ES2357303T3 - STAINLESS STEEL SHEET FERRÍTICO WITH EXCELLENT CONFORMED BY PRESSURE AND SECONDARY CONFORMED AND ITS MANUFACTURING METHOD. - Google Patents

Abstract

A ferritic stainless steel sheet, excellent in pressure forming capacity and secondary forming capacity, which has a composition consisting of 0.02% by mass, or less, of C; 0.8% by mass, or less, of Si; 1.5% by mass, or less, of Mn; 0.050% by mass, or less, of P; 0.01% by mass, or less, of S; 8.0-35.0% Cr mass; 0.05% by mass, or less, of N; 0.05-0.40% by weight of Ti; 0.10-0.50% by mass of Nb, optionally one or more selected from the group consisting of 0.5% by mass, or less, of Ni; 3.0% by mass, or less, of Mo; 2.0% by mass, or less, of Cu; 0.3% by mass, or less, of V; 0.3% by mass, or less, of Zr; 0.3% by mass, or less, of Al; and 0.0100% by mass, or less, of B, and the rest being Fe except the inevitable impurities with a product of (% Ti ×% Nb) less than 0.005, and the precipitating metallurgical structure, of 0.15- 1.0 μm of particle size, selected from the group consisting of TiC, NbC, NbN, Laves phase, and mixtures thereof, but excluding TiN, are distributed at a rate of 5000-50000 / mm2 in a steel matrix.

Description

Ferritic stainless steel sheet with excellent pressure forming and secondary forming and its method of manufacturing.

The present invention relates to a sheet of ferritic stainless steel, which can be formed by pressure up to a default profile without defects such as poor circularity and torsion, and then perform a secondary shaping to a profile final with good hot extrusion, and also refers to a method to manufacture it.

Ferritic stainless steels, represented by SUS430 and SUS430LX, have been used so far in various fields, for example in durable consumer goods, due to its resistance to corrosion and its low value, compared to steels Austenitic stainless containing Ni as an expensive element. The conditions for pressure forming a stainless steel sheet ferritic to give a product profile are becoming more and more severe as the development of its application. A sheet conformed by pressure, often conforms secondarily for the extrusion of, for example, a hole. In response to Application development, there is a high demand for supply of a new ferritic stainless steel sheet, which is quite excellent in its capacity of forming, compared with the plates of conventional ferritic stainless steels, and so that it compliant until giving a product profile without defects, even low severe conditions

There are many reports on the ability to formed of ferritic stainless steel sheets. An improvement representative is the addition of both Ti and Nb to the stabilization of C and N dissolved as carbonitrides. Further, JP2000-192199A shows the distribution of magnesium inclusions, which are effective in the property of avoid the formation of stretch marks, in a ferritic stainless steel that contain both Ti and Nb. JP8-26436B (JP-A-51086015) shows the combination of hot rolling conditions, which are designed to improve the value of Lankford (r) as an index of forming capacity, with the addition of Ti and Nb.

The ability to fix the shape and ability to perform a secondary forming in a steel sheet stainless with a primary forming, which will be shaped up to a final profile, are also important factors, as well as the value of Lankford (r) and property prevent the formation of stretch marks.

Usually a stainless steel sheet ferritic is lower in terms of forming capacity than a Austenitic stainless steel sheet. Specifically, it is reduced significantly the thickness in the primary forming state, and thickness reduction is anisotropic. Therefore, the accuracy dimensional, such as circularity, becomes worse as that the forming conditions are more severe, when the sheet Ferritic stainless steel is formed by pressure up to a cylindrical profile Thickness deviation in the state of primary shaping leads to serious degradation of capacity of secondary forming, such as the extrusion of a orifice.

In the case where the steel plate Ferritic stainless maintain high dimensional accuracy (for example, circularity, width, and anti-torsion), as well as a secondary forming capacity in a state of formed by pressure, a stainless steel plate can be used cheap ferritic as part or member of a steel sheet expensive austenitic stainless, which has been used in view of the severe forming conditions.

