HIGH STRENGTH STAINLESS STEEL SHEET EXCELLENT IN FATIGUE CHARACTERISTICS, AND PRODUCTION METHOD THEREFOR

Abstract

A thin steel sheet contains, in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities; and having a number density of a non-metallic inclusion lining up with an interparticle distance in the rolling direction of 20 mm or less and an interparticle distance in the sheet thickness direction of 10 mm or less that has a length in the rolling direction of 40 mm or more of 3.0 or less per square millimeter on the L cross section.

Claims

1. A stainless steel sheet: having a sheet thickness of from 20 to 500 m; having a chemical composition containing, in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities; and having, with an assumption that in a cross section (L cross section) in parallel to a rolling direction and a sheet thickness direction, a group of non-metallic inclusion particles that line up with an interparticle distance in the rolling direction of 20 m or less and an interparticle distance in the sheet thickness direction of 10 m or less is one non-metallic inclusion, a number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more of 3.0 or less per square millimeter on the L cross section.

2. The stainless steel sheet according to claim 1, wherein the non-metallic inclusion having a length in the rolling direction of 40 m or more contains one or two of (i) and (ii): (i) a TiN based inclusion particle and (ii) a spinel based inclusion particle containing one or more of Al and Mg.

3. The stainless steel sheet according to claim 1, wherein the stainless steel sheet has a tensile strength in the rolling direction of 2,000 N/mm.sup.2 or more.

4. The stainless steel sheet according to claim 1, wherein the stainless steel sheet has a mixed structure of a deformation induced martensite phase and an austenite phase as a matrix (metal basis material).

5. A production method of a stainless steel sheet comprising: a step of providing a molten steel having a chemical composition (A) below, in such a manner that in component control by adding an auxiliary raw material and a slag forming flux to a molten steel having a C content of 0.20% or less having a Cr oxide-containing slag on a molten steel surface after subjecting to a decarburization process by blowing oxygen into a Cr-containing molten iron, a container housing the molten steel, the auxiliary raw material, and the slag forming flux used are selected to make a Ti content in the molten steel of 0.008% by mass or less and an Al content therein of 0.008% by mass or less, at least an FeSi alloy as the auxiliary raw material is dissolved in the molten steel to perform deoxidation, reduction and recovery of Cr in the slag into the molten steel, and control of a Si content in the steel, and a Ca-containing slag forming flux is added to control a slag basicity (i.e., a mass ratio of CaO/SiO.sub.2) to from 1.3 to 1.5; a step of providing a cast piece by casting the molten steel obtained in the preceding step; a step of providing a hot rolled steel sheet by subjecting the cast piece to hot working including at least hot rolling; and a step of providing a cold rolled steel sheet having a sheet thickness of from 20 to 500 m by subjecting the hot rolled steel sheet to annealing and cold rolling one or more times: (A) in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities.

6. The production method of a stainless steel sheet according to claim 5, wherein the container housing the molten steel used is a container having a refractory constituting the inner surface of the container that is not used for housing a molten steel (i.e., a new ladle).

7. The production method of a stainless steel sheet according to claim 5, wherein the FeSi alloy used has an Al content of 0.05% by mass or less, and a Ti content of 0.05% by mass or less.

8. The production method of a stainless steel sheet according to claim 5, wherein the production method further comprises: a step of subjecting the cold rolled steel sheet to an aging treatment.

9. The production method of a stainless steel sheet according to claim 8, wherein the steel sheet obtained by the production method has a mixed structure of a deformation induced martensite phase and an austenite phase for a matrix (metal basis material), and has a tensile strength in the rolling direction of 2,000 N/mm.sup.2 or more.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0039] FIG. 1 is the SEM micrograph of the non-metallic inclusion appearing on the L cross section of the cold rolled steel sheet of Conventional Example No. 1.

[0040] FIG. 2 is the SEM micrograph of the non-metallic inclusion appearing on the L cross section of the cold rolled steel sheet of Invention Example No. 5.

[0041] FIG. 3 is the SEM micrograph of the typical TiN based inclusion observed on the L cross section of the hot rolled steel sheet of Conventional Example No. 4.

