MXPA98010838A - Alloy steel with ferritic chrome, without str - Google Patents

Abstract

Stainless steel is provided, without flute, and process for its elaboration. A chromium alloy steel melt containing sufficient titanium and nitrogen, but a controlled amount of aluminum, is either emptied into an ingot or is commonly cast into a strip or plate having a fine equational grain structure as it is emptied, substantially free of columnar grains. The steel as it is emptied contains 0.08% C, at least about 8% Cr, up to 1.50% Mn, < 0.020% Al, < 0.05% N, < 1.5% of Si, < 2.0% Ni, Ti > 0.10%, the ratio of (Ti.N) / Al > 0.14, all percentages by weight, the rest as Fe and residual elements. Preferably, the titanium is controlled so that (Ti / 48) / (c / 12) + (N / 14) > 1.5. A hot processed sheet can be formed from a continuously poured plate without grinding the surfaces of the plate. The hot processed sheet from a continuously emptied plate without rectifying the surfaces of the plate. The hot-processed sheet can be de-embedded, reduced in cold to a final thickness and annealed by recrystallization. The annealing of the hot-processed sheet before the cold reduction is not required to obtain an annealed sheet essentially free of flutes and having a high aptitude for forming.

Description

ALLOY STEEL WITH FERRITIC CHROME, WITHOUT STRESS ; # ¥ ^ * »» jaíí ¥ a DB IA INVENTION This invention relates to alloyed steel with ferritic chromium formed from a melt having a fine equiaxed grain structure as it is emptied. More particularly, this invention relates to a ferritic chromium alloy steel formed of a melt containing sufficient titanium and nitrogen, but a controlled amount of aluminum to form small inclusions of titanium oxide to provide the nuclei necessary to form the equiaxed grains as it is empty A hot-processed sheet produced from steel having this equiaxed cast grain structure is especially suitable for producing a cold-reduced, recrystallized annealed sheet having excellent non-fluted characteristics and suitability for stretch forming, even with a annealed in hot band or intermediate annealing. It is desirable for a high-strength ferritic stainless steel, in addition to having a high plastic traction ratio, to minimize a phenomenon known as "fluted", "stringed" or "fluted". Unlike austenitic stainless steel, the unpleasant flute can appear on the surfaces of ferritic stainless steel sheets annealed by recrystallization, cold reduced after having been REF: 28729 cold formed in one part, the fluted is characterized by the formation of projections, grooves or corrugations which extend parallel to the rolling direction of the sheet. This effect is not only damaging to the surface appearance of the sheet but also results in an attitude for the lower stretch conformation. Ferritic chromium alloy steels, especially ferritic chrome-alloyed steels at sub-equilibrium, such as the type 409 and 439 stainless steels, regardless of whether they are continuously poured into plates of 50-200 mm thickness or strip cast in thicknesses of 2-10 mm , typically has large columnar grains as it empties. These large columnar grains have a crystallographic texture almost in the form of a cube in the face which leads to a very undesirable fluted characteristic in an annealed, cold rolled final film, used in various manufacturing applications. The appearance of the surface results in this fluted is highly objectionable in exposed shaped parts such as caps, automotive trimmings, tailpipes and end cones, stamping furnaces, oil filters and the like. The striated causes the sheet to have an irregular, rough surface appearance after its formation and a large non-uniform grain structure or "in band" present after cold rolling and annealing is attributed, resulting in the initial presentation of a columnar grain structure in the steel as it empties.

To minimize the presentation of flutes, additional costs are incurred when annealed a hot rolled sheet before cold reduction. This additional annealing step of hot-rolled ferritic stainless steel also results in a reduced forming ability caused by average and lower tensile ratios, ie, R ,,,, which degrades the ability for deep shaping. A hot-rolled sheet that is annealed before cold reduction must be cold reduced by at least 70% to offset the loss of R ,,, caused by hot strip annealing before final annealing. Over the years, there have been numerous attempts to eliminate the aforementioned processing requirements and the expense to remove the flute by modifying the alloy composition of the ferritic stainless steel. It is known that the fluted in ferritic stainless steel originates mainly during hot rolling. There have been attempts to minimize the flute by forming a fine equiaxed grain structure in a cast ingot by controlling the chemistry of the melt, for example, one or more of the impurities of C, N, O, S, P, and by refining the grain structure, by using lower hot rolling temperatures, for example 950-1100 ° C. The control of the chemistry during the refining has produced certain improved characteristics as regards the fluted for ferritic stainless steels due to the formation of a second phase, that is, the austenite at high temperatures which becomes martencite at room temperature, however, the formation of this second phase has been at the expense of the elongation to the traction and the operation in welding of the final products. Temperature control during hot rolling results in operational difficulties as well, since more hot rolling power is required. Consequently, the thicknesses of hot rolled sheet must be greater. Therefore, hot rolling must be followed by cold rolling at least two stages with a second intermediate annealing between the two cold rolled ones. U.S. Patent 5,769,152 recognizes that columnar grains are not desirable in continuously cast stainless steel. This patent suggests that columnar grains should be avoided and that equiaxed grains should be formed instead of these, when casting molten steel using a temperature with a superheat less than 0-15 ° C above the liquids and magnetic stirring of the molten steel in a casting mold. Others have attempted to eliminate the fluted by modifying a ferritic stainless steel alloy composition by the addition of one or more stabilizing elements. U.S. Patent 4,465,525 relates to ferritic stainless steel which has excellent formability and improved surface quality. This patent describes that boron in amounts of 2-30 ppm and at least 0.005% aluminum can increase the elongation and the Rm as well as decrease the characteristics of flute. U.S. Patent 4,515,644 relates to deep drawn ferritic stainless steel having an improved striated quality. This patent describes that - in addition to aluminum, boron, titanium, niobium, zirconium and vanadium, all can increase the elongation of ferritic stainless steel, increase R ,,, and improve anti-stress properties. More specifically, this patent discloses ferritic stainless steel having at least 0.01% Al that has improved anti-stretch characteristics. U.S. Patent 5,662,864 relates to the production of ferritic stainless steel which has good striating characteristics when carefully controlling Ti, C + N and N / C. This patent discloses that striating can be improved due to the formation of carbonitrides by adding Ti in response to the C + N content in a melt. The steel melt contains < . 0.01% C, < 1.0% Mn, < 1.0% Si, 9-50% Cr, < .0.07% of Al, 0.006 < _. C + N =. 0.025%, N / C > 2, (Ti-2S-30) / (C + N) < . 4 y Tí x N < 30 x 10"4. US Pat. No. 5,505,797 relates to the production of ferritic stainless steel having reduced intra-shell anisotropy and excellent grain structure This patent discloses good striating characteristics which are obtained when the molten steel preferably contains 0.0010- 0.008% C, 0.10-1.50% Mn, 0.10-0.080% Si, 14-19% Cr, and 2 or more than 0.010-0.20% Al, 0.050-0.30% Nb, 0.050-0.30% Ti and 0. 050-0.30% of Zr. The steel is emptied into a plate and hot rolled to a sheet having a thickness of 4 mm, the strip is annealed hot, deoxidized to acid, cold rolled and annealed for finishing. The plate is heated to 1200 ° C and subjected to at least one rough hot rolling that passes at a temperature between 970-1150 ° C. The friction between the hot grinding rolls and the hot rolled steel is 0.3 or less, the roll reduction ratio is between 40-75% and the hot rolling finish temperature is 600-950 ° C. The hot-rolled steel is annealed at a temperature of 850 ° C for 4 hours, cold reduced to 82.5% and annealed for finishing at a temperature of 860 ° C for 60 seconds. It is known that when the solubility product of the titanium compounds exceeds the saturation concentration at the liquid temperature, that is to say, to the hyperebalance, for stainless steels stabilized with titanium, the titanium compounds are stable and TiN will precipitate before freezing the metal. The steel sheets produced from plates at the hyper-equilibrium show improved striating characteristics and conformability. However, when freezing, TiN coalesces into large groups and floats on the surface of the cast plate. These non-metallic TiN groups form an unacceptable open surface, a defect known as Ti flutes during hot rolling. These large non-metallic groups must be removed from the plate by expensive conditioning of the surface such as grinding before hot plate processing. U.S. Patent 4,964,926 relates to welded double stabilized ferritic stainless steel having an improved surface quality by eliminating the formation and precipitation of non-metallic titanium oxides and titanium nitrides during casting by forming ferritic stainless steel stabilized with titanium to the subequilibrium. This patent describes that it is known that stringing characteristics can be improved by adding niobium only or niobium; and copper to ferritic stainless steel. However, the addition of niobium only causes fracture in welds. U.S. Patent 4,964,926 describes replacing a portion of a titanium stabilizer with a niobium stabilizer to form double stabilized ferritic stainless steel. The addition of at least 0.05% titanium to steel stabilized with niobium eliminates fracture by welding. The pending North American patent application 08 / 994,382, filed on December 19, 1997, entitled "Non-Ridging Ferritic Chromium Alloyed Steel", incorporated herein by reference, relates to an alloy steel with ferritic chromium, deoxidized with titanium, formed from a melt that has a fine equiaxed grain structure as it empties. This application describes an alloy steel with ferritic chromium formed from a titanium-deoxidized melt and containing no more than 0.01% by weight of aluminum. A hot processed sheet is produced from steel having this equiaxed grain structure as it is emptied is especially suitable for cold-reduced recrystallization annealed sheet having excellent formability, stretch and no flute characteristics, and no additional processing steps, such as hot band annealing or intermediate annealing. The minimization of the flute by the prior techniques has sacrificed the cost and the aptitude for the annealing conformation of hot rolled ferritic stainless steel before the cold reduction. This additional annealing step reduces the formability by decreasing the average Rm. In addition, hot-rolled steel, pre-annealed must be reduced by at least 70% to obtain an R ,,, after a final annealing similar to the Rm for hot-rolled steel that is not otherwise annealed before cold reduction . This higher percentage of cold reduction generally also requires an intermediate annealing step. As evidenced by the endless exhaustive search of other investigators, there still remains for a long time the need for an alloy steel with ferritic chromium annealed, essentially free of striated and having excellent characteristics of fitness for deep shaping such as an R ,, , elevated, a high tensile elongation and an evenly annealed grain structure, furthermore, an additional need remains for a stainless steel ferritic with aptitude for excellent deep shaping that has good flute characteristics and does not require that the hot-processed sheet be annealed before cold reduction. In addition, there is an additional need for a ferritic stainless steel for sub-equilibrium, with excellent deep-shaping ability having good spline characteristics, formed from hot-processed sheets having no surface defects, ie, titanium nitride inlays and titanium oxide striations, without requiring conditioning of the surfaces of a continuously emptied plate prior to the hot processing of the plate.