The document GP-A2-1308532 describes a sheet of ferritic steel manufactured by hot rolling, annealing, optionally double cold lamination and intermediate annealing, and final annealing suitable for deep drawing and resistance to the fragility for a secondary treatment, the steel has a controlled grain size and carbonitrides of Nb and Ti.

The present invention aims at provision of an improved ferritic stainless steel sheet in the dimensional accuracy and secondary forming capacity, in a pressure forming state controlling the size of the particles and distribution of precipitates that are dispersed in the steel matrix.

The present invention, as defined in the claim 1, proposes a new stainless steel sheet ferritic, consisting of 0.02% by mass, or less, of C; 0.8% by mass, or less, of Si; 1.5% by mass, or less, of Mn; 0.050% by mass, or less, of P; 0.01% by mass, or less, of S; 8.0-35.0% Cr mass; 0.05% by mass, or less, of N; 0.05-0.40% by weight of Ti; 0.10-0.50% Nb mass, optionally one or more selected from the group consisting of 0.5% by mass, or less, of Ni; 3.0% by mass, or less, of Mo; 2.0% by mass, or less, of Cu; 0.3% by mass, or less, of V; 0.3% by mass, or less, of Zr; 0.3% by mass, or less, of Al; Y 0.0100% by mass, or less, of B, and the rest being Fe except for unavoidable impurities, with a product of (% Ti \ times% Nb) less than 0.005. Its metallurgical structure is defined by the distribution of precipitates of 0.15 µm or more, of the size of particle, except TiN, at the rate of 5,000-50,000 / mm2.

The ferritic stainless steel sheet is manufactures as described in claim 2, and in particular as follows:

Cast steel with a composition is cast default to form a slab. The hot rolled seedling until obtaining a steel plate with a final temperature of 800 ° C or less, and annealing is done at 450-1080 ° C. The hot rolled steel sheet and Annealing is subjected to pickling and cold rolling along with the minus an intermediate annealing within a temperature range that goes from (the temperature of finish-recrystallization -100ºC) at (temperature finish-recrystallization). Steel sheet cold rolled is finally subjected to a final annealing to a temperature of 1080 ° C or lower.

The hot rolled steel sheet can be annealed for a predetermined period of time of one hour, or shorter. Intermediate annealing and final annealing can be carry out in a continuous annealing oven for one minute or less.

Figure 1 is a view to explain the circularity of a steel sheet, which is shaped cylindrically using a multiplate press.

The inventors have investigated and examined the Manufacturing conditions of ferritic stainless steel sheets for the improvement of dimensional accuracy (for example, the circularity, width, and torsion) from various aspects, and they discovered that circularity and forming ability Secondary of a steel sheet formed by pressure are quite affected by the form and distribution of TiN and others precipitates in an annealing state. The inventors start from the base of the discovery that the properties that are pursued are impart to the ferritic stainless steel sheet by controlling adequately the form and distribution of the precipitates. The formation of precipitates for form and distribution suitable for the purpose pursued is carried out by adding both Ti and Nb to ferritic stainless steel, at a value higher than the stoichiometric ratio for the stabilization of C and N as carbonitrides and subjecting stainless steel ferritic to an optimal thermomechanical treatment.

The effects of the form and distribution of precipitates over the pressure forming capacity and the dimensional accuracy can be explained as follows:

The C and N, in a ferritic stainless steel, they are precipitated mostly as carbonitrides by the addition of Ti and Nb. Precipitated carbonitrides, except TiN, they substantially conform again to give particles very fine in the manufacturing process that goes from annealing a sheet of hot rolled steel, going through a cold rolled, up to A final annealing. Fine particles allow growth predominant of the recrystallized grains with a certain orientation without anchoring action when fabricated steel sheet It is annealed for recrystallization, resulting in formation of a mixed anisotropic grain structure. The anisotropy results in the concentration of tensions along a certain direction during the primary forming of a sheet of steel and the pressure forming capacity and the dimensional accuracy of the steel sheet.

