Abstract
The controlled cooling of previously heated and substantially straight steel wires of diameter more than 3.5 mm to a predetermined temperature including the steps: guiding the wires along individual paths through first coolant bath having bath liquid of water and a stabilizing additive, the bath liquid and the wires create a steam film around each wire along individual paths; directing an impinging liquid immersed inside first coolant bath towards the wires over a length along individual paths to cool down the wires, the impinging liquid decreases the thickness of the steam film or destabilizes the steam film, increasing speed of cooling over the length along individual paths; guiding the wires along individual paths out of the first coolant bath to be cooled down in air; after the further cooling, guiding the wires along individual paths through second coolant bath.
Claims
1. A method of controlled cooling of one or multiple previously heated and substantially straight steel wire/wires to a predetermined temperature range, the previously heated and substantially straight steel wires having a diameter which is more than 3.5 mm and less than 20 mm, the method comprises the steps of: a) guiding the previously heated and substantially straight steel wire/wires along individual path/paths through one or multiple first coolant bath/baths, the first coolant bath/baths comprises a bath liquid, wherein the bath liquid comprises water and a stabilizing additive, wherein the bath liquid and the multiple previously heated and substantially straight steel wires create a steam film around each steel wire itself along each individual path; b) directing an impinging liquid immersed inside the first coolant bath/baths towards the previously heated and substantially straight steel wire/wires over a certain length L along individual path/paths, to cool down the previously heated and substantially straight steel wire/wires, wherein the impinging liquid decreases the thickness of the steam film or destabilizes the steam film, thereby increasing the speed of cooling over the length L along individual path/paths, c) guiding the previously heated and substantially straight steel wire/wires along individual path/paths out of the first coolant bath/baths to be further cooled down in air, d) after the further cooling in air, guiding the previously heated, substantially straight steel wire/wires along individual path/paths through one or multiple second coolant bath/baths, wherein the substantially straight steel wire/wires are subjected to a cooling transformation from austenite to pearlite.
2. The method according to claim 1, wherein the impinging liquid is immersed below each of the previously heated and substantially straight steel wire itself along each individual path; or wherein the impinging liquid is immersed partially below some of the multiple previously heated and substantially straight steel wires along their individual paths.
3. The method according to claim 1, wherein the length of the first coolant bath and/or of the second coolant bath/baths are adjustable.
4. The method according to claim 1, wherein the first coolant bath is provided with partitioning walls separating steel wires in the first coolant bath along the length of the steel wires along which the steam film around the steel wires is affected by the impinging liquid, such that impinging liquids onto a first steel wire do not affect the steam film around a second steel wire.
5. The method according to claim 1, wherein the intensity of the impinging liquids is individually set and/or controlled for each individual steel wire or for subsets of the plurality of steel wires.
6. The method according to claim 1, wherein the first coolant bath(s) has/have a fixed length.
7. The method according to claim 1, wherein the impinging liquid has the same chemical composition as the bath liquid of the first coolant bath.
8. The method according to claim 1, wherein the impinging liquids are continuously recirculated and controlled by a flow rate control system.
9. The method according to claim 8, wherein one or a plurality of sensors are provided to measure the magnetic response of one or more than one of the steel wires; and to provide feedback to adapt in a closed loop control the impinging liquids in the first coolant baths.
10. The method according to claim 1, wherein the cooling transformation from austenite to pearlite starts substantially when the previously heated and substantially straight steel wire is cooled down in air between the first coolant bath and the second coolant bath.
11. The method according to claim 1, wherein each of the steel wire is previously heated above austenitizing temperature and cooled down to a predetermined temperature between 400° C. and 650° C.
Description
BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS
(1) FIG. 1 shows a preferred water air patenting concept 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 shows cooling curves of heated steel wires according to different routines.
(4) FIG. 4 illustrates the influence of flow rate to the cooling speed.
(5) FIG. 5 illustrates the cooling curves of steel wires subjected to a forced cooling at different flow rate according to an example of the present invention.
(6) FIG. 6 illustrates the tensile strength of steel wires subjected to a forced cooling at different flow rate according to an example of the present invention.
