GB2047270A

GB2047270A - Weldable Steel Rods

Info

Publication number: GB2047270A
Application number: GB8000407A
Authority: GB
Original assignee: Stahlwerke Pein Salzgitter AG
Current assignee: Stahlwerke Pein Salzgitter AG
Priority date: 1979-01-05
Filing date: 1980-01-07
Publication date: 1980-11-26
Also published as: NL8000059A; FI794092A; SE451020B; ES8107320A1; FR2445858A1; DE2900271C2; CA1129312A; IT7928312A0; FI69120B; BE881003A; IT1164547B; AU5428780A; LU82058A1; ES487449A0; SE8000069L; DE2900271A1; GB2047270B; FI69120C; JPS55115949A; AU534561B2

Abstract

Weldable steel rods for use in reinforced concrete, so-called concrete steel, are in the form of a concentric core region and a surface layer. The core region is formed of a pure perlite-ferrite mixed structure, in which the proportion of ferrite is from 20 to 80%. The core region is directly adjacent to the surface layer, without any intermediate layer. The surface layer is composed of pure, annealed martensite. A method for the production of such concrete steel is also disclosed.

Description

SPECIFICATION Weldable Steel Rods for use in Reinforced Concrete and Method for the Production of such Steel Rods The present invention relates to weldable steel rods for use in reinforced concrete, said rods being hereinafter referred to as "concrete steel".

Such concrete steel has a carbon content of less than 0.25% and, without additional subsequent treatment, such as cold deformation, patening or surface annealing, has a yield point pro 2 of at least 500 N/mm2 and a tensile strength of at least 550 N/mm2, and is in the form of a concentric core region comprised of a mixed structure of perlite, ferrite and possibly additional components, and a surface layer containing annealed martensite.

The invention further relates to a method of production of such concerete steel.

It is known to use steels having a chemical composition of from 0.35 to 0.45% carbon, up to 1.3% manganese, 0.2 to 0.3% silicon, as well as conventional impurities, as high-quality concrete steels. The production of such steels is indeed cheap, since mainly carbon, manganese and silicon are used as strength formers. However, the deformation ability of these steels is relatively low, more especially they lack any suitable tendency to welding.

Weldable concrete steels have also become known which have a low carbon content (maximum 0.28%) and a silicon content of 0.5%, a maximum manganese content of 1.6% and, as well as conventional impurities, a copper content of at least 0.2% (ASTM Designation: A 440-74, page 336). Such steels, however, have to be subjected to a cold deformation treatment. A substantial disadvantage of these weldable concrete steels reside in that, after a brief storage at temperatures between room temperature and 8000C, they already indicate a substantial loss of yield point or tensile strength. Such temperatures, however, are produced during welding and during hot bending of concrete steels at a building site.

A concrete steel having the aforesaid properties, which does not contain these disadvantages, is known from German Offenlegungschrift No. 24 26 920. By means of a special cooling method, concrete steels are produced which are comprised of a plurality of granular micro-structures. From the edge of the rod, the steel is comprised of highly annealed ma rtensite-bainite, of ferrite-bainite to ferriteperlite in addition to bainite in the rod core. Thus, the proportion of perlite with bainite should preferably be percentually greater than the proportion of ferrite.

Such a steel has the required welding properties and a sufficiently high tensionresistance as well as sufficiently high yield point values. It has, however, been found that the breaking tension of such known steel requires improvement. It tends to initiate fissuring and therefore does not possess optimal fatigue behaviour.

Therefore, it is an object of the invention to provide a steel which possesses the desired properties of known steel, but which, however, is less prone to fissuring, and thus has an improved value for the breaking tension.

According to the present invention there is provided a weldable concrete steel having a carbon content of less than 0.25%, and without additional subsequent treatment, such as cold deformation, patening or surface tempering, has a yield point p0 2 of at least 500 N/mm2 and a tensile strength of at least 550 N/mm2 and is in the form of a concentric core region and a surtace layer, whereby the core region is comprised of a mixed structure of perlite, ferrite and possibly additional components and the surface layer containing annealed martensite, in which the core region is formed of a pure perlite-ferrite mixed structure, in which the proportion of ferrite is from 20% to 80%, and which the core region is adjacent to the surface layer without any intermediate layer, which surface layer, in turn, is comprised of pure, annealed martensite.