[0042] FIG. 4 is the SEM micrograph of the typical TiN based inclusion observed on the L cross section of the hot rolled steel sheet of Invention Example No. 5.

DESCRIPTION OF EMBODIMENTS

Chemical Composition

[0043] The percentage for the chemical composition is percentage by mass unless otherwise indicated.

[0044] The invention targets a steel having the following chemical composition (A).

[0045] (A) In terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities.

[0046] The steel species having the composition forms deformation induced martensite and is increased in strength in cold rolling. Furthermore, in the subsequent aging treatment, the solute atoms, such as C and N, form a Cottrell atmosphere mainly in the martensite phase to fix the dislocation, which causes a function enhancing the strength (strain aging). Moreover, Si present in a large amount in the steel provides solid-solution strengthening of the martensite phase and the residual austenite phase, which contributes to the enhancement of the strength.

[0047] In the invention, for sufficiently providing the benefit of the aforementioned strength enhancing function of Si, a steel having a Si content exceeding 2.00% is targeted. However, with an excessively large Si content, such problems that hot working cracks tend to occur, and the like may be conspicuous. Herein, the Si content is restricted to 4.00% or less.

[0048] C is an element forming an austenite phase and is necessary for strengthening the steel. However, an excessive amount of C may cause deterioration of the corrosion resistance and the toughness. In the invention, a steel having a C content of from 0.010 to 0.200% is targeted, and particularly in the case where a high strength is targeted, the C content is advantageously in a range of from 0.050 to 0.100%.

[0049] N is an element forming an austenite phase and is necessary for strengthening the steel. However, an excessive amount of N may be a factor promoting the formation of a TiN based inclusion. In the invention, a steel having a N content of from 0.010 to 0.200% is targeted. With the range, the particle diameter distribution of the TiN based inclusion can be optimized to the range defined in the invention by the production method of suppressing the incorporation of Ti described later. The preferred range of the N content is from 0.050 to 0.085%.

[0050] Mn is an element, with which the austenite stability can be easily controlled by controlling the content thereof, and the content thereof is controlled in a range of from 0.01 to 3.00%. A large amount of Mn contained may prevent the deformation induced martensite phase from being induced. The Mn content is more preferably controlled in a range of 1.00% or less, and may be managed to a range of 0.50% or less.

[0051] Ni is an element forming an austenite phase, and the content thereof of 3.00% or more is ensured for providing a metastable austenite phase at ordinary temperature. A too large Ni content may prevent the deformation induced martensite phase from being induced, and therefore the Ni content is less than 10.00%. The Ni content is more preferably from 7.00 to 9.50%.

[0052] Cr is an element necessary for ensuring the corrosion resistance. In the invention, a steel having a Cr content of from 11.00 to 20.00% is targeted. Cr is an element forming a ferrite phase, and with a larger amount thereof exceeding the above range, an austenite single phase structure may not be obtained at a high temperature in some cases. The more preferred range of the Cr content is from 12.00 to 15.00%.

[0053] Mo may be contained depending on necessity since Mo has a function enhancing the corrosion resistance, and also has a function contributing the strengthening through the formation of a Mo precipitate in the aging treatment and preventing the structure work-hardened in the cold rolling from being softened in the aging treatment. For sufficiently providing the benefit of the functions, it is preferred to ensure a Mo content of 1.0% or more. In the case where a strength level with a tensile strength in the rolling direction of 2,000 N/mm.sup.2 or more is intended, it is extremely effective that Mo is contained in an amount of 2.00% or more. However, a too large amount of Mo contained may cause formation of a ferrite phase at a high temperature, and therefore in the case where Mo is contained, the content thereof is in a range of 3.00% or less, and may be managed to a range of 2.50% or less.

[0054] Cu may be contained depending on necessity since Cu has a function increasing the strength through the mutual action with Si in the aging treatment. In this case, the Cu content is more preferably 0.01% or more. A too large content of Cu may be a factor decreasing the hot workability. In the case where Cu is contained, the content thereof is in a range of 1.00% or less.