BRIEF DESCRIPTION OF THE INVENTION A main objective of this invention is to provide a sheet of alloy steel with ferritic chrome and with excellent ability for deep shaping, with good spline characteristics without requiring the hot-processed sheet to be annealed before cold reduction. Another object of this invention is to provide an alloy steel sheet with ferritic chromium with good spline characteristics and with an improved grain structure and a high tensile elongation characteristic, without requiring the hot processed sheet to be annealed before the reduction in cold.

Another object of this invention is to provide an alloy steel sheet with ferritic chromium with excellent suitability for deep and stretchable shaping, without requiring multiple cold reductions with annealing between the cold reduction stages. Another object of this invention is to form an alloy steel sheet with ferritic chromium from a continuously emptied plate which does not require surface conditioning before hot processing of the steel plate. Another objective of this invention is to provide an alloy steel sheet with ferritic chromium with excellent suitability for deep and stretchable shaping, with good flute characteristics formed from a continuously emptied plate which does not require surface conditioning before hot processing of the Steel plate. Additional objectives include providing an alloy steel sheet with ferritic chromium with excellent deep shaping ability, with good flute characteristics having improved weldability, corrosion resistance and cyclic oxidation resistance at high temperature. The invention relates to a steel alloyed with ferritic chromium and a process for producing the steel with a structure as it is emptied, having more than 50% equiaxed grains. The steel as it is emptied is deoxidized with titanium and contains up to 0.08% C, at least about 8% Cr, up to 1.50% Mn, < 0.05% N, < _. 1.5% of Si, < 2.0% Ni, Ti 0.10%, where the proportion of (Ti x N) / Al is at least 0.14, all percentages by weight, the rest in Fe and residual elements. The steel as it is emptied is processed hot in a continuous sheet. The sheet can be descaled, cold reduced to a final thickness and then annealed by recrystallization. The annealing of the hot-processed sheet before the cold reduction or the annealing of the sheet between multiple stages of cold reduction is not necessary to eliminate the fluted in the final annealed sheet. Another feature of this invention is that the Ti mentioned above is. 0.15% and aluminum is < 0.02% Another feature of this invention is that the ratio mentioned before (Ti x N) / Al is at least 0.20. Another feature of this invention is that the Ti mentioned above satisfies the ratio (Ti / 48) / [(C / 12) + (N / 14)] > 1.5. Another feature of this invention is that the aforementioned Ti and N are present in subequilibrium amounts. Another feature of this invention is that for the cold-reduced annealed sheet mentioned above it has an R ,,, _ > 1.4 when produced from a hot processed sheet which has not been annealed before cold reduction.

Another feature of this invention is that the equiaxed grains as they are emptied, mentioned above, have a size < 3 mm. The advantages of this invention include an alloy steel with ferritic chromium highly conformable for shaping with excellent flute characteristics that are less expensive to manufacture, which does not require that the hot-processed sheet be annealed before cold reduction, which does not require the annealing of a multistage cold reduction sheet having an improved surface quality and improved weldability, a good resistance to wet corrosion and having a good resistance to cyclic oxidation at high temperature. Another advantage is that it is capable of emptying a plate that does not require surface conditioning, for example, grinding, before hot processing to avoid the formation of open surface defects extending parallel to the rolling direction in a sheet. hot-rolled such as hot-rolled scale and rolled grooves from non-metallic titanium oxide or titanium-nitride group-type precipitates formed near a plated surface during casting. Another advantage of this invention includes an alloy steel sheet with ferritic chromium, highly formable with excellent spline characteristics having a very uniform grain structure in the sheet after annealing.

