DE69824131T2 - Ferritic chrome steel - Google Patents

Description

BACKGROUND THE INVENTION

The The invention relates to a ferritic chromium steel, which consists of a Melt is formed, with a fine equiaxed grain structure in the cast condition. In particular, the invention relates to a ferritic chromium steel, which is formed from a melt containing sufficient titanium and Contains nitrogen, However, a controlled amount of aluminum to form smaller titanium oxide to provide the necessary nuclei for training the equiaxed grains in the condition of steel after casting. A warm processed Sheet metal, made of steel with this structure of equiaxed grains in the Cast condition is specially made for the production of one cold-formed, treated by recrystallization annealing sheet with excellent properties with low ridging and excellent tensile properties suitable, even without hot strip annealing or intermediate annealing.

It is for good moldable ferritic stainless steels desirable - in addition to a high degree of plastic deformation - a phenomenon known as "ridging", "ribbing" or "ribbing" is known. in the Unlike austenitic stainless steel can invisible "ridging" on the surfaces of a cold-formed, recrystallization annealed ferritic stainless steel sheet after cold forming to a component occur. The ridging is through education from vaults, Grooves or corrugations marked, which are parallel to the rolling direction of the sheet metal. This defect is not just for the external Appearance of the sheet harmful, but leads also to inferiority and Walzverformbarkeit.

ferritic Chrome steel, in particular ferritic chromium beam in the lower balance, such as For example, the types 409 and 439 stainless steel typically have - regardless of whether they are continuously in blocks with a thickness of 50-200 mm or in strips with a thickness of 2-10 mm - a pronounced pillar structure in the cast condition. These grains with big columns have almost a crystallographic cube-on-surface texture that produces unwanted ridging in the cube cold-rolled annealed sheet leads, which is used in various applications for the production. The superficial Apparition that results from this "ridging" is especially with molded components that flaunt outwards be put, such. Coffins, Automobile fairings, mufflers, oil filters and the like, extremely disturbing. The "ridging" causes that the sheet a rough, uneven surface appearance after has the deformation and is at a high nonuniform or banded Grain structure attributed to after cold forming and annealing exists and from the initial one Appearance of the grain column structure of the steel in the cast condition results.

To minimize the occurrence of the ridging phenomenon, additional expense is required by annealing a hot rolled sheet prior to cold working. This additional annealing step of the hot-worked ferritic stainless steel also results in reduced ductility caused by the lower average strain rate, ie, R _m , resulting in a deterioration of the thermoforming properties. A hot rolled sheet annealed before cold working must be reduced by at least 70% to compensate for the loss of R _{m produced} by hot strip annealing prior to final annealing.

About the Years have been made various attempts, the above Meet procedural requirements and costs aimed at the "ridging" by modification the alloy composition of ferritic stainless steel to eliminate. It is known that the "ridging" in a ferritic stainless steel primary while of hot rolling arises. Therefore, various attempts have been made the "ridging" through education a fine equiaxed grain structure of the cast block minimize by controlling the chemistry of the melt, e.g. B. by one or more impurities from the group C, N, O, S and P and by refinement of the grain structure by using lesser Hot rolling temperatures of z. B. 950-1100 ° C. The control of chemistry while refining has some improvement in the "ridging" properties of ferritic stainless steel result from the formation of a second phase, d. H. Austenite, at high temperatures, at room temperature becomes martensite. However, the formation of this second phase is up Cost of tensile elongation occurs and at the expense of the welding properties of the finished product. The temperature control during hot rolling has as well led to various operational difficulties, as a higher hot rolling capacity is required. Accordingly, the thickness of the hot-rolled Sheet higher be. After hot rolling must then cold rolling in at least two Stages with a second intermediate annealing between the two cold rolling treatments respectively.

The US-A 5 769 152 recognizes that columnar grains in continuously cast stainless steel not desirable are. The patent beats before, columnar grains too prevent and instead form equiaxed grains by shedding the molten Steel using a low superheat temperature of 0-15 ° C above the liquidus temperature and magnetic stirring of the molten steel in a mold.

Other competitors have attempted to prevent ridging by modifying the alloy composition of a ferritic stainless steel by adding one or more stabilizing elements. US-A 4,465,525 describes a ferritic stainless steel having excellent formability and surface quality. This patent discloses that boron at a level of 2-30 ppm and at least 0.005% aluminum can increase elongation and R _m , as well as a reduction in ridging characteristics. US-A-4 515 644 relates to a deep drawn ferritic stainless steel having improved ridging quality. This patent discloses that the addition of aluminum, boron, titanium, niobium, zirconium and vanadium can improve the elongation of the ferritic stainless steel, the R _m value increase, and thereby the anti- "ridging" can improve properties. More specifically, this patent discloses a ferritic stainless steel having at least 0.01% Al which has improved anti-ridging properties. US-A-5 662 864 relates to the production of a ferritic stainless steel having good ridging properties by careful control of Ti, C + N and the N / C ratio. This patent teaches that "ridging" can be improved by the addition of titanium in response to the C + N content in the melt due to the formation of carbonitrides. The molten steel contains ≦ 0.01% C, ≦ 1.0% Mn, ≦ 1.0% Si, 9-50% Cr, ≦ 0.07% Al, 0.006 ≦ C + N ≦ 0.025%, N / C ≥ 2, (Ti - 2S - 30) / (C + N) ≤ 4 and Ti _x N ≤ 30 x 10 ^-4 . US-A-5 505 797 relates to the production of a ferritic stainless steel having reduced interfacial anisotropy and excellent grain structure. This patent teaches that good ridging properties are obtained when the molten steel is preferably 0.0010-0.080% C, 0.10-1.50% Mn, 0.10-0.80% Si, 14-19% Cr and two or more elements from the group 0.010-0.20% Al, 0.050-0.30% Nb, 0.050-0.30% Ti and 0.050-0.30% Zr contains. This steel is cast into slabs (slabs) and hot rolled into a 4 mm thick sheet, annealed, pickled, cold rolled and finally annealed. The slab (slab) was heated to 1200 ° C and subjected at least one Rohwarmwalzdurchlauf at a temperature between 970-1150 ° C. The friction between the rolls of the hot rolling mill and the hot rolled steel was 0.3 or less, the rolling reduction ratio was between 40 and 75%, and the hot rolling end temperature was 600-950 ° C. The hot rolled steel was then annealed at a temperature of 850 ° C for 4 hours, cold worked to a value of 82.5%, and finally annealed at a temperature of 860 ° C for 60 seconds.

It is known that when the solubility product of titanium compounds at liquidus temperature exceeds a saturation level, d. H. Overbalance for titanium-stabilized stainless steels, the titanium compounds are stable and TiN precipitates before the metal cools. Steel sheets made from these overbalance weight plates have improved ridging properties and improved performance Moldability on. When cooled However, TiN coalesces to large Clusters and floats on the surface of the cast slab on. These non-metallic TiN clusters form unacceptable open ones Surface defects, as Ti strips during of hot rolling are known. These big non-metallic clusters have to from the slab by costly surface treatment, such. B. loops before hot rolling, to be removed. US-A 4,964,926 a weldable one Double stabilized ferritic stainless steel with surface quality the elimination of the formation and excretion of non-metallic Titanium oxides and titanium nitrides during of potting a titanium-stabilized ferritic stainless steel in sub-equilibrium. This document discloses that it was known that groove-forming properties by the addition of niobium alone or niobium and copper to ferritic Stainless steel can be improved. However, that causes sole addition of niobium cracks during welding. This US Pat. No. 4,964,926 discloses replacing a portion of the titanium as a stabilizer with a niobium stabilizer to form a double stabilized ferritic stainless steel. The addition of at least 0.05% Titanium to niobium stabilized steel suppresses cracking during welding.

Of the scientific articles A. Ostrowski and E. W. Langer: "Excretion of titanium carbonitrides in 17% chromium cast stainless steel "in: Scandinavian Journal of Metallurgy 8 (1979) pp. 153-160 describes a mechanism the excretion of titanium carbonitrides in stabilized stainless steels. The Preparation of the samples involves the addition of the alloying element Titanium to the melt in combination with solid oxide particles. The education of titanium carbonitrides is due to interfacial reaction between the oxide particles and in solution in the melt titanium, followed by the precipitation of titanium nitride and growth of titanium nitride, resulting in the formation of a growth layer around the oxide particles.

JP-A 8 296000 describes the production of ferritic stainless steel with minimal ridging by lowering the proportions of carbon and nitrogen as well as the content of Ti to form Ti (C _x , N _y ) with a particular composition.

The JP-A 9 235621 describes a production process for the formation of Ti-containing ferritic stainless steel with improved formability by tailoring of the composition in terms of the proportions of Ti and P in order to comply with Relationship {Ti - 5 × (C + N)} / P in a range of 3 to 20.

Minimizing the "ridging" phenomenon of previous work has been detrimental to cost and formability by annealing the hot rolled ferritic stainless steel prior to cold working. This additional annealing step reduces moldability by reducing the mean R _m value. Also has this pre-annealed hot rolled steel cold reduced at least 70% to obtain an R _m value after the final annealing, the hot rolled steel is similar to the R _m value which is otherwise not annealed prior to cold transformation. This higher percentage of cold reduction basically also requires an intermediate annealing step. As evidenced by the seemingly endless struggle of others, there is a long felt need for annealed ferritic chromium alloyed steel which is substantially free of ridging and excellent thermoforming properties such as galling. B. has a high R _m value, a high tensile elongation and a uniformly distributed grain structure. In addition, there is a need for ferritic stainless steel with excellent low moldability and good ridging properties, which does not require a hot worked sheet to be annealed prior to cold working. In addition, there is a need for a sub-equilibrium ferritic stainless steel having excellent depth ductility and good ridging properties formed from a hot worked sheet having no surface defects, ie, a titanium nitride layer or titanium oxide strips, without surface treatment of the surfaces of the continuously cast slab before the hot working of the slab is needed.

SHORT SUMMARY THE INVENTION

A fundamental The object of the invention is a ferritic chromium-alloyed steel sheet with excellent low formability and ductility and with good ridging properties, without it being necessary for the hot rolled sheet to stand in front of the Cold forming annealed becomes.

