FR2510007A1 - PROCESS FOR THE PRODUCTION OF HIGH STRENGTH MACHINE YARN SUITABLE FOR DIRECT DRAWING - Google Patents

Abstract

THIS PROCESS PROVIDES A SOLUTION TO THE TECHNICAL PROBLEM CONSISTING OF APPLYING THE CONTINUOUS CASTING FOR THE PRODUCTION OF TICKETS FROM WHICH WE CAN OBTAIN HIGH-STRENGTH MACHINE WIRE, LIKELY TO BE THREADED WITHOUT THE NECESSARY SUBMISSION OF THERMIES. INTERMEDIARIES. THE COMPOSITION OF THE STARTING STEEL IS LIMITED TO A RANGE OF CARBON CONTENT OF 0.68 TO 0.76 BY WEIGHT AND TO A RANGE OF CHROME CONTENT OF 0.20 TO 0.50 BY WEIGHT, AND L MELT STEEL IS CONTINUOUSLY CAST IN THE FORM OF BILLETS, WHILE THE SOLID STEEL, IN THE FORM OF POWDER OR GRAINS, IS ADDED TO THE MOLD IN PROPORTIONS BETWEEN 0.1 AND 5 BY WEIGHT OF THE LIQUID STEEL CAST IRON .

Description

The present invention relates to a process for the production

  high strength wire rod steel, suitable for wire drawing

  More specifically, it provides a solution to the technical problem of applying continuous casting for the production of billets from which it is possible to obtain high-strength wire rod, capable of being

  drawn without the need to subject it to

  "High resistance" intermediate thermal elements in the

  present context means resistance to breakage by trac-

  tion of more than 1000 Nxmm-2 for the wire rod.

  We know high strength wire rods, tr.filables directly, but due to their composition, it is very difficult to exploit the advantages offered by continuous casting techniques to produce them For example, the patent of the United Unis N 2 4 123 296 describes a high-strength wire rod, directly wireable, which contains between 0.65 and 0.90% of carbon and between 0.15 and 1.5% of chromium The text of the specification of this patent makes no reference to the techniques of

  casting of steel, and the problems that result from segregation -

  tion of carbon during solidification are taken into account there only very briefly In our opinion, this means that the steel according to the patent of the United States is cast according to conventional techniques in small ingots Indeed, we see on the one hand that the composition of the steel provides for carbon levels which exceed even that of the carbon in the eutectoid (around 0.80% in these steels) and that at the same time, it provides for rates chromium suitable for further lowering the carbon content of eutecto Ide; on the other hand, we know that during the solidification of billets cast in

251 C 007

  continuous, the carbon segregation is much more marked in the axial zone Ie of the billet Indeed, with this type

  of steel, the carbon in the axial area can easily reach

  dre concentrations of more than 1%, this yes makes this area undoubtedly hypereutectoid (Trans ISI J, vol 12, 1972,

page 102 sqq).

  Despite intermediate thermal and mechanical treatments

  diaires, this segregation persists in the wire rod, well

  only in a form that may be attenuated.

  It is a known fact that in the range of speeds

  cooling yes are practicable from a standpoint like-

  cial in the production of wire rod, there is a

  marked fication of the characteristics of transformation by continuous cooling of the composition hypereutecto 1 of, with formation of cementite pro-eutectoid intergranular before perlite This is confirmed, not only by the patent cited, in which it is said that the upper limit of the content in carbon is fixed by the need to avoid the formation of pro-eutectoid cementite, but also by the publication of the University of Michigan, "Transformr: ation and Hardenability of Steels Symposium", 1967, pages 155-157 and p: ar Atlas du Max Planck Institut, vol 2, boards 225 B, 241 E, 120 N and 121 L. The presence of intergranular cementite in a mainly pearlitic matrix prevents the wire rod obtained in this way from being drawn directly: it must be

  lead soaked to remove interranular cementite.

  due to the formation of characteristic chevron cavities in the cementite network during cold drawing These defects result in typical wire-cut fractures during

  of the subsequent wire drawing phase, during other operations

  tions and / or use It is therefore impossible to market and use the product at the desired and required quality levels, given that the high strengths of these steels

  come precisely from the high carbon content, it feels

  would therefore blame that with the marked tendency of carbon to segregation during continuous casting, this process is

very difficult to apply.

  Experiments carried out in the course of research which have led to the present invention have confirmed that there are serious problems of suitability for drawing with high tensile wire rod and therefore with a high carbon content (C 0.80 %), because it is likely to

  break during wire drawing, as a direct consequence of segnega-

axial tion of the material.

  The object of the present invention is to provide a solution that minimizes the problem of carbon segregation during the solidification of continuously cast steel, so

  to avoid the formation of cementite pro-eutectolde intergranu-

wire in the wire rod.

  Another object of the invention is to modify the solidification structure of the continuously cast steel, so as to minimize or completely eliminate the typical difficulties such as "solidification bridges" and porosity.

  central, which also lead to local segregation.

  Yet another object of the invention is to provide a cor m-

  position of steel which, although (being in known ranges of dosage of constituents, makes possible continuous casting, while eliminating the problems resulting from segregation and ensuring high resistance of the wire rod and products

made from it.

  According to one aspect of the present invention, it is necessary to intervene in the continuous casting process, so as to minimize the carbon segregation in the

axial zone of the billet.

