SILICON BASED ALLOY, METHOD FOR THE PRODUCTION THEREOF AND USE OF SUCH ALLOY

Abstract

A silicon based alloy is disclosed having between 45 and 95% by weight of Si; max 0.05% by weight of C; 0.01-10% by weight of Al; 0.01-0.3% by weight of Ca; max 0.10% by weight of Ti; 0.5-25% by weight of Mn; 0.005-0.07% by weight of P; 0.001-0.005% by weight of S; the balance being Fe and incidental impurities in the ordinary amount, a method for the production of the alloy and the use thereof.

Claims

1. A silicon based alloy comprising between 45 and 95% by weight of Si; max 0.05% by weight of C; 0.01-10% by weight of Al; 0.01-0.3% by weight of Ca; max 0.10% by weight of Ti; 0.5-25% by weight of Mn; 0.005-0.07% by weight of P; 0.001-0.005% by weight of S; the balance being Fe and incidental impurities in the ordinary amount.

2. Silicon based alloy according to claim 1, wherein the silicon based alloy comprises between 50 and 80% by weight of Si.

3. Silicon based alloy according to claim 2, wherein the silicon based alloy comprises between 64 and 78% by weight of Si.

4. Silicon based alloy according to any one of the preceding claims, wherein the silicon based alloy comprises max 0.03% by weight of C.

5. Silicon based alloy according to any one of the preceding claims, wherein the silicon based alloy comprises between 0.01-0.1% by weight of Ca.

6. Silicon based alloy according to any one of the preceding claims, wherein the silicon based alloy comprises max 0.06% by weight of Ti.

7. Silicon based alloy according to any one of the preceding claims, wherein the silicon based alloy comprises between 1-20% by weight of Mn.

8. A method for producing a silicon based alloy according to any of the claims 1-7, wherein said method comprises providing a liquid base ferrosilicon alloy and adding a Mn source comprising carbon as an alloying element or as an impurity element into said liquid ferrosilicon thereby obtaining a melt, and refining said obtained melt, the refining comprising removing formed silicon carbide particles before and/or during casting of said melt.

9. Method according to claim 8, wherein the added Mn source is in the form of high carbon ferromanganese alloy, medium carbon ferromanganese alloy, low carbon ferromanganese alloy, Mn metal, or a mixture thereof.

10. Method according to any of claim 8 or 9, wherein the liquid base ferrosilicon alloy comprises: Si: 45-95 wt %; C: up to 0.5 wt %; Al: up to 2 wt %; Ca: up to 1.5 wt %; Ti: 0.01-0.1 wt %; Mn: up to 0.5 wt %; P: up to 0.02 wt %; S: up to 0.005 wt %; the balance being Fe and incidental impurities in the ordinary amount.

11. Method according to any of claims 8 to 10, wherein Al is added to adjust the Al content within the range 0.01-10 wt %.

12. Use of the silicon based alloy according to any of the claims 1-7 as an additive in the manufacturing of steel.

13. Use according to claim 12, in the manufacturing of non-grain oriented electrical steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0026] According to the present invention it is provided a new silicon based alloy that is low in carbon and with a manganese content up to 25% by weight.

[0027] The alloy according to the invention has the following composition:

Si: 45-95 wt %;

C: max 0.05 wt %;

Al: 0.01-10 wt %;

Ca: 0.01-0.3 wt %;

Ti: max 0.10 wt %;

Mn: 0.5-25 wt %;

P: 0.005-0.07 wt %;

S: 0.001-0.005 wt %;

[0028] the balance being Fe and incidental impurities in the ordinary amount.

[0029] In the present application, the terms silicon based alloy and ferrosilicon based alloy are used interchangeably. Si is the main element in this alloy to be added to the steel melt. Traditionally, 75 wt % Si or 65 wt % Si are used. Ferrosilicon with 75 wt % Si gives higher temperature increase of the steel melt when added than 65 wt % Si, which is almost temperature neutral. Ferrosilicon with lower than 50 wt % Si is rarely used in the steel industry today, and mean that a high amount of alloy would have to be added to get to the targeted Si content in the steel and creating challenges during steelmaking. Higher than 80% is seldom used today, as the production cost per silicon unit increases when the silicon content in the Si based alloy increases. Hence, a preferred Si range is 50-80 wt %. Another preferred Si range is 64-78 wt %.

[0030] Carbon is the main unwanted element in NGOES and should be as low as possible in this new alloy according to the invention. A maximum content of carbon in said alloy is 0.05 wt %. A preferred content should be max 0.03 wt % or even max 0.02 wt %, as in current low carbon ferrosilicon grades used in making said steel. It might be difficult to totally remove carbon and therefore normally 0.003 wt % C can be present in the alloy according to the invention. More than the carbon content itself, the carbon to manganese ratio is the one key parameter. With manganese increasing in the alloy, the carbon content in the new silicon based alloy according to the invention can be max 0.05 wt %.