The foregoing and other objects, features and advantages of this invention will become apparent upon consideration of the detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a photograph of a grain structure as emptied, containing 100% large columnar grains for an alloy steel with ferritic chromium containing a proportion of the product of titanium and nitrogen divided by aluminum of 0.13, Figure 2 is a photograph of an empty structure containing approximately 78% fine equiaxed grains for an alloy steel with ferritic chromium having a titanium to nitrogen product ratio divided by 0.16 aluminum, Figure 3 a photograph of a structure as shown in FIG. empty, which contains 100% large columnar grains for an alloyed steel with ferritic chromium having a proportion of the product of titanium and nitrogen divided by aluminum of 0.013, Figure 4 is a photograph of a structure as it is emptied, containing approximately 84% fine equiaxed grains for an alloy steel with ferritic chromium that has a proportion of the product of titanium and nitroge not divided by aluminum of 0.15, Figure 5 is a photograph of an empty structure containing 100% large columnar grains for an alloy steel with ferritic chromium having a titanium to nitrogen product ratio divided by 0.12 aluminum, Figure 6 is a photograph of a structure as it is emptied, containing approximately 92% fine equiaxed grains for an alloy steel with ferritic chromium having a titanium and nitrogen product ratio divided by 0.19 aluminum, Figure 7 is a photograph of a structure as shown in FIG. empty, containing approximately 94% of large columnar grains for an alloy steel with ferritic chromium having a proportion of the product of titanium and nitrogen divided by aluminum of 0.11, Figure 8 is a photograph of a structure as it is emptied, containing about 63% fine equiaxed grains for an alloy steel with ferritic chromium that has a proportion of the tita product nio and nitrogen divided by aluminum of 0.15, Figure 9 is a photograph of a structure as it is emptied, containing approximately 100% of large columnar grains for an alloy steel with ferritic chromium having a proportion of the product of titanium and nitrogen divided between 0.06 aluminum, Figure 10 is a photograph of a structure as it is emptied, containing approximately 100% fine equiaxed grains for an alloy steel with ferritic chromium having a proportion of the product of titanium and nitrogen divided by aluminum of 0.34, Figure 11 is a photograph of a non-uniform grain structure of alloyed steel with ferritic chromium comparative of Figure 9, after cold reduction and annealing by recrystallization, Figure 12 is a photograph of a uniform fine grain structure of alloy steel with ferritic chrome of Figure 10 after cold reduction and annealing by recrystallization, Figure 13 is a graph illustrating the% equiaxed grains (% EQ) in the grain structure as it is emptied, as a function of the product proportion of percentages by weight of titanium and nitrogen divided by aluminum (TNA) for laboratory ingots cast from alloy steel with chromium ferritic, and Figure 14 is a graph illustrating the% equiaxed grains (% EQ) in the grain structure as it is emptied, as a function of the product ratio of the weight percentages of titanium and nitrogen divided by aluminum ( TNA) for continuous plates emptied from alloy steel with ferritic chromium. tr.qrt? tprtprc pKTftTrT? ftT- OF THE PREFERRED MODALITY This invention relates to a highly formable ferritic chromium alloy steel sheet produced from steel having a structure as it is emptied of fine equiaxed grains. Steel is cast from a melt containing enough titanium and nitrogen, but a controlled amount of aluminum to form small inclusions of titanium oxide to provide the cores necessary to form the equiaxed grains structure as they are emptied so that an annealed chrome alloy sheet, produced from this steel has improved characteristics of fluted. By forming a ferrous alloy with chromium rich in small inclusions of titanium oxide instead of large groups of alumina inclusions, a grain structure can be formed as it is emptied having more than 50% equiaxed fine grains (% EQ) . By preventing the formation of large columnar grains in the steel as it is emptied, the striating is minimized in an annealed sheet by recrystallization, cold rolled, produced from the steel, even when a hot-processed sheet from the steel is not annealed before the cold reduction. By alloy steel with ferritic chromium it is meant to include an alloy steel with at least about 8% chromium. The ferritic chromium alloyed steels of this invention are especially suitable for sheets processed in hot, cold reduced sheets, metallic coated sheets and painted sheets. These steels alloyed with ferritic chromium are suitable for stainless steels of the AISI type 400 series containing approximately 10-25% Cr, especially stainless steels type 409, containing approximately 11-13% Cr. For this invention, it will also be understood that by the term "sheets" is meant to include continuous strips or lengths cut or formed from a continuous strip. A ferrous melt is provided in a melting furnace such as an electric arc furnace (EAF). This ferrous melt can be formed in the melting furnace from solid iron that presents waste, carbon steel waste, stainless steel waste, solid iron containing materials including iron oxide, iron carbide, direct reduction iron , hot briquetted iron or melt can be produced upstream of a melting furnace in a blast furnace or in any other iron melting unit capable of providing a ferrous melt. Subsequently, the ferrous melt will be refined in the melting furnace or transferred to a refining vessel such as an argon-oxygen-decarburization (AOD) vessel or a vacuum-oxygen-decarburization (VOD) vessel, followed by a station of conditioning such as a casting metallurgy furnace (LMF) or a wire feeding station.

