Improved resulfurized fine-austenitic-grain steel and process for obtaining it

Description

[0001] The present invention relates to an improved resulfurized, fine-austenitic-grain steel and to the relevant process used to obtain such a type of steel.

[0002] It is well-known that iron sulfide in steel generates an eutectic phase with metal iron, which displays a melting point at 988°C; therefore, the presence of iron sulfide in steel is harmful because it causes steel to display hot-shortness (i.e., high temperature brittleness) -- and steel forging and rolling temperatures are normally higher than 988°C.

[0003] Adding manganese to steel causes manganese sulfides to be formed which do not produce eutectic phases with iron and have higher melting temperatures than steel forging and rolling temperatures.

[0004] From technical literature, it is well-known as well that the minimal amount of manganese to be added to steel must, when expressed as percent level, be eight times as high as the percent level of present sulfur, so as to secure that steel will not display hot-shortness.

[0005] Manganese sulfides can be present in steel in three characteristic forms and are known as Type 1 sulfides, Type 2 sulfides and, respectively Type 3 sulfides.

[0006] Type 1 sulfides display a globular shape and are obtained in the presence of high oxygen levels (see, for example, unkilled steels, semi-killed steels and free-machining steels).

[0007] Type 2 sulfides display a dendritic structure and precipitate at the boundaries of the primary solidification grains; they appear in killed steels, with aluminum amounts which are just sufficient to deoxidize steel.

[0008] Type 3 sulfides are formed as the levels are increased of aluminum or other elements displaying high affinity for oxygen (titanium or vanadium), until such values are reached as to regulate the austenitic grain.

[0009] At present, steels used in the industry of motor vehicles in general and, in particular, in car industry, are resulfurized, fine-austenitic-grain, carbon and alloy steels suitable for casehardening and hardening-tempering treatments, containing additions of aluminum and/or titanium and/or niobium in such amounts as to secure the presence of fine austenitic grains (Type 3 sulfides).

[0010] However, it was demonstrated as well that the levels of grain regulating elements necessary for complete steel deoxidizing and for controlling the austenitic grain structure originate sulfides (Type 3 sulfides), which endanger the machinability properties of the resulting steel.

[0011] From this viewpoint, the most suitable sulfides for improving the machinability of steel are Type 1 sulfides which, on the other hand, cannot be obtained under such conditions as described in the preceding paragraph.

[0012] Consequently, the need arises for the development to be studied of a product which meets both above reminded requirements.

[0013] A proposed solution is adding to steel containing Type 3 sulfides, such metal elements as lead or tellurium, which are known to improve the machinability of steel, but are very dangerous for the health of those attending steel production and, then, users during steel processing.

[0014] The purpose of the present invention therefore is of providing a resulfurized, fine-austenitic-grain steel which obviates the above reminded drawbacks and, in particular, can be used by the motor vehicles industry in general and car industry in particular, because it is such as to allow a complete steel deoxidizing, a fine-austenitic-grain structure and a better machinability of steel to be obtained, as compared to the prior art.

[0015] Another purpose of the present invention is of providing a resulfurized, fine-austenitic-grain steel which is not dangerous to the health of those who produce it at the steel factory and use it during the following processing.

[0016] A further purpose of the present invention is of providing a suitable process for obtaining an improved resulfurized, fine-austenitic-grain steel, as disclosed herein.

[0017] Still a further purpose of the present invention is of providing a low cost, improved resulfurized, fine-austenitic-grain steel, without using complex and expensive technologies.

[0018] Such purposes are achieved by a resulfurized, fine-austenitic-grain steel according to claim 1 and a process for producing said steel according to claim 7, to which claims reference is made herein for the sake of brevity.

[0019] Advantageously, the addition of rare earths as plain metal wire or as a wire sheltered by metals, metal alloys and other deposited coatings (for example, ceramic coatings), to resulfurized, fine-austenitic-grain carbon steels and alloy steels in ingot mould or in tundish, makes it possible a good diffusion and homogenizing of the product, and a homogeneous distribution of the globular sulfides throughout the cast section, to be obtained.

