GB2087927A - Manufacture of rolled steel products having good weldability, a high elastic limit, and toughness at very low temperatures - Google Patents

Abstract

A rolled steel product (concrete- reinforcing rod) containing at most 0.16% C, at most 2.0% Mn, and at least 0.03% Al, and optionally containing up to 0.55% Si, up to 0.05% Nb, up to 0.1 V, and up to 10% Ni, is reheated to austenitization temperature, held at this temperature for a time insufficient to cause grain- coarsening, and subjected to intense surface cooling. As a result the product has an outer region of martensite, which is then tempered as the core transforms to fine ferrite- pearlite.

Description

SPECIFICATION Manufacture of rolled steel products having good eldabilky, a high elastic limit, and toughness at very low temperatures The present invention relates to a method of manufacturing rolled steel products, in particular concrete-reinforcing rods, having good weldability, a high elastic limit, and toughness at very low temperatures.

Concrete-reinforcing rods which are manufactured at present with an elastic limit greater than 400 N/mm2 have a very low degree of toughness. Their transition temperature for the Charpy V-notch toughness test at 35 J/cm2 is approximately +200 C. These products therefore have only a low resistance to brittle fracture at low temperatures.

In the past it has not been required to produce a specification for the Charpy V-notch test at 35 J/cm2 and at temperatures of approximately -1 960C in respect of rolled steel products such as concrete-reinforcing rods. However, recent developments in manufacturing techniques and in particular in the storage of liquefied gases have shown the need for rolled steel products which are resistant to intense cold. For safety reasons it is therefore the practice to provide liquefied gas storage tanks with a reinforced concrete casing in the same way as the safety measures carried out in the field of nuclear reactors.

The concrete-reinforcing rods provided for reinforcing the casings of liquefied gas tanks whose temperature ranges between -500C and -1 960C must have satisfactory weldability in addition to sufficient core toughness, thus requiring a carbon content of less than 0.2%. The known deformed bars which contain 0.16 to 0.2% C are produced by cold twisting, which provides them with a satisfactory elastic limit but leaves them with a low degree of toughness, particularly at low temperatures.

It has therefore been attempted to combine, for example, during the manufacture of concretereinforcing rods, a limitation of their carbon content to 0.20% with a treatment comprising intense surface cooling on discharge from the rolling mill, and self-tempering, which enables the production of weldable and tough reinforcing rods. Deformed bars of this type may have Charpy V-notch (35 J/cm2) transition temperatures of approximately --500C.

It has also been noted that a reinforcing rod manufactured from a 9% Ni steel, which has been subjected to a double normalizing operation followed by tempering, or quenching followed by tempering, has a minimum Charpy V-notch (35 J/cm2) toughness at -1 960C.

What is desired is therefore a method of manufacturing rolled steel products which satisfy the above-mentioned criteria, preferably from a steel containing a minimum of costly alloying elements, thereby reducing the cost price of the rolled steel products.

The present invention provides a method in which use is made of a steel containing at most 0. 16% C, 2.0% Mn, 0.55% Si, and 0.03% Al at least, and possibly up to 0.05% Nb, 0.1% V, and 10% Ni, rolled under normal conditions or preferably having been subjected to thermo-mechanical rolling, and heat treated in the following manner: The rolled product (particularly rod) obtained in this way is reheated in a second heat treatment to austenization temperature in such a manner as to ensure a re-heating speed which is sufficient to obtain a high number of crystal nuclei for grain refinement.

The holding time at the austenization temperature must be very short in order to prevent a reduction in the number of the crystal nuclei and a coarsening of the grain.

The product is then subjected to intense surface cooling in order to obtain a product having a ferrite and pearlite core structure with a very fine grain.

The heat retained in the core of the product is sufficient to re-heat, after the above cooling, the surface zone quenched during the intense surface cooling, so as to provide tempering and to form tempered martensite.

Rapid cooling of the product after the above treatment also enables recrystallization of the grains to be prevented in the core.

The invention will be described further, by way of example, with reference to the accompanying drawing, which schematically illustrates apparatus for heat treating rolled steel rod.

The aim of the heat treatment according to the invention is to achieve an extremely fine grain in order to ensure a high elastic limit and a good degree of toughness at very low temperatures.

Bearing in mind that it is desired to produce a very fine grain it is preferable to use a steel which already has a fine-grained structure in the as-rolled condition.

A heat treatment in accordance with the invention may be carried out by installing a rapid induction-heating device followed by a cooling device. The rolled steel product passes continuously through the two devices in sequence, thereby by being subjected to a thermal cycle consisting of heating to austenization temperature followed by intense surface cooling. It is possible to carry out several such thermal cycles consecutively. In the accompanying drawing, rolled steel rod 1 intended for concrete reinforcement travels continuously through the heat treatment apparatus on rollers 2. In an induction furnace 3 the rod I is rapidly heated throughout its cross-section to austenization temperature.The rod remains at this temperature only briefly, entering a cooling device 4 comprising upper and lower banks of cooling sprays by which the surface of the rod 1 is rapidly cooled into the martensite range. Subsequently, during air cooling, the core of the rod transforms to ferrite-pearlite while the heat from the core tempers the quenched surface. In order to prevent grain-coarsening of the ferrite-pearlite, the rod may be subjected to further rapid cooling.

The concept on which the present invention is based 5 to combine the beneficial effects which may be obtained separately by means of the judicious choice of chemical elements incorporated in the steel in determined amounts with the effects of a particular heat treatment.

It should be stressed that the particular heat treatment which is part of the invention is designed to produce an extremely fine grain structure in the core of the product, given that toughness at very low temperatures must be ensured in the core of the product.

