EP0166239A1 - Process for producing concrete-reinforcing steel bars or rods emerging from the rolling mill - Google Patents

Abstract

For the production of a reinforcing steel with a higher yield point and good weldability and toughness, microalloying elements are alloyed with the steel and their proportion represents 0.02 and 0.06% vanadium and 0.01 to 0.02% nitrogen, said proportions not being sufficient for achieving a higher yield point of at least 450 N/mm2. However, this is reached if the rolling stock undergoes controlled, but relatively limited cooling during or after rolling, in such a way that the compensating temperature of the steel reaches at least 700 DEG C. Due to the fact that the microalloying elements are only alloyed in small quantities and only relatively small water quantities are required, reinforcing steel can be economically produced. In addition, the process permits coiling in the case of wire rolling and can also be used on other rolled steel products.

Description

The invention relates to a method for producing reinforcing steel in the form of bars or wire rod with a yield strength of at least 450 N / mm ² with good weldability and toughness.

When manufacturing reinforcing steel, efforts are made to achieve higher yield strengths while maintaining good toughness and welding properties. In this context, good welding properties are understood to mean the suitability of such reinforcing steels for the welding processes customary here, such as, for example, manual electric arc welding, protective gas welding, flash butt welding and resistance spot welding. A measure for the assessment of the weldability is the carbon content or the carbon equivalent, which values should be as low as possible.

• 1. Naturharte Betonstähle.
  • Sie erreichen ihre Streckgrenze durch Zulegieren folgender Legierungselemente: Kohlenstoff etwa 0,4%, Mangan etwa 1,2%, Silizium etwa 0,5%. Diese Stähle sind wegen des hohen Kohlenstoffgehaltes nicht schweissbar.
• 2. Naturharte Betonstähle mit Zusatz von Mikrolegierungselementen.
  • Eine bedingte Schweissbarkeit wird dadurch erreicht, dass ein Teil des Kohlenstoffes durch beispielsweise Vanadium ersetzt wird, wobei die Legierungselemente folgende Werte aufweisen:
    • Kohlenstoff etwa 0,3%, Mangan etwa 1,2%, Silizium etwa 0,5% und Vanadium etwa 0,03%.
• 3. Naturharte Armierungsstähle mit erhöhtem Zusatz von Mikrolegierungselementen und erhöhten Stickstoffgehalten.
  • Durch die festigkeitssteigernde Wirkung der sich unkontrolliert bildenden Vandiumnitride kann der Kohlenstoffgehalt weiter abgesenkt werden, so dass der Stahl schweissbar wird, Solche Stähle sind beispielsweise in der Firmenschrift der Union Carbide "CARVAN & NITROVAN, Vanadiumträger von Union Carbide für die Stahlherstellung" beschrieben. Sie weisen folgende Legierungselemente auf:
    • Kohlenstoff etwa 0,2%, Mangan etwa 1,2%, Silizium etwa 0,5% und Vanadium etwa 0,08%.
    • Die Schweissbarkeit wird aber erkauft mit höheren Herstellkosten durch den Vanadiumzusatz.
• 4. Kaltverfestigte Betonstähle.
  • Diese Stähle erhalten ihre Eigenschaften durch eine Kaltverfestigung, wie beispielsweise Verwinden, Recken oder Ziehen. Vom Kohlenstoff-Aequivalent her sind sie schweissbar und weisen folgende Legierungselemente auf:
    • Kohlenstoff gleich oder kleiner 0,2%, Mangan etwa 0,6% und Silizium etwa 0,2%.
    • Diese Stähle können sich jedoch beim Schweissen durch zu hohes Wärmeeinbringen wieder entfestigen. Zudem ist der zusätzliche Arbeitsvorgang für die Kaltverfestigung kostensteigernd.
• 5. Aus der Walzhitze vergütete Betonstähle.
  • Es sind Betonstähle bekannt (z.B. DE-AS 2 353 034 und DD-PS 84615, die ihre höhere Streckgrenze dadurch erreichen, dass sie während oder unmittelbar nach dem Walzen aus der Walzhitze vergütet werden. Dabei wird durch eine intensive Wasserabschreckung eine Härtung der Oberflächenzone des Stabes erreicht, die nach dem Verlassen der Kühlstrecke durch die im Stabkern vorhandene Wärme angelassen wird. Es werden also die bekannten Temperaturprofile genutzt, die sich infolge der schlechten Wärmeleitfähigkeit des Stahles im Vergleich zu andern Metallen normalerweise bei Abkühl- oder Aufheiz-Vorgängen einstellen.
  • Wegen des niedrigen Kohlenstoff-Aequivalentes, ähnlich wie beim kaltverfestigten Stahl (Kohlenstoff gleich oder kleiner 0,2%, Mangan etwa 0,6% und Silizium etwa 0,2%), ist dieser Stahl gut schweissbar.
  • Für dieses Verfahren sind jedoch hinreichende Kühlwassermengen und Platz in der Walzstrasse für die Kühlstrecke erforderlich. Die Oberfläche des Walzgutes wird auf eine Temperatur von weniger als 200°C abgekühlt, und nach Auflauf auf das Kühlbett beträgt die Ausgleichstemperatur etwa 600°C. Wegen der niedrigen Oberflächentemperatur werden erhöhte Ansprüche an die Warmschere bezüglich Scherkraft und Messerqualität gestellt, und die Transportvorrichtungen zum Kühlbett verschleissen schneller.
  • Zudem wird dieses Kühlverfahren bei sehr hohen Walzgeschwindigkeiten, wie sie beispielsweise beim Drahtwalzen auftreten, noch nicht beherrscht. Eine weitere Schwierigkeit tritt beim Windungslegen auf, wenn die Oberflächentemperatur weniger als 200°C beträgt und sich auf nur etwa 600°C wieder aufheizt.