(7) FIG. 7 illustrates the cooling curves of steel wires subjected to a forced cooling at different flow rate according to another example of the present invention.
(8) FIG. 8 illustrates the tensile strength of steel wires subjected to a forced cooling at different flow rate according to another example of the present invention.
MODE(S) FOR CARRYING OUT THE INVENTION
(9) A preferred water air patenting cooling method and equipment according to the present invention is schematically shown in FIG. 1. The cooling length with impinging liquid in the first coolant bath (CB1) is fixed and the cooling rate is adjusted by tuning the coolant flow by means of the pressure in front of the jets. A short air gap (AG) is provided to separate the first coolant bath (CB1) and the second coolant bath (CB2). The second coolant bath (CB2) is adjustable in length. The length of first coolant bath, the flow rate of the jets for forced cooling and the length of air gap region are so chosen as to avoid the formation of martensite or bainite.
(10) Preferably as shown in FIG. 1, the first coolant bath is provided with partitioning walls separating steel wires in the first coolant bath along the length of the steel wires along which the steam film around the steel wires is affected by the impinging liquid, such that impinging liquids onto a first steel wire do not affect the steam film around a second steel wire. Preferably, as shown in FIG. 1, the first coolant baths, the impinging liquid generators and the air gaps along each individual path have a fixed length and the length of the second coolant baths is adjustable.
(11) FIG. 2 schematically illustrates a controlled cooling of one substantially straight steel wire according to the present invention. As shown in FIG. 2, a steel wire 10 is led out of a furnace 12 having a temperature T of about 1000° C. The wire running speed can be adjusted according to the diameter of the wire, e.g. about 20 m/min. A first coolant 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 (i.e. perforated plate) 22 immersed inside the first coolant bath are forming an impinging liquid, whose flow rate is controlled by a circulation pump and control system 18 outside the first coolant bath. The impinging liquid under pressure from the holes 20 is jetting towards the steel wire 10. As illustrated in FIG. 2, the first length L.sub.1 is the distance away from the exit of furnace 12 to the impinging liquid. The second length L.sub.2 indicates the length used for forced coolant cooling process—forced coolant cooling length—in the first coolant bath. The steel wire 10 is then led out of the first coolant bath and subjected to an air gap region with a length of L.sub.4 as indicated in FIG. 2. Thereafter, the steel wire 10 is guided into a second coolant bath 17 to further cool down. The immersion length of the steel wire 10 in the second coolant bath 17 is indicated as L.sub.5. The length L.sub.5 can be variable depending on the diameter and the desired tensile strength of the steel wire 10.
(12) FIG. 3 illustrates different cooling curves 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. As an example, a steel wire which is cooled by film boiling in an overflow water bath follows the dotted lines of cooling curve 1′. The dotted line of cooling curve 1′ does not reach the “nose” of the curve S and E. Curves 1-4 illustrate the process described in WO2014118089, wherein curve 1 illustrates the cooling progress in the period of the forced water cooling treatment, curve 2 shows the next stage in a “soft” conventional water air patenting process, curve 3 is the cooling curve during transformation and curve 4 shows further cooling in the post-transformation stage occurs in the air. In comparison with the above two situations, an example of a cooling curve according to the present invention is indicated by curves a-c. Curve a illustrates the cooling occurred in the first coolant bath, where the cooling rate is adjusted by the flow rate, and in the air gap followed by the first coolant bath. Curve b is the cooling curve during transformation and it can be occurred in the second coolant bath without disrupting the steam film. Curve c is the cooling curve showing the post-transformation in the air. The cooling curves a-c can be modified by changing the cooling scheme of steel wire.
(13) The cooling rate of steel wires having different diameter can be well tuned by adjusting the flow rate. Tests on cooling time vs. flow rate have been performed by a probe with 6 mm diameter cooled down from 750° C. to 500° C. The tests are carried out at several flow rates in a range from 1 m.sup.3/h to 16 m.sup.3/h and the results are shown in FIG. 4. An increase of flow rate from 1.15 m.sup.3/h to 15.3 m.sup.3/h can reduce the cooling time from 11.4 second to 5.1 second. It demonstrates that an increase of the flow rate can significantly reduce the cooling time, i.e. accelerate the cooling speed.