[0020] In such a way, the sulfides contained in steel improve the steel machinability, as compared to the traditional techniques, while simultaneously securing the absence of hot-shortness, the regulation of the austenitic grain and the complete deoxidizing of steel.

[0021] According to as discussed hereinabove, adding manganese to steel secures the absence of hot-shortness; the Type 1 manganese sulfides display a globular structure and are obtained in the presence of high oxygen levels, whereas the Type 2 sulfides (which are formed in steels containing minimal amounts of aluminum) display a dendritic structure and precipitate at the boundaries of the primary solidification grains, with the drawback that the resulting steel will display a considerably high hot-shortness.

[0022] By increasing the amount of aluminum or other elements which display high affinity for oxygen (such as titanium or vanadium), up to such levels which allow the austenitic grain to be easily regulated (i.e., at values higher than 0.015% by weight), Type 3 sulfides appear which distribute randomly throughout steel with angular and irregular shapes.

[0023] After hot rolling, the Type 1 sulfides display a lenticular shape, whilst Type 2 and Type 3 sulfides turn into very thin bands or plaques; this feature of Type 2 and Type 3 sulfides contributes to improve the machinability of steel. In fact, during the machining, the cutting edge of the tool applies a force on its contact region of steel, causing microcracks to be formed in it.

[0024] It is evident that the presence of non-metal inclusions considerably reduces the necessary force for generating the microcracks, and the chip shape.

[0025] In those cases when the microcracks are such as not to cause the chip to break into fragments, problems will be met during the mechanical machining (turning, drilling).

[0026] When, on the contrary, said microcracks succeed in breaking the chip into fragments, the necessary force for creating said microcracks decreases and steel displays a better machinability.

[0027] In the subject case, Type 2 and Type 3 sulfides do not cause the chip to break into fragments.

[0028] On the contrary, it was observed that, for globular sulfides of Type 1, which display a coarser morphology, a considerably improved machinability of steel is obtained.

[0029] At present, the cristallographic form of manganese sulfides can be controlled by means of the addition of transition metals of Group III (lanthanides or rare earths); such an addition results in the formation of globular sulfides, but implies the formation of oxides, which obstruct the continuous casting nozzle bores.

[0030] In fact, rare earths display high affinity for oxygen (higher than of aluminum and magnesium), and, if they are present at high levels (0.03%-0.04% by weight), said rare earths undergo oxidation with simultaneously both alumina contained in steel in the form of inclusions and alumina which composes the refractory material of slabs, nozzle bores, and plugs being all reduced to aluminum metal.

[0031] Therefore, the continuous casting of steel cannot be performed if one wants to add to the ladle such an amount of rare earths as to completely bind sulfur, i.e., theoretically, an amount of 0.29 kg/t (kilograms per product ton) per each 0.01% by weight of sulfur contained in steel.

[0032] According to the present invention, on the contrary, the sulfides of Type 3 can be turned into globular sulfides and continuous casting steels can be obtained which display a better machinability, than corresponding steels known from the prior art.

[0033] The lanthanides, according to a preferred, non-limitative embodiment of the invention, are added in a sufficient amount to cause sulfides to turn into globular, as plain metal wires, or metal wires sheltered by metal elements, metal alloys or other deposited materials (for example, ceramic coatings).

[0034] In particular, the steel according to the present invention contains levels of lanthanides which are enough in order to obtain from 20% to 100% of globular sulfides in the solidified steel.

[0035] The desired amount of metal wire is metered to the ingot mould or to the tundish, with, in the latter case, particular refractory materials displaying the property of not chemically reacting with lanthanides being used for manufacturing slabs or nozzle bores.

[0036] So, globular sulfides are obtained in casehardening and hardening-tempering carbon steels and/or alloy steels with fine austenitic grain structure which contain amounts of sulfur equal to or higher than 0.02% by weight, produced by continuous casting, for use by mechanical industry in general and automobile industry in particular.

[0037] The above said steels display a fine austenitic grain structure and are obtained by means of the addition of such levels of metal elements, such as aluminum, titanium, niobium, vanadium, or alloys of these elements, that the end level of these elements in steel is higher than 0.015% by weight.

