8% chromium, up to approx. 0.5% copper, up to approx. 0.15% vanadium, up to approx. 0.06% niobium and traces of titanium and boron individually or in combination. 6. Application of the method according to claims 1 to 5 to the production of hot-rolled steel rods or wires with a smooth surface, which are provided at least at the ends with cold rolled, suitable for screwing on a connecting or anchoring body, in which the fillet in the thread fillets has a much larger radius of curvature than at the thread tips. 7. Application of the method according to claims 1 to 5 to the production of steel rods or wires, which are provided in the way of hot rolling with ribs, which run at least in some areas along a helix, which are arranged on two opposite sides of the rod circumference and Form parts of a thread onto which a connection or anchoring body provided with a corresponding counter thread can be screwed. 8. Application according to claim 7, characterized in that the ribs are designed and arranged so that the ratio of height to medium width to spacing of the ribs is approximately 0.5 to 1 to 4.

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