A Another object of the invention is to provide a ferritic chromium alloyed Sheet steel with good ridging properties and an improved grain structure and high tensile elongation characteristics provide without the need for the hot processed Sheet annealed before cold forming becomes.

A Another object of the invention is to provide a ferritic chromium alloyed Steel sheet with excellent low formability and rolling and with to provide good ridging properties, without the need for multiple cold forming operations with annealing treatments between the cold forming stages.

A Another object of the invention is to provide a ferritic chromium alloyed Produce sheet steel from a continuously cast slab, without it being necessary to finish the surface before hot working to carry out the steel roll plate.

A Another object of the invention is to provide a ferritic chromium alloyed Steel sheet with excellent low formability and rolling and with to provide good ridging properties, made from a continuously cast slab is, where not required, a surface treatment before the hot rolling of the steel roll plate.

Other Objects of the invention include providing a ferritic Chromium-alloyed steel sheet with good ridging properties and excellent deep-forming properties, the improved weldability, corrosion resistance and durability against cyclic oxidation at high temperatures.

A Another object of the invention is a process for the preparation a ferritic chromium alloy steel with the above Specify properties.

These Tasks are performed by an alloyed ferritic steel according to claim 1 and a method for producing such a steel according to claim 8 solved.

The ferritic chromium alloyed steels according to the present Invention have a structure in the cast state with more than 50% equiaxed grains. The steel is deoxidized in the casting condition with titanium and contains up to 0.08% C, at least about 8% Cr, up to 1.50% Mn, ≤ 0.05% N, ≤ 1.5% Si, ≤ 2.00% Ni, Ti ≥ 0.10%, the ratio of (Ti x N) / Al is at least 0.14, where all percentages indicate weight percent and the residual content consists of iron and residual elements. The steel in the cast state becomes too processed a continuous sheet. The sheet can be descaled, to a final strength cold-worked and then annealed for recrystallization. The glow of hot worked sheet before cold working or annealing of Sheet between several cold forming steps for elimination of the ridging phenomenon at the conclusion annealed Sheet metal not necessary.

Another feature of the invention is that the content of titanium is ≥ 0.15% and that of aluminum is <0.02%, wherein said ratio of (Ti × N) / Al may be at least 0.20. Preferably, the content of titanium satisfies the following relationship (Ti / 48) / [(C / 12) + (N / 14)]> 1.5, and Ti and N are present in under-equilibrium parts. An advantageous feature of the cold worked annealed sheet is to have an R _m of ≥ 1.4, being made from a hot worked sheet that was not annealed before cold working. Advantageously, the equiaxed grains in the cast state have a size of ≦ 3 mm and the annealed sheet has a uniform grain structure.

One another ferritic chromium alloyed steel according to the present invention contains ≤ 0.010% Al, until to 0.08% C, up to 1.50% Mn, ≤ 0.05% N, ≤ 1.5% Si, 8-25% Cr, <2.0% Ni and a means of oxidation of the steel, all percentages in Percent by weight and the residual content of iron and Residual elements exists. The deoxidizer is titanium. The as cast steel is hot processed into a continuous one Sheet. The sheet is descaled, cold rolled to a final thickness and then annealed for recrystallization. The glow of the hot worked sheet before cold working to eliminate the ridging phenomenon in conclusion annealed Sheet metal is not necessary.

Further Characteristics of the steel consists of a titanium content of ≥ 0.01%, aluminum ≥ 0.007%, and Ti and N are present in lower balance parts. Another one Feature is that titanium has the relationship (Ti / 48) / [(C / 12) + (N / 14)]> 1.5. The annealed Sheet metal advantageously has an Rm value of ≥ 1.4. Advantageously the equiaxed grains in the cast state a size of <3 mm, and the As-cast microstructure has a high content of fine equiaxed grains on.

advantages The invention consists in a readily formable ferritic chromium alloyed Steel with excellent ridging properties can be produced at a lower cost, it does not require to anneal the hot worked sheet before cold working, and has an improved surface quality, improved weldability, good wet corrosion resistance and a high resistance across from cyclic oxidation at high temperatures. Another Advantage is to be able to cast a slab, the no surface treatment, z. B. grinding, prior to hot processing to prevent formation of open surface defects requires that parallel to the rolling direction in a hot processed Extending sheet such. As hot rolling and rolled strips non-metallic titanium oxide or clusters of titanium nitride precipitates, the near the slab surface during the Vergusses be formed. Another advantage of the invention is in a well malleable ferritic chrome steel, which has an improved weldability, a high wet corrosion resistance and a high resistance across from cyclic oxidation at high temperatures. Another The advantage is that it is possible is to cast a slab for the no surface treatment is necessary, such. B. grinding, before hot processing, for Avoiding the formation of open surface defects that are parallel extend to the rolling direction in a hot-processed sheet, such as z. As hot rolling and rolled strips of non-metallic Titanium oxide or clusters of titanium nitride precipitates formed during the pouring near the surface the rolling plate were formed. Another advantage of the invention consists in providing a ferritic chromium alloyed Steel sheet with high formability and excellent ridging properties a very uniform Having grain structure in the sheet after annealing.

These and other objects, features and advantages of the invention will become apparent with reference to the following detailed description and the accompanying drawings.

SHORT DESCRIPTION THE DRAWINGS

1 FIG. 3 is a graph of a grain structure of a ferritic chromium-alloyed steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.13 added to. FIG Contains 100% columnar grains.

2 FIG. 5 is a graph of a grain structure of a ferritic chromium-alloyed steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.16 containing approximately 78% fine equiaxed grains.

3 FIG. 15 is a graph of a grain structure of a ferritic chromium alloy steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.13 containing 100% columnar grains. FIG.

4 FIG. 12 is a graph of a grain structure of a ferritic chromium alloy steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.15 containing about 84% of columnar grains. FIG.

5 FIG. 12 is a graph of a grain structure of a ferritic chromium-alloyed steel in the as-cast state with a ratio of the product of titanium and nitrogen divided by aluminum of 0.12 containing 100% columnar grains. FIG.

6 FIG. 4 is a graph of a grain structure of a ferritic chromium-alloyed steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.19 containing approximately 92% of fine equiaxed grains. FIG.

7 FIG. 12 is a graph of a grain structure of a ferritic chromium-alloyed steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.11 containing about 94% of columnar grains. FIG.

8th FIG. 15 is a graph of a grain structure of a ferritic chromium alloy steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.15 containing about 63% of columnar grains. FIG.

9 FIG. 12 is a graph of a grain structure of a ferritic chromium alloy steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.06 containing 100% columnar grains. FIG.

10 FIG. 13 is a graph of a grain structure of a ferritic chromium-alloyed steel as cast with a ratio of the product of titanium and nitrogen divided by aluminum of 0.34 containing approximately 100% columnar grains. FIG.

11 is a recording of a non-uniformly banded grain structure of a comparative example, consisting of the ferritic chromium-alloyed steel of 9 after cold forming and recrystallization annealing.

12 is a photograph of a uniform fine grain structure of a ferritic chromium alloy steel 10 after cold forming and recrystallization annealing.

13 Figure 13 is a graph illustrating the percentage of equiaxed grains (% EQ) in the as-cast grain structure as a function of the ratio of the product to titanium times nitrogen, in weight percent divided by aluminum (TNA), for laboratory-scale ferritic chromium-alloyed steel cast plates , and

14 Figure 4 is a graph illustrating the percentage of equiaxed grains (% EQ in the as-cast grain structure as a function of the ratio of the product of titanium times nitrogen, in weight percent divided by aluminum (TNA) for laboratory-scale ferritic chromium-alloyed steel cast plates.

15 Figure 11 is a photograph of a grain microstructure of a low aluminum content ferritic chromium alloy steel according to the present invention.

16 Figure 11 is a photograph of a grain microstructure of a high aluminum content ferritic chromium alloy steel.

17 Figure 11 is a photograph of a grain microstructure in the cast state of another prior art ferritic chromium alloy steel having a high aluminum content.

18 shows a non-uniform structure with large grains typical of high aluminum ferritic stainless steels 17 after annealing is.

19 FIG. 4 is a photograph of a grain microstructure in the potting state of another ferritic chromium alloy steel according to the present invention having a low aluminum content. FIG.

20 FIG. 4 illustrates a uniform grain structure of the low aluminum ferritic stainless steel of FIG 19 after annealing.

21 FIG. 11 is a photograph of a grain microstructure as cast of another ferritic chromium alloy steel of the present invention with low aluminum content; and FIG

22 Figure 13 is a graph illustrating the percentage of equiaxed grains in the as-cast microstructures of ferritic chromium alloyed steels as a function of aluminum content.

Under ferritic chromium-alloyed steel should be understood as a steel, which has at least 8% chromium. The ferritic chromium alloyed steels according to the invention are in particular for hot-processed sheets, cold-reduced sheets, metallic coated Sheets and painted sheets are suitable. These ferritic chromium alloys steels are well suited for stainless steels of the type AISI type 400 series with approximately 10-25% Cr, especially for the type 409 stainless steel, about 11-13% Contains chromium. For the purposes of the invention, the term "sheet metal" is to be understood as meaning continuous Stripes or cut lengths, which are formed from the continuous strip, with should be.

A Iron melt is in a melting furnace, such. B. an electrical Electric arc furnace (EAF), melted. The Molten iron may be scrap containing solid iron, carbon steel scrap, stainless steel scrap, materials containing solid iron, including Iron oxides, iron carbides, direct reduced iron, warm briquetted Iron can be formed in the furnace, or the melt can upstream of the melting furnace in a blast furnace or any other iron melting device produced which is suitable for providing an iron melt. The molten iron is then refined in the furnace or in a refining container, such as As an argon-oxygen decarburization furnace (AOD) or a Vacuum oxygen decarburizing furnace (VOD), followed by a trim station, such as A lance metallurgy furnace (LMF) or a wire management station.

