JPH0218362B2 - - Google Patents

Description

Claim 1 Carbon 0.05-0.20% by weight, manganese 1.20-1.70
wt%, silicon 0.30-0.50 wt%, niobium 0.04
~0.06% by weight, vanadium 0.035-0.05% by weight, molybdenum 0.30-0.50% by weight, copper 0.30-2.00% by weight,
Aluminum 0.04-0.06% by weight, nitrogen 0.015-0.02
% by weight, not more than 0.030% by weight of phosphorus and not more than 0.020% by weight of sulfur is subjected to a thermo-mechanical treatment following reheating from the molten state and by a second heating, and before said thermo-mechanical treatment, The steel is kept at a very low reheating temperature (=second heating below 1150°C) and then the steel is heated to a high degree of deformation (10-45%) at a high deformation rate as well as from 850°C in a few passes. Characterized by controlled rolling at a low deformation temperature slightly above
A method for manufacturing prestressed steel that is high strength, can be welded, and has excellent corrosion resistance and brittle fracture resistance.

2, characterized in that the thermo-mechanical treatment comprises a second stage, in which about 850
2. A process according to claim 1, characterized in that rapid cooling from <0>C to about 650/550<0>C reduces gamma-alpha conversion and retards recrystallization.

3 characterized in that the thermo-mechanical treatment comprises a third stage, in which the temperature is increased in one or several passes from about 650/550°C to a lower final rolling temperature just above the _Ar3 limit. Process according to claim 1 or 2, in which rolling occurs again in a controlled manner with a degree of deformation and high speed, followed by a residence time followed by delayed cooling.

4. Claims 1 to 3, characterized in that the steel is strain hardened following thermal mechanical treatment.
The manufacturing method described in any of paragraphs.

5. The manufacturing method according to any one of claims 1 to 4, characterized in that the steel is deflowed before and/or after scouring.

6. Process according to claim 5, characterized in that before and/or after scouring, the molten steel is subjected to a calcium treatment.

7 Purge with inert gas, vacuum treatment, reduction,
7. The manufacturing method according to claim 5 or 6, further comprising post-processing such as correction using metallic calcium or calcium halide slag or ladle treatment.

8. Claims 1 to 7, characterized in that both solid solution - fine particle - and particle hardening or precipitation hardening, which have a substantial additional effect, occur as strengthening mechanisms during thermomechanical treatment. The manufacturing method described in any of the above.

9. Manufacture according to any one of claims 1 to 8, characterized in that the thermomechanical treatment takes place by controlled rolling of microalloyed fine-grain molten steel to prevent martensite formation. Law.

10. Process according to claim 9, characterized in that recrystallization of the microalloyed fine-grained molten steel results in very fine austenite grains prior to ferrite-pearlite conversion.

11. In microalloyed steels, the rolling process is supplemented by the precipitation of carbides, nitrides and/or carbonitrides, so that solid solution hardening as well as particulate hardening and particularly pronounced grain hardening occur. The manufacturing method according to claim 10.

12. The temperature is controlled in such a way that the γ-α conversion takes place just before and/or during the lowest possible final rolling temperature, just above Ar ₃ The manufacturing method described in any of paragraphs.

13. Claims 4 to 12, characterized in that the thermo-mechanical treatment is followed by stretching (for draw rods) or drawing (for cold drawn wires) as strain hardening means. The manufacturing method described in any of the above.

14 By alloying with the micro alloying elements niobium and/or vanadium and/or molybdenum, in addition to particle hardening and solid solution hardening, carbides, nitrides and/or
14. Process according to claim 1, characterized in that the best possible particle hardening takes place in the form of carbonitrides or carbonitrides.

15 Fine precipitates of aluminum nitride, and especially carbides, nitrides and/or carbonitrides of niobium and vanadium, which are micro alloying elements, with a particle size of 100 to 200 Å and a particle amount of 20 × 10 ⁶ pieces/mm ² Claims 1 to 14, characterized in that the austenite grains are made fine by making the austenite particles present and employing as high a degree and rate of deformation as possible and as low a final rolling temperature as possible. Manufacturing method.

16 Under low final rolling temperatures that elongate the grains and thus increase the surface area of the austenite grains, the proportion of uncrystallized austenite decreases due to the retardation of recrystallization, and in the ferrite-pearlite conversion stage, the nucleus density increases and As a result of the growth inhibition of the particles formed from these nuclei, both by microparticle hardening and by particle hardening, the particles become finer and thus the strength is optimally increased. The manufacturing method according to any one of Items 1 to 15.

17 In addition to controlling the cooling rate, recrystallization of austenite and γ-α conversion are shifted to a lower temperature side by alloying with molybdenum, according to claims 14 to 16. The manufacturing method described in any of the above.

18. Alloying with increased manganese in the baseline analysis ensures the required grain size reduction with optimal strength enhancement through solid solution hardening and increased precipitation hardening. The manufacturing method according to any one of claims 14 to 17, characterized in that:

19. Process according to claim 18, characterized in that, by increasing the manganese content, an optimum retardation of the required austenite conversion and therefore an optimum fine grain formation are simultaneously ensured.

20 By increasing manganese content, i.e. γ-α
Claim 1, characterized in that the required delay in recrystallization is ensured by shifting the conversion to a lower temperature and by the lowest possible final rolling temperature and by carrying out a simultaneous thermo-mechanical treatment. The manufacturing method according to item 18 or 19.

21. Claims 18 to 20, characterized in that an increase in the manganese content ensures an optimum particle precipitation and therefore an optimum particle hardening effect for the greatest possible increase in strength. The manufacturing method described in any of the above.