[0055] Ti is an element forming a TiN based inclusion, and the Ti content is necessarily suppressed to a low level particularly in a large Si content steel, in which TiN is easily formed. As a result of the various investigations, the Ti content is necessarily 0.008% or less, and is preferably 0.006% by mass or less. The Ti content is preferably as small as possible, but in view of the cost in a mass production operation, it is rational that the Ti content is in a range of 0.001% or more.

[0056] Al is a factor generating a spinel based inclusion through the formation of Al.sub.2O.sub.3, and the Al content is necessarily suppressed to a low level particularly in a molten steel of a large Si content steel, in which Al.sub.2O.sub.3 is easily formed. In the invention, the Al content is preferably as small as possible. As a result of the various investigations, the Al content is necessarily 0.008% or less, and is preferably 0.006% by mass or less. The Al content is preferably as small as possible, but in view of the cost in a mass production operation, it is rational that the Al content is in a range of 0.001% or more. However, even with an Al content in the aforementioned range, it may be difficult to make stably the particle diameter distribution of the spinel based inclusion within the range defined in the invention unless the slag basicity after the addition of Si is optimized as described later.

[0057] As the unavoidable impurities, it is preferred that the P content is 0.040% or less, and the S content is 0.002% or less, and it is also preferred that the Mg content is from 0 to 0.002%.

[0058] For controlling the easiness of the formation of the deformation induced martensite phase in the cold rolling, the value Md.sub.30 defined by the following expression (1) is preferably in a range of from 50 to 0.

Md.sub.30=551462(C+N)9.2Si8.1Mn13.7Cr29(Ni+Cu)18.5Mo(1)

[0059] Herein, the element symbols in the expression (1) each represent the content of the corresponding element in terms of percentage by mass.

Non-Metallic Inclusion

[0060] The non-metallic inclusion present in the steel is roughly classified into a soft type having a low melting point and a hard type having a high melting point. In the steel targeted by the invention, the former soft type is mainly a CaOSiO.sub.2 based material. The soft type inclusion is extended in the rolling direction in the hot rolling since it is in a liquid state at the hot rolling temperature, and is then collapsed and finely dispersed in the subsequent cold rolling. The soft inclusion of this type exerts substantially no adverse effect on the fatigue resistance characteristics of the thin sheet material.

[0061] What becomes a problem is the latter hard inclusion. The inclusion of this type remains as a granular material after the hot rolling, and after the cold rolling, hard particles having been crushed to a certain extent remain in the form of lining up in the rolling direction. The proportion of the length in the sheet thickness direction of the inclusion occupied in the sheet thickness is increased with the smaller sheet thickness, and the inclusion particles tend to function as a starting point and a propagation path of cracks due to the stress concentration around the inclusion particles. It has been found that the hard type inclusion that becomes a problem in the steel targeted by the invention is a TiN based inclusion and a spinel based inclusion containing one or more of Al and Mg. In particular, the TiN based inclusion has a tendency of growing associated with the decrease of the solubility of Ti in the process where the temperature of the molten steel is decreased in casting, and thus tends to become a problem.

[0062] According to the investigations by the inventors, when the number ratio of the TiN based inclusion and the spinel based inclusion described above that have a circle equivalent diameter of 6.0 m or more is decreased in the stage of the hot rolled steel sheet, the inclusion distribution mode that is advantageous for improving the fatigue resistance characteristics on repeatedly receiving elastic deformation can be obtained in the form of a thin sheet material having a sheet thickness, for example, of from 20 to 500 m. Specifically, it is extremely advantageous to provide a structure state in the stage of the hot rolled steel sheet, in which the total number density of the TiN based inclusion and the spinel based inclusion containing one or more of Al and Mg each having a circle equivalent diameter of 6.0 m or more on the cross section in parallel to the rolling direction and the sheet thickness direction (L cross section) is 0.05 or less per square millimeter.

