FI72748C - FOERFARANDE FOER FRAMSTAELLNING AV STAOLARMERINGSSTAONG FOER BETONG. - Google Patents

Description

1 72748

Method of making a steel reinforcing bar for concrete Förfarande för framställning av stälarmeringsstang för betong

The invention relates to a method for producing a steel reinforcing bar for concrete with a diameter of 8 to 40 mm, a reinforcing yield strength of 400 to 600 MPa and an elongation of at least 14%, whereby the size of the reinforcing support-5 is applied during or immediately after rolling. surface cooling to a temperature lower than the temperature corresponding to the formation of hardening products to form a surface layer of martensite or bainite on the rod, after which the rod is subjected to subsequent cooling, during which said surface layer is released by heat coming from the unhardened rod core. The object of the invention is to improve economy so that the reinforcing bars have both high yield strength and high formability and, if necessary, also good weldability. The invention is known from the features which appear from the claims. Manufacturing is carried out by means of a rapid cooling treatment during or immediately after rolling.

As is known, the rolling mill operator 20 has to consider several limitations when he wants to solve the above problem. In the first place, the output speed and temperature of the rolled bars depend in practice on the rolling mill. In addition, the rolling mill operator has limited space for possible installation of the refrigeration plant.

25 There are already a number of solutions that allow a trade-off between mechanical properties on the one hand and costs on the other. One first solution is to make "naturally hard" steel bars, the yield strength of which is achieved by adding carbon (e.g. 0.35%) and 30 manganese (e.g. £ 1.3). These steels have an acceptable yield strength (at least 420 MPa), but their elongation and bendability are relatively low and their weldability is rather poor.

2,72748

To improve weldability, it is necessary to reduce the carbon content, which causes a decrease in yield strength.

There are two known kei-5 noas to avoid such a decrease in yield strength.

The first means is to incorporate microalloying agents such as niobine or vanadium into the steel. However, this method is expensive due to the cost of the alloys. Another way is to increase the yield strength of the steel by deforming the tan-10 koa in the cold, e.g. by twisting. In addition to the cost of this measure, an increase in yield strength is achieved at the expense of elongation.

The method according to the present invention is based on a modern technique, according to which the time-limited rapid cooling is applied to the hot concrete reinforcements in order to form a surface layer of martensite or bainite on the bar. This quenching is followed by cooling, during which the core of the rod, i.e. the part of the rod which has not been reached by rapid cooling, is converted into ferrite and carbides. By appropriately limiting the duration of the rapid cooling, it is also possible to retain heat in the core of the rod and to develop in the cross-section of the rod such a temperature gradient that during said further cooling a surface hardening of the maraging or bainite surface layer takes place. Appropriate limitation of the fast o, cooling time length can be ensured by aiming for a certain temperature of the core at the end of the rapid cooling phase. In practice, this type of treatment can be performed by measuring the surface temperature of the rod 30 at the point where reheating is observed as a result of heat from the core.

This type of process, commonly referred to as the "quenching and self-hardening process", can thus be applied - in a particular plant with known conditions for the production of certain reinforcements - on the basis of the characteristics of the rapid cooling phase heart temperature. In addition, it has been suggested that this temperature should be approximately 350 ° C to achieve the combination of 3,72748 armature yield strength and elongation considered optimal.

The use of this combination obviously leads to the production of better quality ones. As the manufacturer has this Terms of Use at his disposal, he is able to choose the means necessary to achieve said 5 required temperatures.

The purpose of the method according to the invention is to determine for each product the parameters which are specific to the quench cooling plant and which give the Army the desired properties. These parameters are, as is known, the average heat flux density on the one hand, presented here in the temperature range 800 to 600 ° C, which flux is generally denoted d and expressed in mW / m, and the length of the quench cooling time denoted in t and expressed in seconds.