The steel in accordance with the invention therefore is characterised by a pure, concentric, two-layer structure, in which both layers are completely free from bainite. The steel in accordance with the invention hence has both good welding properties and a substantially improved break tension, whereby the steel is substantially less prone to initiate fissuring and thus has an improved fatigue behaviour.

In a preferred embodiment, the concrete steel in the core region has ferrite and perlite in substantially equal proportions. It has been found that the concrete steel in accordance with the invention is also very well suited for ribbed reinforcing rods, since both layers adapt themselves to the rib shape in such in manner that the ribs also have the mechanical properties of the unribbed rod.

Furthermore, it is favourable for the proportion of the surface layer at the cross-sectional surface to be at least 20%, preferably 33%.

The concrete steel in accordance with the invention, moreover, has the advantage that it may be produced at favourable costs and at high speed on a rod mill. The method in accordance with the invention for producing the concrete steel in accordance with the invention is characterised by the following method steps:: a) the concrete steel is made on a rod mill; b) after leaving the roll line, the rolled material is subject to an intensive, preferably multi-stage, cooling; c) by such cooling, the surface of the rolled material is cooled to below the martensite start temperature; d) cooling occurs with such intensity that the compensation temperature between core and surface is attained before any conversion into bainite, ferrite or perlite can set in, and that the compensation temperature lies substantially in the temperture range in which an earliest possible conversion of the austenite into ferrite and perlite can occur; and e) on reaching the compensation temperature, the temperature at least to the end of the perlite conversion is kept constant and the rolled material is then subjected to gradual cooling.

In a preferred embodiment, the rolled material is reeled up directly after passing through the cooling stage and in the reeled up state, is cooled in the air. This ensures both the isothermic conversion aimed at with the invention as well as the annealing of the martensite in the edge region directly from the heat used for rolling, i.e. without having to utilise additional measures.

The method in accordance with the invention permits a rapid and reliable production of concrete steel in accordance with the invention without incurring any considerable expenditure. In a surprisingly simple manner, concrete steel may be produced on a rod mill and be so treated that the properties strived after for a long time may be attained already during manufacture without considerable expenditure.

In a preferred embodiment of the method in accordance with the invention, a standard steel having a thickness up to 1 3 mm is used for producing concrete steel. In a standard steel, the sum of all alloy elements (manganese, silicon, sulphur and the like) lies below 1.7%. This standard steel is particularly favourably priced and may be used to produce the concrete steel in accordance with the invention of high quality when using a standard water cooling system, when the thickness of the concrete steel is below 13 mm.

So as not to have to put up with excessive expenditure for the cooling, it is preferable to use a standard steel having a diameter of > 13 mm which is micro-alloyed with a proportion of micro-alloying elements up to 0.08%, for the production of concrete steel.

Alternatively, an alloyed steel may be used for diameters between 13 and 25 mm. In this steel, the sum of the alloy elements lies between 1.7% and 3%. For diameters of more than 25 mm a micro alloy has to be added to the alloyed steel, which then has a proportion of micro-alloying elements of up to 0.03%.

These alloy rules are based on the knowledge that the conversion of the austenite into ferrite, perlite or bainite may be postponed to later periods by the use of alloyed steel and/or microalloyed steel.

It has been found that the method in accordance with the invention may be carried out most economically if the first stage of the cooling is completed within 0.2 seconds.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a photograph of a cross-section through a known concrete steel in accordance with German Offenlegungschrift No, 24 26 920; Figures 2a to 2d are photographic micrographs magnified 500 times of the structure contained in such known concrete steel; Figure 3 is a photographic cross-section through a concrete steel in accordance with the invention; Figures 4a and 4b are photographic micrographs showing the two structures of the concrete steel in accordance with the invention magnified 500 times; Figure 5 is a diagram to elucidate the controlled cooling in accordance with the method of the invention; Figure 6 shows a table to elucidate the cooling in concrete steels of varying diameter and their cooling off behaviour;; Figure 7 is a time-temperature diagram for a standard steel of low carbon content; and Figure 8 is a time-temperature diagram for an alloyed steel of low carbon content.

Figures 1 and 2 show photographic exposures of the concrete steel known from the G-OS 2426 920.