The anchoring action is expected during the Recrystallization annealing by the distribution of the precipitates which have a particle size greater than a certain value. The anchoring action suppresses the orientative growth of grain or the growth to give coarse grains, to improve isotropy and dimensional accuracy of a steel sheet formed by pressure. The effects of the anchoring action on the forming capacity by pressure and dimensional accuracy are indicated, typically, by the distribution of the 0.15 µm precipitates, or more, in particle size, except TiN, at the rate of 5,000-50,000 / mm2, as recognized in the later examples.

Among the precipitates, the TiN is harmful for pressure forming capacity and for accuracy dimensional. In fact, a steel sheet, which has a product of (% Ti x% Nb) greater than 0.005, cracks in a state of formed by pressure. At the starting points of the cracks, they observe thick particles of TiN, which have grown to give a cubic shape The result of the observation tells us that the deformations are concentrated in the cubic vertices during the formed by pressure, and induces the formation of microcracks. The deformation concentration and microcracking around TiN particles are unfavorable for the extrusion of holes in a secondary forming stage.

The ferritic stainless steel sheet of the invention contains alloying components in relationships default, as follows:

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0.02% by mass, or less, of C

C becomes effective carbides for the random growth of recrystallized ferritic grains in the final annealing stage, but degrades the forming ability of the steel sheet due to its hardening effect. The precipitation of carbides also causes a lower corrosion resistance In this regard, the content of C is controls at a level as low as possible, that is 0.02% by mass, or less, for forming capacity and resistance to corrosion. The C content is preferably reduced to 0.015% en masse, or less, for the improvement of forming capacity secondary. However, the reduction of the C content up to extremely lower level needs a refining operation to long term and raises the cost of manufacturing steel. For the therefore, a lower limit of the C content is determined, preferably, in 0.001% by mass. The definition of the limit lower ensures the effect of carbides on growth randomized recrystallized ferritic grains in the stage of final annealing

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0.8% by mass, or less, of Si

Si is an alloying component that is added as a deoxidizing agent during a steelmaking process, But it has a strong hardening effect of the solid solution. Excessive Si, above 0.8% by mass, hardens unfavorably the steel sheet, resulting in a poor ductility. An upper limit of the content of If determined, preferably, in 0.5% by mass, facing ductility and forming capacity.

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1.5% by mass, or less, of Mn

The Mn does not harden the steel plate so much because because its hardening effect of the solid solution is weaker than that of Si. However, an excess of Mn, above 1.5% in mass, causes the discharge of manganese fumes during the process of steelmaking, resulting in a poor productivity.

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0.050% by mass, or less, of P

P is detrimental to the ability to hot work, so that your upper limit is determined in 0.050% by mass.

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0.01% by mass or less of S

The S is a harmful element that is segregated in the grain boundaries and fragilizes the grain boundaries. These defects can be suppressed by controlling the content of S up to 0.01% by mass, or less.

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8.0-35.0%, by mass, of Cr

Cr content is controlled at 8.0% in mass, or more, to ensure the corrosion resistance of a steel stainless. However, the toughness and the ability to form of stainless steel worsens as the Cr content increases so that the upper limit of Cr content is determined in 35.0% by mass. Cr content is preferably controlled in 20% by mass, or less, for a further improvement of the ductility and secondary forming capacity.

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0.05% by mass, or less, of N

N becomes effective nitrides for random growth of recrystallized ferritic grains in a final annealing stage, but they have an effect of hardening. Since the excess of N degrades the ductility of the sheet steel, the content of N is controlled at a level as much as low as possible, that is 0.05% by mass, or less. The content of N is preferably controlled at 0.02% by mass or less for a further improvement of ductility and forming capacity secondary. However, the reduction of the content of N up to extremely lower level needs a refining operation to long term and raises the manufacturing cost of steel. Thus, a lower limit of the content of N is determined, preferably, in 0.001% by mass. The definition of the lower limit ensures the nitride effect on random grain growth recrystallized ferritics in the final annealing stage.