An important feature of this invention is that after refining the melt to a final carbon analysis and during and after conditioning, alloys are added to the melt to meet the final specification, and titanium is added to the melt for deoxidation before casting. The deoxidation of the melt with titanium is necessary to form small inclusions of titanium oxide and thus form the necessary nuclei to form an equiaxed fine grain structure as it is emptied. To provide sufficient quantities of these cores necessary to form an equiaxed fine grain structure as it is emptied, at least about 0.10% Ti is necessary in the melt. Preferably the aluminum is not added to this refined melt as a deoxidant to minimize the formation of alumina inclusions, i.e., aluminum oxide, A1203. An equally important feature of this invention is that sufficient titanium and nitrogen must be present in the melt before it is drained, so that the proportion of the titanium and nitrogen product divided by the residual aluminum (TNA) is at least about 0.14. . By controlling this ratio to less than 0.14, it is considered that the nitrogen in the melt forms small inclusions of titanium oxide coated with titanium nitride, which ensures the small nucleation sites necessary to form the fine equiaxed grains as they are emptied. If the steel is to be stabilized, a sufficient amount of the titanium can be added beyond the necessary for deoxidation, that is, 0.10%, to be combined with carbon and nitrogen in the melt, but preferably lower than that necessary for nitrogen saturation, ie, the subequilibrium, and in this way avoid the precipitation of large nitride inclusions of titanium before solidification. Alternatively, one or more stabilizing elements such as niobium, zirconium, tantalum and vanadium can also be added to the melt. Accordingly, the steel of this invention has at least 0.10% Ti, preferably at least 0.005% N and preferably less 0.02% Al in the melt so that the steel is deoxidized essentially by titanium with small inclusions of titanium oxide constituting the dominant inclusions in the melt, i.e., titanium oxide inclusions > > inclusions of Al203, to provide the necessary nuclei to form an equiaxed grain structure as it is emptied. Ferritic chrome-alloyed steels deoxidized with aluminum instead of titanium can have small inclusions in a melt. Nevertheless, a major difference between the deoxidized chromium and ferritic steels to the prior art aluminum, compared to the titanium deoxidized ferritic chromium steels of this invention is that most of the inclusions of the steel melts of the invention are based on titanium oxide, instead of based on aluminum. We have determined that at least 50% of the inclusions of the steels of this invention have a particle size no greater than about 1 μm and at least 90% of these inclusions have a size no greater than about 1.5 μm. It is not clear to which shape or forms of titanium oxide, ie, TiO, Ti02, Ti203, Ti305 are present, but it is considered that the main inclusions present are TiO. After having been refined and alloyed with chromium in a melting or refining vessel, the ferrous steel alloy with chromium will be deoxidized with titanium and will contain up to 0.08% C, at least about 8% Cr, up to 1.50% from Mn, < 0.03% Al, < 0.05% N, < _. 1.5% of Si, < 2.0% Ni, Ti > . 0.10%, all percentages by weight, the rest as Fe and residual elements. The product ratio of the percentages by weight of titanium and nitrogen, divided by the residual aluminum should be at least about 0.14. The cast alloy steel with chrome can be continuously cast in a sheet, a thin plate of < . 140 mm, a thick plate of < 200 mm or can be emptied into an ingot having a grain structure as it is emptied, formed of more than 50% fine equiaxed grains. Preferably, the steel melt has a product ratio of the weight percentages of titanium and nitrogen divided by the residual aluminum, of at least 0.16, more preferably of at least 0.23, and a void that forms a structure as it is emptied of At least 80% of fine equiaxed grains and essentially all fine equiaxed grains, respectively. We have determined that the proportion of the titanium and nitrogen product divided by the residual aluminum, necessary to obtain an equiaxed grain as it is emptied is also related to the chromium content of the steel. For a T09 stainless steel containing approximately 11% chromium, the ratio of the product titanium and nitrogen divided by residual aluminum to obtain more than 50% equiaxed grains as they are emptied is at least about 0.14, and for get almost 100% equiaxed grains as it is emptied, it is over 0.23. For a T430 stainless steel containing high chromium concentration of at least approximately 16%, and for T439 stainless steel containing high concentration of chromium, of at least approximately 17%, tables 3 and 4 demonstrate the proportion of the product of titanium and nitrogen divided among the residual aluminum to obtain more than 50% equiaxed grains as they are emptied, which is greater than about 0.20, and to obtain nearly 100% equiaxed grains as they are emptied, is greater than about 0.30. The drained steel is processed in cal belly in a sheet. By the term "hot-working" it will be understood that the steel as it is emptied will be reheated, if necessary and then reduced to a predetermined thickness for example by hot rolling.If it is not hot rolled, the steel plate is reheated to 1050-1300 '° C, hot rolled using a finish temperature of at least 800 ° C and rolled up to a temperature < 580 ° C. The hot rolled sheet, for example "hot strip" can be descaled and cold reduced by at least 40%, preferably at least 50% up to a final desired sheet thickness. Subsequently, the cold reduced sheet will be annealed by recrystallization for at least 1 second at a metal peak temperature of 800-1000 ° C. A significant advantage of this invention is that it is not required that the hot processed sheet be annealed before this cold reduction. Another advantage of this invention is that the hot-processed sheet can be cold reduced in one step and thus an intermediate annealing between multiple cold reductions is not required. Annealing by recrystallization followed by cold reduction may be a continuous annealing or annealing in batches. Another advantage of this invention is that the annealed steel sheet, alloy with chrome with excellent flute characteristics has a very uniform fine grain structure with as little as 40% cold reduction. The alloy steel with ferritic chromium of the present invention can be produced from a hot-processed sheet made by numerous methods. The sheet can be produced from plates formed of ingots or plates continuously cast 50-200 mm thick which are reheated to 1050-1300 ° C followed by hot rolling to provide a hot-processed, initial 1-6 mm thick sheet or the sheet can be hot processed from the continuously emptied strip in thicknesses of 2-10 mm. The present invention is also applicable to sheets produced by methods wherein the continuously emptied plates or the plates produced from ingots are fed directly to a hot rolling mill, with or without significant overheating, or the ingots are hot reduced in hot plates. enough temperature to be rolled hot in a sheet, with or without additional reheating. An important feature of this invention is that titanium is used for deoxidation of the melt before casting. Titanium is used for deoxidation, to ensure that the dominant inclusions in the melt are small inclusions of titanium oxide to nucleate the equiaxed ferrite grains as they are emptied. The amount of titanium in the melt will be at least 0.10% and preferably is an amount at the sub-equilibrium. More preferably, the amount of titanium in this steel melt will be j > 0.15% and satisfies the relation (Ti / 48) [(C / 12) + (N / 14) > 1.5. By "unbalance" is meant that the amount of titanium is controlled so that the solubility product of the titanium compounds formed is below the saturation concentration at the liquid temperature of the steel and thus excessive precipitation is prevented of TiN in the melt. If excessive inclusions of TiN are allowed to form, the TiN precipitate grows in large low density groups, which float to solidify on the surface of the plate during continuous emptying. These non-metallic TiN groups form defects in the open surface during the hot processing of the plate. The amount of titanium allowed in the melt to avoid excessive precipitation is inversely related to the amount of nitrogen. The maximum amount of titanium for "underbalance" is generally illustrated in Figure 4 of U.S. Patent No. 4,964,926, incorporated herein by reference. Based on the chromium and nitrogen content of a molten steel alloy, the amount of titanium must be controlled to less than that indicated by the curves in Figure 4 in U.S. Patent No. 4,964,926. The T409 stainless steel containing about 12% Cr and 0.010% N can contain up to about 0.26% Ti. Stainless steel containing approximately 15% Cr and 0.10% N may contain up to about 0.30% Ti. T439 stainless steel containing approximately 18% Cr and 0.10% N may contain up to about 0.35% Ti. Excessive nitrogen is not a problem for those manufacturers that refine ferritic stainless steel melts into an AOD. Nitrogen substantially less than 0.10% can be obtained when the stainless steel is refined in an AOD, and thus an increased amount of titanium is allowed to be tolerated and still be in the sub-equilibrium.