22. Claims characterized by an increased manganese content and the simultaneous presence of niobium and vanadium, as well as a low pallarite content, which increases the reinforcement, i.e. increases the strength-enhancing manganese proportion. The manufacturing method according to any one of items 18 to 21.

23. The manufacturing method according to claims 1 to 22, characterized by reducing the pearlite ratio.

24 In order to prevent rapid particle growth during reheating of propulsion reactors, etc., the aluminum and nitrogen contents are increased within a given reference analysis and the required particle amount is 20 × 10 ⁶ / mm ² , particle size 100
Claim 1 characterized in that it is ~200 Å
The manufacturing method according to Items 23 to 23.

25. Claims 1 to 24, characterized in that by increasing the niobium content within a given reference analysis, the onset of grain growth to be prevented is shifted to higher temperatures. Manufacturing method described.

Description The present invention relates to a method for manufacturing prestressed steel that is high strength, weldable, and has excellent corrosion resistance and brittle fracture resistance.

Currently, prestressed steels are typically manufactured from high-grade structural steels that are unalloyed and have a high carbon content. That is, the composition (weight%) is 0.65 to 0.85 carbon, 0.65 to 0.85 silicon, 1.10 to 1.70 manganese, and sulfur.
0.035 and phosphorus 0.035, and vanadium 0.10
Hot rolled, stretched or drawn and then tempered dimensions 15 to 0.40
40mm composition circular rod and composition carbon 0.60
~0.90, silicon 0.10~0.30, manganese 0.50~0.80,
0.035 sulfur and 0.035 phosphorus with patented or Stelmor treated dimensions
Cold-drawn bracing wires or tension wires are manufactured from circular wire rods of 5.5 to 14.5 mm.

In both cases, the raw materials used are approximately 120
This is a billet semi-finished product of mm4 KT.
Depending on the manufacturing plant and available equipment,
They are heat treated, ie heated to rolling temperature, according to different standards. The semi-finished steels therefore have different structures and structural characteristics, while the final products usually require the mechanical properties required for official certification.

These prestressed steels have the significant drawback that they cannot be welded. For the production of prestressed steel, for example, the known open hearth method, electric furnace method,
Conventional methods such as basic oxygen steelmaking are employed, where the steel is not subjected to any pre- or post-processing. In some cases, the steel is desulfurized as a pre-treatment and post-treated under vacuum. Ingot and continuous casting methods are still in use.

In addition to the disadvantage that known prestressed steels cannot be welded, they also have disadvantages with regard to their mechanical properties, namely their susceptibility to corrosion and, above all, to brittle fracture. These drawbacks are related to the chemical composition, structural composition and production conditions of prestressed steel. Until now, when evaluating prestressed steels, the fact that brittle fracture can begin at temperatures above 0° C. and increases rapidly with decreasing temperature has been overlooked. Safety against brittle fracture is expressed by the so-called transition temperature (Tu¨) at which brittle fracture can occur.
The Tu¨ of conventional prestressed steels is typically well above 20°C. Since most prestressed steel structures can be exposed to temperatures of -40°C and below for months on end, and bridge substructures in particular can be exposed to lower temperatures, prestressed steel is designed and This fact must be taken into account when developing. The susceptibility of brittle fracture to occur is greatly influenced by the internal degree of purity, inclusions such as oxides and sulfides, and the form of inclusion, and these can be greatly improved by systematically post-processing the steel. be able to. The ease with which brittle fracture occurs and, in particular, its temperature dependence, are closely related to the pearlite (cementite) ratio in steel, that is, the carbon content, and carbon is the most significant factor in reducing brittle fracture resistance. To date, low pearlitic, ie low carbon prestressed steels have not been obtained.

Prestressed steel has, for example, pitting, large pitting, crevasses, intergranular corrosion,
Many forms of corrosion occur, including intragranular corrosion. Special attention must be paid to stress corrosion cracking. It is known that copper has corrosion inhibiting properties. However, to date, there has been no example of using copper as an alloy component in prestressed steel.

Therefore, until now, it has been impossible to manufacture high-strength prestressed steel that has excellent corrosion resistance and brittle fracture resistance and is also weldable. The present inventor has aimed at developing such prestressed steel and perfecting a method for manufacturing such prestressed steel.

The object of the invention is achieved by the following method. i.e. carbon 0.05-0.20% by weight, manganese 1.20
~1.70 wt%, silicon 0.30-0.50 wt%, niobium
0.04-0.06% by weight, vanadium 0.035-0.05% by weight, molybdenum 0.30-0.50% by weight, copper 0.30-2.00
wt%, aluminum 0.04-0.06 wt%, nitrogen
The steel containing 0.015-0.020% by weight, up to 0.030% by weight phosphorus and up to 0.020% by weight sulfur is subjected to a thermo-mechanical treatment following solidification from melting and reheating with a second heat source. At this time, before thermal mechanical treatment,
The steel is kept at a very low reheat temperature (second heat below 1150°C) and then the steel is heated to a high degree of deformation (10-45%), high deformation rate as well as 850°C for a limited number of passes. Controlled rolling is performed at a slightly higher, lower deformation temperature.

The reason for keeping the steel at a low reheat temperature is that vanadium and niobium melt at 850 or 950°C, but remelt above 1150°C. Reconstitution should be avoided. 100-200 to achieve the goal
A particle size of Å and a particle amount of 2×10 ⁶ /mm ² are required.

Subsequently, the steel is rolled in a small number of passes with a high degree of deformation (10-45%), high forming speed and controlled low deformation temperature slightly above 850°C. Due to the copper content in the steel, this temperature limit must be observed. This is because the effective action of precipitation hardening of copper can be achieved from approximately 850℃ to approximately 650/550 without rolling.
This is because it is exhibited only when rapidly cooling down to ℃. It is known that copper does not precipitate during rolling at temperatures below 850°C.