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0.05-0.40%, mass, of Ti

Ti is an alloying component that stabilizes the C and N as carbonitrides for forming capacity and corrosion resistance This effect is evidently observed. with a Ti content of 0.05% by mass or more. However the Excessive titanium, above 0.40% by mass, leads to a high cost of steel and induces surface defects originated in titanium inclusions.

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0.10-0.50%, by mass, of Nb

Nb, which has the same effect as Ti on the stabilization of C and N, is an essential component for the precipitation of niobium inclusions of 0.15 µm, or more, of particle size, except TiN. Niobium inclusions are composed, probably, of carbides and Fe 2 Nb. A content of Nb of 0.10% by mass, or more, is necessary for precipitation of These inclusions of niobium. However, an excess of Nb, for above 0.50%, causes excessive rainfall and raises unfavorably the recrystallization temperature of steel ferritic stainless

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0.50% by mass, or less, of Ni

Ni is an optional element for tenacity of hot rolled steel sheet and for resistance to corrosion. But the addition, in excess, of Ni raises the cost of material and hardens the steel sheet, so that a upper limit of Ni content in 0.5% by mass.

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3.0% by mass, or less, of Mo

The Mo is an optional element for the corrosion resistance, but an excess of Mo, above the 3.0% by mass, is unfavorable for its ability to work in hot.

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2.0% by mass, or less, of Cu

Cu is an optional element, which with frequency is included in stainless steel from scrap metal, during a steel manufacturing process. Since the excess Cu causes poor tenacity and degradation of the hot working ability, Cu content is controlled at 2.0% by mass, maximum.

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0.3% by mass, or less, each, of V and Zr

V and Zr are optional elements. Fixed V the free C as carbide in a steel matrix facing the forming ability, while the Zr captures the O free of face to the capacity of conformed and to the tenacity. But nevertheless, the addition of excess V or Zr must necessarily be avoided. face productivity In this sense, a limit is determined higher, for each of them, V and Zr, at 0.3% by mass.

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0.3% by mass, or less, of Al

Al is an optional element that is added as a deoxidizing agent in the steel manufacturing process. Without However, an excess of Al, above 0.3% by mass, causes a increased non-metallic inclusions, which result in a Poor toughness and surface defects.

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0.0100% by mass, or less, of B

B is an optional element that stabilizes the N and improves corrosion resistance and forming ability of a stainless steel. The effects of B become apparent from evident form with 0.0010% by mass, or more, but an excess of B, Above 0.100%, it is detrimental to the ability to work hot and for weldability.

Ca, Mg, Co, REM (earth metals rare), etc., other than the previous elements, can be include from scrap during steelmaking. These elements do not provide significant effects on the circularity of embedded steel sheet or on accuracy Dimension of a steel sheet formed by pressure, unless that are included in extraordinary proportions.

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(% Ti \ times% N) <0.005

The TiN grows to thick particles, or conglomerate form as it increases (% Ti \ times% N). The Coarse particles or TiN conglomerates promote the deformation accumulation during primary shaping, giving as a result the formation of microcracks in an initial step of the stretching stage These harmful effects of the particles Thick or conglomerates of TiN are removed by control of (% Ti \ times% N) at a value lower than 0.005, as Recognize in subsequent examples.

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A rate of distribution of the precipitates, except the TiN, of 0.15 µm, or more, of particle size, from 5000 to 50,000 / mm2

Carbide and nitride precipitates of 0.15 um, or more, of particle size have an anchor action to suppress the growth of indicative grain and also the growth to give coarse grains, to improve the isotropy of the stainless steel sheet, the circularity in a stretch cylindrical and dimensional accuracy in a state of conformed by Pressure.

