JPS6219488B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an economical method for producing steel reinforcement for concrete having both high yield strength and high ductility, as well as good weldability if required. Production is carried out by rapid cooling applied during or immediately after rolling.

A rolling mill operator who wishes to solve the above-mentioned problems must take into account several constraints placed on him. Firstly, the output and temperature of the bar rolled are practically fixed by the rolling mill. Furthermore, rolling mill operators have limited space for possible installation of cooling plants.

There are already some solutions that allow a compromise between mechanical properties on the one hand and cost costs on the other hand. The first solution is to increase the yield strength of carbon (e.g. 0.35%)
and by the addition of manganese (for example 1.3%) to make an inherently hard steel bar. Acceptable yield strength for these steels (at least 420 MPa)
However, their elongation and flexibility are relatively low, and their weldability is not good at all.

To improve weldability it is necessary to reduce the carbon content. However, this reduces yield strength.

There are two known methods to compensate for this loss in yield strength. The first is the incorporation of micro-alloying elements such as niobium or vanadium into the steel. However, this method is expensive due to the cost of alloying elements. A second method is to increase the yield strength of the steel by cold deformation of the bar, such as by torsion. In addition to the expense involved in this method, increased yield strength provides a reduction in elongation.

The method of the invention is based on a recent method consisting in applying rapid cooling (limited in time) to hot-rolled concrete reinforcing bars in order to produce a surface layer of martensite or bainite on the bars. This rapid cooling is followed by cooling that converts the core of the rod, that is, the portion that has not been rapidly cooled, into ferrite and carbide. By appropriately limiting the quenching time, it is also possible to retain heat in the core of the rod and create a temperature gradient across the cross section of the rod so that a martensite or bainite surface layer develops during said subsequent cooling. You can also make Proper control of the rapid cooling time can ensure the purpose of providing a constant temperature of the wick at the end of the rapid cooling phase. In practice, this type of operation can be carried out by observing the surface temperature of the rod at the location where reheating is observed as a result of the heat coming from the wick.

This type of process, which is therefore commonly known as "quenching and self-tempering", can be applied on the basis of the properties constituted by the core temperature at the end of the rapid cooling phase (for the production of certain reinforcements). (in certain plants with known specifications). It is said that this temperature should be about 850°C to give the best possible reinforcement yield strength and elongation combination.

The use of this combination clearly results in the production of reinforcements of improved quality. When this operating condition is available to the operator, he can select the means to be used to obtain the required temperature.

The method of the invention consists in imparting the desired properties to the reinforcement and defining the parameters for each product that are characteristic of rapid cooling. These parameters are
On the one hand, it is expressed in units of MW/m ² and usually denoted by φ, here the intensity of rapid cooling, i.e. the average density of the heat flux, given between 800 and 600 °C, in other words, during the cooling process between 800 °C and 600 °C. the average amount of heat extracted through a unit area of the steel bar surface to reduce the temperature of the steel bar surface, and on the other hand, expressed in seconds,
The rapid cooling time is expressed as t.

This method is particularly applicable to the production of steel reinforcements with mechanical properties that satisfy quality criteria such as a yield strength of between 400 and 600 MPa and an elongation of at least 14%.

In the method for manufacturing steel reinforcement to which the present invention relates, in which the bar is rapidly cooled during or immediately after rolling, the parameters φ and t for adjusting the rapid cooling are
is made to simultaneously satisfy the following condition: 0.45d-0.4<φ·t<0.82d+0.6 logφ≧−0.964·logd+2.098 (where d represents the diameter in mm of the rod to be treated).

The method according to the invention makes it possible to achieve products with the most advantageous combination of mechanical properties. On the one hand, it is not only the volume of the martensite/bainite zones but also the degree of softening that makes them self-tempering, so that they result in high yield strengths. On the other hand, the elongation still remains within the desired limits.

A first example of application of the method of the invention concerns the production of bars with a diameter of 20 mm leaving the rolling mill at a temperature of 1050° C. and a speed of 12 m/s. From the viewpoint of weldability and economy, the chemical composition of steel should be within a certain range, for example 0.10~0.20%C, 0.8~
Contain within the range of 1.3%Mn. In the case in question, the steel contained 0.13% C and 1.2% Mn. In its rolled state it has a yield strength of approximately 365 MPa
It had an elongation rate of 22-24%.

In order to obtain concrete steel reinforcement with improved yield strength and ductility, the rolling mill operator, depending on the equipment and available space, must ensure that the rapid cooling strength and length (and therefore processing time) are within the conditions described above, i.e. φ≧ Select the plant corresponding to 7MW/m ² 8.6<φ・t<17.

Choosing a plant with an average calorific flux density of 10 MW/ ^m2 at 800-600°C and a length (L) of 12 m results in a yield strength of 500 MPa and an elongation of 20.6%. . Choosing a plant with the properties φ = 8.4 MW/m ² and L = 20.5 m provides a steel reinforcement with a yield strength of 550 MPa and an elongation of 18.5%. In this case, the processing time is 1.7 seconds, and the product φ・t is
It is 14.35.

The second example concerns the production of a bar with a diameter of 8 mm leaving the rolling mill at a temperature of approximately 1000° C. and a speed of 18 m/s.
The steel contained 0.18% C and 0.8% Mn. In the as-rolled state it had a yield strength of 325 MPa and an elongation of about 30%.

In this case the limits imposed by the invention are φ≧17 MW/m ² 3.2<φ·t<7.16.

When choosing a plant with φ = 17 MW/m ² and processing length (L) = 4.5 m, bars with a yield strength of 500 MPa and an elongation of 18% were obtained. In this case, the processing time was 0.25 seconds, and the product φ·t was 4.25. φ=
The same properties were obtained at 25 MW/m ² with a processing time of 0.17 seconds or a rapid cooling length of 3 m.

Claims (1)

[Claims] 1. During or immediately after hot rolling, a steel bar is subjected to rapid surface cooling to a temperature lower than the temperature corresponding to the formation of quenched products (Ms point) to form a martensite or bainite surface. forming a layer in the rod and then subjecting the steel rod to subsequent cooling in air, during which time the surface layer is tempered by heat coming from the unquenched core of the steel rod, at 400 MPa.
A method for producing steel reinforcement for concrete having a yield strength of ~600 MPa and an elongation of at least 14%, the method comprising: Determine the minimum value of the average heat flux (φ) that ensures (b) select a valid value of (φ) that is at least equal to the above minimum value, (c) the relationship 0.45d−0.4<φ・t<0.82 d+0.6, determine the application time (t) of the above effective value of (φ) to achieve the required rapid cooling effect; (d) apply the above effective value of the average heat flux during the selected time (t); A method for producing steel reinforcement for concrete, characterized in that the steel rod is rapidly cooled by applying (parameters φ in MW/m ² , t in seconds and d in mm).