GB2102318A - A process for the production of high-strength wire-rod steel suitable for direct drawing - Google Patents

Abstract

In the process the composition of the original steel is restricted to a carbon range of 0.68-0.76% by weight and to a chromium range of 0.20-0.50% by weight, and the molten steel is continuously cast in billets while solid steel in powder or granular form is added to the mould in quantities between 0.1 and 5% by weight of the liquid steel cast.

Description

SPECIFICATION A process for the production of high-strength wire-rod steel suitable for direct drawing The invention relates to a process for the production of high-strength wire-rod steel suitable for direct drawing.

The invention consists essentially in the continuous casting of billets of wire-rod steel lightly alloyed with chromium and manganese, by adopting a technique -- known in itself - of adding to the liquid metal a small quantity of solid metal in powder or granular form, in order to decrease segregation and modify solidification structures. In this way, high-strength rod and wire can be made via a process starting from continuous casting, while avoiding the defects encountered in this type of product when continuous casting is used.

The present invention refers to a process for the production of high-strength wire-rod steel suitable for direct drawing. More precisely, it refers to the solution of the technical problem of using continuous casting for the production of billets from which to obtain high-strength wire rod that can be drawn without need for intermediate heat treatments, such as patenting. High strength in this context means ultimate tensile strengths of over 1000 Nxmm-2 for the wire rod.

Directly drawable high-strength wire-rod steels are known, but because of their composition it is very difficult to exploit the advantages offered by continuous casting techniques to make them. For instance, U.S. Patent Specification No. 4,123,296 describes a high-strength, directly-drawable wire rod containing from 0.65 to 0.90% carbon and from 0.15 to 1.5% chromium. The patent specification makes no mention of the steel casting techniques, nor are the problems deriving from segregation of carbon during solidification considered, except very briefly. This indicates that the steel as per the U.S.

specification is cast conventionally in small ingots. In fact, it is seen, on the one hand, that the composition of the steel provides for carbon values which are even in excess of that of carbon in the eutectoid (around 0.80% in these steels), and at the same time it provides for chromium values such as to lower the carbon content of the eutectoid even further, yet on the other hand it is known that during solidification of continuously cast billets, segregation of carbon in the axial zone of the billet is considerably more marked. Indeed, with this type of steel, carbon in the axial zone can easily reach concentrations in excess of 1%, thus rendering the zone decidedly hypereutectoid (Trans: ISI J vol. 12, 1972, page 102 et seq.).

Notwithstanding intermediate heat and mechanical treatments, this segregation remains in the wire rod, though perhaps in an attenuated form.

It is a known fact that in the range of cooling rates that are commercially practicable in wire-rod production, there is marked modification of the Continuous Cooling Transformation characteristics of the hypereutectoid composition, with formation of intergranular proeutectoid cementite before the pearlite. This is confirmed not only by the patent specification mentioned above, where it states that the upper limit of the carbon content is determined by the need to avoid the formation of proeutectoid cementite, but also by the University of Michigan's "Transformation and Hardenability of Steels Symposium", 1 967, pages 1 55-157, and by the Atlas of the Max Planck Institute, Vol. 2, Plates 225 E,241 E,120Nand121 L.

The presence of intergranular cementite in a prevalently pearlitic matrix prevents the wire-rod obtained in this manner from being direct drawn; it must be patented to eliminate the intergranular cementite, because of the formation of characteristic chevron cavities in the cementite lattice during cold drawing. These defects result in the typical cuppy fractures of the wire in the subsequent drawing stage and during other operations and/or during use. It is thus impossible to market and use the product at the desired and required quality levels. Since the high strengths of these steels derive precisely from the high carbon content, it would seem, therefore, that with the marked tendency of the carbon to segregate during continuous casting, this process would be very difficult to use.

Experiments made during the research that led up to the present invention confirmed that there are serious drawability problems with high-strength and hence high-carbon (C -- 0.80%) wire rod because this is likely to break during drawing, as a direct consequence of axial segregation of the material.

The object of the present invention is to provide a solution that minimises the problem of carbon segregation during solidification of continuously-cast steel, so as to avoid the formation of intergranular proeutectoid cementite in the wire rod.

Another object of the invention is to modify the solidification structure of the continuously cast steel, so as to minimise or completely eliminate such typical difficulties as "solidification bridges" and central porosity which also lead to local segregation.

Yet another object of the invention is to provide a steel composition, albeit within known compositional ranges, which makes possible continuous casting, while avoiding the problems deriving from segregation and ensuring high strengths in the wire rod and in the products made therefrom.

According to one aspect of the present invention, it is necessary to take action in the continuous casting process, so as to minimise carbon segregation in the axial zone of the billet.

According to the invention there is provided a process for the production of high-strength wire-rod steel suitable for direct drawing, wherein the composition of the original steel is restricted to a carbon range of 0.680.76% by weight and to a chromium range of 0.200.50% by weight, and wherein the molten steel is continuously cast in billets while solid steel in powder or granular form is added to the mould in quantities between 0.1 and 5% by weight of the liquid steel cast.

There are also other problems associated with the continuous casting process which arise during the manufacture of wire rod and which may complicate matters. In fact, since the final product is a round bar of small cross-sectional area, it is evident that it would be advantageous to produce continuously cast billets of small cross-sectional area, so as to reduce subsequent rolling costs.

