LU82201A1 - PROCESS FOR PRODUCING LAMINATED STEEL PRODUCTS - Google Patents

Description

I v J Claiming priority of a | Jr patent application filed in Belgium! p February 28, 1979 under No. 874.534.

IBL-2825 / EM / BM

C 1939/7902.

♦ METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Non-profit association - Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Process for manufacturing rolled steel products.

The present invention relates to a process for manufacturing rolled steel products, the elastic limit of which is between 250 and 500 megaPascals (MPa); these laminates can be, for example, concrete reinforcing bars, wire rod, bars of circular or non-circular cross-section, profiles consisting either essentially of a flat iron, or of several plates joined together, strips and even sheets.

The main qualities that users demand from rolled steel products are, among other things, an elastic limit as high as possible for the grade of steel used, as well as, depending on the case and the use to which the product / 3tr 2.- f <is intended, a resistance to fatigue, a ductility, a weldability, even a satisfactory wire drawing.

Obtaining such qualities also requires the implementation of hardening processes either by precipitation

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tion, through the addition of additional alloying elements, either by work hardening, for example by cold drawing or twisting. However, these two known methods have the disadvantage of being relatively expensive.

The subject of the present invention is a process making it possible to avoid the above-mentioned drawback, by adapting a heat treatment to the composition of the steel, with a view to creating suitable metallic microstructures.

The process, object of the present invention, for manufacturing a rolled steel product with elastic limit between 250 and 500 megaPascals, is essentially characterized in that from the austenitization temperature, a rolled steel product containing from 0.20% to 0.40% of Carbon, from 0.30% to 0.70% of Manganese and from 0.10% to 0.30% of Silicon, with intense cooling by means of a fluid, in such a way that a martensitic and / or bainitic quenching of the surface layer of all or part of the product is carried out, while the unhardened part of the rolled product remains at a temperature sufficient to subsequently allow a tempering of the surface layer of martensite and / or bainite.

According to a particularly advantageous form of the invention, intense cooling is applied to the rolled product as soon as it leaves the finishing stand of the rolling mill.

According to the invention, a laminated product is produced „with elastic limit between 400 and 500 megaPascals, from a steel containing from 0.30% to 0.40% C, from 0.50% to 0.70%

Mn and from 0.10% to 0.30% Si.

The cooling fluid used for the implementation of this process is generally water, with or without 4! ' 3.- (* • additives, aqueous solutions of mineral salts, etc ... This fluid can also be a mist, obtained for example by | putting water in suspension in a gas, for example air; it can also be a gas, for example water vapor.

From the practical point of view, the desired rapid cooling of the rolled product is obtained, by choosing the cooling devices and by adjusting, in an appropriate manner, the length and the flow characteristics of the cooling ramps.

The advantages presented by the above process 1 are as follows: - improved qualities, - savings in alloying elements, - reduction in heat treatment costs, - possibility of reducing the number of grades to be produced in the steelworks to obtain the desired qualities, - possibility of saving the production of certain non-standardized steel grades (great flexibility of adjustment).

The following examples are given as an indication, but not limiting, to illustrate the various advantages presented by the process of the present invention.

1. Steel bars with variable diameters (10, 15, 20, 30, 40, 60 mm) and an average chemical composition equal to were used; 0.25% C, 0.40% Mn and 0.20% Si, the balance consisting of Fe containing the usual impurities.

At the exit of the last stand of the rolling mill, these bars were subjected to rapid quenching with self-tempering,

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in accordance with the present invention.

- The bars thus treated had an elastic limit | j of the order of 276 MPa.

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To obtain such an elastic limit with steel bars subjected to a conventional heat treatment (ordinary quenching + tempering), it was necessary to use steels whose average chemical composition is equal to: 0.30% C, 0.62% Mn , 0.15 - 0.30% Si.

This shows the economy in elements C, Mn, Si that the process of the invention can provide.

Comparative tests of the two types of heat treatment (invention, conventional) carried out on steel bars of the same chemical composition (0.25% C, 0.40% Mn, 0.20% Si) showed a difference of elastic limit of the order of 80 MPa in favor of the treatment according to the invention, which clearly shows the improvement in quality that this treatment can provide.

The improvement in quality also generally results in better ductility (increase in elongation and the ability to resist bending).

2. In another series of tests, steel bars of variable diameters (10 to 60 mm) and an average chemical composition equal to: 0.35% C, 0.60% Mn and 0.20% were used If, the balance being made up of Fe containing the usual impurities.

At the exit from the last rolling stand, these bars were also subjected to rapid quenching with self-absorption, in accordance with the process of the present invention.

The bars thus treated had an elastic limit of the order of 414 MPa.

In order to obtain such an elastic limit with steel bars subjected to a conventional heat treatment (ordinary hardening + tempering), steel bars whose chemical composition is equal to: i / 1/1 * 3 had to be used. here 5.- îl

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| C% Mn% Si% Nb% V% - bars of diameters 10 mm 0.43 0.70 to 0.20 0.01 25 mm 0.50 0.90 * - bars of diameters 28 mm to 0.42 0, 90 0.20 0.025 26 mm - bars with diameters 40 mm 0.39 0.70 to 0.20 - 0.035 60 mm O, 43 O, 90 c

We also note the savings in C and Mn elements that the process of the invention leads to and in addition the savings in alloy elements such as Nb and V which are necessary in the case of conventional heat treatment.

As regards the possibilities of reducing the number of grades to be produced in the steelworks to obtain the desired qualities, it can be seen that obtaining such an elastic limit with a conventional heat treatment requires three different chemical compositions to cover the entire range of diameters available, whereas with the treatment of the invention, a single composition is sufficient.

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Claims (3)

1. Process for manufacturing rolled steel products, with elastic limit between 250 and 500 megaPascals, characterized in that from the austenitization temperature of a rolled steel product containing from 0.20% to 0, 40% C, 0.30% to 0.70% Mn and 0.10% to 0.30% Si, this product is subjected to intense cooling, by means of a fluid, in such a way that a martensitic and / or bainitic quenching of the surface layer of all or part of the product is carried out, while the non-quenched part of the rolled product remains at a temperature sufficient to enable subsequent tempering of the surface layer of martensite and / or bainite. 2. Method according to claim 1, characterized in that intense cooling is applied to the rolled product as soon as it leaves the finishing stand of the rolling mill. 3. Method according to either of Claims 1 and 2, characterized in that a laminated product with an elastic limit of between 400 and 500 megaPascals is produced, starting from • x from a steel containing 0, 30% to 0.40% C, 0.50% to 0.70% Mn and 0.10% to 0.30% Si. X>