A METHOD FOR MANUFACTURING A STEEL INGOT

Abstract

A method for manufacturing a steel ingot in a casting arrangement (100) comprising a vacuum vessel (110); an ingot mold (120) arranged within the vacuum vessel and a stirrer (130) arranged to stir liquid steel in the ingot mold, comprising: -providing (1000) a liquid steel melt; filling (2000) the ingot mold (100) with the liquid steel melt; applying (3000) a reduced pressure within the vacuum vessel (110); allowing the liquid steel melt to solidify into an ingot; allowing the liquid steel melt to solidify under stirring within the ingot mold at a reduced pressure during solidification of the steel melt; wherein, the liquid steel melt comprises a predetermined amount of carbon and; incidental impurity elements in the form of oxides, wherein during stirring the oxides are reduced by carbothermic reaction in which oxygen in the oxides and carbon in the steel melt form carbon-monoxide.

Claims

1-18. (canceled)

19. A method for manufacturing a steel ingot in a casting arrangement comprising a vacuum vessel; an ingot mold arranged within the vacuum vessel and a stirrer arranged to stir liquid steel in the ingot mold, comprising the steps of: providing a liquid steel melt; filling the ingot mold with the liquid steel melt; applying a reduced pressure within the vacuum vessel; and allowing the liquid steel melt to at least partially solidify at a reduced pressure into an ingot, wherein the liquid steel melt is stirred within the ingot mold at a reduced pressure during at least a portion of the solidification of the steel melt; wherein the ingot mold is manufactured of steel or cast iron; the liquid steel melt is Fe-based and comprises a predetermined amount of carbon; wherein during stirring incidental impurity elements in the form of oxides are reduced by carbothermic reaction in which oxygen in the oxides and carbon in the steel melt form carbon-monoxide; the step of providing the liquid steel melt includes manufacturing the liquid steel melt outside of the vacuum vessel; and the pressure within the vacuum vessel is ≤1 mbar.

20. The method according to claim 19, wherein the pressure within the vacuum vessel is ≤0.1 mbar.

21. The method according to claim 20, wherein the content of oxides, measured as ppm oxygen, in the solidified ingot is <3 ppm or ≤0.3 ppm or ≤0.1 ppm or ≤0.01 ppm.

22. The method according to claim 19, wherein the initial temperature of the steel melt is 1650-1500° C.

23. The method according to claim 19, wherein the initial content of oxides, measured as ppm oxygen, in the steel melt is ≥3 ppm.

24. The method according to claim 19, wherein the vacuum vessel comprises a closable opening and wherein the ingot mold is filled by supplying liquid steel melt through the closable opening.

25. The method according to claim 19, wherein the ingot mold is filled while a reduced pressure prevails within the vacuum vessel.

26. The method according to claim 19, wherein stirring of the liquid steel melt is performed such that liquid steel is transported in direction from the bottom of the ingot mold towards the top of the ingot mold and from the top of the ingot mold towards the bottom of the ingot mold.

27. The method according to claim 19, wherein the stirrer is an electromagnetic stirrer.

28. The method according to claim 19, wherein the steel melt comprises carbon in an amount or 0.01-1.3 wt %.

29. The method according to claim 19, wherein the steel melt at least comprises one or more of the following alloy elements in (wt %): Si: 0-3; Mn: 0-3; Cr: 0-18; Ni: 0-10; V: 0-2; Mo: 0-3; and N: 0-0.4.

30. An object manufactured from a steel ingot produced by the method according to claim 19.

31. The object according to claim 30, wherein the object is a bar, wire, strip, tube, sheet, ring or plate.

32. The method according to claim 19, wherein the ingot is formed of Fe-based steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1: A diagram showing equilibrium between oxygen and carbon at various atmospheric pressures. FIG. 2a-2d: Schematic drawings show the steps of the method of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0045] The method for manufacturing a steel ingot according to the present disclosure will now be described more fully hereinafter. The method according to the present disclosure may however be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those persons skilled in the art. Same reference numbers refer to same elements throughout the description.

[0046] FIGS. 2a shows a first step 1000 of providing a steel melt. The steel melt may be produced by conventional steel making methods including melting steel raw material such as scrap metal in an electric arc furnace 10. The molten steel is poured into a ladle 20 for oxygen reduction and subsequently into a ladle 30 for refinement. The ladle 30 may provide a container for transporting the steel melt in the method according to the present disclosure. The total weight of the steel in the lade 30 may be 20 tons or more.

