Process for Producing Steel Having Improved Low Temperature Impact Characteristics

This invention relates to a process for the addition of calcium to molten steel and a calcium additive employed therefor.

For the purpose of improving the cold workability and impact characteristics of steel products, either titanium, zirconium or a rare earth element has been added to the molten steel to clean the steel or nodularate sulfides therein. It has been also known that calcium is effective for cleaning steel or nodularating sulfides. When added to molten steel by usual prior art processes, however, the calcium is largely consumed during the passage through the slag layer on the surface of the molten steel because of its strong reactivity and high vapour pressure at the metallurgical operation temperature. Hence, the addition of calcium to molten steel has not been put into practical operation because it is less effective and less stable than is the addition of cerium and other rare earth elements.

We have found that the yield and effectiveness of the calcium added depend largely on the sulfur and total oxygen contents of the molten steel and, as a result, have developed an improved process for the addition of calcium to molten steels.

It is an object of this invention to provide a practical process for adding calcium to molten steel.

It is another object of this invention to improve the yield and effectiveness of the addition of calcium by reducing the sulfur content, the total oxygen content and impurities by treating the molten steel before the addition of calcium or by modifying the slag.

It is a further object of this invention to provide novel additives for use in adding calcium to the molten steel.

Other objects and features of this invention will be apparent from the following detailed description with reference to the attached drawing, wherein:

FIG. 1 is a graphical representation of the relationship between the desulfurization effect (the ordinate) and the ratio CaO/SiO.sub.2 (the abscissa) of an artificial slag according to this invention, when the artificial slag containing calcium fluoride in the order of 20 to 30% by weight is added to a molten steel in an amount of 20 kg/ton of steel.

FIG. 2 is a graphical representation of the relationship between the dissolving speed of the artificial slag into the molten steel (the ordinate) and the content of alumina and magnesia in the slag (the abscissa).

FIG. 3 is a graphical representation of the relationship between the desulfurization effect of the artificial slag (in ordinate) and the added amount of the slag (in abscissa) to molten steel.

FIG. 4 shows a projectile according to this invention which is useful for introducing calcium into the molten steel.

Molten steel which is suitable for the application of the process of this invention contains, by weight, carbon 0.02 to 0.55%, silicon less than 0.50%, manganese 0.20 to 2.50% and conventional amounts of typical alloying elements, the balance being iron and an inevitable amount of impurities.

The lower limit of the carbon content is normally 0.02% in commercial steel-making processes. On the other hand, when the carbon content is larger than 0.55%, the effectiveness of the addition of calcium disappears substantially. The stated silicon content falls within the range specified in the JIS and API standards for hot-rolled steel sheet, seamless and welded steel pipe and tube and preferably ranges from 0.04 to 0.40% by weight. Abbreviations "JIS" and "API" as used herein signify, respectively, "Japanese Industrial Standards" and "American Petroleum Institute". The manganese content imparts mechanical strength to the resulting steel but a content thereof higher than 2.50% by weight reduces the effectiveness of the addition of calcium. The manganese is preferably contained in an amount from 0.75 to 1.70% by weight for improvement of impact characteristics. Other alloying elements may be contained in the steel in the following ranges by weight:

Cu: 0 - 0.50%

Cr: 0 - 1.50%

Mo: 0 - 0.50%

Ni: 0 - 10.0%

Nb: 0 - 0.050%

V: 0 - 0.1%

ti: 0 - 0.05%

In the process of this invention, it is essential to maintain the sulfur content at a value less than 0.010% by weight, preferably less than 0.007%, and the total oxygen content at less than 100 ppm. A sulfur content of higher than 0.010% makes the addition of calcum ineffective and if the suflur content is higher than 0.007%, the addition of calcium is les effective than the addition of cerium for improving the low temperature toughness and impact characteristics of the steel products. However, when the sulfur content is less than 0.007%, the addition of calcium according to this invention exhibits a more significnt effect than he addition of cerium with respect to the above mentioned improvement of the properties of steel products. When the total oxygen content is higher than 100 ppm, the calcium is largely consumed by combination with oxygen due to its strong affinity for the latter. As a result, a sufficient effect can not be expected in cleaning or nodularating sulfides in the molten steel.