[0031] Aluminium is an impurity in the production of silicon based alloy, typically around 1 wt % out of the furnace in standard grade. It can be refined down to a maximum of 0.01 wt % although for NGOES a maximum of 0.03 wt % or even max 0.1 wt % would be good solutions. However, in NGOES, Al is often added in small or large quantities. Therefore, adding aluminium up to 5 wt % or even up to 10 wt % in the alloy according to the invention can in some instances be preferable.

[0032] Calcium is an impurity in the production of silicon based alloys, and should be kept low to avoid problems during steelmaking and casting, such as nozzle clogging. In the alloy according to the invention, the calcium range is 0.01-0.3 wt %. A preferred calcium range is 0.01-0.1 wt %. A preferred content is max 0.05 wt %. If the calcium content in the starting material for producing the alloy according to the invention is higher than the desired calcium content in said alloy, calcium can easily be removed during the production by blowing/stirring with oxygen (from air and/or pure oxygen) thereby forming calcium oxide that can be removed as slag.

[0033] Titanium is an impurity in the production of silicon based alloys, typically around 0.08 wt % out of the furnace in 75 wt % FeSi standard production, but that depends on the raw material mix. However, in NGOES, a low content of titanium is often beneficial, to avoid formation of detrimental inclusions. Therefore, a Ti level of max 0.06 wt % or even max 0.03 wt % in the new alloy according to the invention is preferable. Traces of Ti might be present in said alloy, so that a minimum level of Ti can be 0.005% by weight. It is difficult to refine Ti in the ladle, so good furnace operation and raw material selection are required to succeed in getting low titanium content.

[0034] Manganese is typically an impurity in the production of silicon based alloys. However, the inventors surprisingly found that alloying a silicon based alloy with manganese in the range of 0.5 to 25% while keeping the carbon content low provides an alloy with excellent properties particularly for the use in the production of steel qualities requiring low carbon content such as NGOES. Other possible Mn ranges are 1-20%, or 1-15% or also 2-10%.

[0035] Phosphorous is an impurity in the production of silicon based alloys. In particular, in silicon based alloys without Mn additions, P levels are below 0.04%. However, P is normally higher in Mn alloys, therefore alloying with Mn may lead to a higher P content in the final product. However, P in the steel originating from addition of the silicon alloy of the present invention will be the same or slightly lower than from separate addition of silicon alloy and manganese alloy.

[0036] Sulphur is usually low in silicon alloys production. However, S is normally slightly higher in Mn alloys, so alloying with Mn may lead to higher S in the final product. However, S in the steel originating from addition of the silicon alloy of the present invention will be the same or slightly lower than from separate addition of silicon alloy and manganese alloy.

[0037] A preferred composition of the alloy according to the invention is:

Si: 64-78 wt %;

C: max 0.03 wt %;

Al: 0.1-10 wt %;

Ca: 0.01-0.05 wt %;

Ti: max 0.06 wt %;

Mn: 1-20 wt %;

P: 0.005-0.05 wt %;

S: 0.001-0.005 wt %;

[0038] the balance being Fe and incidental impurities in the ordinary amount.

[0039] The alloy according to the present invention is made by adding a Mn source comprising carbon as an alloying element or as an impurity element into a liquid Si based alloy. The Mn source can be in the form of solid or liquid manganese units, in the form of a manganese alloy or manganese metal or a mixture thereof. The manganese source can comprise normal impurities/contaminants. The manganese alloy can for example be a ferromanganese alloy, such as high carbon ferromanganese, medium carbon ferromanganese, low carbon ferromanganese or a mixture thereof. A commercial manganese alloy, for example as given in table 2 above, or a combination of two or more of such alloys, are suitable for use in the present invention. Preferably the added Mn is in the form of high carbon ferromanganese or medium carbon ferromanganese.

[0040] The added carbon from the manganese source will react with silicon thereby forming solid SiC (silicon carbide) particles that during refining are removed from the melt to the ladle refractory or to any slag that has been formed before or during the casting process, preferably with stirring in the ladle. Slag formers can be added if needed to have a sufficiently large receptor for the formed SiC particles. This results in a Si alloy according to the invention with low carbon content and containing manganese, with the range of elements as indicated above.

[0041] An example of a composition for the starting material could be liquid FeSi from furnace, but many others are possible depending on the final specification to be reached. Remelting any commercial silicon based alloys like standard ferrosilicon or high purity ferrosilicon could also be a possible starting material.

[0042] Thus, a possible starting material can comprise:

Si: 45-95 wt %;

C: up to 0.5 wt %;

Al: up to 2 wt %;

Ca: up to 1.5 wt %;

Ti: 0.01-0.1 wt %;

Mn: up to 0.5 wt %;

P: up to 0.02 wt %;

S: up to 0.005 wt %;

[0043] the balance being Fe and incidental impurities in the ordinary amount.