Figure 1 clearly shows that the concrete steel over its cross-section has at least four concentric layers. The outer layer consists of annealed martensite-bainite, adjoining which inwardly there is a bainite intermediate layer. This is followed by an annularferrite-bainite layer, whilst the core is substantially comprised of ferrite and perlite.

These four types of structures are shown in the micrographs in Figures 2a to 2d in 500-fold magnification. The finely striped outer layer of annealed martensite-bainite differs clearly from the bainite intermediate layer shown in Figure 2b.

The ferrite-bainite layer adjacent thereto, which is shown in Figure 2c, has a coarser structure. The core structure is shown in Figure 2d, wherein the dark patches show the perlite proportions and the bright ones the ferrite proportions.

Figures 3, 4a and 4b show corresponding micrograph of the concrete steel in accordance with the invention. The latter is built up only of two layers. The edge layer is comprised of pure annealed martensite and is directly adjacent to a core layer which is composed of a pure perliteferrite structure. This becomes especially clear from Figures 4a and 4b, in which Figure 4a shows the surface layer of annealed martensite and 4b the sudden transition to the distinctive ferriteperlite structure. The micrographs shown in Figures 4a and 4b also have a 500-fold magnification. The strict two-layer structure of the steel in accordance with the invention leads to the unexpectedly obtained favourable properties which have been described above.

The method of producing the concrete steel in accordance with the invention is described in detail by way of Figures 5 to 8. Figure 5 shows a diagram in which the cooling of a concrete steel is shown, which enters the cooling section at a temperature of about 850 , where it is subject to a three-stage water cooling. Immediately after leaving the cooling section, the steel is reeled up and cooled in the air on the reel. The reeled up rolled material is subjected, in the reel, to an isothermic conversion, whereby the austenite in the core is converted into ferrite and perlite and the surface layer annealed by the released conversion energy of the martensite. These operations are described further below. Figure 5 in the left-hand part shows the gradual cooling of the rolled material during its passage through the finishing roll line.At the period of time characterised by to the rolled material enters the cooling section and remains in the first cooling stage for about 0.15 seconds. The third cooling stage has been passed through after about 0.35 seconds.

In Figure 5, to illustrate the temperature course over the rod cross-section, the rolled material is divided into concentric rings.The outer ring is denoted by 1 and the centre of the rolled material by 4. The ring denoted by 2 extends substantially to half the rod diameter and the ring denoted by 3 has a diameter which corresponds to a quarter of the rod diameter. The ring denoted by 1 a has a radius of about 9/11 of the radius (R) of the rolled material and it substantially characterises the boundary between the martensite layer and core region.

The graphs denoted by 1, 1 a, 2, 3 and 4 show the temperature course on the rings during cooling.

The outer ring is thus cooled down to below the martensite starting temperture, so that an outer layer is formed between the rings 1 and 1 a of martensite. Since the core region, of course, is not cooled down so extensively, the martensite layer between the rings 1 and 1 a is heated up again by the heat located in the core region, whereby on the one hand the martensite is annealed, and on the other hand a compensation temperature TA is attained. Reaching the compensating temperature is synonymous to the fact that the rolled material has a constant temperature over the whole cross section after cooling. This temperature is now maintained until the conversion of the austenite into ferrite and perlite has been concluded. Then a constant cooling may occur.

The compensation temperature TA is chosen so that during the isothermic conversion the bainite area is not traversed. Moreover, it should lie in the region in which the earliest possible conversion occurs. This ensures that the conversion of the austenite into ferrite and perlite occurs in as short a time possible and does not deteriorate into a lengthy process.

Figure 5 shows that in accordance with the invention the forming of bainite is prevented by the fact the compensating temperature has already been reached before a conversion into ferrite can occur and moreover, the conversion occurs isothermally, so that during cooling the bainite area is not traversed.

The conversion graphs chosen in Figure 5 correspond to the conventional time-temperature diagrams, whereby F denotes the area of the ferrite formation, P the area of perlite formation, B the area of bainite formation and M5 the martensite temperature. Austenite which is cooled to below the martensite starting temperature is immediately converted into martensite.

The table shown in Figure 6 shows an embodiment of the possible development of the cooling for different steel diameters of from 5.5 to 30 mm. Thus, an entry temperature into the cooling stage of 8500 is proceeded from, provided there is a standard alloyed steel, i.e. the sum of the alloy elements of the steel does not exceed a proportion of 1.7%.