In order to provide the nucleation sites necessary to form equiaxed ferrite grains as it is emptied, a sufficient time must elapse after the preparation of the titanium addition to the melt to allow the titanium oxide inclusions to form before casting of the melt. If the melt is emptied immediately after adding titanium, the structure as it is emptied of the emptying will be large columnar grains. Ingots cast in the laboratory less than 5 minutes after adding the titanium to the melt have large columnar grains as it is emptied, even when the product of titanium and nitrogen divided by the residual aluminum is at least 0.14. An important feature of this invention is that sufficient nitrogen must be present in the steel before casting so that the ratio of the product of titanium and nitrogen divided by the aluminum is at least about 0.14. By controlling this ratio, it is considered that sufficient inclusions of titanium oxide are formed which ensures the necessary nucleation sites to form the equiaxed grains as it is emptied. The amount of nitrogen present in the melt should be <0.05%, preferably 0.005-0.03% and more preferably 0.007-0.015%. It is considered that small inclusions of titanium oxide coated with titanium nitride are responsible for providing the nucleation sites necessary for the formation of a fine-grained structure equiaxial as it is emptied. By carefully controlling the amounts of titanium and nitrogen in the melt, it is considered that sufficient small inclusions of titanium oxide having a size smaller than 1 μm provide the necessary nucleation sites responsible for the equiaxed fine grain structure as it is emptied. It is possible to control a steel alloy composition with respect to N and the amount at the Ti sub-equilibrium to eliminate the excessive precipitation of TiN and the formation of Ti flutes in the hot-processed sheet. Although concentrations of N after melting in an EAF can be as high as 0.05%, the amount of dissolved N can be reduced during refining with argon gas in an OOD to less than 0.02% and, if necessary, to less than 0.01% Excess precipitation of TiN can be avoided by reducing the amount of Ti unbalance that is added to the melt for any given nitrogen content. Alternatively, the amount of nitrogen in the melt can be reduced by an AOD for an anticipated amount of Ti contained in the melt. For a T409 stainless steel to the sub-equilibrium containing approximately 11-13% Cr and not more than approximately 0.012% N, the steel melt will contain less than approximately 0.25% Ti, to avoid excessive precipitation of TiN before solidification of cast. For a stainless steel T430 or T439 to the sub-equilibrium that contains approximately 16-18% of Cr and not more than approximately 0. 012% N, the steel melt will contain less than about 0.35% Ti to avoid excessive precipitation of TiN before melt solidification. An equally important feature of this invention is to control or minimize the total residual aluminum relative to the amounts of titanium and nitrogen. The minimum amounts of titanium and nitrogen must be present in the melt in relation to aluminum. We have determined that even low amounts of aluminum, that is, no greater than 0.01% will not produce the grains as empty equiaxes pre-requisite the amounts of titanium and especially nitrogen are too low. Apparently, a threshold amount of small precipitates of titanium inclusions is required in the melt even in the absence of alumina inclusions, to form the nucleation sites necessary to form the equiaxed grain structure as it is emptied. We have determined that the proportion of the product of titanium and nitrogen divided by the residual aluminum should be at least about 0.14, preferably at least 0.23 to ensure almost 100% of grains as equiaxed are emptied. In order to minimize the amounts of titanium and nitrogen required in the melt, the amount of aluminum is preferably < 0.020%, more preferably < .0.013% and much more preferably reduced to < .0.010%. If the aluminum is not deliberately alloyed with the melt during refining or pouring, for example by deoxidation immediately before emptying, it can be control or reduce total aluminum to less than 0.010%, especially for stainless steels containing less than 0.14% Cr. For a stainless steel containing high concentration of chromium, ie Cr 15%, which requires the proportion of (Ti x N) / Al > 0.40 to obtain almost 100% fine equiaxed grains as they are emptied, it may be necessary to add nitrogen to the melt by more than 0.01%. Preferably, the aluminum is not inadvertently added to the melt as an impurity present in an alloy addition of another element, for example, titanium. Preferably the use of titanium alloy additions containing an aluminum impurity should be avoided. Titanium alloys can contain up to 20% Al which can contribute up to 0.07% of Al to the melt. By carefully controlling the refining and casting practices, a melt can be obtained that contains < 0.020% aluminum-. Without joining any theory, it is considered that total aluminum, especially for stainless steels containing less than 14% Cr, should be controlled at less than 0.03%, preferably at less than 0.02%, more preferably at not more than 0.013%, and at much more preferable to less than 0.01% to minimize the formation of Al203 inclusions in the melt so that titanium is the main deoxidizer. Steel continuously cast in a thin plate or continuous sheet inherently does not have a fine equiaxed grain structure as desired. It is considered that by carefully controlling the aluminum in this invention, the formation of Al203 inclusions can be minimized. Al203 inclusions contained in a melt tend to coalesce into large groups. By minimizing the formation of alumina inclusions, it is further considered that small inclusions having a size smaller than 5 μm, preferably no greater than 1.5 μm and more preferably no greater than 1 μm titanium oxide become the dominant non-metallic inclusions in the fade. These small inclusions of titanium oxide are considered to provide nucleation sites that allow the formation of a fine equiaxial grain structure as it empties during solidification. Accordingly, titanium is used for deoxidation in order to ensure that the dominant inclusions in the molten and solidified cast steel are small titanium oxides instead of alumina inclusions, ie, the number of titanium oxide inclusions > > with respect to alumina inclusions. The aluminum deoxidized steels of the prior art tend to lock the nozzles during continuous emptying. It is generally required that calcium be added to the steel with high concentration of aluminum to increase the fluidity of the Al203 inclusions in the cast melt to minimize this tendency to gauging the casting nozzles. However, calcium generally adversely affects the formation of a fine equiaxed grain as it is emptied. Consequently, calcium should be limited to < 0.0020%. An important advantage of this invention is to eliminate the need for the addition of calcium to the low concentration aluminum melt since very few inclusions of A1203 are present in the melt when the aluminum is maintained at _ = 0.016%. Large amounts of Al203 inclusions contained in a melt can coalesce rapidly into alumina groups which can cause ingrowth in the nozzle during continuous emptying. The carbon is present in the steels of the present invention in an amount of up to 0.08%, preferably < .0.02%, and more preferably 0.010-0.01%. If the carbon exceeds approximately 0.08%, the suitability for shaping, corrosion and weldability deteriorates. Consequently, carbon must be reduced to as low a quantity as possible. An element for stabilizing carbon and nitrogen may be present in the steels of the present invention in an amount of up to 1.0%, preferably up to 0.6% and more preferably up to 0.3%. If a stabilized steel is desired, sufficient stabilizing element must be present to form a stable carbo-nitride compound effective to make a crystalline grain size to increase the elongation and tenacity of the stainless steel and thus improve the formability for such as the ability of deep stretching after annealing. If the stabilizing element is greater than about 1.0%, the cost of steel production is increased without any corresponding benefit in the properties.

In addition to using titanium for stabilization, other suitable stabilizing elements may also include niobium, zirconium, tantalum, vanadium or mixtures thereof, titanium being preferred alone. If a second stabilizing element is used together with the titanium, for example niobium, the second stabilizing element should be limited to no more than about 0.3% when a deep fitness for shaping is required. Nb above 0.3% adversely affects conformability. Chromium is present in the steels of the present invention in an amount of .8%, preferably _ = 10%. If the chromium is less than about 8%, the wet corrosion resistance of the steel is adversely affected, for example, in automotive exhaust components. If the chromium is greater than about 25%, the ability to form the steel deteriorates. For some applications, it may be desirable to add boron to the steels of the present invention in an amount of .5 ppm, more preferably 20 ppm, and more preferably 40-60 ppm. Having boron at at least 5 ppm improves the brittleness of secondary work of steel so that the steel sheet will not be divided during deep drawing applications and multi-stage forming applications. If the boron is more than about 200 ppm, the ability to form the steel deteriorates.

Oxygen is present in the steels of the present invention and preferably it is present in an amount < 100 ppm. When a steel melt is prepared sequentially in an AOD refining vessel and an LMF alloy vessel, the oxygen in the melt will be within the range of 10-60 ppm, so a very clean steel having small inclusions is provided. of titanium oxide that are necessary for the formation of responsible nucleation sites for the equiaxed fine grain structure as it is emptied. Silicon is generally present in the chromium alloyed steels of the present invention in an amount of < 1.5%, preferably < 0.5% Generally a small amount of silicon is present in a ferritic stainless steel to promote the formation of the ferrite phase. Silicon also improves resistance to high temperature corrosion and provides high temperature strength, for example in automotive exhaust components. Consequently, the silicon must be present in the melt in an amount of at least 0.10%. Silicon should not exceed about 1.5% because the steel is too hard and the elongation is adversely affected. Manganese is present in the steels of the present invention in an amount of up to 1.5%, preferably less than 0. 5%. Manganese improves the ability to work hot when combined with sulfur as manganese sulphide to prevent tearing of the sheet during hot processing. In Consequently, manganese is desirable in amounts of at least 0.1%. However, manganese is an austenite former and affects the stabilization of the ferrite phase. If the amount of manganese exceeds about 1.5%, the stabilization and ability to form the steel is adversely affected. Sulfur is present in the steels of the present invention preferably in an amount of = 0.015%, more preferably < 0.010% and much more preferable < 0.005%. In addition to causing a problem during hot rolling, sulfur adversely affects resistance to wet corrosion, especially in those steels that contain a lesser amount of chromium. Accordingly, preferably the sulfur should not exceed about 0.015%. Like manganese, nickel is an austenite former and affects the stabilization of the ferrite phase. Accordingly, nickel is limited to < .2.0%, preferably < 1.0%. The alloy steel with ferritic chromium of this invention may also include other elements such as copper, molybdenum, phosphorus and the like made either as deliberate additions or present as waste elements, i.e., impurities from the steelmaking process.

Example 1 A ferrous alloy with chromium, comparative, of approximately 25 kg is provided in a vacuum container in the laboratory. After the final conditioning, the elements are added to the container, the melt is deoxidized with titanium. The composition of the cast alloy steel with chromium is 0.006% Al, 0.15% Ti, 0.007% C, 0.26% Mn, 0.36% Si, 11.2% Cr, 0.18% Ni and 0.005% N The proportion of the product of titanium and nitrogen divided by aluminum is 0.125. Approximately 23 minutes after the addition of titanium, the melt is emptied into an ingot having a thickness and width of about 75 mm and about 150 mm, respectively. A grain structure as emptied of a cross-sectional piece shown in Figure 1, cut from a stainless steel ingot having a grain structure that is completely columnar and having an average column size of about 3 mm. This steel shows that by having a low concentration of aluminum alone, ie, < .0.01% is not enough to form a structure as it is emptied of predominantly equiaxed grains. This steel has a ratio of (Ti x N) / Al < 0.14 that illustrates a steel grain structure as it empties that does not contain equiaxed grains.