An important feature of the invention is that after refining the melt to a final carbon content as analyzed and during or after the addition of fine-tuning alloys to the melt to achieve the final specification, titanium is added to the melt for deoxidation prior to casting. Deoxidation of the melt with titanium is necessary to form small titanium oxide inclusions to form the nuclei necessary to form a fine as-cast co-axial grain structure. In order to provide a sufficient number of these seeds necessary to form an equiaxed fine grain structure in the potted state, it is necessary to add at least about 0.10% Ti to the melt. Aluminum is preferably not added to the refined melt as a deoxidizer to minimize the formation of alumina inclusions, ie, Al ₂ O ₃ . An equally important feature of the invention is that sufficient titanium and nitrogen are present in the melt prior to casting so that the ratio of the product of titanium and nitrogen divided by the residual content of aluminum (TNA) is at least 0.14. By controlling this ratio to at least 0.14, it is believed that nitrogen in the melt forms small titanium oxide inclusions coated with titanium nitride, ensuring small nucleators necessary to form the fine equiaxed grains in the casting state. When the steel is stabilized, a sufficient amount of titanium above the level required for deoxidation, ie, 0.10%, should be added to the compound with carbon and nitrogen in the melt, but preferably less than the amount necessary for Nitrogen saturation is needed, ie, underbalance, preventing precipitation of large titanium nitride inclusions prior to solidification. Alternatively, one or two stabilizing elements, such as. As niobium, zirconium, tantalum or vanadium are also added to the melt. Accordingly, the steel according to the invention has at least 0.10% Ti, preferably at least 0.05% N, and preferably less than 0.02% Al in the melt, so that the steel is substantially deoxidized by titanium with small inclusions of titanium oxide containing the represent predominant inclusions in the melt, ie titanium oxide inclusions >> Al ₂ O ₃ inclusions, to provide the nuclei necessary for the formation of the uniform grain structure in the casting state.

Ferritic chromium alloyed steels, which are deoxidized with aluminum rather than titanium, may have low inclusions in the melt. However, a major difference between the aluminum deoxidized ferritic chromium steels of the prior art and the titanium-deoxidized ferritic chromium steels of the present invention is that most of the inclusions of the molten steel of the present invention are titanium oxide and not alumina. We have determined that at least 50% of the inclusions of the steels according to the invention have a particle size of not greater than 1 μm and at least 90% of these inclusions have a size no greater than about 1.5 μm. It is unclear what modification of the titanium oxide, ie TiO, TiO ₂ , Ti ₂ O ₃ or Ti ₃ O ₅ is present, but it is assumed that the primary inclusions are TiO ₂ .

To refining and alloying with chromium in a refining or refining tank becomes the ferrous chromium alloyed steel with deoxidized titanium and titanium contains up to 0.08% C, at least 8% Cr, up to 1.50% Mn, <0.03% Al, ≤ 0.05% N, ≤ 1.5% Si, <2.0% Ni, Ti ≥ 0.10%, all Data in percent by weight, with a residual content of iron and residual elements. The relationship the product of the contents of titanium and nitrogen in weight percent divided by the residual content of aluminum must be at least 0.14. The Chromium-alloyed steel melt can be continuously cast into a sheet metal be, a thin one Plate with a thickness of ≤ 140 mm, a thick plate with a thickness ≤ 200 mm or a block with a grain structure in the casting state, consisting of more than 50% of fine equiaxed grains is formed. Preferably, the molten steel has a ratio of Products divided by titanium and nitrogen in percentage by weight the residual content of aluminum of at least 0.16, more preferably of at least 0.23 and has a potted structure after potting at least 80% of fine equiaxed grains or substantially continuously consisting of fine equiaxed grains.

We have determined that the ratio from the product to titanium times nitrogen divided by the residual content to aluminum, which is necessary to produce an equiaxed grain in the casting condition to get in proportion to the chromium content of the steel. For a stainless steel with approximately 11% chromium of the type T409 The relationship of the product of titanium times nitrogen divided by the residual content Aluminum, which is necessary, more than 50% of equiaxed grains in the Potting state, about 0.14% and that, the necessary to achieve that nearly 100% of the grains in the potted state as equiaxed grains is greater than 0.23. For a stainless steel with high chromium contents of at least about 16% of the Type T430 and a stainless steel with high chromium contents of at least about 17% of the type T439 illustrate Tables 3 and 4, which are related to Achieving a 50% share of equiaxed grains in the Casting condition a ratio from the product of nitrogen times titanium divided by the residual content Aluminum larger than approximately 0.20 needed and that to achieve a proportion of nearly 100% equiaxed grains in the casting condition a ratio of more than 0.30 was.

Of the cast steel is processed into a sheet metal. Under "hot processing" within the meaning of the invention it is understood that the steel is reheated in the casting condition will, if necessary, and then reduced to a predetermined thickness by hot rolling becomes. If hot rolling is performed, the steel roll plate becomes heated to 1050-1300 ° C, hot rolling using a final temperature of at least 800 ° C and a winding at a temperature ≤ 580 ° C performed. The hot-rolled sheet, z. Hot strip, is pickled and cold worked at at least 40%, preferably at 50%, to a desired end sheet thickness. following the cold formed sheet is subjected to recrystallization annealing for at least 1 s at a peak temperature of the metal of 800-1000 ° C subjected. A significant advantage of the invention is that it is not is required, the hot-processed sheet before this cold forming to glow. Another advantage of the invention is that the hot processed Sheet metal can be cold formed in one pass, which prevents it becomes necessary, an intermediate annealing between several cold forming operations. The recrystallization annealing, the Cold forming may be in the form of a continuous annealing or a static glow, z. B. done in a box. Another advantage of the invention is that an annealed Chromium-plated steel sheet with excellent ridging properties a very uniform fine Grain structure with a cold reduction of only 40%.

The ferritic chromium alloyed steel of the present invention can be made from a hot processed sheet by a number of methods. The sheet may be formed of slabs made of blocks or continuously cast slabs of a thickness of 50-200 mm, which are reheated to a temperature of 1050-1300 ° C, followed by hot rolling hot-processed starting sheet to be provided with a thickness of 1-6 mm, or the sheet can be hot worked from continuously cast to a thickness of 2-10 mm tape. The present invention is also applicable to sheets produced by processes in which continuously cast slabs or slabs produced from slabs or without substantial reheating are fed directly to a hot rolling mill, or blocks which are hot-worked into slabs and have a sufficient temperature. to be hot rolled with or without further reheating to a sheet.

An important feature of the invention is that titanium is used to deoxidize the melt prior to casting. Titanium is used for deoxidation to ensure that the predominant inclusions in the melt are small titanium oxide inclusions for nucleating the as-cast equiaxed ferrite grains. The proportion of titanium in the melt will be at least 0.10% and is preferably a portion in the lower balance. More preferably, the content of titanium in the molten steel is ≥ 0.15% and satisfies the relationship (Ti / 48) / [(C / 12) + (N / 14)]> 1.5. By "underbalance" is meant that the content of titanium is controlled so that the solubility of the titanium compounds formed is below the saturation limit at the liquidus temperature of the steel, thereby avoiding excessive precipitation of TiN in the melt. If it is allowed to form excess TiN inclusions, the TiN precipitates grow into large, low density clusters that float on the solidifying slab surfaces during continuous potting. These non-metallic TiN clusters form open surface defects during hot processing of the slab. The proportion of titanium allowed in the melt to avoid excessive precipitation is inversely proportional to the proportion of nitrogen. The maximum amount of under-balance titanium is generally in 4 U.S. Patent 4,964,926. Depending on the chromium and nitrogen content of a molten steel alloy, the proportion of titanium must be below the content in 4 controlled by U.S. Patent 4,964,926. Type T409 stainless steel contains approximately 12% chromium and 0.010% nitrogen and may contain up to 0.26% Ti. Stainless steel contains about 15% chromium and 0.010% nitrogen and may contain up to about 0.30% Ti. Type T439 stainless steel contains approximately 18% chromium and 0.010% nitrogen and may contain up to approximately 0.35% Ti. Excess nitrogen is not a problem for manufacturers refining ferritic stainless steel melts in an AOD furnace. Nitrogen substantially below 0.010% can be obtained when refining the stainless steel in an AOD furnace, thereby allowing an increased level of titanium to be tolerated and still present at underbalance.

to Providing the for the formation of equiaxed in the cast state ferrite grains necessary Nucleation nuclei must be sufficient after adding titanium to the melt Time to allow the titanium oxide inclusions to be present the casting to form the melt. If the melt immediately after the titanium addition is cast, the structure is in the cast state of the molded product in the potted state from large columnar grains. in the Laboratory cast blocks 5 minutes after the addition of titanium to the melt, large columnar grains appeared in the melt potted state, even if the product of titanium times nitrogen divided by the residual content of aluminum was at least 0.14.

One important feature of the invention is that sufficient Nitrogen is present in the steel before casting, so the relationship from the product of titanium times nitrogen divided by aluminum at least about 0.14. It is believed that by controlling this ratio, sufficient titanium oxide be formed to ensure that the necessary germs to form the equiaxed grains in the potted state become. The amount of nitrogen present in the melt should be ≤ 0.05%, preferably 0.005-0.03% and stronger preferably 0.007-0.015% be. It is believed that small titanium oxide inclusions, the coated with titanium nitride are for the provision of for the formation of as-cast fine equiaxed grain structures necessary germs are responsible. Through careful control the proportion of titanium and nitrogen in the melt is assumed that sufficiently small titanium oxide inclusions with a size smaller than 1 μm which represent the necessary germs necessary for the fine, in the potted state equiaxed grain structure responsible are.

A steel alloy composition can be controlled with respect to nitrogen and with respect to the sub-equilibrium Ti content to avoid excess TiN precipitates and Ti streaking in the hot processed sheet. Although nitrogen concentrations after melting in an electric arc furnace (EAF) can reach a significant size of 0.05%, the content of dissolved nitrogen by argon gas refining in an AOD can be reduced to less than 0.02%, and less than 0 if necessary , 01%. The excretion of excess TiN can be reduced by reducing the sub balance of Ti to be added to the melt to any given nitrogen content. Alternatively, the amount of nitrogen in the melt may be reduced in an AOD furnace for a pre-known amount of titanium contained in the melt. For the sub-equilibrium T409 stainless steel, which contains about 11-13% chromium and not more than about 0.012% nitrogen, the molten steel would contain less than about 0.25% titanium to allow excess TiN precipitates prior to solidification of the melt avoid. For a sub-equilibrium T430 or T439 stainless steel containing about 16-18% chromium and not more than about 0.012% nitrogen, the molten steel would become less than about 0.35% Ti to avoid excessive TiN precipitates before solidification of the melt contain.

