IE53019B1

IE53019B1 - Method of manufacturing steel reinforcements for concrete, having improved properties

Info

Publication number: IE53019B1
Application number: IE1651/82A
Authority: IE
Inventors: M Economopoulos; N Lambert
Original assignee: Centre Rech Metallurgique
Priority date: 1981-07-09
Filing date: 1982-07-08
Publication date: 1988-05-11
Also published as: BR8108179A; ES8401530A1; NO822384L; LU84263A1; FI72748B; MX159859A; DK152691B; AT375401B; JPS589920A; AU7862481A; ZA818244B; SE462853B; IE821651L; YU42779B; JPS6219488B2; DK152691C; KR840000653A; NO156903B; FI822426L; ES513811A0

Abstract

According to the process for the manufacture of reinforcements for concrete made of steel which has an elasticity limit of between 400 and 600 MPa and an elongation of at least 14 %, the steel reinforcing rod is cooled abruptly during or immediately after the rolling. The control parameters of the abrupt cooling, namely the mean heat flow density between 800 and 600 DEG C, denoted by phi and expressed in MW/m<2>, and the cooling time, denoted by t and expressed in seconds, simultaneously satisfy the following conditions: 0.45 d - 0.4 < phi .t < 0.82 d + 0.6 log phi >/= - 0.964 log d + 2.098 in which d denotes the diameter of the rod, expressed in mm. [CH639137A5]

Description

The present invention relates to a method for the economical manufacture of steel reinforcements for concrete, having both a high yield strength and a high ductility as well as, if required, good weldability. Manufacture is carried out by a rapid cooling treatment applied during or immediately after rolling.

It is known that the rolling mill operator who wishes to solve the problem stated above must take into account several constraints which are imposed on him. In the first instance, the output speed and temperature of rolled bars are in practice fixed by his rolling mill. In addition, the rolling mill operator has limited space for the possible installation of a cooling plant.

There are already several solutions enabling a compromise between mechanical properties, on one hand, and cost price, on the other hand. A first solution is to produce “naturally hard steel bars whose yield strength is obtained by the addition of carbon (for example 0.35%) and manganese {for example 1.3%). These steels have an acceptable yield strength (at least 420 MPa), but their elongation and bendability are comparatively low and their weldability is not good at al1.

In order to improve weldability it is necessary to decrease the carbon content, which causes a decrease in the yield strength. - 2 S3019 There are two known ways of offsetting this decrease in the yield strength. The first is to incorporate microalloying elements, such as niobium or vanadium, in the steel. This technique is costly, however, as a result of the price of the alloying elements. The second way is to increase the yield strength of the steel by cold deformation of the bar, for example by torsion. In addition to the costs which this operation involves, the yield strength increase is provided to the detriment of the elongation.

The method of the present invention is based on the recent technique which consists in applying rapid cooling (which is limited in time) to the hot rolled concrete reinforcements in order to produce a surface layer of martensite or bainite in the bar. This quenching is followed by cooling during which the core of the bar, i.e. the portion which has not been reached by the rapid cooling, is converted into ferrite and carbides. By suitably limiting the duration of the rapid cooling it is also possible to preserve heat in the core of the bar and to create a temperature gradient in the cross-section of the bar such that, during the said further cooling, tempering of the martensitic or bainitic surface layer takes place. A suitable limitation of the duration of the rapid cooling may be ensured by aiming for a given temperature of the core at the end of the rapid cooling state. In practice, an operation of this type may be carried out by observing the temperature of the surface at the location of the bar where re-heating is observed as a result of the heat coming from the core. - 3 19 A method of this type, commonly known as “quenching and sel f-tempering may therefore be applied - in a given plant with known specifications for manufacturing given reinforcements - on the basis of the characteristic constituted by the core temperature at the end of the rapid cooling phase.

It has been claimed in addition that this temperature should be approximately 850°C in order to provide a combination, considered to be optimum, of the yield strength and elongation of the reinforcement.

The use of this combination obviously leads to the production of reinforcements of improved quality. As the operator I has available this operating condition, he is able to select the means to be used to obtain the said required temperature.