The following reinforcing steels with higher yield strengths are known:

• 1. Naturally hard reinforcing bars.
  • They reach their yield strength by alloying the following alloying elements: Carbon about 0.4%, manganese about 1.2%, silicon about 0.5%. Because of the high carbon content, these steels cannot be welded.
• 2. Naturally hard reinforcing steel with the addition of micro-alloying elements.
  • Conditional weldability is achieved by replacing part of the carbon with, for example, vanadium, the alloying elements having the following values:
    • Carbon about 0.3%, manganese about 1.2%, silicon about 0.5% and vanadium about 0.03%.
• 3. Naturally hard reinforcing steels with increased addition of micro-alloying elements and increased nitrogen contents.
  • Due to the strength-increasing effect of the uncontrolled formation of vandium nitrides, the carbon content can be further reduced, so that the steel can be welded. Such steels are described, for example, in the Union Carbide company publication "CARVAN & NITROVAN, vanadium carrier from Union Carbide for steel production". They have the following alloying elements:
    • Carbon about 0.2%, manganese about 1.2%, silicon about 0.5% and vanadium about 0.08%.
    • However, the weldability is bought with higher manufacturing costs through the addition of vanadium.
• 4. Work hardened reinforcing steel.
  • These steels acquire their properties through work hardening, such as twisting, stretching or drawing. They are weldable from the carbon equivalent and have the following alloying elements:
    • Carbon equal to or less than 0.2%, manganese about 0.6% and silicon about 0.2%.
    • However, these steels can soften again when welding due to excessive heat input. In addition, the additional work process for work hardening increases costs.
• 5. Reinforced reinforcing steel from the rolling heat.
  • Reinforcing steels are known (for example DE-AS 2 353 034 and DD-PS 84615) which reach their higher yield strength by tempering them from the rolling heat during or immediately after rolling. Intensive water quenching causes the surface zone of the When the rod is reached, which is left on by the heat present in the core of the rod after leaving the cooling section, the known temperature profiles are used which, due to the poor thermal conductivity of the steel compared to other metals, normally occur during cooling or heating processes.
  • Because of the low carbon equivalent, similar to that of work hardened steel (carbon equal to or less than 0.2%, manganese about 0.6% and silicon about 0.2%), this steel is easy to weld.
  • However, sufficient cooling water quantities and space in the rolling mill for the cooling section are required for this process. The surface of the rolling stock is cooled to a temperature of less than 200 ° C, and after it has reached the cooling bed, the compensation temperature is about 600 ° C. Because of the low surface temperature, there are increased demands on the hot shear in terms of shear force and knife quality, and the transport devices to the cooling bed wear out faster.
  • In addition, this cooling process is not yet mastered at very high rolling speeds, such as those that occur in wire rolling. Another difficulty arises when winding the surface if the surface temperature is less than 200 ° C and heats up to only about 600 ° C.