This table shows that the first stage of cooling never lasts longer than 0.2 seconds. Whilst for a diameter of 5.5 mm one cooling stage suffices, with larger diameters up to eight cooling stages may be provided. The entire cooling process is thus concluded at the latest after three seconds.

In the next column the time taken to attain the compensating temperature is given. This permits the concrete steels to be divided into three groups l, ll, lil 111 differing in diameter. The first group includes the thicknesses from 5.5 to 13 mm, the second group from 13 to 25 mm and the third group from 25 to 30 mm.

Within the first group, the compensating temperature is attained in two seconds. In the second group the compensating temperature is attained within 10 seconds, whilst in the third group the compensating temperature is attained within 14 seconds. These connections are of considerable importance for the usability of the water cooling carried out herein, which is described in detail below.

In the further columns of Figure 6, the core temperature at the end of each cooling step is given for the different rolled material diameters.

By core there is understood herein r=O. Moreover, the compensating temperature attained for each rod diameter is given.

Figures 7 and 8 show the significance of the already mentioned division into three diameter groups. Figure 7 shows the time temperature diagram for a standard steel of low carbon content (C < 0.25%). The earliest possible conversion of the austenite into ferrite accordingly is possible after about 2 seconds at a temperature of about 5000C. In accordance with the theory of the invention, the compensating temperature should be attained by this period of time. This shows, that when using the water cooling system characterised in Figure 6, standard steels up to a diameter of 13 mm may be used. The compensating temperature then lies approximately above 5000C.

In comparison thereto, Figure 8 shows the time-temperature diagram of a steel low in carbon content, in which the sum of the alloying elements lies between 1.7% and 3%. It is thus clear that the earliest possible conversion of the austenite into ferrite is possible in an order of magnitude of 10 seconds. Furthermore, it should be noted that the compensating temperature has to be chosen substantially higher, since the earliest possible conversion into ferrite sets in at approximately 7000C. By the addition of alloying elements therefore, the time period of the earliest possible conversion of the austenite into ferrite can be postponed, so that for attaining the compensating temperature more time is available.

An identical effect, namely the displacement of the time period of earliest possible conversion of the austenite into ferrite to a later time period, is attainable by the addition of micro-alloying elements, such as niobium, vanadium or molybdenum. In distinction to the use of alloyed steels (Figure 8) merely the conversion graphs of Figure 7 are displaced by about 1 decade to the right without thereby changing the position or the shape of the conversion graphs. Therefore by adding micro-alloying elements - in contrast to the addition of other alloying elements - the compensating temperature is not varied.

With the retention of the water cooling system indicated in Figure 6, the necessity arises during the production of concrete steel having diameters +13 mmthatthere must be used either an alloyed steel (sum of alloy elements between i .7% and 3%) or a micro-alloyed steel (vanadium, niobium, molybdenum up to 0.8%).

With a diameter of > 25 mm in an alloyed steel, the sum of alloying elements would have to be increased beyond 3%. This will generally not be advisable, so that for these diameters additionally or on their own micro alloys are to be provided.

Instead of varying the proportion of the alloy in the steel, it would also be possible to cause an intensification of the cooling so that the compensating temperature would be attained sooner. Such a cooling, however is very costly.

The diagrams in Figures 7 and 8 also show that the proportion of ferrite to perlite in the core can be influenced by the selection of the compensating temperature. In the embodiments hitherto described, an entry temperature of the rolled material into the cooling section of 8500C is proceeded from. Other temperatures are also conceivable, but the entry temperatures must at least be high enough so that the austenite is still stable and selected to be so low that cooling the rolled material to the compensating temperature is still possible within the required period. This means that especially with small rolled material diameters, a higher entry temperature of the rolled material into the cooling section may be accepted. All in all, however, 8500C has proved to be particularly expedient for these purposes.

The isothermal conversion of the austenite into ferrite and perlite may be attained by incorporating a furnace behind the cooling section. It is, however, much more favourable to reel up the uncut concrete steel arriving from the rod will immediately after the exit from the cooling section. By remaining in the reel, the temperature of the concrete steel does not drop for a sufficiently long period, since due to the release of environmental heat, it rises through the reel so that a reduced heat removal accurs in the atmosphere, Moreover, this kind of cooling permits the most rapid production process which is known per se from the rod mill, but has not yet been applied for producing concrete steels.

