Forced water cooling of thick steel wires

Abstract

A method of and an equipment for controlled cooling of one or multiple previously heated, straight, and thick steel wire to a predetermined temperature range between 400 C. and 650 C. Each of the thick steel wires is subjected to a controlled cooling-transformation treatment from austenite to pearlite, which occurs substantially after the wire leaves a forced water cooling length.

Claims

1. A method of controlled cooling of one or multiple previously heated and substantially straight steel wires to a predetermined temperature range, the method comprising the steps of: a) guiding the one or multiple previously heated and substantially straight steel wires along individual paths through a coolant bath, the coolant bath comprising a bath liquid and a stabilizing polymer, the bath liquid comprising water and having a temperature of more than 80 C., the bath liquid creating a steam film around each of the one or multiple previously heated and substantially straight steel wires itself along each individual path; b) directing an impinging liquid immersed inside the coolant bath towards the steam film over a length L along the individual paths such that a thickness of the steam film is decreased or the steam film is destabilized, thereby increasing a speed of cooling over the length L along the individual path; wherein the impinging liquid is immersed below one previously heated and substantially straight steel wire itself along the individual path, or the impinging liquid is immersed partially below some of the multiple previously heated and substantially straight steel wires along their individual paths.

2. The method according to claim 1, wherein the length L along each individual path is smaller than a length of the coolant bath.

3. The method according to claim 2, wherein the impinging liquid has a same chemical composition as the bath liquid.

4. The method according to claim 3, wherein the impinging liquid is taken from the coolant bath.

5. The method according to claim 4, wherein the impinging liquid is continuously recirculated.

6. The method according to claim 1, wherein a diameter of each of the previously heated and substantially straight steel wires ranges from 5.5 mm to 20 mm.

7. The method according to claim 6, wherein the diameter of each of the previously heated and substantially straight steel wires ranges from 6.5 mm to 13.5 mm.

8. The method according to claim 1, wherein each of the previously heated and substantially straight steel wires is subjected to a controlled cooling-transformation treatment from austenite to pearlite.

9. The method according to claim 8, wherein each of the previously heated and substantially straight steel wires is previously heated above austenitizing temperature and cooled at a predetermined temperature between 400 C. and 650 C.

10. The method according to claim 9, wherein a transformation from austenite to pearlite occurs substantially after the one or multiple of the previously heated and substantially straight steel wires leave the length L.

11. The method according to claim 1, wherein the method comprises controlled cooling of multiple previously heated and substantially straight steel wires, and wherein longitudinal directions of the multiple previously heated and substantially straight steel wires are substantially parallel to each other.

12. An equipment for controlled cooling of one or multiple previously heated steel wires to a predetermined temperature range, said equipment being adapted to carry out a method according to claim 1.

13. An equipment according to claim 12, said equipment comprising: a) a coolant bath, said coolant bath comprising water and a stabilizing polymer as bath liquid, said bath liquid having a temperature of more than 80 C.; b) guiding means for guiding one or multiple previously heated steel wires continuously along individual paths through said coolant bath; c) an impinging liquid generator immersed inside said coolant bath being adapted to jet impinging liquid towards each steel wire along individual path.

14. The method according to claim 3, wherein the coolant bath and the impinging liquid consists of the water and the stabilizing polymer.

15. The method according to claim 1, wherein the stabilizing polymer comprises alkalipolyacrylates or sodium polyacrylate.

16. A method of controlled cooling of a straight steel wire to a predetermined temperature range, the method comprising the steps of: a) guiding the straight steel wire, which has been previously heated, along an individual path through a coolant bath, the coolant bath comprising a bath liquid and a stabilizing polymer, the bath liquid comprising water and having a temperature of more than 80 C., the bath liquid creating a steam film around the straight steel wire along the individual path; b) directing an impinging liquid immersed inside the coolant bath towards the steam film over a length L along the individual path such that a thickness of the steam film is decreased or the steam film is destabilized, thereby increasing a speed of cooling over the length L along the individual path; wherein the impinging liquid is immersed below the straight steel wire itself along the individual path.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1 shows a cooling curve of a process according to the present invention;

(2) FIG. 2 gives schematic representation of carrying out a cooling process according to the present invention;

(3) FIG. 3 gives a cross-section along plane A-A of FIG. 2;

(4) FIG. 4 illustrates the influence of pump flow rate to start of transformation;

(5) FIG. 5 and FIG. 6 give two embodiments of holes with different distributions;