The precipitates are carbides and nitrides of Ti and Nb, Laves phase and mixtures thereof. TiN particles, which precipitate in cubic form, are excluded from precipitates effective for pressure forming capacity and for dimensional accuracy, since the cubic particles of TiN are susceptible to concentrate deformations in their vertices and act as a starting point for microcracks. The distribution of precipitates with particle size of 0.15 µm, or more, except the TiN, at a rate of 5000-50000 / mm2, ensures a effective anchoring action for pressure forming capacity and for the dimensional accuracy of the steel part formed by pressure, as indicated in the examples below.

The effects of the precipitates on the pressure forming capacity and dimensional accuracy of a piece of steel formed by pressure are revealed with a particle size of 0.15 µm or more, and more are made large as the particle size increases. But nevertheless, coarse precipitates, above 1.0 µm in size particle are unfavorable, since thick particles promote the accumulation of deformations and the formation of microcracks during pressure forming, resulting in a poor ability to fix the shape. The anchoring action of precipitates is evidently revealed with a rate of distribution of 5000 / mm2 or more, but excess distribution of precipitates, above 50,000 / mm2, greatly degrades the Ductility and drawing capacity of the steel sheet. The excess distribution unfavorably raises the temperature of recrystallization of the steel sheet, so that it is barely count to the recrystallized state.

The manufacturing conditions, which are necessary to control the form and distribution of precipitates, will be understood from the following Explanation.

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Hot rolled at a final temperature of 800 ° C or lower

A steel sheet is hot rolled ferritic stainless at a temperature relatively lower than final temperature in order to induce nucleation sites to the precipitates, which will be distributed in a steel sheet with a final annealing The limits of the ferritic grains and the internal deformations in the state of hot rolling serve as nucleation sites. The final temperature of the laminate in hot is determined at 800 ° C, or lower, in order to induce as many nucleation sites as possible.

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Annealing of a hot rolled steel sheet, to 450-1080 ° C

The precipitates in the rolled steel sheet hot moderated until they have a proper way to control the precipitates, which will be distributed on the steel plate with final annealing, in a particle size of 0.15 µm or more, by annealing the hot rolled steel sheet to 450-1080 ° C. If the annealing temperature is below 450 ° C, effective precipitates are hardly formed. Yes, for him Otherwise, the hot rolled steel sheet is heated to a temperature above 1080 ° C, precipitates, except TiN, They dissolve again in a steel matrix.

Annealing is completed in one hour to adequately control the distribution rate of precipitates without growing to thick particles.

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Intermediate annealing at a temperature within the range that ranges from (recrystallization temperature -100 ° C) to (the recrystallization temperature)

During cold rolling, the steel sheet it is counted at a relatively lower temperature in order to inhibit the redisolution of precipitates that have formed annealing the hot rolled steel sheet. To release the stresses that are introduced into the steel sheet by means of the cold rolled, an annealing temperature is preferable intermediate just below the temperature recrystallization-finish. The steel plate can be soften without the redisolution of the precipitates, independently that some of the laminate texture remains that is not recrystallized yet, as long as the temperature of annealing is maintained within a range from (the recrystallization temperature -100 ° C) at (the temperature of recrystallization).

The intermediate annealing period is completed in one minute in order to avoid the redisolution of the precipitates, considering the faculty of a continuous annealing furnace conventional.

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Final annealing at a temperature of 1080 ° C, or lower

The laminate texture is removed by final annealing But a heating temperature above 1080 ° C is not only inconvenient for serial productivity, but it promotes the redisolution of the precipitates and the growth to thick grains, resulting in poor tenacity.

The final annealing is completed in a minute, considering the faculty of a continuous annealing furnace conventional.

The other features of this invention will obviously be understood from the following examples, although the scope of the present invention is not Restricted by these examples.