In this context, it must be pointed out that the use of reinforcing steel in the form of profiled wire rod in rings, in particular as starting material for bending shops, is becoming increasingly popular.

• - nach dem dieser Betonstahl infolge niedriger Gehalte an Mikro- und anderen Legierungselementen kostengünstig herstellbar ist,
• - das keine grossen Wassermengen und Investitionen zur Anwendung im Walzwerk benötigt,
• - das Warmschere und Kühlbettzulauf nicht übermässig beansprucht, und
• - nach dem der Betonstahl in einfacher Weise in Form von profiliertem Walzdraht in Ringen herstellbar ist.

The object of the present invention is to find a method for producing a reinforcing steel of the type described at the outset with a higher yield strength and good toughness and welding properties,

• after that this reinforcing steel can be produced inexpensively due to the low contents of micro and other alloying elements,
• - that does not require large amounts of water and investments for use in the rolling mill,
• - The warm shear and cooling bed inlet are not excessively stressed, and
• - After which the reinforcing steel can be produced in a simple manner in the form of profiled wire rod in rings.

This object is achieved according to the invention in that the steel is micro-alloyed with nitrite-forming elements and nitrogen and, moreover, is subjected to controlled cooling during and / or after rolling, which brings about an average compensation temperature of greater than 700 ° C., so that a preferred nitride precipitation below the area of gamma-alpha conversion.

• - beim Stabwalzen die Warmschere und der Kühlbettzulauf gegenüber der Herstellung von aus der Walzhitze vergüteten Betonstählen geschont werden und
• - beim Drahtwalzen ein Windungslegen möglich ist.

It has been shown that in a steel with a low carbon equivalent, to which microalloying elements such as vanadium and nitrogen are only added in small amounts, the precipitation process of the vanadium (carbo) nitrides is preferably effectively achieved if the steel is used during and / or after the rolling is additionally cooled rapidly by controlled cooling to the temperature range below the area of the gamma-alpha conversion. To produce a reinforcing steel with a yield strength greater than 500 N / mm ² , the vanadium content with a low carbon equivalent only needs to be 0.04%. A temperature range of greater than 700 ° C. has been shown to be the temperature range which is advantageous for the elimination. With this low cooling, the surface of the rolling stock has a temperature greater than 600 ° C directly at the exit of the cooling section and heats up because of the low temperature The cooling layer thickness quickly increases to a temperature greater than 700 ° C. This ensures that

• - During rod rolling, the hot shear and the cooling bed inlet are spared compared to the production of heat-treated rebars and
• - It is possible to lay turns in the case of wire rolling.

• Fig. 1 ein Diagramm, das den Zusammenhang zwischen Streckgrenze und Ausgleichstemperatur von Betonstahl und verschiedenen Anteilen an Mikrolegierungselementen darstellt,
• Fig. 2 das Schliffbild eines nach der Erfindung hergestellten Betonstahls mit einem Durchmesser von 8 mm, einem V-Gehalt von 0,04% und einer Ausgleichstemperatur von 710°C und
• Fig. 3 das Schliffbild desselben Betonstahls, bei dem jedoch durch eine intensive Wasserabschreckung eine Oberflächenhärtung bei einer Ausgleichstemperatur von 655"C erreicht wird.

The invention is described below on the basis of some operating results, for example, and illustrated in the figures. Show it:

• 1 is a diagram showing the relationship between the yield strength and the compensation temperature of reinforcing steel and various proportions of microalloying elements.
• Fig. 2 shows the micrograph of a reinforcing steel made according to the invention with a diameter of 8 mm, a V content of 0.04% and a compensation temperature of 710 ° C and
• Fig. 3 is the micrograph of the same reinforcing steel, but in which a surface hardening is achieved at an equalization temperature of 655 "C by intensive water quenching.