Example 2 A ferrous alloy ferrous alloy of the invention, of about 25 kg, is provided in the same laboratory vacuum vessel as described in Example 1. After the final conditioning, alloy elements are added to the vessel, and the melt is deoxidized. with titanium. The composition of the cast alloy steel with chromium is 0.007% Al, 0.28% Ti, 0.008% C, 0.05% Mn, 0.36% Si, 11.1% Cr, 0.18% Ni and 0.004% N. The proportion of the product of titanium and nitrogen divided by aluminum increases to 0.16. Approximately 17 minutes after the addition of titanium, the melt is emptied into an ingot having a thickness and a width of about 75 mm and about 150 mm, respectively. Figure 2 shows a grain structure as it is emptied of a cross section piece cut of a stainless steel ingot having a fine grain structure of approximately 78% equiaxed grains and an average diameter size of about 2 mm This steel has a ratio (Ti x N) / Al _ > 0.14, which illustrates that a steel grain structure as it is emptied will contain .50% fine equiaxed grains.

Example 3 Another ferrous alloy with chromium, comparative, of the invention is produced in a manner similar to that of Example 1 having a composition of 0.013% Al, 0.19% Ti, 0. 007% of C, 0.26% of Mn, 0.36% of Si, 11.0% of Cr, 0.24% of Ni and 0.009% of N. The proportion of the product of titanium and nitrogen divided by aluminum is 0.13. Approximately 19 minutes after the addition of titanium, this steel melt is emptied into an ingot. A grain structure as it is emptied, of a cross section piece cut of the stainless steel ingot has a grain structure that is completely columnar and having an average column size of about 2 mm, as shown in Figure 3 This steel has a ratio of (Ti x N) / Al _ 0.14, which illustrates that a steel grain structure as it is emptied will contain < 50% equiaxed grains.

Example 4 Another ferrous alloy with chromium, of the invention, is produced in a manner similar to that of example 2, and has a composition of 0.013% Al, 0.24% Ti, 0.007% C, 0.26% Mn, 0.37. % of Si, 11.1% of Cr, 0.25% of Ni and 0.008% of N. The proportion of the product of titanium and nitrogen divided by aluminum increases to 0.15. This steel melt is emptied into an ingot in the next approximately 14 minutes after the addition of titanium is made. The structure as it is emptied of the cut of piece in cross section from the ingot of stainless steel has a structure of fine grain of about 84% equiaxed grains and an average diameter size of about 3 mm, as shown in Figure 4. This steel illustrates that a steel grain structure as it is emptied will contain _ > 50% fine equiaxed grains although the steel has a high concentration of aluminum, i.e., j > 0.01%, if the ratio (Ti x N) / Al 0.14. The compositions, TNA and% EQ of ingots as they are emptied for the comparative type 409 stainless steel melts of the invention of the preceding examples 1-4 as well as many additional type 409 stainless steel laboratory melts, comparative and of the invention, produced and cast in ingots in a manner similar to that described for Examples 1-4 are summarized in Table 1. In Figure 13% EQ is shown as a TNA function for these ingots. Figure 13 generally demonstrates that Ti of at least about 0.10% and a TNA, ie, (Ti x N) / Al of about 0.14, or more are necessary to obtain a steel grain structure as it is emptied containing at least 50% of fine equiaxed grains. st O) s 8 co co SJ CM CM? 8 8 8 8 3 8 8 8 i Table 3 summarizes the compositions, TNA and% EQ for other laboratory ingots as additional vacuums, for stainless melts with high concentration of type 430, type 439 and type 439Mo chromium comparative and of the invention, produced and cast similar to the ingots of examples 1-4 . Table 3 shows that Ti is required of at least about 0.10% and a TNA, ie (Ti x N) / Al, of at least about 0.20 to obtain a steel grain structure as it is emptied containing at least minus 50% fine equiaxed grains. This increase in TNA is apparently needed due to the increase in chromium from about 11% for the type 409 stainless in Table 1 to a composition with high chromium concentration of approximately 17% or more for stainless steels with high concentration of chromium type 430 , type 439 and type 439Mo in table 3.

Example 5 A ferrous alloy with chromium, comparative, of approximately 125 metric tons is provided in a refined AOD container. After the carbon is reduced to the final specification, the melt is transferred to an LMF where the final conditioning alloy elements are added. Subsequently, the melt is deoxidized with titanium. The final composition of the melt is 0.009% Al, 0.21% Ti. 0. 007% of C, 0.26% of Mn, 0.32% of Si, 11.2% of Cr, 0.14% of Ni and 0.005% of N. The proportion of the product of titanium and nitrogen divided by aluminum is 0.12. The steel melt is then transferred to a melter in the next approximately 40 minutes and continuously melted into thin plates having a thickness of 130 mm and a width of 1200 mm. Cross-section pieces are cut from a medium width position and several other positions along the length of the thin plate. In Figure 5 a typical voided grain structure of one of these pieces cut from a plate of this steel is illustrated, and having a columnar grain structure having an average column size of about 4 mm. This steel, like that of example 1, demonstrates that by only having low aluminum, that is, < .0.01%, it is not enough to form a structure as empty of predominantly equiaxed grains. Figure 5 illustrates a ferritic stainless steel having a ratio of (Ti x N) / Al j > 0.14, resulting in a steel grain structure as it empties that does not contain equiaxed grains.

Example 6 A chromium-alloyed ferrous melt of the invention of about 125 metric tons is produced in a manner similar to that described above for example 5, except for the following changes in the composition. The composition of the melt is 0.23% Ti, 0.008% Al, 0.010% C, 0.27% Mn, 0.31% Si, 11.1% Cr, 0.13% Ni and 0.007% N. Unlike the example 5, the proportion of the product of titanium and nitrogen divided by aluminum increases to 0.19. Subsequently the steel melt is transferred to a melter and emptied into thin plates in a manner similar to that described above for example 5. In figure 6 a grain structure is illustrated as it is emptied of a plate of this steel stainless having a fine grain structure of approximately 84% equiaxed grains and an average size of approximately 2 mm. Figure 6 illustrates a ferritic stainless steel having a ratio (Ti x N) / Al 2.0.14, which results in a steel grain structure as it is emptied containing > 50% equiaxed grains. The plates of this steel contain inclusions mainly of titanium oxides.

Example 7 Another comparative example of ferrous alloy with chromium is produced, similar to that of example 5. The composition of the melt is 0.20% Ti, 0.014% Al, 0.011% C, 0.28% Mn, 0.31% Si, 10.9% of Cr, 0.12% Ni and 0.0087% N. In a similar manner to Example 5, the proportion of titanium and nitrogen product divided by aluminum is only 0.11.

Subsequently the steel melt is transferred to a melter and emptied into thin plates in a manner similar to that described above for example 5. In figure 7 a grain structure is illustrated as it is emptied of a plate of this steel stainless having approximately 94% large columnar grains having an average column size of about 5 mm. Figure 7 illustrates ferritic stainless steel having a ratio (Ti x N) / Al 0.14, which results in a steel grain structure as empty that contains very few equiaxed grains.

Example 8 Another ferrous alloy with chromium of the invention is produced, similar to that of example 6. The composition of the melt is 0.21% Ti, 0.016% Al, 0.006% C, 0.23% Mn, 0.27% Yes, 11.3% Cr, 0.11% Ni and 0.011% N. The proportion of the product of titanium and nitrogen divided by aluminum is 0. 15. Subsequently the steel melt is transferred to a melter and emptied into thin plates in a manner similar to that described above for example 5. In figure 8 a grain structure is illustrated as being emptied of a cut piece of a plate of this stainless steel having a predominantly fine equiaxed grain structure. Figure 8 illustrates a ferritic stainless steel having a ratio of (Ti x N) / Al _ > 0.14, resulting in a steel grain structure as it empties that contains 63% fine equiaxed grains and that has a size of approximately 3 mm. This steel illustrates that a steel grain structure as it is emptied can contain j > 50% fine equiaxed grains although the steel has high concentration of aluminum, i.e., j > 0.01%, if the ratio is (Ti x N) / Al j 0.14. The plates of this steel contain inclusions mainly of titanium oxides.