(14) By adjusting the flow rate, the starting point of the transformation from austenite to pearlite of the steel wire can be controlled. The transformation can start in the first coolant bath (CB1), in the air gap region (AG), or in the second coolant bath (CB2).
(15) As an example shown in FIG. 5, a steel wire having a diameter of 6.5 mm and a carbon content of 0.62 wt % is cooled from 950° C. The heated steel wire is quickly guided from the furnace into the first coolant bath (CB1), subsequently subjected to an air gap region (AG), and followed by a second coolant bath (CB2). The temperature vs. cooling time of the steel wire at a different flow rate of 3 m.sup.3/h, 9 m.sup.3/h, 12 m.sup.3/h and 15 m.sup.3/h are respectively measured and the cooling curves are respectively shown as curve A, B, C and D in FIG. 5. Herein, the same cooling equipment installation is applied except the flow rates are different. The length for the forcing cooling is 160 cm, for the air gap region is 65 cm and for the second coolant bath is 200 cm. When the flow rate is set at 3 m.sup.3/h, as shown in curve A, the transformation starts at a temperature of about 580° C. in the second coolant bath. Using higher flow rate, i.e. at 9 m.sup.3/h, 12 m.sup.3/h and 15 m.sup.3/h, the transformation starts in the first coolant bath at a temperature between 500° C. and 550° C. and continues in the air gap region.
(16) Consequently, the cooling rate and cooling process determine the microstructure of the cooled steel wires and thus the ultimate tensile strength of the steel wire. The tensile strength of the steel wires having a diameter of 6.5 mm and a carbon content of 0.62% by weight as a function of flow rates are illustrated in FIG. 6. The steel wire cooled at a forced cooling rate of 3 m.sup.3/h, 9 m.sup.3/h, 12 m.sup.3/h and 15 m.sup.3/h respectively has a tensile strength (Rm) of 1012 N/mm.sup.2, 997 N/mm.sup.2, 1077 N/mm.sup.2 and 1151 N/mm.sup.2. Thus, the tensile strength of the steel wires can be adjusted by selecting the flow rate during the forced cooling in the first coolant bath.
(17) Another example is shown in FIG. 7: a steel wire having a diameter of 3.6 mm and a carbon content of 0.70% by weight is cooled from 950° C. The heated steel wire is quickly guided from the furnace into the first coolant bath (CB1), subsequently subjected to an air gap region (AG), and followed by a second coolant bath (CB2). The temperature vs. cooling time of the steel wire at a different flow rate of 3 m.sup.3/h, 9 m.sup.3/h, 11 m.sup.3/h and 14 m.sup.3/h are respectively measured and the cooling curves are respectively shown as curve A, B, C and D in FIG. 7. Herein, the same cooling equipment installation is applied except the flow rates are different. The length for the forced cooling is 160 cm, for the air gap region is 65 cm and for the second coolant bath is 120 cm. When the rate is set at 3 m.sup.3/h, as shown in curve A, the transformation starts at a temperature slightly higher than 560° C. in the second coolant bath. Using higher flow rate, i.e. at 9 m.sup.3/h, 11 m.sup.3/h and 14 m.sup.3/h, the transformation starts in the first coolant bath at a temperature around 500° C. and continues in the air gap region.
(18) Consequently, the cooling rate and cooling process determine the microstructure of the cooled steel wires and thus the ultimate tensile strength of the steel wire. The tensile strength of the steel wires having a diameter of 3.6 mm and a carbon content of 0.70 wt % as a function of flow rates are illustrated in FIG. 8. The steel wire cooled at a forced cooling rate of 3 m.sup.3/h, 9 m.sup.3/h, 11 m.sup.3/h and 14 m.sup.3/h respectively has a tensile strength (Rm) of 1084 N/mm.sup.2, 1094 N/mm.sup.2, 1164 N/mm.sup.2 and 1252 N/mm.sup.2. It demonstrates that the tensile strength of the steel wires can be adjusted by selecting the flow rate during the forced cooling in the first coolant bath.