[0047] In a substep 1500, see FIG. 2b, the ladle 30 is transported to a casting arrangement 100 having a vacuum vessel 110, an ingot mold 120 arranged within the vacuum vessel and a stirrer 130 arranged to stir liquid steel in the ingot mold. The vacuum vessel may be manufactured from steel sheet and has a doom-shaped housing 111 which is arranged such that it's interior may be completely air and gastight sealed off from the exterior. It is obvious that the vacuum vessel may have any suitable form. The vacuum vessel comprises a closable and airtight sealable opening 112 for allowing the mold to be filled with steel from the ladle outside the vacuum vessel.

[0048] The vacuum vessel further comprises a vacuum opening 113 which is connected to a vacuum pump (not shown) which allows the pressure within the vacuum vessel to be reduced. The ingot mold 113 is manufactured of austenitic steel or cast iron in dimensions 600×600×2000 mm and is open at its top 120. Typically, the mold may accommodate ingots weighing 4.2 tones. It is possible to arrange more than one ingot mold within the vacuum vessel. The stirrer 10 may be an electromagnetic stirrer and may be arranged to circulate liquid steel from the bottom to the top of the mold and vice-versa. The stirrer may be strand stirrer of the ORC 1100/400M-serie, which is commercially available from the company ABB.

[0049] The liquid steel in the ladle may have composition of C: 0.1%; Mn: 0.2%; Si 0.2%;Cr 1.5% and balance Fe. The oxygen content in the liquid steel may be approximately 3 ppm tied up as oxides.

[0050] In a second step 2000, see FIG. 2c, the ingot mold 120 is filled with liquid steel melt. This may be achieved by positioned the ladle 30 above the closable opening 122 in the vacuum vessel, opening the closable opening and lowering the ladle such that its outlet tube 31 enters through the closable opening and into the top 110 of the ingot mold 120. The steel in the ladle is then released through the outlet tube into the mold. When the mold is filled, the ladle is removed and the closable opening is closed.

[0051] Subsequently, in a third step 3000, see FIG. 2d, the pressure is reduced in the vacuum vessel 110 by activating the vacuum pump (not shown). The pressure may be reduced to 0.1 mbar or less.

[0052] Next or simultaneous, in a fourth step 4000, the stirrer 130 is activated to circulate the liquid steel in the mold. Stirring is continued until at least a portion of the steel melt is solidified. For an ingot mold of the present dimension the time for complete solidification of the steel melt into an ingot may be 2 hours. During stirring, the oxygen content is reduced by reaction with carbon in the steel melt as described hereinabove. In the described embodiment, stirring is applied to the side of the ingot mold. However, it is possible to apply stirring to other positions. For example, to the upper part of the mold or on the top of the mold or the bottom of the mold. Stirring may also be applied to multiple positions of the mold.

[0053] In a subsequent step 5000, not shown, the ingot is removed from the ingot mold. The ingot may subsequently be subjected to additional working steps such as heat treatment and forming by e.g. rolling, forging or drawing into objects such as bars, wires, strip, sheet or plates. These steps are not shown.

[0054] Although a particular embodiment has been disclosed in detail, this has been done for purpose of illustration only, and is not intended to be limiting. In particular it is contemplated that various substitutions, alterations and modifications may be made within the scope of the appended claims. For example,

[0055] The casting arrangement may be arranged such that the ingot mold may be filled with liquid steel while a reduced pressure prevails in the vacuum vessel 110. In an embodiment this may be achieved by arranging a further vacuum chamber around the casting arrangement. Filling of the mold may be performed by: placing the ladle in the vacuum chamber, evacuating both vacuum chamber and vacuum vessel, filling the mold through the closable opening 112 and closing the opening.

[0056] In another embodiment, the closable opening 122 may be provided with an air-lock.

[0057] It is also possible to combine the described alternatives.

[0058] Moreover, although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Furthermore, as used herein, the terms “comprise/comprises” or “include/includes” do not exclude the presence of other elements. Finally, reference signs in the claims are provided merely as a clarifying example and should not be construed as limiting the scope of the claims in any way.