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Example one

Fundamental experiments

The inventors have investigated the effects of TiN that frequently precipitates in a stainless steel matrix ferritic as well as the effects of the form of the precipitates on dimensional accuracy and forming ability secondary of the steel part formed by pressure under the following conditions.

Several steels were melted in an oven experimental and sneaked into slabs, in which each steel was adjusted to give a composition of 0.007% by mass of C, 0.40% by mass of Si, 0.25% by mass of Mn, 0.030% by mass of P, 0.0005 by mass of S, 0.05% by mass of Cu, 16.50 by mass of Cr, 0.04 in mass of Al, except Faith and the inevitable impurities, with the condition that the contents of Nb, Ti and N be varied within 0.02-0.30% mass intervals, 0.05-0.30% in mass and 0.005-0.035% in mass, respectively.

Table 1 shows the contents of Nb, Ti and N together with a product of (% Ti \ times% Nb) and a temperature of recrystallization-finish.

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TABLE 1 Contents of Nb, Ti and N (% by mass) together with a product of (% Ti x% Nb) and a temperature of recrystallization-finish Rf (° C)

one

The numbers underlined are values that are outside the present invention.

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Each slab was hot rolled to a thickness of 4 mm at a final temperature of 750 ° C.

Hot rolled steel sheets, numbers 1-7, were annealed at 800 ° C for 60 seconds, they were pickled and then cold rolled to a thickness 2 mm The steel sheets were cold rolled again until a final thickness of 0.5 mm, together with an intermediate annealing at a temperature of (the temperature of finish-recrystallization -50ºC) for 60 seconds. The cold rolled steel sheets suffered a final annealing to 1000 ° C for 60 seconds.

Hot rolled steel sheets, numbers 8 and 9, were annealed, stripped and then rolled cold to a thickness of 2 mm. The steel plates suffered a intermediate annealing and additional cold rolling to a thickness 0.5 mm end. The cold rolled steel sheets were undergoing final annealing. Table 2 shows conditions of annealed from hot rolled steel sheets, annealed intermediate and final annealing.

2

Rate of distribution and form of precipitates

A chemical attack was performed on a piece of test taken as a sample of each steel plate annealed in a 10% acetylacetone non-aqueous electrolyte - chloride 1% tetramethylammonium - methyl alcohol, under one condition potentiostatic, and then it was observed by a microscope scanning electronic to investigate the distribution of precipitates A cross section was inspected, parallel to the rolling direction, at 50 arbitrary points, and the maximum length of each precipitate and was evaluated as the size of particle.

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Dimensional accuracy of a piece of steel made up of Pressure

A brute piece taken under pressure was formed of each annealed steel sheet to give a cylindrical profile (shown in Figure 1) using a multiplate press. Be measured the radii by means of a laser displacement meter maximum and minimum of a cylindrical part C in a separate position 5 mm of the flanged part F. The ratio was calculated (maximum diameter - minimum diameter) / (minimum diameter) and was considered as circularity to assess the dimensional accuracy of the sheet pressure formed steel.

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Secondary forming capacity

Another test piece was combined to a height 10 mm, using a 103 mm diameter punch with a radius of curvature of the rounded edge that was 10 mm and a matrix of 105 mm in diameter and a radius of curvature of the rounded edge that was 8 mm, under the condition that a flange with a trim was fixed. A sample of the bottom of the tested test piece was taken as gross piece of 92 mm in diameter, and a 10 mm hole was formed in diameter in the center of the blank with a free space of 10% The blank was then subjected to a capacity test of secondary forming as follows:

The brute piece was held between a punch 40 mm diameter flat head with a radius of curvature of the rounded edge that was 3 mm and a matrix of 42 mm in diameter with radius of curvature of the rounded edge that was 3 mm, of so that burrs are formed around the hole directed towards matrix. The hole was extruded by the punch until the appearance of cracks on its edge, while a piece flange was fixed Gross with a piping. The angle of the hole in the initiation of cracks. The extrusion ratio (%) was calculated secondary = (extruded hole diameter - hole diameter without extruded) / (hole diameter without extruded) \ times 100