The chemical composition of the product is selected in accordance with results obtained from numerous tests which have shown that the lower the desired transition temperature the lower the carbon content should be.

The carbon content is for example preferably limited to a maximum of 0.08% for a Charpy V-notch (35 J/cm2) transition temperature down to -1 400 C.

A manganese content of approximately 1 .7% provides the steel with the adequate strength whilst improving its toughness, whilst 0.3% of silicon may be added to increase its strength.

Fine-grain steel is killed with aluminium in order to improve weldability and to considerably diminish its ageing tendency. Grain refinement also increases the elastic limit as well as the degree of toughness.

Niobium (up to 0.05%) and/or vanadium (up to 0.1 ó) may possibly be added to ensure a high elastic limit, in particular when large diameters are involved.

Nickel may be added to ensure transition temperatures of less than -1 400C. To obtain a Charpy V-notch (35 J/cm2) toughness down to -1 96 C, the Ni content is preferably 5 to 10%.

In accordance with the invention the product is subjected after rolling to a particular heat treatment designed to refine the grain.

On discharge from the rolling mill the product is preferably subjected to intense surface cooling.

The advantages of the method of the invention are clearly shown from the six experiments described below: EXPERIMENT 1 A naturally hard steel for concrete reinforcing rods (C = 0.35% approx.) provided satisfactory tensile characteristics, in particular an elastic limit exceeding 400 MPa. However, the Charpy V-notch transition temperature at a power level of 35 J/cm2 was +200 C. This steel did not therefore have any toughness at low temperature. Its weldability was mediocre.

EXPERIMENT 2 A steel having a carbon content limited to G. 18%, which had been subjected to heat treatment in accordance with the invention, also had satisfactory tensile characteristics. A clear improvement with respect to Experiment 1 was observed with respect to elongation and in particular with respect to the transition temperature, which decreased to -650C. The steel was observed to be weldable.

EXPERIMENT 3 The use of a steel having a chemical composition according to the invention, but without the application of the heat treatment according to the invention, led to mechanicai properties which were inadequate from the point of view or elasticity and strength. The elongation was increased. The transition temperature, even without the treatment, had the same ievel as that of Experiment 2, in which the treatment according to the invention was applied.

EXPERIMENT 4 The combination of a chemical composition according to the invention and the heat treatment according to the invention enabled a double improvement, i.e.

- satisfactory mechanical properties and an increased elongation, - a transition temperature decreasing to a very low temperature (--1400C).

EXPERIMENT 5 A 9% Ni steel provided a Charpy V-notch (35 J/cm2) transition temperature of -500C in the asrolled condition.

EXPERIMENT 6 The same 9% Ni steel, which may normally only be provided with toughness down to -1 960C by means of a costly heat treatment (double normaiising + tempering or quenching + tempering), enabled the achievement of a transition temperature of-i 960C when the much less costly heat treatment according to the invention was applied.

The six experiments are summarised in the following Table.

Example No. 1 2 3 4 5 6 C=0.35% C=0.18% Semi- Semi- C = 0.08% C = 0.08% 9% Ni 9% Ni Type of Steel killed killed Killed Killed Killed Killed Heat treatment No Yes No Yes No Yes in acc. with invention Elastic limit 435 485 330 500 850 700 (MPa) Tensile strength 640 580 510 590 980 920 (MPa) Elongation* 19 32 33 36 14 21 (%) Transition temp. +20 -65 -65 -140 -50 -196 ( C) Charpy V toughness at 35 J/cm2 * Gauge length = 5 x diameter.

Although the above description is mainly directed to the production of concrete-reinforcing rods, the method described may be equally well applied to other merchant bars such as smooth, flat, square, and angle bars, to profiled sections, and to sheet, when it is desired to combine the properties of weldability, high elastic limit, and toughness at very low temperatures in the same product.

Claims (10)

1. A method of manufacturing a rolled steel product having good weldability, a high elastic limit, and toughness at very low temperatures, the method comprising the steps of (a) providing a rolled steel product consisting of a steel containing a maximum of 0.1 6% C, a maximum of 2.0% Mn, a minimum of 0.03% Al, optionally a maximum of 0.55% Si, optionally a maximum of 0.05% Nb, optionally a maximum of 0.1 % V, and optionally a maximum of 10% Ni; and (b) subjecting the said product to a grain-refining heat treatment comprising reheating the rolled product to austenization temperature, holding the product at this temperature only for a period which is sufficiently short to prevent grain-coarsening, and immediately thereafter subjecting the product to intense surface cooling.

2. A method as claimed in claim 1 , further comprising repeating the said heat treatment at least once.

3. A method as claimed in claim 1 or 2, including lowering the transition temperature by lowering the carbon content of the steel.

4. A method as claimed in any of claims 1 to 3, including lowering the transition temperature by increasing the nickel content of the steel.

5. A method as claimed in any of claims 1 to 4, in which the product to be subjected to the said heat treatment is provided by hot rolling in a rolling mill followed by intense surface cooling on discharge from the rolling mill.

6. A method as claimed in any of claims 1 to 5, further comprising, after the said heat treatment, allowing the product to cool so that the core of the product transforms to ferrite and pearlite while heat from the core tempers the surface zone of the product, and subjecting the product to rapid cooling in order to prevent recrystallization of the core.

7. A method as claimed in any of claims 1 to 6, in which the steel contains a maximum of 0.08% C.

8. A method as claimed in any of claims 1 to 7, in which the steel contains 5 to 10% Ni.

9. A method as claimed in any of claims 1 to 8, in which the product is steel concrete-reinforcing rod.

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