However, because of the use of constant discharge nozzles, and hence without the aid of stopper rods, the casting of small-section billets calls for a degree of overheat (AT) vis-a-vis the solidification temperature, that is much greater than that needed for casting large-sized blooms. This overheat increases the solidification times of the billet and owing to the solidification trend in continuous casting, aggravates the segregation problem.

It has been found that in the specific case of wire-rod billets, use of the technique of adding metallic powders or grains to the mould -- known, for example, from the Italian Patent Applications Nows. 48510 A/73, 49007 A/77 and 49008 A/78 made by some of the same applicants -- not only accelerates solidication, as indicated in these applications, by absorbing part or all of the overheat, but also provides a considerable number of solidification nuclei which permit an equiaxed fine grain structure to be obtained, thus preventing the formation of large columnar crystals extending from the skin of the billet towards the centre.In this way it is possible to avoid the formation of the "solidification bridges", which are the cause of segregation in restricted zones and to decrease markedly the formation of axial porosity and cavities. The overall result is a notable reduction in the intensity and distribution of segregation, plus - as mentioned above - the formation of a solidification structure that is more favourable to the attainment of a better end product.

Different quantities, types and compositions of powder or granular additions can be adopted, depending on the type of continuous casting machine, the format of the end product and the quality thereof.

By and large, it can be said that the size of the powders can be between 0.01 and 1 mm, while the grains can be in the 0.1--2.5 mm range. The amount of metal added is in the range of 0.1 to 5% by weight of steel cast, while the carbon content of the powder or granular additions is between 0.05 and 2% by weight.

The particle size ranges given have no influence from the process point of view. The lower figure in the case of the powders merely indicates a limit for economic, easy use, while the upper figure for grains indicates the limit of easy solubility of the solid in the molten metal.

The range given for the amount of metal added is significant in the sense that less than 0.1% has little effect on the quality of the product, while over 5% of solid metal with normal billet formats and steel overheats, is not wholly dissolved in the molten mass, and so points of discontinuity are formed.

The small quantity of powder added does not substantially alter the composition of the steel.

However, if marked carbon segregation seems likely owing to billet format, steel composition or overheat, the addition of low or very low carbon solid additions towards the upper limit indicated greatly favours redistribution and homogenisation of the carbon in the billet.

Finally, in the manner already known, solid additions can also be used to introduce alloy elements such as Cr and Al, for instance, to adjust composition or even for alloying proper.

According to another aspect of the present invention, it is desirable to have a particular molten steel composition, so as to further reduce segregation problems, while still maintaining high mechanical strength in the wire rod and in products derived therefrom. Hence the carbon content is restricted to the 0.680.76% range and the high mechanical strength is restored by the addition of between 0.20 and 0.50% chromium.

The reasons for these analytical limits stem from the fact that below 0.68% carbon unsatisfactory mechanical strength values are obtained, while over 0.76% carbon can give rise to undesirable segregation phenomena in the wire rod. As regards the chromium, less than 0.20% does not ensure the desired results, while with over 0.50% the composition of the carbon in the eutectoid drops further, so the inevitable, albeit minimal, segregation of carbon could lead to the formation of intergranular proeutectoid cementite when the wire rod is cooled, thus nullifying the advantages of the present invention.

A number of commercial heats were prepared and drawn into 11.5 mm diameter wire rod and about 4 mm diameter wire. The steel compositions were:

Heat C% Cr% Mn% Si% Al% S% P% A 0.65 0.40 0.80 0.25 0.065 0.010 0.019 B 0.70 0.30 0.78 0.25 0.062 0.011 0.018 C 0.71 0.50 0.80 0.25 0.064 0.010 0.019 D 0.75 0.40 0.80 0.26 0.061 0.010 0.020 E 0.75 0.60 0.80 0.25 0.064 0.010 0.019 F 0.80 0.30 0.81 0.30 0.050 0.013 0.018 The results obtained after rolling and direct drawing were::

Wire rod Wire Strand 1/2 Draw- Drawing Heat R Nxmm- Z% R Nxmm- R Nxmm- abillty defects A 980 43 1750 1750 Good None B 1065 43 1845 1835 V. good None C 1130 33 2020 2010 V. good None D 1180 33 2040 2020 V. good None E 1200 30 - - Poor Cuppy fracture F 1210 30 - - Poor Cuppy tracture As is evident, outside the very narrow range of analysis indicated earlier, it is not possible to obtain either the desired quality results nor to direct draw the wire rod. In particular, it can be seen that with 0.65% carbon, even with a relatively high chromium content, the desired strength is not achieved either in the wire rod or in the drawn wire, while with more than 0.76% carbon or more than 0.50% chromium, drawability becomes poor and wire-breaks can occur.

Claims (4)

1. A process for the production of high-strength wire-rod steel suitable for direct drawing, wherein the composition of the original steel is restricted to a carbon range of 0.68-0.76% by weight and to a chromium range of 0.20-0.50% by weight, and wherein the molten steel is continuously cast in billets while solid steel in powder or granular form is added to the mould in quantities between 0.1 and 5% by weight of the liquid steel cast.

2. A process according to claim 1, wherein the particle-size range selected from the solid steel is in the 0.01-1 mm range for powders and 0.1-2.5 mm range for grains

3. A process according to claim 2, wherein the carbon content of the solid steel added is in the 0.05-2% by weight range.

4. A process for the production of high-strength wire-rod steel according to claim 1 and substantially as hereinbefore described.