One just as important feature of the invention is the total content to control or minimize residual aluminum in relation to the proportions of titanium and nitrogen. The minimum content of titanium and nitrogen in the ratio to aluminum be present in the melt. We found that myself low levels of aluminum, d. H. not more than 0.01%, not produce the necessary equiaxed grains in the cast state, if the proportions of titanium and in particular of nitrogen are too low. A limiting proportion of small precipitates of Titanoxideinschlüssen is even in the absence of alumina inclusions, apparently in the melt to form the necessary germs for the education of the spilled State equiaxed grain structure needed. We have found, that the ratio of the product of titanium times nitrogen divided by the residual content of aluminum at least 0.14, preferably at least 0.23 must to ensure that nearly 100% of the grains in the potted state as equiaxed grains available. To minimize the proportions of titanium and nitrogen, which are required in the melt, the aluminum content is preferably <0.020%, more preferably ≤ 0.013% and most preferably reduced to ≤ 0.010%. If aluminum is not while refining or potting intentionally alloyed with the melt, as directly for deoxidation before casting, The total content of aluminum can be controlled or set to values less than 0.010%, especially for stainless steels, which contain less than 14% chromium. For a stainless steel high chromium content, d. H. Cr ≥ 15%, is a relationship required by (Ti × N) / Al> 0.40 around the as-cast state, a structure with nearly 100% fine equiaxial grains to achieve, it may be necessary, nitrogen to melt add to values greater than 0.01%. Aluminum is preferably not intentionally melted add as an impurity in the form of an additive consisting of an alloy, another element, e.g. As titanium, is present. The use of titanium alloy additions with impurities Aluminum should preferably be avoided. titanium alloys can contain up to 20% Al, giving a content of 0.07% of the total aluminum content may correspond to the melt. By careful control of refining and the casting process can be a melt that is less than 0.020% Contains aluminum, to be obtained.

While not being bound by theory, it is believed that the total content of aluminum, especially for stainless steels with less than 14% chromium, is less than 0.03%, preferably less than 0.02%, more preferably not more must be limited to 0.013%, most preferably less than 0.01%, to minimize the formation of Al ₂ O ₃ inclusions in the melt such that titanium is the primary deoxidizer. The steel is continuously cast into a thin slab or continuous sheet that does not inherently have a coaxially fine equiaxed crown structure. It is believed that by careful control of the aluminum according to the present invention, the formation of Al ₂ O ₃ inclusions can be minimized. Al ₂ O ₃ inclusions present in the melt coalesce into large clusters. By minimizing the formation of alumina inclusions, it is further believed that small inclusions having a size of less than 5 microns, preferably no larger than 1.5 microns and most preferably no larger than 1 microns of titanium oxide to the dominant non-metallic inclusions in the Melt. It is believed that these small titanium oxide inclusions are the nucleating agents that allow the formation of an as-cast equiaxed grain structure during solidification. Accordingly, titanium is used for deoxidation to ensure that the predominant inclusions in the melt and solidified cast steel are predominantly titanium oxide rather than alumina inclusions, ie, the number of titanium oxide inclusions is significantly greater than the number of alumina inclusions.

Prior art deoxygenated steels tend to clog the nozzles during continuous casting. It has generally been necessary with high aluminum steels to add calcium to the melt to increase the fluidity of the Al ₂ O ₃ inclusions in the cast melt to minimize this tendency to clog the casting nozzle. However, calcium basically has a detrimental effect on the formation of as-cast equiaxed grains. Accordingly, calcium should be limited to levels ≤ 0.0020%. A significant advantage It is an object of the invention to avoid the need to add calcium to low aluminum-containing melts, since there are very few Al ₂ O ₃ inclusions in the melt when the aluminum content is maintained at values of ≤ 0.016%. High levels of Al ₂ O ₃ inclusions contained in the melt can rapidly coalesce into alumina clusters, which can cause the nozzle to clog during continuous potting.

carbon is in the steels according to the invention with a content of up to 0.08%, preferably ≤ 0.02%, and more preferably between 0.0010-0.01%. If the carbon content exceeds 0.08%, moldability, the corrosion resistance and the weldability deteriorated. Accordingly, carbon should be on a salary be reduced as low as possible.

One Element for the stabilization of carbon and nitrogen may be included the steels the present invention in an amount of up to 1.0%, preferably 0.6% and stronger preferably up to 0.3%. If a stabilized steel he wishes is, should have a sufficient content of stabilizing element present to the formation of a stable carbonitridic compound to enable for forming a crystal size to increase elongation and toughness stainless steels is effective, whereby the shaping properties such. B. the Thermoformability, after annealing, is improved. When the stabilizing element is containing is greater than 1.0%, the cost of producing the Steel increases, without achieving corresponding advantages in the properties. In addition to Use of titanium for stabilization may include other suitable stabilizing elements, such as As niobium, zirconium, tantalum, vanadium or mixtures thereof can be used, with titanium alone is preferably used. If used a second stabilizing element together with titanium is, for. As niobium, should the second stabilizing element on contents not higher be limited to 0.3% if deep drawability is required. niobium in contents greater than 0.3% is detrimental to moldability.

chrome lies with the steels according to the present Invention with a content of ≥ 8%, preferably ≥ 10% in front. If the chromium content is less than 8%, the wet corrosion behavior, z. B. in exhaust components from the automotive sector, adversely affected. If the chromium content is higher than 25%, the formability of the steel is deteriorated.

For some Applications may be desirable Bor, to the steels according to the present invention with a share of ≥ 5 ppm, more preferably containing ≥ 20 ppm, and most preferably between 40-60 ppm. With a boron content of at least 5 ppm the resistance increases embrittlement by reworking the steels to, so that the steel sheet itself during thermoforming and shaping in several steps not parted. If boron is present at a level greater than 200 ppm the moldability of the steel deteriorates.

oxygen is preferred in the steels of the invention with a content <100 ppm included. In the production of a molten steel step by step in an AOD refining tank and a LMF alloy container, is the oxygen present in the melt with a content in the range of 10-60 ppm, thereby providing a very clean steel with small titanium inclusions which are necessary as nucleating agents who are responsible for the fine equiaxed grain structure in the cast state.

silicon is basically in the steels according to the invention a proportion ≤ 1.5%, and preferably with a content ≤ 0.5% available. A low content of silicon is basically in ferritic stainless steels to promote the formation of the ferritic phase present. Silicon also benefits the high temperature corrosion resistance and carries for high-temperature strength, z. B. in exhaust gas components for the automotive sector at. Accordingly, silicon should be in the melt with a proportion of at least 0.10%. Silicon should be grades of Do not exceed 1.5%, because then the steel is too hard and affects the elongation negatively becomes.

manganese is in the steels according to the invention with a proportion of up to 1.5% and preferably at levels of less than 0.5% available. Manganese improves hot workability Compound with sulfur to manganese sulfide to prevent tearing of the Sheet metal during the hot working. Accordingly, a manganese content of at least 0.1% desirable. However, manganese acts as an austenite former and affects stabilization the ferritic phase. When the content of manganese exceeds about 1.5%, the stabilization and the formability of the steels become negative affected.

Manganese is present in the steels of the invention at a level of up to 1.5%, and preferably with Levels of less than 0.5% available. Manganese improves hot workability by combining with sulfur as manganese sulfide to prevent the tearing of the sheet during hot working. Accordingly, a manganese content of at least 0.1% is desirable. However, manganese acts as an austenite former and affects the stabilization of the ferritic fiber. When the content of manganese exceeds about 1.5%, the stabilization and moldability of the steels is adversely affected.

sulfur is preferred in the steels of the invention with a content <0.015%, more preferably with a content of <0.010% and the strongest preferably with contents of <0.005% contain. additionally to cause a problem during hot rolling affects sulfur the wet corrosion resistance in a negative sense, especially for those steels that are low in content have on chrome. Accordingly, sulfur should preferably Do not exceed levels of 0.015%.

Similar to Manganese, nickel is an austenite former and influences the stabilization the ferritic phase. Accordingly, nickel is on contents ≤ 2.0% and preferably <1.0% limited.

Of the Ferritic chromium steel of the present invention may also be further Elements, such as As copper, molybdenum, phosphorus and the like either in the form of intended additions or as a residual element, d. H. as an impurity from the steel production process.

example 1

As a comparative example, a chromium alloyed molten iron of about 25 kg was set in a laboratory vacuum vessel. After the final addition of fine tuning alloying elements into the vessel, the melt was deoxidized with titanium. The composition of the chromium-alloyed molten steel was 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 ratio from the product of titanium times nitrogen divided by aluminum was 0.125. About 23 minutes after the titanium addition, the melt was cast into a block having a thickness and a width of about 75 mm and 150 mm, respectively. The grain structure in the potted state of a sample in cross section is in 1 and has a grain structure that was completely columnar with an average column size of about 3 mm. This steel illustrates that a low aluminum content alone, ie ≤ 0.01%, is insufficient to form a predominantly equiaxed as-cast structure. This steel, which has a ratio of (Ti × N) / Al <0.14, illustrates a as-cast steel grain structure containing no equiaxed grains.

Example 2

A chromium-alloyed molten iron of about 25 kg according to the present invention was charged to the same laboratory vacuum vessel as described in Example 1. After the final addition of the fine tuning alloying elements into the vessel, the melt was deoxidized with titanium. The composition of the chromium-alloyed molten steel was 0.007% Al, 0.28% Ti, 0.008% C, 0.25% Mn, 0.36% Si, 11.1% Cr, 0.18% Ni and 0.004% N. The ratio of titanium product times nitrogen divided by aluminum was 0.16. About 17 minutes after the addition of titanium, the melt was cast into a block having a width of about 75 and 150 mm, respectively. An as-cast grain structure of a sample cut out of the stainless block had a fine grain structure of about 78% of the equiaxed grain grains having an average diameter of about 2 mm as in FIG 2 shown. This steel had a ratio (Ti × N) / Al ≥ 0.14, which illustrates that a grain structure of a as-cast steel has a proportion of ≥50% of fine equiaxed grains.