At the time of the addition of calcium according to this invention, the temperature of molten steel is maintained in the range of 1480.degree. to 1800.degree. C for the following reason: At a temperature lower than 1480.degree. C, the ingot-making operation becomes difficult and calcium contaminates are not sufficiently able to float up to the surface of the molten steel, thus resulting in dirty steel ingots. On the other hand, if the temperature exceeds 1800.degree. C, the vapour pressure of the molten steel becomes so high that the calcium which has been added is evaporated out before the reaction with the sulfides in the molten steel can occur thereby causing a decrease in the yield from the calcium addition and a prominent loss in ladle refractories by fusion.

According to this invention, calcium is added in an amount ranging from 0.05 to 2.0 kg per ton of molten steel so that the calcium content becomes 0.05 to 0.8 times the sulfur content in the resulting steel product. The addition of calcium in a net amount of less than 0.05 kg/ton of steel is insufficient to clean or nodularate the sulfides which the molten steel contains at a sulfur content of about 0.010%. On the other hand the addition of calcium in a net amount of larger than 2.0 kg/ton of steel esults in a saturation effect and, therefore, the addition of an excessive amount makes the operation uneconomical.

According to this invention, there are provided pretreatments of the molten steel to which calcium is to be added for the purpose of enhancing the yield and effectiveness of the addition of calcium. The pretreatments include vacuum degassing, inert gas bubbling, aluminum deoxidizing and slag modifying processes.

The molten steel is preferably an aluminum killed steel and an acid soluble aluminum content higher than 0.005% by weight is preferable, because the soluble aluminum content decreases the ferrous oxide content of the slag and thus results in a decrease in the consumption of the slag and ladle refractories by oxidation. Further, the addition of calcium desulfurizes the molten steel in the presence of aluminum by forming contaminates of the Ca-Al-O-S system according to the following equation:

accordingly, the higher the aluminum concentration is, the lower the sulfur content in the molten steel becomes by the addition of calcium and the more the impact characteristics of the resulting steel products are improved.

When the steel product is specified not to contain aluminum, the molten steel should be subjected to a vacuum degassing process before the addition of calcium to decrease the total oxygen content to less than 100 ppm. The aluminum killed steel may be also subjected to the vacuum degassing process. When aluminum killed steel is vacuum degassed by the DH process, the degassing is preferably carried out under the condition of a final vacuum of less than 0.5 mm Hg in a vacuum degasser and a circulation ratio of higher than 1.5, with the result that the content of active oxygen becomes less than 10 ppm.

According to the present invention, the molten steel is preferably subjected to a bubbling process with an inactive gas, such as argon, to agitate the molten steel so that the remaining contaminates float up. When the molten steel is contained in a ladle of 25 to 300 tons capacity, the inactive gas bubbling process is carried out under the following conditions:

Pressure of inactive gas: 2.5 to 5.0 kg/mm.sup.2 gauge

Flow rate of inactive gas: 10 to 80 Nm.sup.3 /hr

Duration time of bubbling: 20 to 40 minutes

This inactive gas bubbling process, of course, may be carried out solely or in combination with an aluminum deoxidizing and/or vacuum degassing process.

According to this invention, it is preferable to effect a slag modifying process before the addition of calcium to suppress the formation of highly oxidizing slag. The slag modifying process is carried out by putting an artificial slag in an empty ladle and thereafter pouring molten steel into it. Such an artificial slag is of high basicity and comprises, by weight, lime 40 to 60%, magnesia 7 to 9%, alumina 15 to 25%, silica 3 to 5% and calcium fluoride 20 to 30%. The artificial slag of this composition is prepared from the slag which forms at the final stage of steel refining in an electric furnace.