The results are shown in Table 3. There are to indicate that the steel plates cracked during the formed by pressure with a product of (% Ti x% N) by above 0.005. Steel sheets with a lower Nb content at 0.02% by mass, they were formed with poor circularity, regardless of manufacturing conditions. The results of the observation of any of the cracked steel sheets and of the steel sheets formed with poor circularity prove than precipitates of particle size of 0.15 µm, except the TiN, only one was distributed in the steel matrix little bit.

On the other hand, circularity was improved with a increase in the number of precipitates, which were distributed in the steel matrix with an Nb content of 0.3% by mass or more, together with the conditions of thermomechanical treatment. However, the excessive distribution of the precipitates was inadequate for the circularity.

Steel sheets with a product of (% Ti % N) of more than 0.005 were extremely inferior in their secondary forming capacity. There was also a poor secondary forming capacity in steel sheets with 0.02% in mass of Nb.

The improvement of forming capacity secondary (i.e. hole extrusion) was recognized as a increase in the number of precipitates, which were distributed in the Steel matrix with an Nb content of 0.3% by mass, or more. Without However, the excess distribution of the precipitates was unsuitable for secondary forming capacity.

The previous results prove that the dimensional accuracy and the secondary forming ability of a steel sheet formed by pressure, depends on the distribution of precipitates of 0.15 µm, or more, of particle size, except the TiN. That is, the optimal thermomechanical treatment for controlled distribution of these precipitates at the rate of 5000-50000 / mm2 is effective for accuracy dimensional and secondary forming capacity.

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TABLE 3 Circularity and capacity of secondary forming in relationship with the distribution of the precipitates except the TiN

3

The precipitates * are 0.15 µm, or more, of particle size, except the TiN.

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Example 2

Several stainless steels were cast, with compositions shown in Table 4, in a vacuum oven and sneaked in the form of slabs. A-H steels belong to the present invention while steels I-L does not meet the compositional definitions of The present invention.

Each slab was hot rolled to a 4.0 mm thick, annealed, stripped and cold rolled until a thickness of 2 mm. The cold rolled steel sheet was subjected at an intermediate annealing, cold rolled again to a thickness 0.5 mm and then subjected to a final annealing. Table 5 shows the conditions of the final hot rolling temperature, the Annealing of hot rolled steel sheets, annealing intermediate and final annealing.

4

5

6

Each steel plate was examined the same so as in Example 1, to investigate the form and distribution of precipitates, as well as dimensional accuracy and secondary forming capacity of a shaped steel sheet by pressure

The results shown in Table 6 prove that the ferritic stainless steel plates, in which distributed precipitates of 0.15 µm, or more, in size of particle, except TiN, in the steel matrix, at the rate of 5000-50000 / mm2, formed by pressure until giving a good profile, with a circularity of 2.5% or less.

On the other hand, comparative steel sheets (Examples numbers A2, B2, C2 and D2), which satisfied the conditions of composition of the present invention, but which were manufactured under inadequate conditions, they had poor dimensional accuracy and secondary forming capacity in a forming state by pressure, because the metallurgical structure of that number of distribution of the precipitates, except the TiN, was out of 5000-50000 / mm2.

The steel sheet I was too hard due to the excess C and cracked during pressure forming. The plate K steel was too strong due to excess Nb and cracked during pressure forming. The steel plate L with a product of (% Ti x% N) above 0.005 also cracked during pressure forming, in which the cracks are started near the thick TiN particles. Steel sheet J, with a shortage of Nb, was formed by pressure with poor circularity.

It is understood, from the comparison above, that the plates of ferritic stainless steels can be conform by pressure to desired profiles, with a high dimensional accuracy and excellent forming ability secondary, through controlled distribution of precipitates 0.15 µm, or more, of particle size, except TiN.