Example 3

Another comparative example of a chromium alloyed molten iron of the invention was prepared in a manner similar to that described in Example 1 and had a composition of 0.013% Al, 0.19% Ti, 0.007% C, 0.26% Mn, 0.36 % Si, 11.0% Cr, 0.24% Ni and 0.009% N. The ratio of the product of titanium to nitrogen divided by aluminum was 0.13%. About 19 minutes after the addition of titanium, the melt was poured into blocks. A as-cast grain structure of a cross-section sample cut from the stainless steel ingot had a grain structure that was completely columnar and had an average column size of about 2 mm, as in FIG 3 shown. This steel with a ratio of (Ti × N) / Al <0.14 illustrates that a grain structure of the as-cast steel has less than 50% of equiaxed grains.

Example 4

Another chromium alloyed molten iron according to the present invention was prepared in a manner similar to that described above in connection with Example 2 and had a composition of 0.013% Al, 0.24% Ti, 0.007% C, 0.26% Mn, O, 37% Si, 11.1% Cr, 0.25% Ni and 0.008% N on. The ratio of the product of titanium to nitrogen divided by aluminum was increased to 0.15. This molten steel was cast into a block approximately 14 minutes after the addition of titanium. A structure of a as-cast cross-section sample cut from the stainless steel block had a fine grain structure of about 84% equiaxed grains and an average diameter of about 3 mm as in FIG 4 shown. This steel illustrates that a as-cast steel grain structure contains ≥ 50% of fine equiaxed grains even when the steel has high aluminum contents, ie, ≥ 0.01%, when the ratio (Ti × N) / Al ≥ 0.14 is.

The compositions, the (Ti x N) / Al ratio and the percentage of equiaxed grains of the as-cast blocks of the Comparative Examples and inventive Examples 409 stainless melt examples of Examples 1-4, as well as the numerous additional comparative examples and inventive examples A 409 type stainless steel melt, prepared on a laboratory scale and cast into blocks in a manner similar to Examples 1 to 4 described above, is summarized in Table 1. The percentage of equiaxed grains as a function of the (Ti x N) / Al ratio for these blocks is in 13 shown. 13 shows that a titanium content of at least 0.10% and a (Ti × N) / Al ratio of about 0.14 or higher is necessary to obtain a grain structure containing at least 50% of fine equiaxed grains in the as-cast state.

The compositions, the (Ti x N) / Al ratio and the percentage of equiaxed grains for additional as-cast laboratory ingots as comparative examples and examples of the present invention for high chromium type 430, 439 and 439 Mo stainless steels were prepared in a similar manner and cast into blocks, such as Examples 1 to 4, are summarized in Table 3. Table 3 illustrates that a titanium content of at least 0.10% and a (Ti x N) / Al ratio of at least 0.2% to obtain a as-cast steel grain structure containing at least 50% equiaxed fine grains. necessary is. Increasing the ratio (Ti × N) / Al was apparently necessary due to the increase in chromium content of about 11% in the stainless steel of the 409 stainless type composition in Table 1 to high chromium containing compositions of about 17% or higher in the high chromium containing ones stainless steels of the type 430, 439 and 439 Mo, which are shown in Table 3.

Example 5

As a comparative example, a chromium-alloyed molten iron of about 125 t was placed in an AOD refining tank. After reduction of the carbon to the final specification, the melt was converted to an LMF, in which the final fine addition of alloying elements took place. Subsequently, the melt was deoxidized with titanium. The final melt composition was 0.009% Al, 0.21% Ti, 0.007% C, 0.26% Mn, 0.32% Si, 11.2% Cr, 0.14% Ni and 0.005% N. The ratio of Product of titanium times nitrogen divided by aluminum was 0.12. The molten steel was then transferred to a potting device within about 40 minutes and cast continuously into thin slabs 130 mm thick and 1200 mm wide. Cross-section samples were cut from a middle width region and from several other locations along the length direction of the thin slab. A typical as-cast grain structure of one of these samples cut out of the slab is shown in FIG 5 and has a columnar grain structure with an average column size of about 4 mm. This steel, like Example 1, shows that low levels of aluminum alone, ie ≤ 0.01%, are not sufficient to produce a predominantly unidirectional grain as cast structure. 5 illustrates that a ferritic stainless steel having a ratio of (Ti x N) / Al <0.14 results in a cast steel structure of the steel that does not contain unidirectional grains.

Example 6

A chromium alloyed molten iron of about 125 t according to the present invention was prepared in a manner similar to Example 5 described above except for the following compositional changes. The composition of the melt was 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. In contrast to Example 5 was the ratio of the product of titanium times nitrogen divided by aluminum increased to a value of 0.19. The molten steel was then transferred to a potting device and cast into thin slabs in a manner similar to that described for Example 5 above. A grain structure of the as-cast slab of this stainless steel had a fine grain structure with about 84% of equiaxed grains and an average size of 2 mm as in FIG 6 shown. 6 illustrates that a ferritic stainless steel having a ratio (Ti × N) / Al ≥ 0.14 results in a as-cast steel grain structure containing more than 50% equiaxed grains. The slabs of this steel contained inclusions predominantly of titanium oxide.

Example 7

Another comparative example of a chromium-alloyed molten iron was prepared in a manner similar to Example 5. The composition of the melt was 0.20% Ti, 0.014% Al, 0.011% C, 0.28% Mn, 0.31% Si, 10.9 % Cr, 0.12% Ni and 0.0087% N. Similar to Example 5, the product of titanium times nitrogen divided by aluminum was only 0.11. The molten steel was then transferred to a potting device and cast into thin slabs in a manner similar to that described for Example 5 above. An as-cast grain structure of the slab of this stainless steel had a proportion of about 94% of large columnar grains having an average column size of 5 mm, as shown in FIG 7 shown. 7 illustrates that a ferritic stainless steel with a ratio (Ti × N) / Al <0.14 results in a as-cast steel grain structure containing very few equiaxed grains.

Example 8

Another chromium alloyed molten iron according to the present invention was prepared in a similar manner to Example 6. The composition of the melt was 0.21% Ti, 0.016% Al, 0.006% C, 0.23% Mn, 0.27% Si, 11 , 3% Cr, 0.11% Ni and 0.011% N. The ratio of the product of titanium to nitrogen divided by aluminum was 0.15. The molten steel was then transferred to a potting device and poured into thin slabs in a manner similar to that described above for Example 5. A grain structure of a as-cast specimen cut out of the slab of this stainless steel predominantly had a fine equiaxed crown structure as shown in FIG 8th is shown. 8th illustrates that a ferritic stainless steel with a ratio (Ti × N) / Al ≥ 0.14 resulted in a as-cast steel grain structure accounting for 63% of fine equiaxed grain Grains with a size of about 3 mm. This steel illustrates that when the ratio (Ti × N) / Al ≥ 0.14, a grain structure of a as-cast steel having more than 50% of fine equiaxed grains may have, although the steel has a high aluminum content, ie ≥ 0.01%. The slabs of this steel contained inclusions predominantly of titanium oxide.

Example 9

Another comparative example of a chromium-alloyed iron melt was prepared in a manner similar to Example 5. The composition of the melt was 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 ratio of the product of titanium to nitrogen divided by aluminum was only 0.08. The molten steel was then transferred to a potting device and poured into thin slabs in a manner similar to that described above for Example 5. The grain structure of a rolled plate of this as-cast steel showed a coarse grain structure consisting of 100% columnar grains having an average column size of about 4 mm, as shown in FIG 9 shown. 9 illustrates that a ferritic stainless steel with a ratio (Ti × N) / Al <0.14 results in a as-cast steel grain structure containing no equiaxed grains.

Rolled slabs from this melt were reheated to 1250 ° C, hot worked to a thickness of 3.3 mm at a final temperature of about 800 ° C, and wound at a temperature of about 700 ° C. The hot worked sheet was then descaled, pickled in nitric acid and hydrofluoric acid and cold worked 58% to a gauge of 1.4 mm. This hot processed sheet was not annealed prior to cold working. The cold worked sheet was annealed at a peak metal temperature of 870 ° C for about 60 seconds. After rolling, the ridging characteristic of the sheet was 3 to 4 and it had a grain structure of 1.22 to 1.27. A ridging characteristic of 3 or more means low to significant ridging on a scale of 0 to 6. A high ridging characteristic of 3 or more and a low grain size of less than 1.3 are for many applications inferior to ferritic stainless steel in terms of thermoformability or exterior display. The mechanical properties of this steel are summarized in Table 5. A cold rolled annealed grain structure of this steel having a nonuniform "banded" crown structure typical of steels that tend to "ridge" is disclosed in US Pat 11 shown. This non-uniform banded grain structure can not be accepted for outwardly exhibited ferritic steel parts requiring high formability. Annealed cold-formed sheets made from a slab having a columnar structure are subjected to significant ridging when a sheet hot-rolled from the slab is not annealed prior to cold-working.

Example 10

Another inventive chromium alloyed iron melt was prepared in a similar manner to Example 8. The compositions of the melt were 0.19% Ti, 0.005% Al, 0.008% C, 0.12% Mn, 0.16% Si, 10.7%. Cr, 0.13% Ni and 0.011% N. The ratio of the product of titanium to nitrogen divided by aluminum was 0.34. The molten steel was then transferred to a casting facility and cast into thin slabs in a similar manner as described above for Example 5. 10 Figure 4 illustrates that this ferritic stainless steel having a ratio of (Ti x N) / Al ≥ 0.23 results in a as-cast grain structure having 100% of fine equiaxed grains about 1 mm in size. The slabs of this steel contained inclusions predominantly of titanium oxide.

These thin slabs were then reheated to 1250 ° C, hot worked to a thickness of 3.3 mm at a final temperature of 800 ° C, and then wound up at a temperature of 700 ° C. The hot worked sheet was descaled, pickled in nitric acid and hydrofluoric acid and cold worked 58% to a thickness of 1.4 mm. This hot processed sheet was not annealed prior to cold working. The cold formed sheet was annealed at a peak metal temperature of 870 ° C for 60 seconds. After elongation, the ridging characteristic of the annealed sheet decreased to a value of 1 and the R _m value increased to 1.45. A ridging characteristic of 1 means excellent ridging properties and the steel is almost free of ridging. A ridging characteristic of 2 or less and an R _m of at least 1.4 are acceptable for most deep drawing applications of outwardly exposed ferritic stainless steel. The mechanical properties of these sheets according to the invention are summarized in Table 6. The cold rolled and annealed grain structure used in 12 is shown shows a very uniform, fine grain structure. This annealed cold-worked sheet according to the present invention was made from a slab having a fine equiaxed grain structure and had excellent "ridging" characteristics, although the hot rolled sheet was not annealed prior to cold working.