As shown in FIG. 1, the desulfurization effect (which is represented by a ratio of the test result to the maximum desulfurization achieved all of the tests) begins to increase at a CaO/SiO.sub.2 ratio of 8 and reaches a maximum at a CaO/SiO.sub.2 ratio of 20. This experiment was conducted while maintaining the content of calcium fluoride at 20 to 30%. The desulfurization effect also reaches a saturation point when the slag contains from 40 to 60% by weight of lime, alumina and magnesia are included to lower the melting point of the artificial slag and the dissolving rate of the slag into the molten steel, as shown in FIG. 2.

It is preferable to control the size distribution of the artificial slag as follows:

The slag modifying process is also effective for the desulfurization of the molten steel and the desulfurization effect becomes significant at an addition of the artificial slag of 5 kg/ton of steel and reaches a saturation point at 20 kg/ton, as shown in FIG. 3. The artificial slag may have a chemical composition of line 55 to 70%, alumina 10 to 25% and calcium fluoride 3 to 14%.

This invention provides a method for the addition of calcium, wherein the calcium is not consumed while passing through the slag layer but at a sufficient depth in molten steel to have the desired effect.

According to a preferred embodiment of this invention, the addition of calcium is carried out by shooting projectiles of a calcium additive at an initial velocity of 20 to 100 m/sec from a launcher into molten steel in a ladle of 25 to 300 tons capacity. Such a ladle is 1 to 7 meters in height. At an initial velocity lower than 20 m/sec, the projectiles of calcium additive cannot penetrate into the molten steel to a sufficient depth, therefore, the projectiles float upwards to the slag layer before being completely dissolved, with the result that the effectiveness and yield of the addition of calcium is decreased. On the other hand, an initial velocity higher than 100 m/sec is unfavorable in the described ladle since the projectile collides against and damage the referactories in the bottom part of the ladle.

The projectiles of calcium additive which is suitable for use in the method described in the above has a diameter of 5 to 100 mm, preferably 25 to 50 mm, a length of 100 to 800 mm and comprises a shell and has a calcium additive contained therein. The shell of the projectile is composed of any one of the following materials:

Aluminum of a thickness--0.5 to 20 mm

Iron of a thickness--0.2 to 15 mm

Copper of a thickness--0.2 to 15 mm

Organic material of a thickness--0.2 to 20 mm

Fire-proof paper of a thickness--1.0 to 20 mm

The calcium additives contained in the projectiles include metallic calcium and calcium alloys such as Ca-Si or Ca-Ba-Si alloy. Representative compositions for the calcium additive are shown by the following examples by weight: ______________________________________ i) Ca More than 40% Si More than 40% Ba Less than 20% ii) Ca More than 40% Mg More than 10% Si Less than 10% ______________________________________

Other ingredients: Coating material of a high molecular weight organic compound ______________________________________ iii) Ca More than 25% Mg More than 7% Rare earth elements More than 15% Al More than 20% Si More than 20% iv) Ca 25 - 35% Mg 5 - 15% Rare earth elements 10 - 20% Si More than 10% ______________________________________

FIG. 4 illustrates one embodiment of a projectile according to this invention. The projectile 10 consists of a tip 11 made of a metal such as iron having a higher density than the materials of the other portions, a middle portion 12 of the shell which is made of the above-mentioned materials and has the calcium additive contained therein, and a plurality of rudders 13 attached to the end portion of the cannon ball 10 for stabilizing the flight. As mentioned above, the tip portion 11 is preferably made of a material of a higher density to maintain the posture of the projectile 10 perpendicular to the surface of the molten steel when the projectile is shot from a launcher.

According to another embodiment of this invention, the addition of calcium is performed by continuously feading the calcium additive in the form of a wire at a velocity of 10 to 100 m/sec into molten steel in a ladle having a capacity of 25 to 300 tons. The wire of calcium additive according to this invention comprises a hollow cylindrical shell having an outer diameter of 5 to 100 mm, preferably 25 to 50 mm and has the calcium additive contained therein. The material of the hollow cylindrical shell and its thickness are the same as in the case of the calcium additive of the projectile type. For the same reasons as with the projectile type, the feeding velocity of the wire is limited to the range of 10 to 100 m/sec.