The method of the present invention is designed to specify the parameters for each product which are characteristic of a rapid cooling plant and which confer the reinforcements the desired properties. It is known that these parameters are, on one hand, the intensity of the rapid cooling, i.e. the mean density of calorific flux, given here at between 800 and 600°C, commonly designated by $ and expressed in 2 MW/m and, on the other hand, the duration of the rapid cooling designated by_t and expressed in seconds.

This method may in particular be applied to the production of steel reinforcements having mechanical properties satisfying quality standards, for example a yield strength of between 400 and 600 MPa and an elongation of at least 14%. - 4 S3O18 In the method of manufacture of steel reinforcements to which the present Invention relates, in which the bars are rapidly cooled during or immediately after rolling, t the parameters ji and t for regulating the rapid cooling simultaneously satisfy the following conditions: 0.45 d - 0.4 * ψ . t < 0.82 d + 0.6 !> - 0.964 . log d + 2.098 in which _d represents the diameter of the treated bar in mm.

The method of the invention enables the achievement of the most advantageous product with respect to the combination of the mechanical properties. On one hand the volume of the martensitic/bainitic area as well as its degree of softening by self-tempering are such that they lead to a high yield strength. On the other hand, the elongation still remains within the desired limits.

A first example of application of the method of the invention relates to the manufacture of bars having a diameter of 20 mm which are discharged from the rolling mill at a speed of 12 m/s and at a temperature of 1050’C. For reasons of weldability and economy, the chemical composition of the steel is contained within a given range, for example 0.10 - 0.202 C, 0.8 - 1.32 Mn. In the case in question, the steel contained 0.132 C and 1.22 Mn. In the as-rolled condition it had a yield strength of approximately 365 MPa and an elongation of 22 to 242. - 5 In order to obtain steel reinforcements, for concrete, having an improved yield strength and ductility, the rolling mill operator selects, in accordance with the equipment and space available, a plant whose rapid cooling intensity and length (therefore duration of treatment) correspond to the above conditions, i.e.

MW/m2 8.6 <17.

The selection of a plant whose average calorific flux density between 800 and 600°C is 10 MW/m and whose length, L, is 12 m, leads to a yield strength of 500 MPa and an elongation of 20.6%. The selection of a plant whose characteristics are = 8.4 MW/m2 and L = 20.5 m would provide a steel reinforcement having a yield strength of 550 MPa and an elongation of 18.5%. In this case, the duration of treatment would be 1.7 seconds and the product $. t would be 14.35.

A second example relates to the manufacture of bars of 8 nm in diameter which are discharged from the rolling mill at a speed of 18 m/s and at a temperature of approximately 1000°C. The steel contains 0.18% and 0.8% Mn. In the as-rolled condition, it has a yield strength of 325 MPa and an elongation of approximately 30%.

In this case, the limits imposed by the method of the invention are: - 6 53019 / Π WAtm 3.2 t < 7.16.

If a plant is selected having f = 17 MW/mm2 with a length of treatment L = 4.5 m, bars are obtained having a yield strength of 500 MPa and an elongation of 18%. In this case, the duration of treatment would be 0.25 s and the product ! . t would be 4.25. The same properties were obtained with ψ = 25 MW/m2 and a duration of treatment of 0.17 seconds, i.e. a rapid cooling length of 3 m.

Claims

1. A method of manufacturing a steel reinforcement for concrete, having a yield strength of between 400 and 600 MPa and an elongation at least equal to.14%, in which steel 5 reinforcing bars are rapidly cooled during or immediately after rolling, the mean calorific flux density between 800 and 600°C, designated by ψ and expressed in MW/m 2 , and the duration of cooling, designated by t and expressed in seconds, simultaneously satisfying the following conditions: 10 0.45 d - 0.4 t -<0.82 d + 0.6 1og^ => - 0.964 log d + 2.098 in which ji represents the diameter of the bar expressed in mm.

2. A method as claimed in claim 1, substantially as described 15 herein.