Example 11

Another inventive chromium alloyed iron melt was prepared in a similar manner as Example 10. The composition of the melt was 0.19% Ti, 0.006% Al, 0.007% C, 0.13% Mn, 0.31% Si, 11.0% Cr, 0.16% Ni and 0.008% N. The ratio of the product of titanium to nitrogen divided by aluminum was 0.24. The molten steel was then transferred to a potting device and cast into thin slabs in a manner similar to that described for Example 5 above. This ferritic stainless steel with a ratio of (Ti × N) / Al ≥ 0.23 resulted in a structure of the as-cast steel having 100% fine equiaxed grains about 1 mm in size. The slabs of this steel contained inclusions mainly composed of titanium oxide.

These slabs were reheated to 1250 ° C, hot worked at a finishing temperature of 800 ° C to a thickness of 3 mm, and wound at a temperature of 700 ° C. The hot-processed sheets were descaled and pickled in nitric acid and hydrofluoric acid. The hot worked sheets were then cold worked 53% to a thickness of 1.4 mm. These hot processed sheets were not annealed prior to cold working. The cold-formed sheets were annealed at a metal tip temperature of 940 ° C for 10 seconds. After rolling, the ridging characteristics of the annealed sheets were 1 to 2, and had an R _m of 1.39 to 1.48. A ridging characteristic of 2 means good ridging properties. The mechanical properties of these sheets according to the invention are summarized in Table 7.

Example 12

Another 130 mm thick slab having a composition described in connection with Example 11 was reheated to 1250 ° C, hot worked at a final temperature of 830 ° C into 4.1 mm thick sheets and at a temperature of 720 ° C wound up. The hot worked sheets were descaled, pickled in nitric acid and hydrofluoric acid and then cold worked from 66, 76 and 85%, respectively, to a thickness of 1.4, 1.0 and 0.6 mm, respectively. These hot-processed sheets of the invention were not annealed prior to cold working. The cold formed sheets were annealed at a peak metal temperature of 940 ° C for 10 seconds. After rolling, the ridging characteristics of the annealed sheets were basically 2 or better and had an R _m of 1.76 to 1.96. An R _m value ≥ 1.7 is considered to be excellent for ferritic stainless steel, and it was previously not believed that it would be possible to obtain such values for a ferritic stainless steel which was not subjected to an annealing treatment before cold working. The mechanical properties of these sheets according to the invention are summarized in Table 8.

The composition, the (Ti × N) / Al ratio and the percentage of equiaxed grains of the as-cast slabs for comparative examples and inventive examples of type 409 stainless melts of Examples 5-11 as well as additional comparative examples and examples of stainless melts of the invention Type 409, which was prepared in a manner similar to that described for Examples 5 to 11 above and cast into thin slabs, are summarized in Table 2. The percentage of equiaxed grains as a function of the (Ti × N) / Al ratio for these slabs is in 14 shown. 14 illustrates in principle that steels according to the present invention require a Ti content ≥0.10% and a (Ti × N) / Al ratio of about 0.14 or higher so that a structure of the as-cast steel having more than 50% fine equiaxial Grains may be included. An exception to this was only a 980460 slab, lot 880459, lot 880463, lot 980655, and lot 980687. Lot 980655 and 980687 had nozzle clogging problems, ie, excessive alumina inclusions leading to low tundish temperatures of the molten steel below 1545 ° C led. Accordingly, to prevent clustering of large alumina inclusions, the inventive melts were preferably potted continuously at an overheat temperature of at least 40 ° C, preferably at least 55 ° C. Charge 880459 was again blown due to excessive carbon contents after deoxidation with titanium, ie, titanium inclusions were likely to be discharged into the slag. With regard to batch 880463, nothing extraordinary was observed.

The Compositions containing (Ti x N) / Al ratios and the percentages of equiaxed grains of slabs in the as-cast state for comparative examples and stainless according to the invention Melting of the type 430, 439 and 439 Mo, the similar manner as above in In connection with the examples 5 to 11, manufactured and shed are summarized in Table 4. table 4 illustrates that a Ti content of at least 0.10% and a (Ti × N) / Al ratio of at least 0.30 to a grain structure of the steel as cast leads, the principle significantly more than 50% of fine equiaxed grains in high chromium alloyed steels contains.

The good moldable ferritic chromium alloyed steel sheets of the present Invention made of a steel having a structure in the casting state consisting of fine equiaxed grains, is cast from a melt containing sufficient titanium and nitrogen, However, a controlled proportion of aluminum to form smaller titanium oxide has to provide the necessary germs to form a equiaxed crown structure in the cast state, so that an annealed chromium alloyed Sheet metal made from this steel has improved ridging characteristics. By forming a chromium-alloyed molten iron, in contrast too big Clusters of alumina inclusions rich in titanium oxide inclusions is, a grain structure as cast can with more than 50% of the grains as equiaxed fine grains be formed. By avoiding the formation of large columnar grains in The steel in the cast state becomes the "ridging" phenomenon in a cold rolled recrystallization annealed sheet made from this Steel is minimized, even if a hot-processed, sheet formed from this steel is not annealed prior to cold working.

at other preferred embodiments according to the present Invention, which have lower Al contents, becomes an iron melt in a reflow oven, such. B. an electric arc furnace (EAF). This molten iron can be made in a smelting furnace Hard iron containing scrap, carbon steel scrap, stainless Steel scrap, solid iron-containing materials such. Iron oxide, Iron carbide, directly reduced iron, or hot briquetted iron formed be, or the melt can upstream of a melting furnace in one Blasofen or any other Eisenaufschmelzeinheit produced which is capable of providing an iron-containing melt. The iron-containing melt is then refined in the smelting furnace or in a refining container, such as An argon-oxygen decarburization vessel (AOD) or a vacuum-oxygen decarburization vessel (VOD), followed by a trim station, such as B. a lance metallurgy furnace (LMF) or a wire feeding station. An important feature of this invention It is that after refining the melt to a final content at carbon and during or after the addition of fine-tuning alloys of the melt to achieve the final specification a means of deoxidizing the Melt before pouring is added. This deoxidizer is titanium. One Another important feature of the invention is that aluminum is not as a deoxidizer added to this refined melt becomes. If the steel is to be stabilized, sufficient quantities may be available Titan, the for exceed the amount required to deoxidize, to the compound be added with carbon and nitrogen in the melt. Preferably, the content of added titanium is less than that for the Equilibrium with nitrogen required, eliminating the excretion titanium nitride is avoided prior to solidification of the melt. Alternatively, a or more stabilizing elements, such as. Niobium, zirconium, tantalum and vanadium are also added to the melt. Accordingly has the steel according to the invention with a low aluminum content, preferably 0.01% to the melt titanium, so that the steel is essentially titanium is deoxidized to the formation of a microstructure in the cast state ensure that from a fine equiaxed grain structure is formed. Low aluminum contents are understood to mean that the steel contains up to 0.010% total Al. For steels containing more than 0.010% Al contained, it was observed that band-shaped structures occur indicating that the microstructure of the slab in the as-cast condition is columnar.

After the melt is refined and alloyed with chromium in a smelting or refining vessel, the low alloy chromium-alloyed molten iron is deoxidized with titanium and contains up to 0.08% C, ≤ 0.05% N, up to 1.50% Mn , ≤ 1.5% Si, 8-25% Cr, <2.0% Ni, all figures in percent by weight, the remainder being iron and residual elements. The chromium-alloyed molten steel is continuously cast into a sheet, a thin slab having a thickness ≤140 mm, a thick slab having a thickness ≤200 mm or cast in block with a as-cast microstructure consisting of a fine equiaxed grain structure with a proportion greater than 50%, preferably at least 60%, more preferably at least 80% and most preferably consists of a microstructure with almost exclusively fine equiaxed grains which is free of large columnar grains. The cast steel is then hot worked into a sheet of continuous length. By "hot working" it is meant that the steel is reheated in the as-cast state, if necessary, and then reduced to a predetermined thickness, such as by injection molding. B. during hot rolling. When the steel is hot rolled, the steel roll plate is reheated to 1050-1300 ° C, hot rolled at a final temperature of at least 800 ° C and wound at a temperature ≤ 580 ° C. Additionally, the hot rolled sheet may then be descaled and cold reduced to at least 40%, preferably at least 50%, to the desired final thickness of the sheet. Subsequently, the cold-rolled sheet is recrystallized for 1 second at a peak metal temperature of 800-1000 ° C. A significant advantage of the invention is that the hot-processed sheet to avoid the formation of "ridiging" phenomena does not need to be annealed prior to cold forming, ie no hot strip annealing must be performed. The recrystallization annealing, which follows the cold forming may be a continuous annealing or annealing in a box. Another advantage of the invention is that the annealed alloy steel having excellent ridging characteristics has a very uniform grain structure with a low cold working of 40%.

Of the Ferritic chromium alloyed steel of the present invention can from a hot processed sheet by a number of methods getting produced. The sheet can be made from slabs, the castings or continuously cast slabs are made, the to a temperature of 1050-1300 ° C again are heated, followed by hot rolling, to produce a hot processed Starting sheet with a thickness of 2-6 mm, or the sheet can are processed from a continuous strip, the with a thickness of 2-10 mm was poured. The present invention is also applicable on sheets, which were produced by processes in which continuous potted slabs or slabs produced from blocks led directly to a hot rolling mill be, with or without significant heating, or on blocks that were heat reduced to slabs, with a suitable temperature for hot rolling the sheet with or without additional heating, or on Cases, where the molten metal directly to one for the other Processing suitable metal sheet is poured.

One important feature of the invention is that the total aluminum content on contents of not greater than 0.010%, preferably <0.010%, stronger preferably ≤ 0.007% and most preferably ≤ 0.005% is limited. If aluminum is not intentionally with the melt is alloyed while refining or potting, such as B. for deoxidation immediately before the potting, the Total content of aluminum to contents less than 0.010% controlled become. Preferably, aluminum should not melt unintentionally be added as an impurity in an alloy addition another element, such as As titanium, is included. That is, the Use of titanium alloy additives as impurity Aluminum should be avoided. Titanium alloys can be high Contain aluminum up to 20%, which leads to a high Total aluminum content of 0.07% in the melt can contribute. By careful Control of refining and the casting process can be a melt obtained with an aluminum content of not higher than 0.010%.