15 This method is particularly suitable for the production of steel reinforcements whose mechanical properties meet certain quality standards, such as a yield strength in the range of 400 to 600 MPa and an elongation of at least 14%.

In the method according to the invention for producing steel reinforcement by rapidly cooling the bars during or immediately after rolling, the parameters d and t for controlling the rapid cooling simultaneously meet similar conditions: 0.45 d to 0.4 <d. t <0.82 d + 0.6 logdi £ - 0.964. log d + 2,098 where in equations d is the diameter of the treated bar in millimeters.

Thanks to the method according to the invention, the most advantageous products in terms of the combination of mechanical properties can be obtained. On the other hand, the volume of the martensii11i / bainii11i region as well as its degree of softening by self-hardening is such that it results in a high yield strength. On the other hand, the elongation still remains within the desired limits.

A first application example of the method according to the invention relates to the production of bars with a diameter of 20 mm and leaving the rolling mill at a speed of 12 m / s and a temperature of 1050 ° C. From the weldability and economy, the chemical composition of the steel * 72748 is maintained in a certain range, e.g. 0.10 ... 0.20% C, 0.8 ... 1.3% M n. In this case, the steel had 0.13% C and 1.2% Mn. When rolled, the yield strength of the steel was about 363 MPa and the elongation was 22 ... 24%.

3 In order to obtain steel reinforcements for concrete with improved yield strength and formability, the rolling mill operator selects, on the basis of the equipment and space available, a plant whose rapid cooling intensity and time length (i.e. processing length) meets the above conditions, i.

d ^ 7 mW / m2 8.6 <¢ 5. t <17.

The choice of plants with an average heat flux density of 800 mW / m2 at 800 ... 600 ° C and a length L of 15 12 m results in a yield strength of 500 MPa and an elongation of 20.6%.

The choice of a plant with properties of 0 = 8.4 mW / m and L = 20.5 m would result in a reinforcing steel with a yield strength of 550 MPa and an elongation of 18.5%. In this case, the processing length would be 1.7 seconds and the input d x t 20 would be 14.35.

Another example relates to the production of bars with a diameter of 8 mm which leave the rolling mill at a speed of 18 m / s and have a temperature of about 1000 ° C. The steel contains 0.18% C and 0.8% Mn. When rolled, the product 25 has a yield strength of 325 MPa and an elongation of about 30%.

In this case, the limits set by the method according to the invention are d> 17 mW / m2 3.2 <d. t <7.16.

2 30 If a plant with ¢ (= 17 mW / m and a handling distance of L = 4.5 m is selected, rods with a yield strength of 500 MPa and an elongation of 18% are obtained, in which case the treatment length should be 0.25 s and the input dxt would be 4.25 The same property would be achieved when d = 25 mW / m and a treatment length of 0.17 seconds, i.e. a fast cooling distance length = 3 m.

Claims (1)

72748 CLAIMS Method for producing a steel reinforcing bar for concrete with a diameter of 8 to 40 mm, a reinforcing yield strength of 400 to 600 MPa and an elongation of at least 14%, in which case the fastening bar is subjected to rapid surface cooling during or immediately after rolling to a temperature lower than the temperature corresponding to the formation of hardening products to form a surface layer of martensite or bainite on the rod, after which the rod is subjected to screen air cooling, said surface layer being released by heat from the unhardened rod core, characterized in any of the above limits; with the varying diameter of the rod: (a) a minimum value of the average density of the heat flux (ei) is determined, which ensures the required cooling of the rod at a ratio of 1. g 0 ^ - 0.964 log d + 2.098; (b) selecting an effective value for (i $) that is at least equal to said minimum value; (c) determining the operating time of said value of effective (tf) 20 to provide the required cooling in the ratios 1a: 0.45 d to 0.4 <d.t <0.82 d + 0.6; (d) controlling the quench cooling by applying selected values of (d) and (t); 25 t is expressed in seconds, tf in millimeters and ^ in mW / m. 30