Method for producing a low-alloy steel ingot

Abstract

A method of fabricating a low alloy steel ingot, the method including a) melting all or part of an electrode by a vacuum arc remelting method, the electrode, before melting, including iron and carbon, the melted portion of the electrode being collected in a crucible, thus forming a melt pool within the crucible; and b) solidifying the melt pool by heat exchange between the melt pool and a cooling fluid, the heat exchange applied serving to impose a mean solidification speed during step b) that is less than or equal to 45 μm/s and to obtain an ingot of low alloy steel.

Claims

1. A part made of low alloy steel comprising iron and carbon, the part extending along a longitudinal axis, the part being such that when evaluated using the D method of the ASTM E 45-10 standard, the following results are obtained for analysis along the longitudinal axis: the number of fields including type D inclusions of severity level equal to 0.5 is less than 5; no field including type D inclusions of severity level equal to 1 is obtained; and no field including type B inclusions of severity level equal to 0.5 is obtained, wherein the part is fabricated by solidifying a melt pool comprising iron and carbon such that a mean solidification speed of the melt pool is less than or equal to 45 micrometers per second.

2. A part according to claim 1, wherein when the part is evaluated using the method D of the ASTM E 45-10 standard, the following result is obtained when summing three measurement results obtained along the longitudinal axis of the part and along two axes perpendicular to the longitudinal axis: the total number of fields including type D inclusions of severity level equal to 0.5 is less than or equal to 15.

3. A part according to claim 1, wherein carbon is present at a content by weight lying in the range 0.09% to 1.00%.

4. A part according to claim 1, further comprising chromium at a content by weight lying in the range 0.05% to 5.00%.

5. A part according to claim 1, further comprising molybdenum at a content by weight less than or equal to 5.00%.

6. A part according to claim 1, comprising iron together with: carbon at a content by weight lying in the range 0.09% to 1.00%; manganese at a content by weight less than or equal to 5.00%; nickel at a content by weight less than or equal to 5.00%; silicon at a content by weight less than or equal to 3.00%; chromium at a content by weight lying in the range 0.05% to 5.00%; molybdenum at a content by weight less than or equal to 5.00%; and vanadium at a content by weight less than or equal to 5.00%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the invention appear from the following description of particular implementations of the invention given as non-limiting examples and with reference to the accompanying drawings, in which:

(2) FIGS. 1 and 2 are diagrammatic and fragmentary views showing the implementation of a method of the invention.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

(3) As shown in FIG. 1, the electrode 1 that is to be melted is present in the inside volume defined by the crucible 10. The electrode 1 may previously have been prepared by any conventional means, such as preparation in air or preparation by induction in a vacuum. As shown, before being melted, the electrode 1 may be cylindrical in shape. As explained above, it would not go beyond the ambit of the present invention if an electrode were used that presents some other shape before melting.

(4) By way of example, the crucible 10 is made of copper. The crucible 10 extends along a longitudinal axis X. A generator G imposes a potential difference between the crucible 10 and the electrode 1. As shown, a first terminal of the generator G may be connected to the electrode 1 and a second terminal of the generator G may be connected to the bottom 11 of the crucible 10, as shown. The potential difference imposed between the crucible 10 and the electrode 1 by the generator G serves to create electric arcs 3 in the gap 2 in which there exists a vacuum. These electric arcs 3 serve to melt the electrode 1 and to perform step a).

(5) The melted portion of the electrode 1 is collected in the crucible 10 and thus forms a melt pool 20. The melt pool 20 has a liquid portion 21 situated beside the electrode 1 and a pasty portion 22 situated between the liquid portion 21 and the ingot 30. The ingot 30 is obtained by cooling the melted portion of the electrode. The solidification front 34 lies between the resulting ingot 30 and the melt pool 20, and during step b) it propagates towards the free surface of the melt pool 20. Water flows around the crucible 10 so as to cool the crucible 10 continuously and also cool the melt pool 20 so as to ensure that it solidifies.