(6) FIG. 7 illustrates the working principle of a movable steel plate for controlling the numbers of the holes;

(7) FIG. 8 and FIG. 9 and FIG. 10 are reference microstructures of sample 1 and sample 2 and sample 3 according to the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

(8) General description of influence of diameter on cooling speed with respect to TTT diagram of FIG. 1. FIG. 1 shows a cooling curve 1-4 in a so-called TTT diagram (Temperature-Time-Transformation). Time is presented in abscissa and temperature forms the ordinate. S is the curve which designates the start of the transformation from austenite (A) to pearlite (P), E is the curve which designates the end of this transformation. A steel wire with a diameter of about 6.50 mm which is cooled by film boiling in an overflow water bath (a conventional WAP process) follows the full dotted lines of cooling curve 1. The dotted lines of cooling curve 1 do not reach the nose. It takes a much longer time to start transformation, which will result in too coarse a pearlite structure. Such a structure takes a high risk of yielding a desired ultimate tensile strength of the steel wire. So the cooling speed of the pre-transformation stage of curve 1 has to be accelerated so as to enter the nose of the transformation curve at a suitable place in order to have a fine pearlite structure. The concept of forced water cooling according to this invention is particularly aimed at having a rapid cooling speed at a pre-transformation stage. Curve 1 illustrated the cooling progress in the period of the forced water cooling treatment and curve 2 showed the next stage in a soft conventional WAP process. Curve 3 is the cooling curve during transformation (also in the soft conventional WAP process). Further cooling in the post-transformation stage occurs in the air and is shown by cooling curve 4.

(9) Referring now to FIG. 2 and as a matter of another example, a steel wire 10 with a diameter D of 10 mm (S3) is led out of a furnace 12 having a temperature T of about 1000 C. The wire speed V is about 10 m/min. A water bath 14 of an overflow-type is situated immediately downstream the furnace 12. A plurality of jets 16 from the holes 20 of a hollow plate (perforated plate) 22 immersed inside said coolant bath are forming an impinging liquid, whose flow rate is controlled by a circulation pump 18 outside the coolant bath. As illustrated in FIG. 2, the impinging liquid under pressure is rushing up from the holes 20 jetting towards said steel wire 10.

(10) The first length l.sub.1 is due to the positioning of the forced water cooling equipment. The forced water cooling equipment might be installed just at the exit of the furnace (l.sub.1=0) or a small distance away from the exit. The length l.sub.1 can be adjustable as required. The second length l.sub.2 indicates the length used for forced water cooling processforced water cooling length. The third length l.sub.3 is the remaining cooling length in the same water coolant bath 14. FIG. 2 illustrates the setup with this wire (S3) running through the whole cooling installation and FIG. 3 is the cross-section according to plane A-A.

(11) The magnetic point, indicating the start of the austenite to pearlite transformation was measured using a magnet and is indicated in table 1 (Magtransdefined as the distance away from the exit of the furnace). The tensile strength was also measured and indicated in table 1 together with other four samples (S1 and S2 and S4 and S5, S1 is the reference wire through a conventional WAP while S2 to S5 are the wires through the inventive processforced water cooling treatment).

(12) TABLE-US-00001 TABLE 1 Sample V D, m.sup.3/min D, mm % C T, C. l.sub.1, m l.sub.2, m Flow m.sup.3/h l.sub.3, m Rm, N/mm.sup.2 Magtrans, m S1 100 10 0.6 1000 0.5 0 0 0.6 960 4.30 S2 100 10 0.6 1000 0.5 1.45 8.5 0.1 970 2.20 S3 100 10 0.6 1000 0.5 1.45 8.5 1.7 990 2.50 S4 100 10 0.6 1000 0.5 0.6 6 2.6 990 3.00 S5 100 10 0.6 1000 0.5 0.6 17 2.6 1000 2.30

(13) For the present examples starting product is a plain carbon steel wire rod. This steel wire rod has following steel composition: a carbon content of 0.60%, a manganese content of 0.50%, a silicon content of 0.202%, a sulphur content of 0.013%, a phosphorus content of 0.085%, all percentages being percentages by weight.

(14) A typical steel wire rod composition for high-tensile steel wire has a minimum carbon content of around 0.80 weight %, e.g. 0.78-1.02 weight %, a manganese content ranging from 0.30% to 1.10%, a silicon content ranging from 0.15% to 1.30%, a maximum sulphur content of 0.15%, a maximum phosphorus content of 0.20%, all percentages being percentages by weight. Additional micro-alloying elements may also be added, such as chromium from 0.20% to 0.40%, copper up to 0.20%, vanadium up to 0.30%.