Example 9 Another ferrous alloy with chromium, comparative, similar to that of Example 5 is produced. The composition of the melt is 0.18% Ti, 0.022% Al, 0.007% C, 0.22% Mn, 0.17% Si, 10.6% Cr, 0.14% Ni and 0.010% N. The proportion of the product of titanium and nitrogen divided by aluminum is only 0.08. Subsequently the steel melt is transferred to a melter and emptied into thin plates in a manner similar to that described above for example 5. A grain structure as it is emptied of a plate of this stainless steel has a grain structure large which is a 100% columnar grain structure having an average column size of about 4 mm, as illustrated in Figure 9. Figure 9 illustrates a ferritic stainless steel having a < rj X! < rj In (Ti x N) / Al < 0.14, which results, in a steel grain structure as it empties that does not contain equiaxed grains. The plates emptied from this melt are reheated to 1250 ° C, hot processed to a thickness of 3.3 mm with a finishing temperature of about 800 ° C and rolled at a temperature of about 700 ° C. The hot-processed sheet is descaled, deoxidized to acid in nitric and hydrofluoric acid and cold reduced to 58%, to a thickness of 1.4 mm. This hot-processed sheet is not annealed before cold reduction. The cold reduced sheet is annealed at a peak metal temperature of 870 ° C for about 60 seconds. After stretching, the fluted characteristic of the sheet is 3-4 and has a grain structure of 1.22-1.27. A striated characteristic of 3 or more means moderate to severe striata on a scale of 0-6. A high flute characteristic of 3 or more and a low grain structure of less than 1.3 are unacceptable for many exposed ferritic stainless steel applications, with deep formability. Table 5 summarizes the mechanical properties for this steel. Figure 11 shows an annealed, cold rolled steel structure of this steel, which shows a non-uniform "in band" grain structure, characteristic of striated steels. This non-uniform band grain structure is not acceptable for exposed ferritic stainless steel applications that require high fitness for conformation. Cold annealing reduces the sheet produced from a plate having a columnar grain structure which will experience severe flute characteristics unless the sheet is hot rolled from the plate and annealed before cold reduction .

Table 5 Example 10 Another ferrous alloy with chromium of the invention is produced, similar to that of Example 8. The melt composition is 0.19% Ti, 0.005% Al, 0.008% C, 0.12% Mn, 0.16% Si, 10.7% of Cr, 0.13% Ni and 0.011% N. The proportion of the product of titanium and nitrogen divided by aluminum is 0.34. Subsequently, the steel melt is transferred to a melter and emptied into thin plates, in a manner similar to that described above for example 5. Figure 10 illustrates this ferritic stainless steel having a ratio of (Ti x N) / A1 > 0.23 resulting in a steel grain structure as empty that contains 100% equiaxed grains having a size of approximately 1 mm. The plates of this steel contain inclusions mainly of titanium oxides. These thin plates are reheated to 1250 ° C, hot processed to a thickness of 3.3 mm with a finished temperature of 800 ° C and rolled at a temperature of 700 ° C. The hot-processed sheet is descaled, deoxidized to acid in nitric and hydrofluoric acid and cold reduced to 58%, to a thickness of 1.4 mm. This hot-processed sheet is not annealed before cold reduction. The cold reduced sheet is annealed at a peak metal temperature of 870 ° C for 60 seconds. After stretching, the annealed sheet flute characteristics decrease to 1 and it has an increase of ^ of 1.45. A fluted characteristic of 1 means excellent flute and the steel is essentially striated free. A fluted characteristic of 2 or less and an R ^ of at least 1.4 are acceptable for most applications of exposed ferritic stainless steel, with deep formability. The mechanical properties of the sheets of the invention are summarized in Table 6. Figure 12 shows a cold-rolled and annealed grain structure, showing a very uniform fine grain structure. This cold-reduced and annealed sheet of the invention produced from a plate having a fine equiaxed grain structure has excellent flute characteristics although the hot rolled sheet has not been annealed before cold reduction.

Table < ? Example 11 Another ferrous alloy with chromium of this invention is produced, similar to that of Example 10. The melt composition is 0.19% Ti, 0.006% Al, 0.007% C, 0.13% Mn, 0.31% Si, 11.0% of Cr, 0.16% Ni and 0.008% N. The proportion of the product of titanium and nitrogen divided by aluminum is 0.24. The steel melt after transfer to a melter and is emptied into thin plates in a manner similar to that described above for example 5. This steel ferritic stainless has a ratio of (Ti x N) / A1 = 0.23 resulting in a steel structure as empty that contains 100% fine equiaxed grains of a size of approximately 1 mm. The plates of this steel contain inclusions mainly of titanium oxides. These plates are reheated to 1250 ° C, hot processed to a thickness of 3.0 mm with a finished temperature of 800 ° C and rolled at a temperature of 700 ° C. The hot-processed sheets are descaled and deoxidized to the acid in nitric and hydrofluoric acid. The hot-processed sheets are cold reduced to 53% to a thickness of 1.4 mm. These hot-processed sheets are not annealed before cold reduction. The cold reduced sheets are annealed at a metal peak temperature of 940 ° C for 10 seconds. After stretching, the characteristic flute in the annealed sheets is 1-2 and has an R ,,, of 1.39-1.48. A characteristic flute of 2 means good flute characteristics. Table 7 summarizes the mechanical properties of the sheets of the invention.

Table 7 Example 12 Another thin 130 mm thick plate of the composition described in example 11 is reheated to 1250 ° C, hot processed into sheets having a thickness of 4.1 mm with a finishing temperature of 830 ° C and rolled to a temperature of 720 ° C. The sheets processed in hot were descaled, deoxidized to acid in nitric and hydrofluoric acid and then reduced in cold 66%, 76% and 85% corresponding to thicknesses of 1.4, 1.0 and 0.6 mm, respectively. These hot-processed sheets of the invention are not annealed prior to cold reduction. The cold reduced sheets are annealed at a metal peak temperature of 940 ° C for 10 seconds. After stretching, the fluted feature of the annealed sheets is generally 2 or better, and has an F ^ of 1.76-1.96. A Rra of 1.7 is considered outstanding for Ferritic stainless steel and previously it was not considered possible for ferritic stainless steel which has not been provided, an annealing before cold reduction. Table 8 summarizes the mechanical properties of the sheets of the invention.

Table 8 The compositions, TNA and% EQ of the plates as they are emptied for the comparative type 409 stainless melts and of the invention of the above examples 5-11, as well as the additional, produced and cast stainless type 409 comparative melts and of the invention. in plates in a manner similar to that described in Examples 5-11, are summarized in Table 2. In Figure 14 the% EQ is shown as a function of TNA for these plates. Figure 14 generally demonstrates that the steels of the invention require Ti = 0.10% and a TNA, ie, (Ti x Na) / Al, of about 0.14 or greater to obtain a steel structure as it is emptied containing more than 50% fine equiaxed grains. Exceptions to this were plates on Heat 980460, Heat 000459, Heat 880463, Heat 980655 and Heat 980687. Heats 980655 and 980687 that experienced nozzle locking problems, ie, excessive inclusions of alumina, and resulted in steel temperatures melted in the low refractory arches, you infer at 1545 ° C. Accordingly, the melts of the invention are preferably continuously emptied to have a superheat of at least 40 ° C more preferably at least 55 ° C, to prevent the clumping of large inclusions of alumina, Heat 880459 is blown again to remove the excess carbon after being deoxidized with titanium, that is, the titanium oxide inclusions are probably removed to the slag. Nothing unusual is observed for Heat 880463. Table 4 summarizes the compositions, TNA and% EQ for other plates as they are additionally emptied, for high concentration chromium type 430, Type 439 and Type 439Mo, comparative and the invention, produced and cast similar to the plates of Examples 5-11. Table 4 shows that Ti of at least about 0.10% and a TNA, ie (Ti x N) / Al of at least about 0.30 results in a steel grain structure as it is emptied which generally contains an excess of 50. % of equiaxed fine grains for alloy steels with high chromium concentration.