This first step of thermo-mechanical processing can produce the wire rod properties necessary for the production of cold drawn wire, 3-wire strands, 7-wire strands and tie rods. These properties meet European Standard 138, but
Furthermore, it has additional properties (excellent corrosion resistance and brittle fracture resistance, and weldability). Due to these excellent properties, costly cooling deformation (stretching) and subsequent tempering can be omitted when manufacturing draw bars. This constitutes an important advantage of the present invention.

The actual curing process in the reinforcing system employed in the present invention occurs from 850° C. to residence times approaching the AR ₃ limit. In subsequent steps of the method of the invention,
Rapid cooling to about 650/550° C. without rolling causes simultaneous recrystallization retardation with reduced gamma-alpha conversion.

Employing the first and second stages of the present invention provides a hardened and tempered wire strength rating that corresponds to European Standard 138 without costly hardening and tempering. These advantages of the present invention are due to the chemical composition of the present invention. In draw rods and wires, the yield strength is increased by at least 20% compared to conventional ones, and cold drawn wires with correspondingly increased strength can be produced from these strands.

According to the invention, in the third processing stage, approximately 650/
From 550° C., controlled rolling is carried out again in one or several passes, ie with a high degree of deformation and a high deformation rate. Residence time and delayed cooling, eg due to still air, need to be considered. As precipitation hardening increases,
Strength is increased by at least 40% compared to traditional prestressed steel. The diagram shown in the accompanying drawings illustrates this process sequence.

If higher strength is required, it is also possible to further strain harden the steel following the thermo-mechanical treatment and the associated strengthening mechanism.

The strength enhancement mechanism in the thermomechanical treatment of the present invention includes chemical composition and microalloying elements.
The effect of planned measurement of elements) is given. The mechanisms by which the alloying element copper precipitates in a particularly effective manner are in particular particulate hardening, solid solution hardening and especially precipitation hardening. This shows that thermomechanical treatment with the necessary chemical composition to cause melting and hardening of the fine particles achieves the objectives of the present invention, namely, high strength, excellent corrosion resistance and brittle fracture resistance, This means that it is the most important step to obtain weldable prestressed steel. The alloying elements have been weighed in such a way as to provide not only a significant increase in strength, but also an increase in toughness, which occurs in particular through particulate hardening. If the alloying elements are measured systematically, the strength will be maximized through precipitation. Deposition or precipitation in ferrite is most effective for increasing strength.

Precipitation hardening has the greatest strength-enhancing effect, as a result of rapid cooling and low final rolling temperature, high deformation degree and high deformation rate, followed by final formation with residence time and delayed cooling, so this thermo-mechanical treatment step is It should be given maximum significance. Because, as a result of the planned weighing of alloying elements,
This step, together with the above-mentioned effects, provides maximum protection against brittle fracture, especially due to the interaction of manganese and molybdenum.

The premise for effective strength enhancement in the present invention is, after all, fine particle hardening, and fine particle melting is also necessary to perform fine particle hardening most efficiently.
Particulate melting simultaneously enhances toughness. According to American Society for Testing and Materials Standards (ASTM) 112, the particle size amounts to at least 9, preferably 12. Contributing to this is the increased manganese content to 1.45% on average.

This requires very fine austenite grains. This is because this determines the particle size of the ferrite particles. For this purpose,
Trace alloying elements predetermined in standard analysis,
In particular, it is necessary to introduce aluminum, nitrogen, niobium and vanadium into the austenitic structure by fine precipitation to inhibit grain growth and to form strength-enhancing obstacles to dislocations.

According to the invention, microparticle melting consists of the following steps.

1 Pre-treatment of steel: Substantial deflow occurs through the calcium-treated CAB, for example by the TN process.

2 Post-treatment of the steel: in particular purification with inert gases, vacuum treatment, reduction, sedation and, if possible, correction of inclusions and/or ladle treatment with metallic calcium or calcium halide slags.

In the present invention, it is preferable to adopt a continuous casting method. The continuous casting method is the most economical, and due to its nature, it is the most suitable casting method for melting and hardening the raw material, ie, the billet used in the production of prestressed steel, to form steel. To ensure the high quality standards required for prestressed steel and to prevent serious internal and surface defects such as central separation crosslinking and solidification crosslinking, special measures such as reoxidation protection, submerged casting methods, and electromagnetic stirring are required. In the present invention, standard analysis determines the carbon content by approximately
The low carbon content of 0.1% sufficiently prevents the above-mentioned drawbacks, and eliminates the costly measures required for the production of conventional prestressed steels with high carbon content, making continuous casting possible in the production of prestressed steels. Further enhance the economic efficiency of the law. Moreover, high purity, high uniformity, and high quality can be ensured.

In considering the first problem mentioned above, namely improving the strength of prestressed steel, the most important strength limiting factors must be considered. This factor includes, among other things, obstacles to metastatic movement. The structure of prestressed steel has a particularly large influence on its strength properties. This is because, in order to increase strength, transfer motion must be prevented. Such disorders are subdivided as a function of their dimension as follows: : Zero dimension: i.e., point obstacles such as impurity atoms in mixed crystals - increase the strength by hardening the mixed crystal; One dimension: linear obstacles such as dislocations - strengthen by cold working; 2 Dimensions: Planar hindrances such as grain limits - strengthen by finely graining or granules; 3D: steric hindrances such as precipitates or precipitates - strengthen by particle hardening or dispersion hardening.