(15) Table 1 further illustrates the effect of low and high pump flow rates in the installation. The situation acted on the last sample S5 is extreme since in normal conditions the flow rate is between 6 and 10 m.sup.3/h. During the last two trials (S4, S5), with the same forced cooling length l.sub.2=0.6 m and the same soft water cooling length l.sub.3=2.6 m, the position of the start of transformation was measured respectively using a magnet for different pump flow rates. A clear correlation between the distance from the furnace to the transformation point and the flow rate was found as shown in FIG. 4.

(16) However, according to this invention, the parameterthe pump flow rate is calculated as the sum of the jets from all the holes. If the size of the holes is fixed, the more the holes, the higher the flow rate; if the number of the holes is fixed, the bigger the holes, the higher the flow rate. Further, the higher the pump flow rate, the higher the forced cooling speed.

(17) Ideally the system should provide the same cooling speed irrespective of the travelling path of the steel wires. Indeed the steel wires may change somewhat from travelling path. In case only one set of holes is provided for one steel wire, a changing travelling path may cause changing cooling speeds and this is to be avoided. This can be avoided by providing various types of distributions of the holes. For example, there may be an at random distribution of holes.

(18) FIG. 5 and FIG. 6 show two kinds of distributions of holes. W.sub.1 to W.sub.I represents the width between each line of holes; the width can be different from each other or the same as each other.

(19) In FIG. 5 the widths W.sub.1 to W.sub.i2 may vary while in FIG. 6 the diameter of the holes may vary.

(20) The diameter of the holes preferably ranges from 0.5 mm to 5.0 mm, e.g. 1.0 mm, 2.5 mm, 4.0 mm, and the length between two adjacent holes along the same line are preferably larger than 5.0 mm, e.g. 6.8 mm, 8.2 mm, 10.6 mm. The holes 52 shown in FIG. 5 share the same diameter d1=3 mm. The length l.sub.01 between two adjacent holes along each line is the same: l.sub.01=15 mm; the width between each line of holes (W.sub.1 to W.sub.i2) is different from each other. Comparatively, as shown in FIG. 6, there are two kinds of holes 62 and 64 with different sizes respectively: d1=3 mm and d.sub.2=4 mm. The length between two adjacent holes along each line is different from each other in this figure: l.sub.02=5.5 mm and l.sub.03=15.0 mm and l.sub.04=20.8 mm; the width between each line of holes is the same: W.sub.i1=W.sub.i. The number of holes is also different in each individual line in order to have different cooling speed of individual travelling path of the steel wires. It is obvious that such a design is applied to cool a plurality of previously heated steel wires with different diameters at the same time.

(21) As illustrated in FIG. 5 and FIG. 6, the holes might be located just below the steel wire or wires. For a forced water cooling equipment used for a plurality of previously heated steel wires, holes might be different from individual line to line (as shown in FIG. 6) in order to have different flow rates, further contributes to different cooling speeds, which needs to be well calculated and controlled. Different flow rates may be useful to treat wires of a different diameter. Another feasible way is to use steel plates to cover some of the holes to reduce the total number of the jets further to control the forced water cooling length in a necessary path in order to meet the needs of a slower flow rate and further a decreased cooling speed.

(22) FIG. 7 illustrates the working principle of a movable steel plate 70 which is put above the holes 72 of a hollow plate (perforated plate) 74 thus to control the numbers of the holes and further the jets and further the forced water cooling length. Such a forced water cooling equipment is quite flexible, which can realize the transformation cooling of thick steel wires with different diameters in different individual travelling paths within the same coolant bath.

(23) FIG. 8 is a reference microstructure for S1 cooled with a short length in the WAP (l.sub.3 of S1). FIGS. 9 and 10 are micrographs corresponding to S2 and S3, respectively. The observation of samples showed that more lamellar pearlite was present in the reference S1. In the region close to the surface, in samples S2 and S3 less lamellar pearlite was present, due to the faster cooling via the forced water cooling process.

(24) The tensile properties of other samples cooled with the prototype are significantly higher than those of reference S1 and are close to the expected tensile strength of a 10 mm lead-patented wire rod with 0.6 wt % C (target value 1010 N/mm.sup.2).