Under identical conditions the breaking tension, which in a concrete steel produced according to the G-OS 24 26 920 amounts to 5.

2% is 10.1% in the concrete steel in accordance with the invention. This produces the improvements with respect to resistance to fissure initiation and approved fatigue strength.

Under favourable conditions the breaking tension for the concrete steel in accordance with the invention can be still further increased. The mean breaking tension, for example, may be between 13.9% and 17.4% whereby the values required according to DIN 488/sheet 1 may be considerably exceeded.

Claims

1. Weldable concrete steel having a carbon content of less than 0.25%, and without additional subsequent treatment, such as cold deformation, patening or surface tempering, has a yield point O,2 of at least 500N/mm2 and a tensile strength of at least 550 N/mm2 and is in the form of a concentric core region and a surface layer, whereby the core region is comprised of a mixed structure of perlite, ferrite and possibly additional components and the surface layer containing annealed martensite, in which the core region is formed of a pure perlite-ferrite mixed structure, in which the proportion of ferrite is from 20% to 80% and in which the core region is adjacent to the surface layer without any intermediate layer, which surface layer, in turn, is comprised of pure, annealed martensite.

2. A concrete steel as claimed in claim 1, in which ferrite and perlite are contained in the core region in substantially equal proportions.

3. A concrete steel as claimed in claim 1 or 2 in which the concrete steel rods are ribbed.

4. A concrete steel as claimed in claim 1,2 or 3 in which the proportion of the surface layer at the cross-sectional surface is at least 20%, preferably 33%.

5. A concrete steel as claimed in any one of claims 1 to 4, in which, with a diameter of 413 mm, the sum of all the alloying elements is A1.7%.

6. A concrete steel as claimed in any one of claims 1 to 4 in which, with a diameter of A13 mm, the sum of all the alloying elements is 41.7%. and in which the steel contains a proportion of micro-alloying elements up to 0.08%.

7. A concrete steel as claimed in any one of claims 1 to 4 in which, with a thickness between 13 mm and 25 mm, the sum of all the alloying elements is from 1.7% to 3%.

8. A concrete steel as claimed in any one of claims 1 to 4, in which, with a thickness of more than 25 mm, the sum of the alloying elements is from 1.7% to 3% and in which the steel has a proportion of micro-alloying elements up to 0.03%.

9. A concrete steel, as claimed in any preceding claim, substantially as hereinbefore described and illustrated.

10. A method for producing a concrete steel as claimed in any preceding claim, characterised by the following process steps: a) the concrete steel is made on a rod mill; b) after leaving the roll line, the rolled material is subjected to an intensive, preferably multistage, cooling; c) by such cooling, the surface of the rolled material is cooled to below the martensite start temperature; d) cooling occurs with such intensity that the compensation temperature between core and surface is attained before a conversion into bainite, ferrite or perlite can set in, and that the compensation temperature lies substantially in the temperature range in which an earliest possible conversion of the austenite into ferrite and perlite can occur; and e) on reaching the compensation temperature, the temperature at least to the end of the perlite conversion is kept substantially constant and the rolled material is then subject to gradual cooling.

11. A method as claimed in claim 10, in which the rolled material is reeled up directly after passing through the cooling stage and in the reeled up state is cooled in the air.

12. A method as claimed in claim 10 or 11, in which the first stage of cooling is completed within 0.2 seconds.

13. A method as claimed in claims 10, 11 or 1 2 in which for producing comcrete steel having a diameter of 413 mm, standard steel (sum of all alloying elements = 1.7%) is used.

14. A method as claimed in claims 10, 11 or 1 2 in which, for the production of concrete steel having a thickness of 413 mm standard steel is used which has a proportion of micro-alloying elements up to 0.08% microalloyed therewith.

15. A method as claimed in claim 10, 11 or 12, in which for producing concrete steel having a thickness of from 13 to 25 mm, an alloyed steel (sum of all alloying elements between 1.7% and 3%) is used.

16. A method as claimed in claims 10, 11 or 12, in which for producing concrete steel having a thickness of more than 25 mm, an alloyed steel is used which is micro-alloyed with a proportion of micro-alloying elements up to 0.03%.

1 7. A method as claimed in any one of claims 10 to 16, substantially as hereinbefore described and illustrated.