GB2161831A - Steel composition for wire rod - Google Patents

Abstract

The steel comprises 0.90 to 1.20 wt.% carbon, 0.5 to 1.0 wt.% manganese, 0.05 to 0.25 wt.% vanadium, and optionally 0.15 to 1.0 wt.% chromium or 0.20 to 2.0 wt.% silicon or both, the balance being iron and incidental elements or impurities.

Description

SPECIFICATION Steel composition for wire rod This invention relates to steels of high carbon content, particularly but not solely for the production of hot rolled bar and wire rod which is cooled in a controlled manner from the rolling temperature or is subsequently heat-treated and cooled in a controlled manner, typically by the process known as patenting, or both.

Steels for the production of high tensile strength wire are prepared for cold working by thermal treatments designed to produce the metallurgical structures of ferrite and pearlite or ferrite and bainite or mixtures of both. In this condition the strength of the steel is a function of the method of heat treatment and of the carbon content. As the carbon content is increased so is the strength raised due to an increase in the amount of pearlite or bainite and a commensurate reduction in the amount of ferrite in the microstructure. At the eutectoid composition the structure consists of pearlite and/or bainite only and any further increase in carbon content causes the formation of pro-eutectoid cementite at grain boundaries which has a deleterious effect on mechanical properties.

It is known that suitable modifications to the thermal treatment, particularly the manner of cooling, can suppress the formation of pro-eutectoid cementite, thus enabling advantage to be taken of the continued increase in strength which occurs with increase in carbon content above the nominal eutectoid composition. There is, however, a limit to which the suppression of the pro-eutectoid cementite can be achieved in practice by thermal treatment alone. Thermal treatment, typically by the patenting process, causes a refinement of the pearlite so that steel so treated has an increased strength in comparison to that obtained by controlled cooling from the rod rolling temperature. Such treatment, however, is expensive and a means of increasing the strength of controlled cooled rod by the addition of small amounts of alloying elements to the steel has been widely investigated.

The addition of chromium is known to cause an increase in strength by delaying the commencement of transformation of the steel from austenite to pearlite so that, during continuous cooling, the transformation occurs at a lower temperature at which a more refined pearlite is produced. The addition of silicon has a similar effect to chromium and additionally raises the strength of the ferrite component of pearlite by its effect in solid solution.

Vanadium has a lesser influence on transformation characteristics but causes an increase in strength by a precipitation hardening mechanism. This mechanism offers the particular advantage that, in addition to the increase in strength by the refinement of pearlite, whether by thermal treatment by patenting or by the use of chromium and/or silicon additions to the steel, a further increment in strength may also be obtained.

In our investigations of the effectiveness of the addition of these elements to steel we have discovered that vanadium is also effective in preventing the formation of pro-eutectoid cementite at even higher carbon contents than the maximum for which thermal treatment alone is effective. The addition of vanadium can thus permit the exploitation of the higher strengths which can be obtained by these higher carbon contents in addition to the increase in strength obtained from pearlite refinement and from the precipitation hardening effect of the vanadium.

This invention provides a high carbon steel containing vanadium in addition to conventional levels of manganese and silicon. In addition the steel may also contain an increased silicon content or chromium or both.

We have discovered that vanadium is beneficial in inhibiting the formation of pro-eutectoid cementite in steel particularly at carbon contents greater than 0.85 wt %.

According to the invention, there is provided a steel comprising 0.90 to 1.2 wt.% carbon, 0.5 to 1.0 wt.% mangese, and 0.05 to 0.25 wt.% vanadium, the balance being iron and incidental elements and impurities.

The steel may further comprise 0.15 to 1.0 wt.% chromium (e.g. 0.2 to 1.0 wt.%) and/or 0.20 wt.% silicon (e.g. 0.25 to 2.0 wt.%). The vanadium content may, for example, be 0.1 to 0.2 wt. %.

With steel compositions according to the invention it is possible to obtain hot rolled bar or wire rod which after controlled cooling or subsequent heat treatment has an increased strength and refined microstructure in which the unwanted cementite phase is suppressed.