[0063] Examples of the more preferred structure state of the hot rolled steel sheet include a metal structure, in which in addition to the aforementioned provision of the number density, the maximum particle diameter of the circle equivalent diameter of the TiN based inclusion and the spinel based inclusion containing one or more of Al and Mg on the L cross section is 10.0 m or less. In this case, the measurement area of the L cross section for determining the maximum particle diameter suffices to be 200 mm.sup.2 or more.

[0064] It has been found that in a thin sheet material having a sheet thickness of from 20 to 500 m, the number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more is particularly effectively 3.0 or less per square millimeter on the L cross section for improving the fatigue resistance characteristics on repeatedly receiving elastic deformation. Herein, a group of the non-metallic inclusion particles that line up with an interparticle distance in the rolling direction of 20 m or less and an interparticle distance in the sheet thickness direction of 10 m or less is assumed to be one non-metallic inclusion, as described in the foregoing.

[0065] As for the fatigue resistance characteristics, adjacent inclusion particles that line up closely to each other to a certain extent function as a starting point of occurrence of cracks, as similar to the case where the particles are present as a continuous one group of particles. As a result of the various investigations, a group of particles formed of plural non-metallic inclusion particles that line up while maintaining an interparticle distance in the rolling direction of 20 m or less and an interparticle distance in the sheet thickness direction of 10 m or less (which is assumed to be one non-metallic inclusion), particularly having a length in the rolling direction of 40 m or more tends to be a starting point of cracks on repeated application of elastic deformation in the high strength steel as the target of the invention. However, even with the non-metallic inclusion of this type, the fatigue resistance characteristics can be improved by decreasing the number density thereof to 3.0 or less per square millimeter on the L cross section. It is estimated that the mechanism therefor is that in the case where the density of the non-metallic inclusion having a length in the rolling direction of 40 m or more is sufficiently decreased, the non-metallic inclusion is prevented from exerting a function as a propagation path of cracks.

[0066] The number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more on the L cross section that is as low as possible is advantageous for the enhancement of the fatigue resistance characteristics of the thin sheet material. In the case where a steel is manufactured by using raw materials having high purities without the use of scraps, for example, with an experimental melting furnace, it is considered that a thin sheet material containing an extremely small amount of a non-metallic inclusion can be produced. However, in the case where a steel sheet having a thickness of from 20 to 500 m is manufactured in mass production sites, the complete prevention of the formation of the non-metallic inclusion having a length in the rolling direction of 40 m or more increases the load in the steel manufacturing, which leads to increase of the cost. Therefore, it is rational that in the thin sheet material having a sheet thickness of from 20 to 500 m, the number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more is in a range of from 0.1 to 3.0 per square millimeter on the L cross section.

Production Method

[0067] The stainless steel sheet having the optimized particle size distribution of the hard non-metallic inclusion described above can be produced by utilizing an ordinary manufacturing equipment for a stainless steel. Representative examples thereof include a VOD process and an AOD process. In any process, a molten steel having a C content of 0.20% or less having a Cr oxide-containing slag on a molten steel surface after subjecting to a decarburization process by blowing oxygen into a Cr-containing molten iron is produced firstly. The steel making process up to this stage may be performed according to the ordinary method except that such a container is selected as the container housing the molten steel that Ti and Al are substantially or completely not incorporated from an attachment and the like of the container.

[0068] The molten steel in this stage is the molten steel having been subjected to decarburization by blowing oxygen, and therefore easily oxidizable elements, Ti, Al, Mg, and Si, are substantially oxidized and removed. That is, Ti, Al, Mg, Si are substantially not present in the molten steel. Furthermore, a part of Cr, which is contained in a large amount in the molten steel, is oxidized to form a slag as a Cr oxide on the molten steel surface. The slag formed mainly of a Cr oxide contains oxides of Ti, Al, Mg, and Si having been removed from the molten steel. The molten steel also contains a large amount of oxygen dissolved therein having been blown for the decarburization. Accordingly, deoxidation is necessarily performed before casting. In the invention, Si is necessarily contained in the steel for producing a large Si content steel having a Si content exceeding 2.00%. Furthermore, it is desirable to perform a process of returning Cr escaping from the molten steel in the decarburization from the slag to the steel (reduction and recovery of Cr). In the invention, accordingly, the deoxidation, the control of the Si content, and the reduction and recovery of Cr are performed at one time by adding an FeSi alloy to the molten steel. Furthermore, the component control is performed by adding other auxiliary raw materials depending on necessity.