Without being dictated by theory, it is believed that the aluminum content should not exceed 0.010% to suppress the formation of Al ₂ O ₃ particles in the melt. Steel cast continuously to thin slabs or a continuous sheet does not inherently have a fine equiaxed grain microstructure as cast. By carefully controlling the aluminum content to not higher than 0.010 wt%, it is believed that in the invention, the formation of Al ₂ O ₃ particles can be minimized. It is further believed that by suppressing the formation of Al ₂ O _3, small particles having a size smaller than 10 μm, preferably smaller than 5 μm, and most preferably smaller than 1 μm consisting of complex oxides of titanium are the predominant non-metallic particles in the melt. It is believed that these small complex titanium oxide particles provide the nucleating agents that allow the formation of a fine co-axial as-cast grain structure during solidification.

Prior art deoxidised aluminum melts tend to clog the nozzles during continuous potting. It was generally necessary to add calcium to high aluminum steels in order to increase the flow properties of the Al ₂ O ₃ particles in the cast melt to minimize the tendency for the nozzle to clog. However, in general, calcium adversely affects the formation of as-cast fine equiaxed grains. Accordingly, the calcium content should be limited to 0.0020%. An important advantage of the invention is the elimination of the need to add calcia to the low aluminum content melt since very few alumina particles are present in the melt if the aluminum content is limited to less than 0.010%. A large number of Al ₂ O ₃ particles in the melt can rapidly coalesce into large clusters of Al ₂ O ₃ , which can cause clogging of the nozzle during continuous potting.

Another feature of the invention is that only titanium is used to oxidize the melt prior to potting, which melt preferably has a lower equilibrium content of titanium of at least 0.01%. Preferably, the content of titanium in this molten steel should satisfy the relationship (Ti / 48) / [(C / 12) + (N / 14)]> 1.5. Underbalance is understood to mean that the content of titanium is controlled so that the solubility products of the titanium compounds are below the saturation level at the liquidus temperature, thereby preventing the precipitation of TiN in the melt. When the formation of TiN particles is allowed, the TiN precipitates coalesce into large clusters density, which rise during the continuous casting to the surface of the solidifying roll plate. The content of titanium allowed in the melt to prevent TiN precipitates is conversely correlated with the content of nitrogen. The maximum content of titanium for under-equilibrium conditions is in the 4 No. 4,964,926, incorporated by reference. That is, depending on the chromium and nitrogen contents of a molten steel alloy, the content of titanium must be lower than that in 4 shown curve are controlled. The presence of a sub-equilibrium content of titanium to prevent TiN precipitates as inclusions in the melt is important for preventing the formation of a surface defect known as "titanium tape". When the precipitates of non-metallic titanium nitride inclusions in the melt are allowed, ie, overbalanced, open surface defects form during hot rolling as these titanium inclusions precipitate near the slab surface during solidification of the slab. These non-metallic titanium nitride inclusions must be removed from the slab by surface treatment such. B. by grinding before hot processing of the slab.

Nitrogen is present in the steels of the present invention at a level of ≤ 0.05%, preferably ≤ 0.03%, and more preferably at levels of ≤ 0.012%. In the invention, it is desirable to control the level of nitrogen to avoid TiN precipitation in the melt, ie, underbalance, thereby favoring the formation of titanium oxides instead. It is believed that small particles of complex titanium oxides are responsible for forming the necessary nucleators to form a fine cast-iron equiaxed grain structure. By carefully controlling the proportions of nitrogen and titanium in the melt to levels below the solubility limit of TiN, small TiO ₂ particles smaller than 1 μm in size form the necessary nucleators for the fine equiaxed crown microstructure in the as-cast state responsible for.

For any Vergusstemperatur may relate to a steel alloy composition on the nitrogen content and the sub-equilibrium content of Ti for avoidance controlled by TiN precipitates. Although nitrogen concentrations assume high values of 0.05% after melting in an EAF oven can, can the content of dissolved Nitrogen during Inert gas refining in an AOD oven to values less than 0.02% and, if necessary, smaller to values than 0.01%. The precipitation of TiN can be given for any one Nitrogen content by reducing the underbalance weight Ti, which must be added to the melt, is avoided. Alternatively, the underbalance content of nitrogen in the melt in an AOD furnace for an anticipated Content of Ti contained in the melt can be reduced. For one Bottom weight stainless steel type T409 with approximately 11-13% chromium and not more than about 0.012% N, would the molten steel to prevent TiN precipitates from solidification the melt less than about 0.25% Ti. For a sub-equilibrium type T439 stainless steel, the about 16-18% chromium and not more than about Contains 0.014N, would the Molten steel to prevent TiN precipitation from solidification the melt less than about 0.35% Ti.

carbon is in the steels according to the invention with a level of up to 0.08%, preferably ≤ 0.02%, and most preferably with 0.0010-0.01% contain. If the carbon content exceeds 0.08%, it will deteriorate formability, corrosion resistance and weldability. Accordingly, carbon should be reduced to a content the lowest possible is.

One Element for the stabilization of carbon and nitrogen can be found in with the steels according to the invention a content of 0.05-1%, preferably 0.10-0.45%, more preferably 0.15-0.25% and most preferably at a level of 0.18-0.25%. If a stabilized steel is desired, the stabilizing element at least have a content of 0.05% to form a stable carbonitridic compound which is effective for producing a Size of the crystalline grains to increase stretching and toughness of stainless steel, whereby the moldability, such as. B. the thermoformability after the annealing, is improved. If the stabilizing element with contents greater than approximately 1.0% is present, the formability of the steel is not further improved and the cost of producing the steel is increasing. In addition to Titanium may be a suitable stabilizing element niobium, zirconium, tantalum, Vanadium or mixtures thereof, with titanium alone preferred becomes. If a second stabilizing element is used, at which is not titanium, such as As niobium, the content should be at second stabilizing element to values not higher than 0.25% are limited. Niobium influences the contents in amounts ≥ 0.25% Moldability in a negative way.

Chromium is present in the steels according to the invention with a proportion of ≥ 8%, preferably 10%. If the chromium content is less than 8%, the wet corrosion resistance of the steel is adversely affected. If the chromium content is higher than about 25%, the moldability of the steel is deteriorated.

silicon is the chromium alloy according to the invention toughen of the present invention at a content of ≦ 1.5%, preferably ≦ 0.5%. A low content of silicon is in a ferritic stainless Steel basically for the benefit the formation of the ferritic phase present. Silicon also benefits the high temperature corrosion resistance and has a favorable effect on the high-temperature strength. Accordingly, silicon should be present in the melt with a content of at least 0.1%. Silicon should not exceed levels of about 1.5%, otherwise the steel will become too hard and the strain will be negative being affected.

manganese is involved in the steels according to the invention a content of up to 1.5% and preferably less than 0.5% in front. Manganese improves by combining with sulfur as manganese sulfide the hot workability for preventing the tearing of the Sheet metal during hot working. Accordingly, manganese contents of at least 0.1% desirable. However, manganese is an austenite former and affects stabilization the ferritic phase. When the content of manganese exceeds about 1.5%, stabilization and malleability of the steel become negative affected.

sulfur is with the steels of the present invention preferably in an amount of <0.015%, more preferably with <0.010% and most preferably <0.005% available. additionally influenced to cause a problem during hot rolling the sulfur negatively affects the wet corrosion resistance, especially with such steels, which have a low content of chromium. Accordingly, should Sulfur preferably does not exceed a content of about 0.015%.

Similar to Manganese, nickel is an austenite former and influences the stabilization the ferritic phase. Accordingly, nickel is ≤ 2.0% and preferably <1.0% limited.

Of the Ferritic according to the invention Chromium-alloyed steel may also contain other elements, such as. As copper, molybdenum, phosphorus and the like, either as intentional additions or as remnants present, d. H. as impurities from the steelmaking process.

Example 13

A chromium alloyed molten iron of about 25 kg according to the present invention was placed in a laboratory vacuum vessel. After the final addition of fine tuning elements to the alloy, the melt was deoxidized with titanium. The composition of the chromium alloyed melt was 0.009% Al, 0.18% Ti, 0.0068% C, 0.26% Mn, 0.51% Si, 11.1% Cr, 0.20% Ni and 0.0081% N. The molten steel was cast into blocks having a thickness of about 75 mm and 150 mm, respectively. The as-cast microstructure of a cross-section sample cut from the stainless steel block had a fine grain structure with approximately 80% equiaxed grains, with an average size of approximately 1 mm, as in FIG 15 shown. These slab samples contained inclusions predominantly of TiO ₂ . A comparative example of a prior art steel containing more than 0.010% Al is shown in FIG 16 shown. These high aluminum-containing, prior art, as-cast steel microstructures generally contain less than 10% of equiaxed grains.

Example 14

A Chromium-alloyed molten iron of approximately 125 t was placed in an AOD refining tank. After reducing carbon to a value according to the final specification was transferred, the melt was transferred to a LMF container, in which the final addition made of alloying elements for fine tuning.

After it was determined that the melt was within the final chemical composition, the melt was deoxidized with titanium. The composition of the melt was 0.18% Ti, 0.022% Al, 0.007% C, 0.22% Mn, 0.17% Si, 10.6% Cr, 0.14% Ni, 0.01% N, 0 , 0010% Ca, 0.10% Cu, 0.03% Mo and 0.029% V. The molten steel was then transferred within 40 minutes in a Vergusseinrichtung and continuously to thin slabs with a thickness of 130 mm and a width of 1200 mm shed. Cross-section samples were cut from a central area at various locations along the length of the thin slab. The as-cast microstructure of these high-aluminum-content stainless steel samples cut from the slab had a large-columnar microstructure Grains on, as in 17 shown. These 17 illustrates that a 0.022% Al ferritic stainless steel had a nearly 100% microstructure of large columnar grains. These large columnar grains of the 17 had an average diameter of about 3 mm.