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8

According to the present invention and according to the above, ferritic stainless steel sheets are provided, which can be conform by pressure with high dimensional accuracy and excellent secondary forming capacity, due to the distribution of precipitates of 0.15 µm, or more, of particle size, except the TiN, at a rate of 5000-50000 / mm2 in a matrix of steel with controlled composition. The form and distribution of these precipitates, suitable for this purpose, are carried out properly controlling the final temperature of the laminate in hot and heat treatment conditions for annealing of a hot rolled steel sheet, for annealing intermediate during cold rolling, and for final annealing of a cold rolled steel sheet. Stainless steel plates ferritic manufactured in this way are useful as members or parties, which demand strict dimensional accuracy, in various fields, such as members for airtight closure in organic electroluminescent devices, pressed parts with precision, sinks, utensils, stove burners, pipes Oil filling for fuel tanks, carcasses engines, covers, sensor covers, injector tubes, valves thermostats, bearing seals, flanges and so on, instead of expensive austenitic stainless steel sheets.

Claims (2)

1. A ferritic stainless steel sheet, excellent in the capacity of forming by pressure and in the secondary forming capacity, which has a composition consisting of 0.02% by mass, or less, of C; 0.8% by mass, or less, of Si; 1.5% by mass, or less, of Mn; 0.050% by mass, or less, of P; 0.01% by mass, or less, of S; 8.0-35.0% Cr mass; 0.05% by mass or less of N; 0.05-0.40% by weight of Ti; 0.10-0.50% by mass of Nb, optionally one or more selected from the group consisting of 0.5% by mass, or less, of Ni; 3.0% by mass, or less, of Mo; 2.0% by mass, or less, of Cu; 0.3% in mass, or less, of V; 0.3% by mass, or less, of Zr; 0.3% by mass, or less, of Al; and 0.0100% by mass, or less, of B, and the rest being Fe except the inevitable impurities with a product of (% Ti \ times % Nb) less than 0.005, and the metallurgical structure that precipitates from 0.15-1.0 µm particle size, selected of the group consisting of TiC, NbC, NbN, Laves phase, and their mixtures, but excluding TiN, are distributed at the rate of 5000-50000 / mm2 in a steel die.

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2. A method of manufacturing a sheet of ferritic stainless steel, excellent in forming capacity by pressure and in the secondary forming capacity, which It comprises the stages of: Provide a slab of a stainless steel ferritic having the composition consisting of 0.02% by mass, or less, of C; 0.8% by mass, or less, of Si; 1.5% by mass, or less, of Mn; 0.050% by mass, or less, of P; 0.01% by mass, or less, of S; 8.0-35.0% Cr mass; 0.05% by mass or less of N; 0.05-0.40% by weight of Ti; 0.10-0.50% by mass of Nb, optionally one or more selected from the group consisting of 0.5% by mass, or less, of Ni; 3.0% by mass, or less, of Mo; 2.0% by mass, or less, of Cu; 0.3% in mass, or less, of V; 0.3% by mass, or less, of Zr; 0.3% by mass, or less, of Al; and 0.0100% by mass, or less, of B, and the rest being Fe except the inevitable impurities with a product of (% Ti \ times % Nb) less than 0.005; hot rolling the slab to a final temperature of 800 ° C or lower; anneal the hot rolled steel sheet to a temperature within a range of 450-1080 ° C, for a period of time that avoids the redisolution of the precipitates; cold rolled the annealed steel sheet together with at least one intermediate annealing at a temperature within the interval that goes from (the temperature of finish-recrystallization - 100ºC) at (temperature finish-recrystallization), during a period of time that avoids the redisolution of the precipitates; and later make an final annealing of the steel sheet cold rolled, at a temperature of 1080 ° C or lower, during a period of time that avoids the redisolution of the precipitates.