Some operating results are described below and shown in the attached diagram.

und den üblichen Begleitelementen eines Elektrostahls sowie Vanadium- und Stickstoff-Gehalten von

wurden an Betonstählen mit kleinen Durchmessern (8 - 12 mm) die in dem Diagramm dargestellten Ergebnisse erzielt:

• Während ohne kontrollierte Abkühlung (Ausgleichstemperatur etwa 900°C) Streckgrenzen von 350, 420 und 450 N/mm erreicht wurden, nehmen die Werte bei kontrollierter Abkühlung zu und betragen bei einer Ausgleichstemperatur von 700°C 440, 530 und 560 N/mm² (Fig. 1). Bei Ausgleichstemperaturen von kleiner als 700°C machen sich bereits Härtungseffekte bemerkbar (Fig. 3). Diese Temperaturen lägen aber auch im Sinne der Erfindung zu niedrig.

On three melts with the composition

and the usual accompanying elements of an electric steel as well as vanadium and nitrogen contents of

the results shown in the diagram were achieved on rebars with small diameters (8 - 12 mm):

• While yield limits of 350, 420 and 450 N / mm were reached without controlled cooling (compensation temperature about 900 ° C), the values increase with controlled cooling and are 440, 530 and 560 N / mm ² at a compensation temperature of 700 ° C Fig. 1). At compensation temperatures of less than 700 ° C, hardening effects are already noticeable (Fig. 3). However, these temperatures would also be too low for the purposes of the invention.

According to the invention, the upper compensation temperature is determined by the gamma-alpha transition temperature (A _r3 point). The A _r3 point depends on the austenitizing temperature and in particular on the steel composition. In the example given, it is around 825 ° C.

After rolling, the gamma-alpha conversion should also take place as quickly as possible in the core. It is therefore advisable to control the cooling so that the Gamma-alpha conversion is accelerated in the core, but on the other hand the temperature of the rod surface does not _- below the MS point, in the example cited 450 ° C. Compensation temperatures up to 760 ° C have proven to be well applicable.

The average heat flux density was determined as a measure of the controlled cooling, which was approximately 11 MW / m ² for the rod diameters of 8-12 mm and approximately 6 MW / m ² for the rod diameter 20 mm.

The mean heat flow density is understood to mean the amount of heat dissipated by the cooling medium, based on the rod surface cooled in the cooling system during the cooling time.

The test results show that with controlled cooling, despite the low carbon equivalent and the low content of microalloying elements (vanadium and nitrogen), the required high yield strengths of reinforcing steel equal to or greater than 500 N / mm ² can be set easily and inexpensively.

A vanadium content of 0.04% with a nitrogen content of 0.012% (120 ppm) is sufficient for this; an increase in the vanadium content to 0.06% has only a comparatively minor effect.

Of course, the method can also be applied to products and / or types of steel other than reinforcing steel in bars or wire rod, e.g. on steel bars and flat products.

Claims (7)

1. A process for the production of reinforcing steel in the form of bars or wire rod with a yield strength of at least 450 N / mm ² with good weldability and toughness, which has a low carbon equivalent, characterized in that the steel micro-alloys with nitride-forming elements and nitrogen and also during and / or after the rolling is subjected to a controlled cooling, which brings about an average compensation temperature of greater than 700 ° C., so that a preferred nitride precipitation takes place below the area of the gamma-alpha conversion. 2. The method according to claim 1, characterized in that the average heat flux density when cooling the rolling stock in the form of bars or wire rod about 11 MW / m ² with a diameter of 8 mm and about 6 MW / m ² with a diameter of 20 mm is. 3. The method according to claim 1 or 2, characterized in that the reinforcing steel has the following analysis, in% by weight, to achieve a yield strength of greater than 500 N / mm ² : Carbon 0.10 - 0.25%, manganese equal or greater than 0.6%, silicon about 0.2%, vanadium 0.02% - 0.06%, nitrogen 0.01% - 0.02% for the usual levels of trace elements, the rest iron. 4. Process according to claim 3, characterized in that the vanadium content is 0.03% - 0.05%. 5. The method according to any one of claims 1 to 4, characterized in that for the production of wire rod this is laid at a temperature of greater than 700 ° C to turns. 6. The method according to any one of claims 1 to 4, characterized in that the rolling stock is sheared at a temperature of greater than 700 ° C. 7. Application of the method according to claim 1 to 6 on other products and / or types of steel.

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