Mixed crystal hardening or solid solution hardening occurs due to the nature of the chemical composition, and the influence of impurity atoms in the substituted mixed crystal and the impurity atoms dissolved in its interstitials is particularly important. In this connection, there are a number of figures and tables showing individual alloying elements and their tensile strength enhancing effects. The influence of individual alloying elements can be revealed from the strains caused by these elements in the lattice. The greater the strain, the greater the strength increasing effect.

Of the four different types of reinforcement, particulate hardening is the most important. This is because the resulting reinforcement mechanism is characterized not only by an improvement in strength, but also by a simultaneous increase in toughness. Moreover, the two-dimensional obstacles that prevent dislocation migration are such strong obstacles that even increased toughness cannot overcome. Dislocations are no longer possible, and the grains accumulate a large number of dislocations at their boundaries, concentrating considerable stress and affecting strength. However, it is precisely the average particle size that influences the lower limit of tensile strength.

It must be emphasized that for grain hardening by precipitation, the greatest strength improvement occurs when the grain size and spacing are large enough so that no cutting or crossing occurs. The precipitation process in particle hardening is strongly influenced by the degree of supersaturation, deformation, transformation and finally recrystallization and is of particular importance in connection with thermomechanical treatments for strength enhancement. When forming high-strength prestressed steel, attention must also be paid to the precipitation of carbides, nitrides or carbonitrides due to particle hardening. It must also be remembered that when special carbides or carbonitrides are deposited, a particularly higher hardening effect is caused by the minor alloying elements niobium and vanadium than, for example, by depositing copper. According to the invention, these individual reinforcement mechanisms can be combined with each other and also with strain hardening. Although their effects are additive, their ratio in individual cases can vary considerably as a function of the given conditions. However, according to the invention, it has been found that the basic mechanism of the individual curing process is only optimized after the most important subsequent processing step, the so-called thermomechanical processing step.

The concept of thermo-mechanical treatment involves forming processes carried out under a number of special controls, which are related to the best improvement of the steel properties, and which control the deformation temperature, degree of deformation, deformation rate, deformation time and final deformation. temperature, cooling rate, γ
Limiting factors such as −α conversion, residence time after deformation and subsequent cooling play an important role. Thermomechanical treatments can affect all mechanical properties, but especially strength and toughness properties, transition temperatures, and sensitivity to brittle fracture.

Within the scope of the present invention, the thermomechanical treatment is carried out by controlled rolling of specially developed microalloyed and micromelted steel in a specific sequence. In particular, since a low final rolling temperature, rapid cooling prior to the final rolling pass and a high degree of final deformation are set, the recrystallization of the ferrite
Very fine austenite grains are produced prior to pearlite conversion. When the inventive steel is rolled in a controlled manner after reference analysis in the inventive sequence, this microalloy steel is subject to both solid solution hardening and particle hardening similar to that due to precipitation of carbides, nitrides or carbonitrides. caused. From an alloying and rolling standpoint, the temperature is controlled so that the gamma-alpha conversion occurs as soon as possible before and/or after the final rolling temperature and before AR ₃ . Martensite formation must be prevented in any case.

In low pearlitic trace alloy steels, it is important that the carbides and nitrides of the minor alloying elements niobium and vanadium have a face-centered cubic lattice, and that they are homogeneous and completely mixed. However, the maximum strength-enhancing effect of the above-mentioned strengthening mechanism is
Occurs in a body-centered cubic lattice. Attention must also be paid to the shape and size of the carbonitride precipitates. The factors influencing the mechanical properties are decisive: the size and quantity of the particles or their spacing, the shape and arrangement of the precipitates/precipitates and their strength.

These amounts are influenced by the chemical composition and especially the temperature-time conditions under which the precipitate/precipitate forms. As a function of temperature, carbonitrides exhibit γ-
It precipitates in austenite or ferrite during α conversion. The latter is most effective in improving strength. The kinetics, extent and temperature conditions for precipitation depend not only on the thermodynamic conditions but also on the diffusivity of the alloying elements, the degree of rapid cooling and the nucleus condition of the precipitate.

When carrying out the alloying according to the invention, the following practical steps must be taken in order to optimize the rate of particulate hardening associated with the thermo-mechanical treatment required for this development.

Γ Low furnace temperature for fine particle retention, in particular to prevent or limit redissolution of carbide, nitride and/or carbonitride precipitates Γ High degree of deformation under a low number of passes Γ Low deformation temperature Reducing the γ-α transition temperature by rapid Γ cooling and/or alloying and/or retardation of recrystallization. In the thermo-mechanically treated state, the optimum finest grain size and Accordingly, the most favorable effects on toughness and transition temperature are obtained.

A significant portion of the ratio of particulate hardening to precipitation hardening in the present invention, and the associated potential strength enhancement, is determined by the production conditions, i.e., the thermo-mechanical treatment.
greatly affected. High deformation degrees and deformation rates, consistent with the production of thermo-mechanically treated cold-strained draw rods or wires, required for the production of cold-drawn tension wires and strands, before the final rolling pass. and/or subsequent rapid controlled cooling followed by delayed cooling is required.

The final rolling temperature and degree of deformation, especially in the final pass, determine the achievable mechanical properties. As the final rolling temperature decreases, the pearlite proportion decreases, so that in the final controlled rolled low carbon microalloyed structure, only a small amount of pearlite or no pearlite is present. As a result, more favorable mechanical properties are obtained.