The invention will be described further, by way of example only, with reference to the accompanying drawings and photographs, in which: Figure 1 is a graph of hardness versus transformation temperature for Steels 6 to 10 in Table 1 below: Figure 2 shows optical micrographs (x800) of Steels 6,8, and 10 (Table 1) isothermally transformed at 560"C (a) zero V. (b) 0.10 V, and (c) 0.21 V; Figure 3 shows optical micrographs (x550) of steels transformed at 650 C: (a) Steel 11, (b) Steel 12, (c) Steel 13, (d) Steel 14, (e) Steel 15, and (f) Steel 9; Figure 4 shows optical micrographs (x630) of continuously cooled steels: (a) Steel 16, (b) Steel 17; Figure 5 shows optical micrographs (x630) of isothermally transformed steels: (a) Steel 16, (b) Steel 17; Figure 6 shows optical micrographs (x630) of continuously cooled steels: (a) Steel 18, (b) Steel 19; Figure 7 shows optical micrographs (x630) of isothermally cooled steels; (a) Steel 18, (b) Steel 19; and Figure 8 shows optical micrographs (x630) of furnace cooled steels: (a) Steel 18, (b) Steel 19.

Initial trials were carried out on wire rods having the compositions (in wt.%) identified by steel numbers 1 to 5 in Table 1 below. The wire rods were produced by hot rolling and cooling on a commercial rod rolling line incorporating state-of-the-art conventional controlled cooling practices. A production plain carbon, unalloyed steel (steel 1) was used as a basis for comparison and the mechanism properties obtained on these initial trial rods are given in Table 2 below.The tensile strengths of the rods increased in the order: plain carbon (steels 1 and 2); chromium alloyed (steel 3); vanadium alloyed (steel 4); and chromium plus vanadium alloyed (steel 5). The effect of pearlite refinement by the chromium was less than the effect of precipitation hardening by the vanadium and the combination of the two effects resulted in the greatest increment in tensile strength.These relative effects were confirmed by metallographic examination of the microstructures of the rods and it was noted in this examination that in the vanadium containing steels the higher tensile strengths had been obtained despite the presence of a greater amount of pro-eutectoid ferrite than the non-vanadium containing steel.

The trial rods were subsequently drawn to 4.22 mm diameter wire using conventional wire drawing practices. No significant differences in work hardening behaviour between the rods were noted and the wires had tensile strength differences directly comparable with those of the rod. Physical properties obtained with the wire are given in Table 3 below and show that no loss in ductility was observed in the alloyed materials in comparison with the plain carbon steel.

The effect of increasing the vanadium content in hypoeutectoid steels of otherwise carefully controlled composition was studied using steels 6 to 10 of Table 1. After austenitising at 10500C for 300 seconds these samples were isothermally transformed at seven different temperatures between 430"C and 680"C.

Hardness measurements of the steels were then made, the results for which are shown graphically in Figure 1. These results showed a clear relationship between the vanadium content and the hardness obtained at each transformation temperature and that the greatest hardness is obtained at a temperature of 5603C. Micro-examination of the samples showed the microstructures after transformation at this temperature to consist of pearlite, of which less than 5% could be resolved by optical metallography at a magnification of x550, and grain boundary ferrite the amount of which increased with increased vanadium content. At lower transformation temperatures the structure contained bainite in addition to pearlite. At higher transformation temperatures the pearlite was more coarse.

The observation of an increase in the amount of grain boundary ferrite with increase in vanadium content is shown in the photomicrographs in Figure 2. This confirmed that the presence of a greater proportion of grain boundary ferrite than expected in the vanadium containing steels used in the initial trials was caused by the presence of vanadium.

The level of carbon in steels for wire rod is chosen to be not greater than that which would cause the formation of pro-eutectoid cementite, which is deleterious to mechanical properties.lt is known, however, that if the level of carbon is increased above the eutectoid composition the strength of pearlite is raised.