[0044] If the aluminium content is to be increased in the final product (up to 10%), addition of solid or liquid aluminium units can be made in the ladle. Alternatively, aluminium from the furnace can be increased by selection of raw materials to the furnace. Al can be added to adjust the Al content within the range 0.01-10 wt %.

[0045] To produce the alloy according to the invention, additional steps involving slag refining, skimming and/or stirring according to generally known techniques can be performed, in particular to reach the low levels of carbon claimed by the present invention. Such steps can be performed before or during the casting process or in combination.

[0046] The following Examples illustrate the present invention without limiting its scope.

Example 1

[0047] In two separate trials, ferrosilicon was tapped as normal into a tapping ladle (Ladle 1 and Ladle 2) with bottom stirring with air. The amount of ferrosilicon that was tapped was about 5900 kg into each of Ladle 1 and Ladle 2. Table 3 shows the starting material composition in the two ladles used.

TABLE-US-00003 TABLE 3 Starting materials (wt %) Starting material Al Si P Ca Ti Mn C Ladle 1 0.78 77.26 0.012 0.16 0.058 0.172 0.0533 Ladle 2 1.60 75.25 0.011 0.98 0.057 0.234 0.3794

[0048] After tapping, lumpy FeMn, with 75.7 wt % Mn and 6-8 wt % C; the balance being Fe and incidental impurities in the ordinary amount, was added into the liquid ferrosilicon in each ladle in an amount equal to 246 kg of Mn unit to reach 4.5 Mn in the final product. As the Mn yield was not known, FeMn was added gradually over a period between 20-25 minutes until the Mn target of 4.5% was reached. (Additions can be done in a shorter or longer time). The bottom stirring was kept during the whole addition process, ensuring good Mn dissolution and that formed SiC particles were removed from the Si alloy melt to the slag formed and the ladle walls. After the refining step, the ladles were taken to the casting area where final liquid sample was taken before casting into cast iron moulds.

[0049] Samples of the new alloy produced according to the invention were taken at the end of the liquid stage, just prior to casting. Results of the two ladles are shown in table 4.

[0050] All samples were analyzed with XRF (Zetium from Malvern Panalytical) for Al, Si, P, Ca, Ti, Mn, and for C, LECO CS-220 (combustion analysis) was used.

TABLE-US-00004 TABLE 4 Analysis (wt %) at the end of liquid stage Al Si P Ca Ti Mn C Ladle 1 0.27 74.18 0.016 0.02 0.057 4.43 0.018 Ladle 2 0.22 73.47 0.015 0.01 0.058 4.74 0.008

Example 2

[0051] Liquid ferrosilicon was tapped as normal into a tapping ladle with bottom stirring with air. The amount of ferrosilicon that was tapped into the ladle was about 6000 kg. The starting material composition can be seen in table 5.

[0052] During tapping, lumpy FeMn, with 78.4 wt % Mn and 6.85 wt % C; the balance being Fe and incidental impurities in the ordinary amount, was added into the liquid ferrosilicon in an amount equal to 950 kg. Together with FeMn, 100 kg of quartz was added to the melt to increase the volume of receptors to support the capture of the formed SiC. The bottom stirring was kept during the whole addition process, ensuring good Mn dissolution and that formed SiC particles were removed from the FeSi alloy melt to the ladle walls and slag formed. After the refining step, the ladle was taken to the casting area where final liquid sample was taken before casting into cast iron moulds.

[0053] Samples of the new alloy produced according to the invention were taken at the end of the liquid stage, just prior to casting, and on final product after casting. Results are shown in table 5.

[0054] All samples were analyzed with XRF (Zetium from Malvern Panalytical) for Al, Si, P, Ca, Ti, Mn, and for C, LECO CS-220 (combustion analysis) was used.

TABLE-US-00005 TABLE 5 Chemical composition (wt %) at different steps of the experiment Al Si P Ca Ti Mn C Starting material 0.57 75.66 0.008 0.33 0.017 0.21 0.030 Before casting 0.10 68.71 0.021 0.03 0.018 9.04 0.004 Final product 0.09 68.76 0.019 0.03 0.018 8.91 0.005

[0055] By applying such method, the inventors achieved a low carbon level, which can be explained by the low solubility of carbon in high silicon alloys. It was however surprising that it was possible to reach carbon levels as low as in current low carbon ferrosilicon grades (see table 1).

[0056] The alloy according to the invention is a cost-efficient alternative to separately adding the required alloying elements Si and Mn separately as ferrosilicon and manganese alloy or a manganese metal, by improving process time and quality. Said alloy could also help NGOES producers to decrease the overall carbon content in the steel and reach a lower level than by adding ferrosilicon/Si based alloy and manganese in the form of low carbon manganese alloy or manganese metal separately. Further, said alloy could allow electrical steel producers to make new grades with higher Mn level and at the same keep the carbon content low in the steel using only one alloy additive.

[0057] Having described different embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.