As the rolling reduction per pass increases and the number of passes decreases, the austenite particles become smaller and correspondingly smaller ferrite particles are obtained, resulting in more favorable mechanical properties. An increase in the reduction rate of 10 to 45% per pass has a particularly favorable effect on making the ferrite grains finer, improving their insusceptibility to transition temperatures or brittle fracture. The reduction rate per pass and the final rolling temperature as well as the possible holding times are determined by the final product (thermo-mechanically treated and cold drawn) draw rod and (to produce cold drawn wire) The desired properties and dimensions of the wire must be met. This is to ensure the desired metallurgical behavior as well as economic performance of rolling. Rapid rolling and cooling after final rolling thus have a decisive influence on the mechanical properties achieved. Lower temperatures affect the size of ferrite particles. This is because the .gamma.-.alpha. conversion is displaced to lower temperatures by rapid cooling. Moreover, low temperatures significantly accelerate the precipitation process that occurs during the subsequent slow cooling.

Recrystallization, γ-α transformation and precipitation of trace alloying elements determine the structure formation in prestressed steel.
These processes not only can occur in rapid succession within just a few minutes, but they also influence each other. In order to develop prestressed steels with high strength, corrosion resistance and brittle fracture resistance based on mechanical properties and dimensional ranges, it is necessary to accurately understand the relationship between the evolving process set and the evolving structure. decided,
It is necessary to optimize the properties obtained by them.

The final step associated with the thermomechanical treatment is strain hardening, which in particular consists of stretching or drawing. As a result of this subsequent cold working, which is employed in the production of all prestressed steels and is particularly suitable for the steel of the invention, a considerable strength increase compared to conventional prestressed steels occurs once again. This is a result of the degree of deformation adopted.

In order to further improve the properties of the prestressed steel according to the invention in connection with thermomechanical treatment, trace alloying elements may be included as alloying components. Of the elements that can be used as microalloys, niobium is the most effective for particulate hardening and precipitation hardening in thermomechanical treatments, ie, increasing strength, followed by vanadium.

When micro-alloying niobium and vanadium,
With a small amount of pearlite, as the content of manganese and silicon increases, their contribution to strength improvement increases.

In the presence of vanadium, increasing nitrogen content also increases the yield point. Since the tensile strength is also improved, the yield point is increased by about 70-90%. This is particularly important for prestressed steel.

In steels alloyed with niobium, the proportion of particulate hardening is significantly higher than that of precipitation hardening, and therefore not only higher strengths than alloyed with titanium or vanadium are obtained, but also, in particular, very favorable low transitions. temperature is obtained. The main reason why niobium is used more preferably is that the addition of niobium increases the ratio of particulate hardening to precipitation hardening and at the same time provides the greatest reduction in transition temperature.

Regarding improvement of transition temperature or brittle fracture insensitivity, it was found that there is a relationship between improvement of strength and improvement of transition temperature by using niobium and vanadium as alloy components, regardless of the fine alloy element group. Don't forget. For the same yield strength, substantially the same brittle fracture insusceptibility or transition temperature is achieved regardless of the niobium or vanadium content. Manganese and nickel as well as silicon shift the transition temperature towards lower temperatures when the content is below about 0.5%.

Refining the grain size achieves a reinforcing effect and clearly improves the toughness and significantly lowers the transition temperature. Reducing the pearlite proportion also strengthens the desired influence. Therefore, low pearlitic steels are usually particularly insensitive to brittle fracture in the case of fine ferrite particles.

Regarding the cold rolling tensile properties of prestressed steel in the claims, particular attention must be paid to the chemical composition of the prestressed steel. Sulfur content determines the anisotropy of toughness, which is a major limiting factor in cold forming. The required low sulfur content, ie a reduction in the number of sulfided inclusions, significantly improves the toughness with respect to fracture reduction. Fracture reduction is very important for prestressed steels. Shortening the length of the sulfide is also particularly effective for more favorable fracture reduction. The addition of calcium makes it possible to achieve a strong deflow which normally occurs in ladle metallurgy processes. In this case, the high vapor pressure of calcium is of particular importance. Calcium melting temperature 1600
Its atmospheric pressure at °C is 1.86 bar, where calcium has a high affinity for oxygen. That is, measures must be taken to prevent calcium evaporation. Even when the sulfur content is 0.008%,
Manganese sulfide in aluminum-killed steel cannot be detected; instead, spherical inclusions consisting of calcium and aluminum oxides with a small amount of dissolved sulfur attached to the surface are present. As a result of the favorable conditions of the calcium aluminate for precipitation from the melt, the purity of the oxygen content is also improved. The mechanical properties obtained by calcium treatment clearly show a reduction in the spatial anisotropy of toughness. The reduction of fractures, which is very important for ensuring the quality value of prestressed steels, is improved by calcium treatment and reduction of the sulfur content. Avoid drainage as much as possible
This should be done until it is below 0.020%.

As the most suitable combination of trace alloying elements, a molybdenum-niobium alloy structure has the most excellent properties. Subjecting the combination of niobium, vanadium, molybdenum, and copper to thermomechanical treatment according to the present invention provides additional property improvements. The best results are obtained with low final rolling temperatures and high final deformation degrees.

In addition to the results of heat treatment, rolling and cooling rates and on-bed cooling play a role in the production of prestressed steel. When the temperature drops to 750℃,
Both strength and toughness improvements are established. alloyed with molybdenum, and between 650 and 550°C, i.e.
By controlling the rolling speed to maximize the reduction in γ-α conversion in the range where the precipitation hardening strength enhancement mechanism is most effective, the effectiveness of the means responsible for strength enhancement is considerably improved. .

However, the most effective means of obtaining optimal mechanical properties is to form extremely fine particles. The fineness of the grains allows an improvement in the transition temperature and at the same time an increase in the yield point. In practice very fine austenite grains are required. This is because the austenite particles help determine the size of the ferrite particles. As a general experience, as the austenite grain size decreases, the ferrite grain size decreases.
Decreased by a factor of 0.3. The main process of austenite grain growth is not the dissolution of precipitates, but their agglomeration and thereby effective grain formation.