The following specific examples are included merely to aid in the understanding of the invention, and variations therefrom may be made by those skilled in the art without departing from the spirit and scope of the invention.

Aluminum killed steels having the chemical compositions shown in Table 1 were prepared for use as the material of high strength line pipe in arctic sites. Additions of Ca, Ti, Zr and Ce were made respectively to the steel in the molten state. The resulting steels were rolled to plate of 11 mm in thickness under the same low, temperature controlled condition.

The mechanical properties of the rolled steel samples were determined and are shown in Table 2.

As seen from Talbe 2, all of the steel samples exhibit mechanical properties generally exceeding the values required for the line pipe materials. Particularly, Sample Nos. 6 to 8 exhibit values of shelf energy about twice those of Sample Nos. 1 to 5. The fracture transition temperature for Sample Nos. 6 to 8 is also excellently low as compared with Sample Nos. 1 to 5. Namely, at a sulfur level of 0.05%, the addition of calcium according to this invention is prominently effective for the improvement of the absorption shelf energy characteristic which cannot be achieved by the conventional addition of elements, such as titanium, zirconium and cerium.

Samples of molten steel each having the chemical composition shown in Table 3 were prepared in a high frequency induction furnace and had a calcium additive added thereto. The resulting steel samples were rolled to 17 mm thickness under the same low, temperature controlled conditions.

The mechanical properties of the rolled steel samples are shown in Table 4.

As may be readily seen from Table 4, the addition of calcium according to this invention (Sample Nos. 9 and 13) remarkably improved the impact characteristics. At the same sulfur level of 0.005% by weight, Sample No. 9 exhibited a larger absorption energy at -80.degree. C than did Sample No. 12. Namely, the addition of calcium in a net amount of less than 100 g/ton of steel did not improve the impact properties across the rolling direction. At a sulfur level of 0.010%, the addition of calcium in a net amount of higher than 100 g/ton of steel (Sample No. 18) was less effective than a cerium addition (Sample No. 16).

Steel samples were prepared each having the chemical composition as shown in Table 5. Calcium was added to Sample Nos. 21 and 25, and cerium was added to Sample No. 24. Thereafter, each of the samples was hot rolled at a finishing temperature of 800.degree. C and a coiling temperature of 570.degree. C, thereby obtaining a steel sheet of 6.0 mm thickness.

The following Table 6 shows the mechanical properties of the resulting sheets, which properties were determined across the rolling direction.

Charpy tests were conducted by means of half subsized test pieces. Notch elongation tests were conducted using test pieces of the same shape as the test pieces for the tensile test according to JIS No. 5 and cut with V-notches of 1 mm on the both sides of the center. The gauge length of the notch elongation test was 25 mm.

As seen from Table 6, remarkable improvements in notch elongation and charpy properties were achieved by the addition of calcium according to this invention.

Molten steel was prepared having a chemical composition of, by weight, carbon 0.05%, silicon 0.33%, Manganese 1.46%, phosphorus 0.016%, sulfur 0.005%, copper 0.006%, chromium 0.02%,, niobium 0.023%, soluble aluminum 0.038% and the balance being iron. The samples of this molten steel were respectively maintained at various temperatures as shown in Table 7 and had calcium added thereto by means of projectiles under the following conditions:

Calcium additive; metallic calcium

Size of the additive; 40 mm in outer diameter and 250 mm in length

Initial velocity; 50 m/sec

Net amount of added Ca; 200 g/ton of steel

Calcium content of the steel after the addition; 0.0031% by weight

Ca/S ratio of the steel after the addition; 0.6

Microcleanliness tests for nonmetallic inclusions in the resulting steel were conducted in accordance with JIS and the results obtained are shown in Table 7.

At temperatures of molten steel below 1480.degree. C, the effect of the addition of calcium is too fluctuant to put the calcium addition into practice.