[0069] By controlling the Si content to more than 2.00% by adding the FeSi alloy to the molten steel, the deoxidation in the steel proceeds with the Si source added in a large amount. The oxygen concentration in the steel by the deoxidation is determined through the chemical equilibrium of the chemical reaction of the expression (2) below.

Si(in metal)+2O(in metal)=SiO.sub.2(in slag)(2)

[0070] The equilibrium constant K is shown by the expression (3) below.

K=A(SiO.sub.2)/A(Si)/A(O).sup.2(3)

[0071] In the expression, A(X) represents the activity of the component X. As understood from the expression (3), when the Si activity (i.e., the Si concentration) in the molten steel is larger, the oxygen activity (i.e., the oxygen concentration) in the molten steel shows an equilibrium at a lower level. Therefore, in the molten steel having a large amount of the Si source added targeted by the invention, the oxygen concentration in the molten steel becomes smaller than a small Si content steel (for example, an ordinary steel, such as SUS 304).

[0072] The chemical equilibrium based on the following expression (4) is established between the Al oxide in the slag and oxygen in the molten steel.

2Al(in metal)+3O(in metal)=Al.sub.2O.sub.3(in slag)(4)

[0073] According to the chemical equilibrium, the equilibrium is retained by increasing the Al concentration in the molten steel when the oxygen concentration in the molten steel is small. The relationship is also applied to Ti and Mg. Consequently, when the oxygen concentration in the molten steel is small, the Al concentration, the Ti concentration, and the Mg concentration in the molten steel are increased.

[0074] When the Al concentration and the Mg concentration in the molten steel are larger, the spinel based inclusion tends to be formed and grown. When the Ti concentration in the molten steel is larger, the TiN based inclusion tends to be formed and grown. Therefore, for suppressing the formation and growth of the inclusions, the decrease of the oxygen concentration in the molten steel associated with the increase of the Si concentration in the molten steel is necessarily suppressed as much as possible. For suppressing the decrease of the oxygen concentration in the molten steel, it is advantageous that the SiO.sub.2 concentration in the slag is larger. Accordingly, the invention uses a measure for controlling the slag basicity (i.e., the mass ratio of CaO/SiO.sub.2) to a low level. Specifically, the amount of the Ca-containing substance added as a slag forming flux is controlled. The slag forming flux added may be quick lime CaO. Ca is also supplied to the slag from CaF.sub.2, which is added as a flux component depending on necessity. Ca that is supplied from CaF.sub.2 is converted to the CaO amount and added to the CaO value used for calculating the basicity. As a result of the various investigations, it has been found that it is effective to make the slag basicity in a range of 1.3 or more and 1.5 or less in the slag present on the molten steel surface after completing the addition of the FeSi alloy. The slag basicity is more preferably 1.3 or more and 1.45 or less, and further preferably 1.3 or more and 1.4 or less. With a smaller slag basicity, the decrease of the oxygen concentration in the molten steel is suppressed, and Ti, Al, and Mg are prevented from being incorporated into the molten steel from the slag. However, with a too small slag basicity, other inclusions, such as Cr.sub.2O.sub.3, are formed in a large amount. The desulfurization capability is also lowered. Therefore, it is extremely effective to control the final slag basicity to a range that is not lower than 1.3.