(6) In addition, as shown in FIG. 1, a cooling channel 13 is present within the side wall 12 of the bottom wall 14 of the crucible 10. A cooling liquid may flow within the cooling channel 13 in order to contribute likewise to solidifying the melt pool 20.

(7) As shown, during step b), the ingot 30 is present between the melt pool 20 and the bottom 11 of the crucible 10 and also between the melt pool 20 and the side wall 12 of the crucible 10. In addition, at least a fraction of the peripheral surface 31 of the ingot 30 need not be in contact with the side wall 12 of the crucible 10, being separated therefrom by a gap 33. In some circumstances, it is possible to inject a gas (e.g. He, Ar, N.sub.2) into this gap 33 in order to improve cooling.

(8) At the end of step b), the resulting ingot 30 may be cylindrical in shape.

(9) FIG. 2 is a simplified representation showing certain details of the method of the invention. Before melting, the electrode 1 includes inclusions 40. These inclusions 40 may be non-metallic inclusions. As shown, during melting, the end 1a of the electrode 1 is melted by the energy of the electric arcs 3. Drops 5 of melted electrode are produced, which are collected by the crucible 10. As explained above, the crucible 10 is cooled with water. The melt pool 20 has a diameter d equal to the inside diameter of the crucible 10.

(10) As shown, throughout all or part of step b), the melt pool 20 may be hemispherical in shape. By way of example, such a shape may be obtained when the crucible 10 that is used is cylindrical in shape. The melt pool 20 may have other shapes, e.g. a semi-quasi-ovoid shape. Such a shape may be obtained for example when using a crucible of rectangular parallelepiped shape.

(11) The distance e between the free surface 25 of the melt pool 20 and the electrode 1 is advantageously kept constant during step b). This distance e may be controlled either by controlling the voltage (V), or by controlling pulses associated with the drop rate of the drops 5. In the example shown, during step b), the electrode 1 is moved along the longitudinal axis X of the crucible 10 in order to keep the distance e constant.

(12) During melting of the electrode 1, the drops 5 drop and they are collected by the crucible 10. The drops 5 may include inclusions 40 that were initially present in the electrode 1. Once the inclusions 40 have been taken to the melt pool 20, they may be entrained towards the bottom 26 of the melt pool 20 (i.e. the point of the melt pool 20 that is closest to the bottom 11 of the crucible and that is in contact with the solidification front 34).

(13) From a thermal point of view, the melt pool 20 presents an axial portion of temperature that is greater than the temperature of its peripheral portion. This leads to natural convection, corresponding to the buoyancy forces that are engaged going from the bottom 26 of the melt pool 20 to the free surface 25 of the melt pool 20 and then going towards the edge 27 of the melt pool 20. This convection is represented in FIG. 2 by arrows 28a and 28b.

(14) During remelting, solid or liquid inclusions 40 of density lower than the density of the melt pool 20 will tend to rise to the surface 25 at a certain speed as a result of buoyancy mechanisms, as explained above.

(15) Aggregates 41 made up of agglomerated inclusions 40 are present on the free surface 25 of the melt pool 20. These aggregates 41 are entrained towards the periphery of the ingot 30, where they become frozen.

(16) In FIG. 2, there can be seen the solidification front 34 propagating from the bottom 11 of the crucible 10 towards the free surface 25 of the melt pool. The solidification front 34 propagates during step b) along the longitudinal axis X of the crucible 10 as represented by arrow 35. As shown, the solidification front 34 may retain its shape throughout all or part of step b). The mean speed at which the solidification front 34 rises is controlled so as to be less than the speed at which all or some of the inclusions 40 rise to the surface, as explained above. Specific positions P.sub.1 and P.sub.2 occupied by the bottom 26 of the melt pool 20 are shown in FIG. 2. The distance d.sub.1 traveled by the bottom 26 of the melt pool is measured along the longitudinal axis X of the crucible 10.