One method of controlling austenite grain size is to incorporate fine precipitates into the austenite structure, which inhibit grain growth. Apart from aluminum, which has this effect as aluminum nitride, niobium, which is a trace alloying element with a grain size of 100 Å to 200 Å,
vanadium and titanium, their carbides,
It acts similarly as a nitride or carbonitride.
When reheating in a kiln-type furnace for rolling, the most favorable conditions to prevent strong grain growth are a higher aluminum content (0.050%
) and nitrogen content (up to 0.020%). As the niobium content increases, the temperature at which rapid particle growth begins shifts toward higher temperatures.

Another way to prevent or limit the re-melting of such precipitates when heating prior to rolling is to keep the kiln temperature as low as possible. A higher degree of deformation can also reduce the size of the austenite grains. The most significant grain size reduction effect occurs at low final deformation temperatures.

If the γ-α conversion is shifted to lower temperatures as a result of rapid cooling, the conversion temperature will be lower and therefore the nucleation frequency will be higher as well as the grain boundary mobility will be lower. This reduces the size of the ferrite particles.

In addition to reducing grain size, austenite recrystallization can also be delayed. The unrecrystallized austenite portion is then deformed during the final rolling temperature, resulting in elongated grains and therefore austenite grains whose surface area is increased. This structural transformation in the ferrite-pearlite stage increases the density of the nuclei as well as inhibits the growth of the grains formed from these nuclei, resulting in a considerable reduction in grain size.

Apart from controlling the cooling rate, the retardation of austenite recrystallization can be favorably influenced by alloying a small amount of molybdenum into the microalloyed low pearlitic structure, so that the γ-α transformation can occur in the lower temperature range. Move to. This possibility is exploited in thermo-mechanical treatments so that even finer grain structures are obtained with additional improvements in the transition temperature.

Due to the greater reduction in grain size, the preferred transition temperature remains unchanged or, in niobium, vanadium, or niobium and vanadium alloy structures, an improvement in the transition temperature may even occur. The reasons for doing so will be explained. Apart from strengthening, this reduction in particle size also results in improved toughness, as required by the present invention, and at the same time significantly lowers the transition temperature. This is helped by reducing the pearlite proportion. Therefore, in the case of fine ferrite particles, the low pearlite structure is usually insensitive to brittle fracture.

In relation to micro alloy components and microparticle hardening,
It should be noted that the effective size and amount of incoherent niobium, vanadium and titanium carbonitrides in the grains will affect the size of the ferrite grains in different ways. In the thermomechanically treated state, vanadium produces only a slight particle size reduction. Basic composition plays a role in that higher carbon and nitrogen contents result in finer secondary structures due to stronger or faster precipitation before or during the γ-α conversion. It has been pointed out that optimum particle size reduction occurs at niobium contents between 0.04 and 0.10%, while increasing titanium and vanadium contents become more effective.

The carbon and nitrogen contents of the steel affect ferrite particle size to a lesser extent in steels containing niobium than in steels containing vanadium. As the carbon content decreases, the effect of nucleation by precipitated particles on particle size decreases, replaced by recrystallization inhibition by dissolved niobium, which is very significant and required in the present invention. be done advantageously. Therefore, in the thermo-mechanically treated state, low pearlitic steels have smaller ferrite particles than steels with higher carbon content.

By delaying the desired austenite conversion, the dissolved vanadium, niobium or titanium exhibits a further atomization effect. An increase in the manganese content also lowers the transition temperature, ensuring optimum particle precipitation and therefore optimum effect of particle hardening.

A recrystallization delay usually occurs in connection with time-transfer austenite conversion. That is, recrystallization occurs later at lower temperatures. This is due to a decrease in γ-α conversion,
The requirements of recrystallization retardation and therefore of setting a low final rolling temperature are met and, for example, copper is optimally deposited. At the same time, it increases the strength to the maximum. The structure becomes finer and finer due to the increased nucleus density and growth inhibition of newly formed ferrite grains.

In connection with precipitation hardening, it must be remembered in thermo-mechanical treatments that precipitation hardening is at its maximum at temperatures in the range 550-650°C. This is manifested by the action of chemically undetectable coherent precipitates (clusters) of niobium, carbon and nitrogen atoms (also titanium).
These coherent precipitates precede the unconformable precipitates. After the precipitation has reached its maximum, attention must be paid to the resulting decrease in yield point.
This decrease is brought about by increasing the temperature or exceeding the holding time, resulting in a decrease in coherent tension as the particles progress from coherent to misaligned and subsequent increases in particle size and particle quantity. Constrained by.

Starting material (crude steel) for carrying out the method of the present invention
should be a steel with a standard analysis consisting of the following alloying elements:

Carbon 0.05-0.20% by weight Manganese 1.20-1.70% by weight Silicon 0.30-0.50% by weight Niobium 0.40-0.06% by weight Vanadium 0.035-0.05% by weight Molybdenum 0.30-0.50% by weight Copper 0.30-2.00% by weight Aluminum 0.04-0.06% by weight Nitrogen 0.15-0.02% by weight Phosphorus 0.030% by weight Sulfur 0.020% by weight Each element will be explained below.