[0075] As described above, the molten steel having been subjected to the decarburization by blowing oxygen contains substantially no Ti, Al, and Mg, and these elements are present as oxides in the slag formed mainly of Cr oxide. Ti, Al, and Mg in the slag include those incorporated from the raw materials and the refractory and those incorporated from a slag, a metal, and the like of the previous batch attached to the facilities, such as an electric furnace and a converter furnace. For optimizing the particle diameter distribution of the hard inclusion in the steel sheet by controlling the slag basicity as described above, it is necessary that Ti, Al, and Mg are prevented from being newly incorporated after the time when the decarburization by blowing oxygen is completed, i.e., after the time when the FeSi alloy is added. In particular, it has been confirmed that when a slag attached in the previous charge remains in the container housing the molten steel, a coarse hard inclusion tends to be formed due to small amounts of Ti and Al incorporated from the attachment. For preventing the incorporation thereof from the container housing the molten steel, it is most preferred to use a container having a refractory constituting the inner surface of the container that is not used for housing a molten steel (i.e., a new ladle). It has also been confirmed that an FeSi alloy that is generally used in the production site of stainless steels contains impurities, such as Al and Ti, and Al and Ti incorporated therefrom become a factor forming a coarse hard inclusion. Therefore, in the invention, it is necessary to use an FeSi alloy having a high purity. Specifically, it is preferred to use an FeSi alloy having an Al content of 0.05% by mass or less and a Ti content of 0.05% by mass or less. It is also desirable that Ti and Al are prevented as much as possible from being incorporated from the other auxiliary raw materials and slag forming flux.

[0076] It is important that the container housing the molten steel, the auxiliary raw material, and the slag forming flux used are selected to make a Ti content in the molten steel of 0.008% by mass or less and an Al content therein of 0.008% by mass or less finally. In the case where the final Ti content and the final Al content in the steel exceed the aforementioned values, it is difficult to achieve stably the aforementioned intended particle diameter distribution of the hard inclusion even though the slag basicity is controlled in the aforementioned manner. While the content of Mg is desirably controlled to 0.002% by mass or less in the molten steel finally, it has been confirmed that when the container housing the molten steel, the auxiliary raw material, and the slag forming flux used are selected to make the Ti and Al contents within the aforementioned range, the particle diameter distribution of the spinel based inclusion is in the aforementioned intended state even though the Mg content in the steel is not particularly restricted, and no problem occurs.

[0077] It is more preferred that the Ti content in the molten steel is 0.006% by mass or less, and the Al content therein is 0.006% by mass or less.

[0078] The casting may be performed according to an ordinary method. In general, a cast piece is obtained by a continuous casting method. In the description herein, a steel material obtained by casting (i.e., a material having a solidification structure) is referred to as a cast piece. Accordingly, a steel ingot obtained by an ingot making method is encompassed in the cast piece referred herein for the sake of convenience.

[0079] The resulting cast piece is subjected to hot working including at least hot rolling, so as to provide a hot rolled steel sheet. In the case where the ingot making method is employed, after performing blooming and hot forging, hot rolling is performed. The heating temperature in the hot rolling may be from 1,100 to 1,250 C., and the sheet thickness of the hot rolled steel sheet may be, for example, from 2.5 to 6.0 mm. According to the manners, the stainless steel hot rolled sheet, in which the total number density of the TiN based inclusion and the spinel based inclusion containing one or more of Al and Mg each having a circle equivalent diameter of 6.0 m or more on the cross section in parallel to the rolling direction and the sheet thickness direction (L cross section) is 0.05 or less per square millimeter, can be obtained.

[0080] Subsequently, the hot rolled steel sheet is subjected to annealing, cold rolling, and an aging treatment, and thereby a thin sheet material of the high strength stainless steel sheet can be obtained. The step of cold rolling may be performed plural times including an intermediate annealing step. After each of the heat treatments, acid pickling may be performed depending on necessity. The condition for the annealing applied to the hot rolled steel sheet (i.e., the hot rolled sheet annealing) may be, for example, from 1,000 to 1,100 C. for from 40 to 120 seconds, the final cold rolling reduction ratio (which is the cold rolling reduction ratio after the final intermediate annealing in the case where the intermediate annealing is performed) may be, for example, from 40 to 70%, and the condition for the aging treatment may be, for example, from 400 to 600 C. for from 10 to 60 minutes. For the purpose of a thin sheet spring material, the final sheet thickness is, for example, preferably 150 m or less, and more preferably less than 100 m. For example, a thin sheet material having a thickness of from 20 to 70 m may be provided. According to the manners, the thin sheet material of the high strength stainless steel having a mixed structure of a deformation induced martensite phase and an austenite phase as the matrix (metal basis material) can be obtained. The ratio of an area ratio M of the deformation induced martensite phase and an area ratio A of the austenite phase in terms of M/A is generally in a range of from 30/70 to 50/50. In the material containing Mo in an amount, for example, of 2.00% or more, a high strength with a tensile strength in the rolling direction of 2,000 N/mm.sup.2 can be obtained. In the thin sheet material, assuming that a group of the non-metallic inclusion particles that line up with an interparticle distance in the rolling direction of 20 m or less and an interparticle distance in the sheet thickness direction of 10 m or less is one non-metallic inclusion, a structure state, in which the number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more is 3.0 or less per square millimeter on the L cross section can be obtained, and the thin sheet material exhibits good fatigue resistance characteristics in the purpose of a spring material, which repeatedly receives elastic stress.