A very important advantage of the present invention relates to an annealing, recrystallized, cold-reduced end product. The ferritic stainless steels of the prior art were not only adversely affected in appearance by the striated but also have a poor conformability, ie, a low R ,,,. One reason that ferritic stainless steels have a limited ability to form is because the structure, after annealing, consists of large, non-uniform or "in-band" grains. Figure 11 illustrates a typical non-uniform grain structure after the annealing of ferritic stainless steel of the prior art, comparative, having a proportion of the product of titanium and nitrogen, divided by aluminum, less than 0.14 and having a structure like empty that contains < 50% equiaxed grains. This invention allows a fine equiaxed grain to be formed in the steel as it is emptied, so that a uniform, fine recrystallized grain structure can be formed consistently from the cold reduction. A sheet of alloy steel with ferritic chromium having a uniform, fine recrystallized grain, without annealing the steel before cold reduction, and with only a cold reduction can be formed. It will be understood that various modifications to this invention can be made without departing from the spirit and scope thereof. Therefore, the limits of this invention should be determined from the appended claims.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional method for manufacturing the objects to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (27)

1. A ferritic steel alloyed with chromium, characterized in that it contains: = 0.08% of C, > 8% Cr, = 1.50% Mn, = 0.05% N, = 1.5% of Si, < 2.0% Ni, Ti > 0.10%, a ratio of (Ti x N) / Al > 0.14, all percentages by weight, the rest as Fe and residual elements, and the steel is deoxidized with titanium and has a structure as it is emptied > 50% equiaxed grains.

2. A sheet of ferritic steel alloyed with chromium, characterized in that it comprises: = 0.08% C, = 8% Cr, = 1.50% Mn, = 0 .05% N, = 1.5% Si, < 2.0% Ni, Ti > 0.10%, a ratio of (Ti x N) / Al > 0.14, all percentages by weight, the rest as Fe and residual elements, the sheet is annealed by recrystallization and is essentially free of striated when it is formed in one part, the annealed sheet is cold reduced from a hot-processed sheet , and the hot processed sheet is formed from deoxidized steel with titanium and having a structure as it is emptied > 50% equiaxed grains.

3. The steel sheet according to claim 2, characterized in that Ti = 0.15%.

4. The steel sheet according to claim 3, characterized in that Ti and N are in subequilibrium quantities and Ti satisfies the ratio (Ti / 48) / [(C / 12) + (N / 14)]. > 1.5.

5. The steel sheet according to claim 3, characterized in that N = 0.012% and Ti = 0.25%.

6. The steel sheet according to claim 2, characterized in that the equiaxed grains have a size = 3 mm.

7. The steel sheet according to claim 2, characterized in that the Al < 0.020% 8.

The steel sheet according to claim 7, characterized in that the Al = 0.013%. .

The steel sheet according to claim 8, characterized in that it includes a second stabilizing element of the group consisting of niobium, zirconium, tantalum and vanadium.

10. The steel sheet according to claim 7, characterized in that 0 is 10-60 ppm.

11. The steel sheet according to claim 2, characterized in that it includes = 20 ppm of B.

12. The steel sheet according to claim 7, characterized in that the structure as it is emptied is > 60% equiaxed grains.

13. The steel sheet according to claim 8, characterized in that the structure as it is emptied is > 80% equiaxed grains.

14. The steel sheet according to claim 2, characterized in that the proportion of (Ti x N) / Al = 0.23 and the structure as it is emptied is substantially free of columnar grains.

15. The steel sheet according to claim 2, characterized in that the Al = 0.010, the ratio of (Ti x N) / Al = 0.23 and the structure as it is emptied is substantially free of columnar grains.

16. The steel sheet according to claim 2, characterized in that the steel as it is emptied has inclusions of titanium oxide with a greater part of the inclusions that have a size < 1.5 μm.

17. The steel sheet according to claim 2, characterized in that Cr > 16% and the ratio of (Ti x N) / Al = 0.30.

18. The steel sheet according to claim 2, characterized in that the annealed sheet has a value R ", de = 1.4.

19. The steel sheet according to claim 13, characterized in that the annealed sheet has a value P ^ of = 1.7.

20. A sheet of ferritic steel alloyed with chromium, characterized in that it comprises: = 0.013% of Al, 0.15-0.25% of Ti, = 0.02% of C, = 1.50 of Mn, 0.005-0.012% N, __l.5% Si, 8-25% Cr, < 2.0% of Ni, an amount to the Ti unbalance, the proportion of (Ti x N) / Al = 0.16, all percentages by weight, the rest of Fe and residual elements, the sheet has been annealed by recrystallization and has a value of B ^ of = 1.4 and is essentially free of striated when it is conformed In one part, the annealed sheet is cold-reduced from a hot-processed sheet that has not previously been annealed before the cold reduction, and the hot-processed sheet formed from titanium-deoxidized steel and having a structure as described above. empty = 80% equiaxed grains.

21. A process for making chromium alloy steel, characterized in that it comprises the steps of: providing a steel melt containing = 0.08% C, >; 8% Cr, = 1.50% Mn, = 0.05% N, = 1.5% Si, < 2.0% Ni, all percentages by weight, the rest as Fe and residual elements, deoxidize the melt with an amount of Ti that satisfies the ratio (Ti x N) / Al = 0.14 and Ti = 0.10%, empty the melt in a steel that has a structure as it is emptied = 50% equiaxed grains, processing the steel in a hot sheet, descaling the sheet, cold reducing the sheet to a final thickness, and annealing by recrystallization the cold reduced sheet, wherein the annealed sheet is essentially free of fluted when it is formed into a part.

22. The process according to claim 21, characterized in that Al < 0.020%, Ti = 0.15%, (Ti x N) / Al = 0.23 and satisfies the relation (Ti / 48) / [(C / 12) + (N / 14)] > 1.5.

23. The process according to claim 21, characterized in that the melt is continuously emptied into a thin plate having a thickness = 140 mm, the additional step of reheating the plate to a temperature of 1050-1300 ° C before rolling in Warm the plate in the continuous sheet.

24. The process according to claim 21, characterized in that the hot-processed sheet is cold reduced without prior annealing.

25. The process according to claim 24, characterized in that the hot-processed sheet is cold reduced in a single step.

26. The process according to claim 21, characterized in that the cold reduced sheet anneal at a temperature of 800-1000 ° C for at least 1 second.

27. A process for producing alloy steel with chromium, characterized in that it comprises the steps of: providing a steel melt containing = 0.013% Al, 0.15-0.25% Ti, = 0.02% C, = 1.50 Mn, 0.005 -0.012% N, = 1.5% Si, 8-25% Cr, < 2.0% Ni, the ratio of (Ti x N) / Al > 0.16 and (Ti / 48) / [(C / 12) + (N / 14)] > 1.5, an amount to the unbalance of Ti, all the percentages by weight, the rest as Fe and residual elements, empty the melt in a steel that has a structure as empty of = 80% equiaxed grains, hot process the steel in a sheet, descaling the sheet, cold reducing the sheet to a final thickness without prior annealing, and annealing by recrystallization the cold reduced sheet, wherein the annealed sheet is essentially free of fluted when formed into a part.