  There are also other problems associated with the continuous casting process, which arise during the production of wire rod and which can complicate matters. Indeed, being given that the final product is a round bar of small cross section, it is obvious that it would be advantageous to produce continuously cast billets of small cross section, so as to reduce the subsequent rolling costs, however, due to the use of constant discharge nozzles and, consequently, without the using cattails, casting billets

  small section requires, in relation to the solidification temperature

  cation, a degree of superheating (/ T) which is much higher than what is necessary for the casting of large caliber blooms This overheating prolongs the times of solidification of the billet and, because of the tendency to solidification

  in continuous casting, aggravates the segregation problem.

  It has been discovered that, in the particular case of billet-

  for wire rod, the application of the consistent technique

  to add powders or grains of metal to the technical mold

  that which is known for example from Italian patent applications No. 48510 A / 73, 49007 A / 77 and 49008 A / 78 filed by some of the authors of the present invention does nothing but accelerate solidification, according to this yes is indicated in these patent applications, by absorbing some - or all of the

  superheat heat but also provides a considerable number

  ble of solidification nuclei, this yes allows to obtain a fine grain structure Eéoiaxes, thus avoiding the formation of ros crystals of basaltic structure, extending from the skin of the billet towards the center In this way, it is possible to avoid the formation of the% "bridges: solidification" yes are at the origin of segregation in restricted zones, and to reduce in a marked way the formation of axoral porosity and cavities The overall result is a notable reduction of the intensity and distribution of segregation more like

  mentioned above the formation of a solidification structure

  cation which is more favorable for obtaining a better product

final.

  Different choices can be made, as to the quantities, types and compositions of the powder or mash additions, depending on the model of the continuous casting machine, the

  format of the final product and the quality of it.

  In general, it can be said that the size of the powders can be between 0.01 and 1 mm, while the greases: can be included in the range from 0.1 to 2.5 min. The proportion of metal added is of the order of 0.1 to 5% by weight relative to

  to cast iron, while the carbon content of additives

  powder or grain is between 0.05 and 2% by weight.

  The size ranges indicated have no influence

  from a process point of view The lower number, in the

  in the case of powders, simply indicates a limit for an application

  easy economic tion, while the upper figure relating to the grains indicates the limit of easy solubility of the material

solid in molten metal.

  The range given for the proportion of metal added is significant, in the sense that an addition of less than 0.1%

  has little effect on the reality of the oroduit, whereas an addi-

  tion of more than 5% solid metal, with the usual formats of bi Ilettes and under normal conditions of overheating, does not dissolve fully in the melt, which

  where discontinuity points are formed.

  The small amount of powder added hardly changes the composition of the steel. However, if a marked carbon segregation is to be feared, due to the size of the billets, the composition of the steel or the overheating, the addition of metal solid with low or very low carbon content, near the upper limit, greatly promotes the

  redistribution and homogenization of carbon in the billet.

  Finally, already known per se, the solid additions can also be used to introduce alloying elements, such as Cr and A 1 for example, in order to adjust the

  composition or even to get a good alloy.

  According to another aspect of the present invention, it is desirable to give a particular composition to the molten steel, so as to further reduce the problems of segregation,

  while maintaining high mechanical strength of the machined wire

  ne and products derived therefrom The carbon content is therefore limited to the range of 0.68 to 0.76% and the high resistance

  mechanics is restored by the addition of chromium in a pro-

  portion between 0.20 and 0.50%.

  The reasons for these analytical limits come from the fact that below 0.68% of carbon, unsatisfactory values of the mechanical resistance are obtained, while beyond

  0.76% carbon, undesirable phenomena may occur

  wheat segregation in the wire rod For chromium, an addition of less than 0.20% does not guarantee the desired results, while with more than 0.50%, the share of carbon in the composition of the eutectoid drops further, from which it follows that carbon segregation, inevitable although

  minimal, could lead to the formation of pro-eutectic cementite

  intergranular roof when the wire rod is cooled,

  which would negate the advantages of the present invention.

  A number of commercial baths have been prepared and then drawn into 11.5 mm diameter wire rod and approximately 4 mm diameter wire. The compositions

steels were as follows.

  Fusion bath O% Cr%? N% Sif A 1 S% J%

  A 0.65 0.40 0.80 0.25 0.065 0.010 0.019 Q

  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, CO O

  B 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 the following Bain del Machine wire R Nxmm 2 Wire R Nxmm-2

33-2020

  Strand 'R Nxmm-2 Wire drawing ability Good Very good Very good Very good Poor e Bad Wire drawing faults None None None Cup fracture Cup fracture As can be seen' obviously, very analytical is not possible, narrow which has been indicated hii get the results outside the ganmme previously it vo-ulus as to the

  quality, nor to draw the wire rod directly In particular

  bind, it is visible that with 0.65% carbon, even with a relatively high chromium content, the desired resistance is not achieved, both in the wire rod and in the drawn wire, while with more rle 0.76% carbon or more than 0.50% chromium, the drawing ability becomes poor and may

produce wire breaks.

fusion A B C D F

Claims (3)

CLAIM -   1. Process for the production of high-strength wire rod suitable for direct drawing, characterized in that the composition of the starting steel, generally known per se, is limited to a range of the carbon content of 0.68 to 0.76% by weight and at a range of chromium content from 0.20 to 0.50% by weight, and in that the molten steel is continuously cast in the form of billets, while solid steel, in the form of powder or grains, is added to the mold in proportions of between 0.1 and 5% by weight of the cast iron of liquid steel.   2. Method according to claim 1, characterized in that   that the particle size range chosen for solid steel is   kills between 0.01 and 1 mm for powders and between 0.1 and 2.5 mm for grains.   3 Method according to claim 2, characterized in that the carbon content of the added solid steel is in the range of 0.05 to 2% by weight.