Through cementite (pearlite), carbon provides significant strengthening and plays an important role in this connection. However, carbon content, as a pearlite component, has the most significant negative effect on brittle fracture protection (transition temperature) and weldability, and the negative effects become more pronounced as the carbon content increases, so carbon content provides increased strength and improved corrosion resistance. The ratio must be such that both of the following occur, and on the other hand, it is possible to protect brittle fracture at temperatures around -40°C and improve weldability.
It must also be remembered that the carbon content has a significant influence on the required optimal particle formation. As the carbon content decreases, the effect of nucleation by precipitated particles on particle size decreases, favoring recrystallization inhibition by dissolved niobium, which is very significant and necessary in the present invention. It will be done. Low pearlite structures in the thermo-mechanically treated state have smaller ferrite grain sizes than structures with higher carbon content.

It is preferable to fix the manganese content at an upper limit, since manganese acts in particular by reducing the particle size and at the same time strengthening the solid solution and increasing precipitation hardening. This is because the strength improvement due to manganese is highly dependent on the pearlite content, and a suitably low pearlite proportion ensures a favorable transition temperature and thus prevents brittle fracture. The increase in manganese content greatly contributes to the retardation of austenite conversion desired in the present invention, thus resulting in optimal particulate formation. The simultaneous presence of niobium and vanadium as microalloying elements increases their strengthening effect in the case of low pearlite structures with increasing manganese content.

What has been said above regarding manganese also applies to silicon. When the silicon content is below about 0.5%, the transition temperature moves to lower levels. However, even above 0.5% silicon has a reinforcing effect. But at the same time,
Prestressed steels exhibit high embrittlement effects that must be avoided.

Niobium has the most effective effect on particulate hardening and precipitation hardening by thermo-mechanical treatment, ie the strength increases that can be achieved, followed by vanadium and titanium. Niobium provides the greatest transition temperature drop. Structures containing niobium undergo a much higher rate of particulate hardening than precipitation hardening, and therefore not only have higher yield strength than structures alloyed with titanium or vanadium, but also have particularly low transition temperatures. It's lower, which is very advantageous. Niobium significantly reduces the size of ferrite particles. When niobium is added, the ratio of particulate hardening to precipitation hardening in the structure (particulate hardening/precipitation hardening)
becomes higher. This is the main reason why niobium is more preferred. At the same time, even at low pearlite contents, niobium exhibits an additive strengthening effect with increasing manganese content.

Like niobium, vanadium forms special carbide precipitates, which contribute on the one hand to grain formation and grain hardening, and on the other hand to precipitation hardening and thus to strength improvement. Like niobium, vanadium contributes to the control of austenite grain size through the combination of fine precipitates in the austenite structure, so that grain growth is inhibited. Like niobium, vanadium also contributes to solid solution strengthening, but both are insoluble in ferrite. Precipitation of both into ferrite is most effective in improving strength. Therefore, carbides and nitrides of vanadium and niobium have a face-centered cubic lattice,
They have the same shape and can be mixed without any gaps. Unlike titanium, they do not contribute to the formation of sulfides. With increasing nitrogen content, vanadium has the greatest extent of influence on the formation of fine ferrite grain dimensions and additionally realizes an increase in the yield point. Like niobium, dissolved vanadium affects this micronization and hardening through a delay to austenite conversion.

Retardation of austenite recrystallization is aided by alloying small amounts of molybdenum into the microalloyed low pearlite structure, thus shifting the γ-α transformation to lower temperatures. This possibility is exploited when the thermomechanical treatment is carried out at even lower final rolling temperatures, so that an even finer grain structure is obtained and at the same time the transition temperature is improved. By having molybdenum as an alloying component and the possibility of moving the gamma-alpha transformation to lower temperatures, the great reinforcing properties of copper can also be fully exploited.
In microalloy structures of the type described herein, when the pearlite proportion is low and the copper content is high, during the precipitation of mixed crystals and the formation of carbonitrides, especially at 650-550 °C, Both precipitation hardening mechanisms are in play.

High manganese and molybdenum contents can achieve additional strength enhancement in high strength prestressed steels, such as steels with copper alloy structures, through high dislocation density and particulate hardening other than grain hardening.

Copper is used for the purpose of the present invention because of its two effects: strength-enhancing action through precipitation hardening and strong corrosion-inhibiting action. The corrosion-inhibiting properties of copper are particularly well exploited in high-strength structures produced by thermo-mechanical processing. Because, in addition to its corrosion-inhibiting effect, copper, together with the precipitation-hardening elements used simultaneously in the present invention, under low final rolling temperatures that yield the greatest strength improvement.
It also acts as a precipitation hardening element at 650-550℃. With a low pearlitic structure and with the thermomechanical treatment shown here, rapid cooling from the gamma region of about 840 DEG C. can dissolve about 2% of the copper. This precipitates copper-rich, face-centered cubic mixed crystals in the form of mismatched spherical particles.
The crystals produce significant precipitation hardness effects, for example as a result of looping, depending on the particular particle size. In the presence of niobium, two precipitation effect mechanisms operate due to mixed crystals and carbonitride precipitation when the pearlite proportion of the microalloy structure is low and the copper content is high. With high copper content, the copper alloy structure has a content of 1%
Nickel must be added so that the This is to prevent solder embrittlement caused by copper. When the content of manganese and molybdenum is high as in this case, the strength of the copper alloy structure can be additionally improved by high dislocation density and fine grain hardening in addition to grain hardening. The corrosion inhibition activity of copper is
It is very effective when the copper content is as low as 0.25-0.40%. Therefore, by adjusting the copper content appropriately,
On the one hand, the addition of nickel should be avoided in order to exhibit its corrosion-inhibiting and strength-improving effects in an optimal manner, and on the other hand, to avoid solder embrittlement, which is unacceptable for prestressed steels, and, if possible, to prevent this. We must make sure that we are able to do so.