EXAMPLES

Example 1

(17) An electrode having the following chemical composition: C 0.42%-Mn 0.82%-Ni 1.80%-Si 1.70%-Cr 0.80%-Mo 0.40%-V 0.08% and the balance Fe (the percentages are weight percentages) was melted by a vacuum arc remelting method.

(18) Before melting, the diameter of the electrode was 920 mm.

(19) The conditions applied during vacuum arc remelting were as follows: applied voltage: 25 volts (V); applied current: 9 kiloamps (kA); and pulses: 250 circuit-cutout drops of molten electrode produced per minute.

(20) Those conditions enable a molten electrode drop rate to be obtained that was equal to 9.5 kilograms per minute (kg/min).

(21) The molten electrode drops were collected in a crucible having a diameter of 975 mm and they formed a melt pool within the crucible that was made of copper.

(22) Thereafter, the melt pool was solidified by exchanging heat between the melt pool and water flowing at a rate of 3000 L/min at a thermostatically-controlled temperature of 38° C. at the inlet and with continuous injection of He at 20 millibars (mbar).

(23) The resulting heat exchange enabled a mean solidification speed of 24 μm/s to be imposed during step b).

(24) After solidification, a low alloy steel ingot was obtained having the following chemical composition: C 0.410-Mn 0.800-Ni 1.800-Si 1.700-Cr 0.800-Mo 0.400-V 0.080, with the balance being Fe (the percentages are by weight).

(25) The results obtained in terms of inclusion cleanliness using method D of the ASTM E 45-10 standard are given below in terms of number of fields along the longitudinal axis:

(26) TABLE-US-00002 Severity level Size A B C D 0.5 Thin 0 0 0 3 0.5 Heavy 0 0 0 1 1 Thin 0 0 0 0 1 Heavy 0 0 0 0 1.5 Thin 0 0 0 0 1.5 Heavy 0 0 0 0

(27) The sum of the fields including type D inclusions in all three directions was 7.

Example 2 (Comparative)

(28) An electrode having the following chemical composition: C 0.420-Mn 0.830-Ni 1.810-Si 1.720-Cr 0.850-Mo 0.380-V 0.09% and the balance Fe (the percentages are weight percentages) was melted by a vacuum arc remelting method.

(29) Before melting, the diameter of the electrode was 550 mm.

(30) The conditions applied during vacuum arc remelting were as follows: applied voltage: 25 V; applied current: 11 kA; and pulses: 330 circuit-cutout drops of molten electrode produced per minute.

(31) Those conditions enable a molten electrode drop rate to be obtained that was equal to 12 kg/min±0.6 kg/min.

(32) The molten electrode drops were collected in a crucible having a diameter of 600 mm and they formed a melt pool within the crucible that was made of copper.

(33) Thereafter, the melt pool was solidified by exchanging heat between the melt pool and water flowing at a rate of 1500 L/min at a thermostatically-controlled temperature of 38° C. at the inlet and without injecting gas.

(34) The resulting heat exchange enabled a mean solidification speed of 49 μm/s to be imposed during step b).

(35) After solidification, a low alloy steel ingot was obtained having the following chemical composition: C 0.41%-Mn 0.81%-Ni 1.82%-Si 1.73%-Cr 0.85%-Mo 0.38%-V 0.09%, with the balance being Fe (the percentages are by weight).

(36) The results obtained in terms of inclusion cleanliness using method D of the ASTM E 45-10 standard are given below in terms of number of fields along the longitudinal axis:

(37) TABLE-US-00003 Severity level Size A B C D 0.5 Thin 0 5 0 28 0.5 Heavy 0 1 0 15 1 Thin 0 1 0 2 1 Heavy 0 0 0 0 1.5 Thin 0 0 0 0 1.5 Heavy 0 0 0 0

(38) The sum of the fields including inclusions of type B or type D in all three directions was 87. Such an ingot presents mechanical properties that are significantly lower than those of the ingot of the invention.

(39) The term “including/comprising a” should be understood as “including/comprising at least one”.

(40) The term “in the range . . . to . . . ” should be understood as including the limits.