Samples were prepared from molten steel consisting of, by weight, carbon 0.09%, silicon 0.28%, molybdenum 1.30%, phosphorus 0.017%, sulfur 0.006%, copper 0.02%, uranium 0.03%, soluble aluminum 0.041% and the balance being iron. Samples of the molten steel were respectively maintained at the temperatures shown in Table 8 and the addition of calcium was conducted by feeding into the samples a wire of a calcium additive under the following conditions:

Ca-ingredient: Ca alloy consisting of Ca 33%, Si 50% and the balance Fe.

Size of wire: 35 mm in diameter

Feeding velocity of wire: 35 m/sec

Net amount of added Ca: 250 g/ton of steel

Ca/S ratio of the steel after the addition: 0.5 .about. 1.5

Microcleanliness tests for nonmetalic inclusions in the resulting steel were conducted on each sample in accordance with the method of JIS and the results are shown in Table 8.

Samples of molten steel were prepared each having a chemical composition as shown in Table 9. Some of the samples were subjected to vacuum degassing by the DH method under the conditions shown in Table 10 thereby reducing the active oxygen content a.sub.o to less than 1 ppm. The active oxygen content in the molten steel was determined by an oxygen probe in accordance with the solid zirconia electrode method.

During the vacuum degassing, the samples of molten steel had the following alloys added thereto:

Low-carbon Si-Mn alloy---10 kg/ton

Low-carbon Fe-Mn alloy:--3 kg/ton

Fe-Nb:--0.4 kg/ton

Thereafter, samples of the molten steel had calcium added to them under the following conditions:

Ca additive, projectile comprising a shell of iron with a calcuim element contained therein.

Initial velocity; 50 m/sec

Net amount of added Ca; 270 g/ton of steel

The mechanical properties of each samples are shown in Table 11.

From these results, it can be seen that the low temperature impact characteristics were markedly improved in Sample Nos. 34 and 35 wherein the vacuum degassing had been performed before the addition of calcium as compared with Sample Nos. 36 and 37 wherein vacuum degassing had not been performed. Thus the vacuum degassing enhanced the effectiveness of the addition of calcium.

A ladle was charged with molten steel having a chemical composition of carbon-0.08%, silicon-0.30%, manganese-1.41%, phosphorous-0.018%, sulfur-0.008%, copper-0.02%, chromium-0.03%, niobium-0.023%, vanadium-0.06T, soluble aluminum-0.027%, the balance being iron. The molten steel was stirred by blowing argon gas therethrough for 20, 30, 40 or 50 minutes under the following conditions:

Blowing pressure: 3.5 kg/cm.sup.2

Flow rate of argon: 35 Nm.sup.3 /hr

Thereafter, the calcium addition was made to the steel samples which had been subjected to the gas bubbling under the above conditions and to a steel sample which had not been subjected thereto.

Additive: projectile

Temperature of molten steel: 1630.degree. C

Initial velocity: 50 m/sec

Net amount of added Ca: 270 g/ton of steel

Ratio of Ca/S: 0.33

The mechanical properties and cleanliness of the resulting steel samples are shown in Table 12.

It may be readily seen from Table 12 that the yield and effectiveness of the calcium addition are remarkably enhanced by the gas bubbling.

Molten steel was prepared having a chemical composition of by weight, carbon-0.07 to 0.09%, silicon-0.28 to 0.33%, manganese-1.33 to 1.41%, phosphorus-0.011 to 0.023%, sulfur-0.004 to 0.007%, niobium-0.020 to 0.021%, copper-0.02%, chromium-0.02 to 0.03%, soluble aluminum-0.018 to 0.045% and the balance being iron.

In addition, artificial slag having a composition of lime-45%, magnesia-7%, alumnia-20%, silica-35 and calcium fluoride-25% was put in a ladle in an amount of 5.0, 10.0, 20.0 or 30 kg/ton of steel. Thereafter, the ladle was charged with molten steel of the above composition and the calcium was added thereto under the same conditions. Table 13 shows the yield from the calcium addition, the cleanliness degree and the impact property of the resulting steel.