As a result of the aluminum content, rapid grain growth due to heating of the feedstock reaches temperatures as high as 1150°C, in which case holding time is also important. Similar to aluminum, which produces this effect through aluminum nitride, the micro alloying elements niobium and vanadium exhibit similar effects through their carbides, nitrides, or carbonitrides. To prevent or limit the re-melting of such precipitates due to heating prior to rolling, it is important that the temperature of the thruster be as low as possible. A high aluminum content exhibits the most favorable correlation in preventing strong grain growth due to reheating for rolling. Aluminum also contributes to solid solution hardening.

In addition to aluminum, the rapid grain growth due to reheating for rolling also raises temperatures to higher than about 1150°C due to nitrogen. Increased nitrogen content due to increased nitrides also contributes significantly to increased strength. In particular, the presence of vanadium significantly improves the yield point strength. Tensile strength also increases. Therefore, it is particularly important for prestressed steels that the yield point is increased by 70-90%.

In the present invention, the yield point increases as the phosphorus content increases, but at the same time it significantly embrittles the steel, so the phosphorus content must be limited. Oxygen injection/
The combination of inert gas scavenging makes it possible to reduce the phosphorus content and significantly prevent its embrittling effects. A corresponding reduction in the phosphorus content can also be achieved by metallurgical operations in the ladle.

According to the invention, it is important to keep the phosphorus content as low as possible.

The sulfur content plays the most decisive role in relation to the toughness anisotropy, which is the most important limiting factor for the cold workability of prestressed steels.

Lowering the sulfur content, ie reducing the number of sulfided inclusions, improves toughness with respect to brittleness reduction. This is very important for prestressed steel. Reducing sulfide length is also very effective in reducing fracture. A high degree of desulfurization can be achieved by adding calcium, which is customary in ladle metallurgy.

Regarding titanium, which is not included in the present invention,
Unlike niobium and vanadium, it participates in the formation of sulfides and first combines with total nitrogen to form nitride, i.e., TiN, and then combines with sulfur to form titanium carbonitride, i.e., Ti ₄ C ₂ S ₂ I would like to point out that. Therefore, for these two reasons,
There is no need to take titanium into account. This is because, inter alia, the effect of growing austenite grains and, together with other microalloying elements, increasing strength, is eliminated as the nitrogen content increases.

The production of high-strength, corrosion-resistant and brittle-fracture-resistant prestressed steel according to the invention eliminates all the problems that have to be taken into account when producing conventional prestressed steel grades with high carbon content. . In particular, the important production drawbacks of continuous wire casting processes, namely those caused by center separation and surface imperfections which impair drawability, can be avoided. The economic advantage of continuous casting over ingot casting is that it is well tolerated both in terms of cost and quality. It is possible to prevent carbon enrichment in the center of the cast body, which could conventionally occur. Carbon enrichment forms eutectic precipitates in the cementite lattice, thus significantly reducing not only the structure and its properties, but also the brittle fracture resistance.

Due to the high oxygen affinity of carbon, measures that had to be taken in the entire production process, namely both melting (e.g. scouring of the melting or remelting step and subsequent post-treatment of the steel and particularly expensive reoxidation protection) can also be omitted.A high degree of microscopic purity and a substantial avoidance of oxide and sulfide inclusions are facilitated.In conjunction with continuous casting processes, electromagnetic It also eliminates the high cost of stirring.
This prevents extremely disadvantageous center separation, solidification bridging, directional solidification structures, internal and surface defects.

The prestressed steel according to the invention has very high strength values, significantly reduced internal stresses, significantly improved resistance to brittle fracture, very high wear resistance, great usability due to its suitability for welding and very good performance. It has excellent corrosion resistance.

Regarding the latter two advantages, it is also pointed out that in terms of improved corrosion resistance, the present invention can take into account the more economical elements that interact in many respects with chromium in stainless steels. can. In addition, such corrosion-inhibiting elements can be used particularly advantageously and satisfactorily in high-strength steels produced by thermomechanical treatment. This is because, in addition to the corrosion-inhibiting effect due to precipitation hardening, these elements simultaneously contribute to strength improvement at low final rolling temperatures, which provide the highest strength improvement. However, apart from the increased strength in high-strength prestressed steels, if weld compatibility is achieved, great potential for structural simplification and improvement of current tensioning or fixing mechanisms is provided. For example, as is known in the construction of bridges, the composite members represent the most sensitive points of weakness where damage can occur due to the penetration of corrosion-promoting media into the steel. According to current technological possibilities, such combination parts are usually arranged very closely to each other. As the number of coupling couplings increases, the number of weak points also increases.

By using the high-strength, prestressed steel with excellent corrosion resistance and brittle fracture resistance of the present invention, longer tension rods can be manufactured.
The number of combined members and weak points can be reduced.
The suitability of these prestressed steels for welding greatly reduces the susceptibility of the fastening structure to damage, provided that the fastening structure is structurally simplified and improved.

Other advantages are that the diameter of the tension wire, draw rod or strand can be reduced and easily controlled, and the higher strength properties result in structures with smaller concrete thicknesses. This reduces the weight of the structure and offers great possibilities for structural design that could not be realized for technical or economic reasons with conventional prestressed steels, which have lower strength. this is,
It involves a reduction in the weight of movable structures (bridge buiding, member builder) and a reduction in the transportation costs of movable structures and prestressed steel.

Despite the increase in corrosion resistance and brittle fracture resistance due to the use of microalloying elements and improved post-treatment of the steel for strength enhancement, the price level of the prestressed steel of the present invention is limited by its manufacture and use. Due to its considerable advantages, it is possible to maintain or even improve the price of the current product. The additional structural possibilities resulting from the welding of fixed structures significantly improve the economic situation; however, the overall economic advantage is
This should more than compensate for the disadvantage of higher prices.