If the formation of pro-eutectoid cementite could be prevented, nominally hypereutectoid rod steels could be produced with higher strengths.The effect of vanadium on nominally hypereutectoid steels was therefore studied using steels 11 to 15 to Table 1. These high purity steels were prepared without the addition of chromium in order to provide a comparison with the plain carbon rod steels normally used for wire production. These steels, together with steel 9 as a control, were austenitised at 1050it for ten minutes and subsequently isothermally transformed at 650cm to 620or. The microstructures of the steels transformed at 6503C are shown in Figure 3.Similar but finer microstructures were observed in the samples transformed at 620so. The steels which contain no vanadium (steels 11 and 12) contained pro-eutectoid cementite distributed as grain boundary films.The steels which obtain 0.15 wt.% vanadium contained no pro-eutectoid cementite. In view of these results further additions of vanadium, chromium, and silicon, both singly and in combination and in a range of carbon contents from the nominal eutectoid composition of 0.85 wt % to 1.02 wt.% were made.

The transformation products of these steels were examined following austenitisation at 1050 C and continuous cooling at a rate of between 10 C and 23"C per second at 725-C and also following austenitisation under the same conditions and isothermal transformation at 560 C. These conditions were chosen to give structures consisting of a refined pearlite of optimum hardness and strength which would represent the characteristic requirement for practical application of the steels.

Steels 16 and 17 were of nominal eutectoid composition, but with the addition of 0.15% vanadium in steel 17. Figures 4a and 4b show the continuous cooling transformation microstructures and Figures 5a and 5b show the isothermal transformation microstructures, after etching to reveal ferrite and pearlite and also after etching to reveal the presence of grain boundary pro-eutectoid cementite. No grain boundary pro-eutectoid cementite was present in either of the steels, but the ferrite stabilising effect of the vanadium is illustrated by the presence of a greater proportion of grain boundary pro-eutectoid ferrite in the microstructures of steel 17 under both conditions of transformation.

In the nominally hypereutectoid steel which contains no vanadium (steel 18), grain boundary pro-eutectoid cementite was present in both the continuously cooled and isothermally transformed condition as illustrated in Figures 6a and 7a. The nominally hypereutectoid steel containing 0.15 wt.% vanadium (steel 19) contained no grain boundary cementite in either the continuously cooled or the isothermally trans formed condition and, indeed, the ferrite stabilising effect of vanadium resulted in the presence of some grain boundary pro-eutectoid ferrite, particularly in the isothermally transformed condition.These microstructural features are illustrated in Figures 6b and 7b.

As further confirmation of the effect of vanadium on the suppression of cementite formation, specimens of steels 18 and 19 were also furnace cooled to maximise the available time for carbide rejection at the grain boundaries. Figure 8a shows the complete and substantial networks of cementite present in steel 18, whereas only intermittent, discontinuous grain boundary cementite existed in steel 19, Figure 7b. Without the addition of vanadium steel 19 would also be expected to exhibit continuous grain boundary cementite networks as the slight difference from steel 18 in carbon content would be compensated for by the difference in manganese level.

The trials carried out as described indicated that microstructures could be obtained in steels of 0.85 wt.% carbon and greater which, by the addition of vanadium, were free from deleterious grain boundary cementite. Hardness tests carried out on the samples confirmed that an increase had been obtained above that of plain carbon steel of nominal eutectoid composition. Also, the microstructures were such as to provide a material suitable for drawing into wire without the limitations imposed by the presence of pro-eutectoid cementite.

To ascertain whether even higher hardness and tensile strength could be achieved whilst retaining the optimum microstructure, samples of steels 20 to 25 were made with controlled additions of silicon and chromium, with and without combination with vanadium and higher carbon content. Additional increments in hardness were obtained by the addition of the silicon and chromium and, in combination with the vanadium and higher carbon, steels with hardnesses well in excess of those obtained from a plain carbon steel of eutectoid composition were achieved. The hardness and equivalent tensile strength obtained after isothermal transformation at 560"C for steels 11 to 25 inclusive are given in Table 4 below.In all cases in which the steels containing vanadium had been transformed to produce structures of fine pearlite, either by continuous cooling or by isothermal transformation, no harmful networks of grain boundary cementite were in evidence even at carbon contents in excess of the nominal eutectoid composition.