Rolled plates cast from this melt were reheated to 1250 ° C, hot worked to a thickness of 3.3 mm at a finishing temperature of about 800 ° C, and wound at a temperature of about 700 ° C. The hot worked sheets were descaled, pickled in nitric acid and hydrofluoric acid and cold worked 58% to a thickness of 1.4 mm. These hot processed sheets were not annealed prior to cold working. The cold formed sheet was annealed at a peak metal temperature of 870 ° C for approximately 60 seconds. After rolling, the ridging characteristics of the sheet were 3 to 4 and it had an r _m of 1.22 to 127. A ridging score of 3 or more is equivalent to medium to significant ridging on a scale of 0 to 6. A high ridging score of 3 or greater and a low r _m of less than 1, 3 are unacceptable for many applications of deep formability and outward appearance of ferritic stainless steel. The mechanical properties of these steels are summarized in Table 5. The cold rolled and annealed grain structure is in 18 shown and has a non-uniform grain structure.

Example 15

Another inventive chromium alloyed molten iron was prepared in a manner similar to that described for Example 14, except that the melt had a low aluminum content and the final alloy addition was for fine tuning in the LMF furnace after the melt was blown Titanium was deoxidized. The composition of this melt was 0.19% Ti, 0.005% Al, 0.008% C, 0.12% Mn, 0.16% Si, 10.7% Cr, 0.13% Ni, 0.009% N, 0.001% Si , 0.09% Cu, 0.03% Mo, 0.025% V and 0.0009% Ca. The molten steel was continuously cast into slabs having a thickness of 130 mm, as described for Example 14. The microstructures of as-cast cross-section samples cut from these thin slabs are shown in FIG 19 and illustrate that a ferritic stainless steel according to the present invention having an Al content of 0.005% has a microstructure of nearly 100% of fine equiaxed grains having a size of approximately 1 mm.

These thin slabs were reheated to 1250 ° C, hot worked at a final temperature of 800 ° C to a thickness of 3.3 mm, and then wound up at a temperature of 700 ° C. The hot worked sheet was descaled, pickled in nitric acid and hydrofluoric acid and cold reduced by 58% to a thickness of 1.4 mm. This hot processed sheet was not annealed prior to cold working. The cold formed sheet was annealed at a peak metal temperature of 870 ° C for 60 seconds. After rolling, the ridging characteristic of the annealed sheet was 1 and had an r _m of 1.44 to 1.45. A ridging characteristic of 1 equals excellent ridging properties and the steel is virtually free of ridging phenomena. A ridging characteristic of 2 or less and an r _m of at least 1.4 are acceptable for most applications of deep forming and the display of ferritic stainless steel. The mechanical properties of these sheets according to the invention are summarized in Table 6. The cold rolled and annealed grain structure is in 20 shown and has a uniform grain structure.

A very important advantage of the invention relates to the recrystallization annealed end product. Ferritic stainless steels of the prior art were not only adversely affected by ridging, but also had low moldability, ie low r _m values. One reason that ferritic stainless steels have limited formability is that the grain structure is not uniform after annealing. 18 Figure 12 illustrates a typical non-uniform grain structure of a comparative example of prior art ferritic stainless steel after annealing containing 0.022% aluminum. 20 illustrates a uniform grain structure of a ferritic stainless steel after annealing according to the present invention. As in 20 is shown, the grain structure of a ferritic stainless steel according to the invention after annealing with less than 0.01% total content of aluminum after the recrystallization annealing is much smaller and considerably more uniform than that of a ferritic stainless steel of the prior art.

Example 16

Another chromium alloyed iron melt of the present invention was prepared in a similar manner to that of Example 15. After the final addition of the alloying elements for fine tuning in the Container, the aluminum melt was deoxidized with titanium. The composition of the melt was 0.19% Ti, 0.006% Alm, 0.007% C, 0.13% Mn, 0.31% Si, 11.0% Cr, 0.16% Ni, 0.008% N, 0.001% S , 0.10% Cu, 0.03% Mo, 0.026% V and 0.0012% Ca. The molten steel was continuously cast into thin slabs with a thickness of 130 mm. A as-cast microstructure of a cross-section sample cut from these thin slabs is shown in FIG 21 It is shown that a 0.006% aluminum ferritic stainless steel of the present invention has a near 100% microstructure of equiaxed grains about 1 mm in size.

The slab was reheated to 1250 ° C, hot worked at a final temperature of 800 ° C to a thickness of 3.0 mm and wound at a temperature of 700 ° C. The hot worked sheet was descaled and pickled in nitric acid and hydrofluoric acid. The hot worked sheet was cold reduced by 53% to a thickness of 1.4 mm. This hot processed sheet was not annealed prior to cold working. The cold worked sheet was annealed at a metal tip temperature of 940 ° C for 10 seconds. After rolling, the ridging characteristics of the annealed sheet were 1 to 2, and it had an r _m of 1.39 to 1.48. A ridging score of 2 equals good ridging scores. The mechanical properties of these sheets according to the invention are summarized in Table 7.

Example 17

Another 130 mm thick thin slab of the composition of Example 16 was reheated to 1250 ° C, hot worked at a final temperature of 830 ° C into sheet of 4.1 mm thickness and wound at a temperature of 720 ° C. The hot processed sheets were descaled, etched in nitric acid and hydrofluoric acid and then cold worked 66%, 76% and 85%, respectively, in accordance with the thicknesses of 1.4, 1.0 and 0.6 mm. These hot-processed sheets of the invention were not annealed prior to cold working. The cold formed sheets were annealed at a peak metal temperature of 940 ° C for 10 seconds. After rolling, the ridging characteristic of the annealed sheet was basically 2 or better, and it had an r _m of 1.76 to 1.96. An r _m value of 1.7 is excellent for ferritic stainless steel and it has not previously been thought that this can be achieved. The mechanical properties of these sheets according to the invention are summarized in Table 8.

22 illustrates the percentage of equiaxed grains in the as-cast microstructure as a function of aluminum content for ferritic chromium alloyed steels deoxidized with titanium. The potted microstructures of the ferritic chromium alloyed steels for these embodiments correspond to those containing <0.010% Al. For steels containing less than 0.01% Al, all microstructures contain at least 60% of fine equiaxed grains to levels as high as 80% or greater. For steels containing about 0.02% or more aluminum, the as-cast microstructure does not contain more than about 20% equiaxed grains, ie, it is essentially columnar.

An important advantage of the present invention relates to a cold-worked recrystallization annealed end product. Ferritic stainless steels of the prior art were not only adversely affected by the occurrence of "ridging" but also had low moldability, ie low R _m values. One reason why ferritic stainless steels have limited formability is that the structure after annealing consists essentially of nonuniform or "banded" large grains. 11 FIG. 12 illustrates a typical non-uniform grain structure after annealing of a comparative example of a ferritic stainless steel of the prior art having a ratio of the product of titanium to nitrogen divided by aluminum of less than 0.14 and having an as-cast structure less than 50%. contains on equiaxed grains. The invention makes it possible to form a fine equiaxed, ie rectified grain in the steels in the casting state, so that a uniform, fine, recrystallized grain structure can be formed continuously after cold working. A ferritic chromium alloy steel sheet having a fine uniform recrystallized crown structure can be formed without annealing the steel before cold working and with only cold working.

It is self-evident, that various modifications of the invention are made can, without from the fundamental To deviate idea and the scope of the invention. Therefore, the should Limits of the present invention are defined by the following claims become.

Claims (10)

A chromium alloyed ferritic steel with, in% by weight, C ≤ 0.08%, Mn ≤ 1.5%, Si ≤ 1.5%, Cr ≥ 8%, Ni <2%, 0.1-1.0% Ti, N <0.05%, optionally S ≤ 0.015%, Ca ≤ 0.002%, 10-60 ppm O and 5-200 ppm B, as a second stabilizing element, optionally additionally up to 1 % of one or more elements) from the group of niobium, zirconium, tantalum and vanadium and a residual content of Fe and unavoidable impurities, wherein the steel has a ratio of (Ti × N) / Al ≥ 0.14 and is deoxidized with titanium, which results in a as-cast steel microstructure in which titanium is in the form of titanium oxide inclusions and has more than 50% equiaxed grains. Steel according to claim 1, wherein the ratio of (Ti × N) / Al> 0.23. Steel according to claim 1 or 2, wherein Al <0.03%, Ti 0.15-0.35% and N <0.03%. Steel according to at least one of the preceding claims, at which Ti and N are present in under-equilibrium proportions, and Ti satisfies the relationship (Ti / 48) / [(C / 12) + (N / 14)]> 1.5. Steel according to at least one of the preceding claims, wherein the proportion of grains which are rectified in the cast state at least 60%. Steel according to at least one of the preceding claims, wherein a majority of the titanium oxide inclusions has a size of <1.5 μm. Process for producing an alloyed steel with the steps - Provide and refining a molten steel, in wt%, C ≤ 0.08%, Mn ≤ 1.5%, Si ≤ 1.5%, Cr ≥ 8%, Ni <2%, 0.1-1.0 % Ti, N <0.05%, optional S ≤ 0.015%, Ca ≤ 0.002%, 10-60 ppm O and 5-200 ppm B, residual iron and unavoidable impurities, - Deoxidize the melt by adding titanium as a deoxidizer with a Proportion of 0.1-1% Ti satisfying the relationship (Ti × N / Al ≥ 0.14) - after this titanium oxide as a nucleating agent for the equiaxed ferrite grains in the casting state during the deoxidation of the melt were formed, shedding the melt to a steel with a microstructure in the cast state more than 50% of equiaxed grains, - hot forming of steel to a sheet, - descaling the sheet, - cold forming of the sheet to a final thickness and Recrystallization annealing of cold-formed sheet, leaving the annealed sheet substantially free by "ridging" is when it's too a part is formed. Process according to claim 7, characterized by continuous Shed the melt into a thin one Flat goods of thickness ≤ 140 mm and the additional Step of reheating of the slab to a temperature of 1050-1300 ° C before hot rolling of the slab to a continuous sheet. Method according to claim 7 or 8, characterized that the hot-formed sheet is cold-worked without prior annealing, advantageously in a one-step process. Method according to any one of claims 7 to 9, characterized in that the cold-formed sheet at a Temperature of 800-1000 ° C for at least Annealed for 1 second becomes.