[0038] The presence of the above said metal elements in larger amounts than 0.015% by weight in steel causes, as already mentioned, sulfides (of Type 3) to be formed, the shape of which has a negligible influence on the machinability characteristics of steel.

[0039] The amount of rare earths to be added to the sulfides in the form of metal wire of any shapes (with circular, square, hexagonal cross-section, and so forth), to be added to the tundish or to the ingot mould, is comprised within the range of from 0.05 kg/t to 0.35 kg/t per each 0.01% by weight of sulfur.

[0040] The addition of rare earths as individual elements, or as alloys, in the form of metal wire to the ingot mould makes it possible lanthanides to be perfectly diffused an homogenized on the product and an equally homogeneous distribution of globular sulfides to be obtained throughout the cast section.

[0041] All the more reason, this homogeneousness of distribution is obtained if the addition is performed to tundish.

[0042] From the above disclosure, the characteristics will be clear of the improved, resulfurized, fine-austenitic-grain steel and of the process for obtaining it, which are the subject-matter of the present invention, as well as the advantages thereof will be clear.

[0043] In particular, said advantages are represented by:

• possibility of obtaining simultaneously resulfurized, fine-austenitic-grain, casehardening and hardening-tempering carbon steels and alloy steels which display a highly enough machinability, to be used by the mechanical industry and, in particular, by the automobile industry;
• reduced costs as compared to the prior art, on considering the obtained advantages.

[0044] Finally, it will be clear that a large number of modification can be supplied to the product and process which are the subject-matter of the present invention, without thereby departing from the principles of novelty of the inventive idea, as well as it is clear that when practicing the invention, the materials, shapes and forms, and size of disclosed products can be any, according to the requirements, and that the same can be replaced by other equivalent materials, and product shapes, forms and size.

Claims

1. Resulfurized, fine-austenitic-grain steel of the type of carbon steel and alloy steel suitable for casehardening and hardening-tempering treatments, produced by continuous casting, containing sulfur in amounts equal to or higher than 0.02% by weight and manganese in an amount which (as expressed as percent by weight) is equal to, or higher than, 8 times as high as the value of the sulfur percentage by weight, with the addition of metal elements in such amounts as to secure the fine-austenitic-grain structure, characterized in that said steel also contains transition metals of Group III (rare earths or lanthanides) in such amounts as to obtain from 20% to 100% of globular sulfides in the solidified steel.

2. Steel according to claim 1, characterized in that said metal elements comprise aluminum, titanium, niobium, vanadium or mixtures of the latters.

3. Steel according to claim 1, characterized in that said metal elements are present in a total amount equal to or higher than 0.015% by weight.

4. Steel according to claim 1, characterized in that the transition metals of Group III are present as metal wires of any shape, plain, or as a metal wire of any shape sheltered by metals, metal alloys or other deposited coatings, for example, ceramic coatings.

5. Steel according to claim 1, characterized in that the transition metals of Group III are added to the continuous casting tundish or to the continuous casting ingot mould.

6. Steel according to claim 1, characterized in that the transition metals of Group III are contained in amounts comprised within the range of from 0.05 kg/t (kilograms per ton of product) per each 0.01% by weight of sulfur to 0.35 kg/t per each 0.01% by weight of sulfur.

7. Process for obtaining carbon steels and alloy steels suitable for casehardening and hardening-tempering treatments with a fine austenitic grain structure in a percent equal to or higher than 0.015% by weight, produced by continuous casting, characterized in that said process comprises the following steps:

- addition of manganese to steel, so as to cause manganese sulfides to be generated;

- addition of aluminum, titanium, niobium, vanadium or mixtures of these elements in a total amount equal to, or higher than, 0.015% by weight, so as to regulate the austenitic grain;

- addition of transition metal of Group III (rare earths or lanthanides) in the form of metal wires of any shape, either plain or sheltered by metals or other deposited coatings, for example, ceramic coatings, to the continuous casting ingot mould or continuous casting tundish in amounts comprised within the range of from 0.05 kg/t (kilograms per ton of product) per each 0.01% of sulfur, to 0.35 kg/t per each 0.01% of sulfur.