From the tests which have been carried out it appears that a preferred steel composition has a carbon content of 0.9 to 1.20 wt.%, a manganese content of 0.5 to 1.0 wt.%, and a vanadium content of 0.05 to 0.20 wt.%. In addition, the steel may advantageously contain 0.15 to 1.0 wt.% chromium and/or 0.20 to 2.00 wt.% silicon. The balance will be iron and incidental elements and impurities, silicon being considered as an incidental element when present in an amount below 0.20 wt.%.

TABLE I STEEL C Mn Si Cr V No.

1 0.79 0.60 0.22 0.05 2 0.80 0.73 0.27 0.03 3 0.77 0.73 0.28 0.39 4 0.77 0.75 0.29 0.03 0.12 5 0.76 0.68 0.26 0.39 0.12 6 0.75 0.61 0.23 0.36 7 0.76 0.61 0.23 0.36 0.05 8 0.77 0.60 0.23 0.36 0.10 9 0.76 0.60 0.23 0.36 0.15 10 0.77 0.59 0.23 0.36 0.21 11 1.06 - - - 12 1.10 0.80 0.33 - 13 0.95 0.60 0.23 - 0.15 14 1.05 0.60 0.23 - 0.15 15 1.15 0.60 0.23 - 0.15 16 0.86 0.67 0.21 0.05 17 0.87 0.74 0.24 - 0.15 18 1.02 0.52 0.19 0.15 19 0.95 0.72 0.24 - 0.15 20 0.96 0.69 0.27 0.18 0.15 21 0.95 0.79 0.27 - 0.20 22 0.85 0.74 0.39 - 23 0.84 0.72 0.48 - 24 0.94 0.71 0.50 - 0.15 25 0.94 0.79 0.48 0.19 0.20 TABLE 2 Steel Number PHYSICAL PROPERTY 2 1 3 1 4 1 5 Tensile Strength (N/mm2) 1070 1071 1127 1024 1234 1064 1274 1053 Elongation (50mm) (%) 15 15 16 15 15 15 15 17 Elongation (250mm) (%) 9 9 8 9 8 9 8 9 R of A (%) 30 - 36 - 32 - 38 R of A = reduction of area at fracture.

TABLE 3 Steel Number PHYSICAL PROPERTY 2 3 4 5 Tensile Strength(N/mm2) 1746 1847 1903 1956 0.2% P.S.(N/mm2) 1307 1563 1583 1634 Elongation (50mm) (%) 5 5 5 4 Elongation (250mm) (%) 2.0 1.8 1.6 1.4 R of A (%) 45 48 43 45 Torsions to failure 24 39 18 19 Bends to failure 14 14 12 11 TABLE 4 Steel Hardness Tensile Conversion Number HV5 Nlmm2 11 287 927 12 293 948 13 403 1320 14 380 1242 15 370 1205 16 367 1198 17 387 1265 18 378 1236 19 395 1292 20 437 1429 21 429 1405 22 382 1248 23 382 1248 24 433 1417 25 485 1603

Claims (8)

1. A steel comprising 0.90 to 1.2 wt.% carbon, 0.5 to 1.0 wt.% manganese, and 0.05 to 0.25 wt.% vanadium, the balance being iron and incidental elements and impurities.

2. A steel as claimed in claim 1, further comprising 0.15 to 1.0 wt.% chromium.

3. A steel as claimed in claim 2, in which the chromium content is at least 0.2 wt.%.

4. A steel as claimed in any of claims 1 to 3, further comprising 0.20 to 2.0 wt.% silicon.

5. A steel as claimed in claim 4, in which the silicon content is at least 0.25 wt.%.

6. A steel as claimed in any of claims 1 to 5, in which the vanadium content is 0.1 to 0.2 wt.%.

7. Hot rolled bar or wire rod made from steel according to any preceding claim.

8. Wire produced from bar or rod according to claim 7.