Examples

[0081] The steels shown in Table 1 were manufactured by a VOD process. In each of the steels, the final decarburization process by blowing oxygen into the Cr-containing molten iron was completed in the VOD equipment, and a molten steel having a C content of 0.10% or less having a Cr oxide-containing slag on the molten steel surface was obtained. In this stage, the C content is substantially equal to the final C content shown in Table 1.

TABLE-US-00001 TABLE 1 Chemical composition (% by mass) Class No. C Si Mn P S Ni Cr Mo Cu N Ti Al Md.sub.30 Conventional 1 0.080 2.64 0.25 0.020 0.001 8.32 13.42 2.22 0.10 0.068 0.023 0.010 12.8 Example 2 0.086 2.74 0.23 0.020 0.001 8.25 13.43 2.22 0.10 0.066 0.018 0.010 13.5 3 0.086 2.65 0.42 0.029 0.001 8.34 13.57 2.23 0.18 0.066 0.010 0.009 21.3 4 0.072 2.60 0.28 0.030 0.001 8.28 13.71 2.14 0.07 0.073 0.010 0.006 11.7 Invention 5 0.086 2.73 0.22 0.028 0.001 8.32 13.73 2.23 0.28 0.069 0.004 0.004 26.3 Example

[0082] In the final decarburization in the VOD equipment, the container housing the molten steel used was a ladle, and the same ladle was used in the process up to casting. As the ladle, a ladle that had been used for manufacturing a Ti-containing stainless steel as the previous charge was used for Conventional Examples Nos. 1 and 2, a ladle that had been used for manufacturing a Ti-free stainless steel as the previous charge was used for Conventional Examples Nos. 3 and 4, and a ladle having a refractory constituting the inner surface of the ladle that was not used for housing a molten steel (i.e., a new ladle) was used for Invention Example No. 5.

[0083] An FeSi alloy was added to the molten steel having been completed for the final decarburization, so as to control the Si content in the molten steel to the target value, and to perform deoxidation, and reduction and recovery of Cr in the slag. In this stage, the Si content in the molten steel is substantially equal to the final Si content shown in Table 1. As the FeSi alloy, a material corresponding to the ferrosilicon No. 2 defined in JIS G2302:1998 was used for Conventional Examples Nos. 1 to 4. As a result of the analysis, the material corresponding to the ferrosilicon No. 2 contained approximately 1.0% by mass of Al, approximately 0.07% by mass of Mg, and approximately 0.08% by mass of Ti, while there was a certain fluctuation depending on the product lot. On the other hand, a high quality FeSi alloy having an Al content having been extremely decreased was used for Invention Example No. 5. As a result of the analysis, the Al, Mg, and Ti contents of the high quality FeSi alloy were 0.009% by mass for Al, less than 0.001% by mass for Mg, and 0.012% by mass for Ti.

[0084] Subsequent to the addition of the FeSi alloy, industrial quick lime (CaO) was added as a slag forming flux to the slag. Thereafter, the slag was collected and subjected to compositional analysis. As a result, the slag basicity was from 1.60 to 1.65 for Conventional Examples Nos. 1 to 4, and 1.33 for Invention Example No. 5.

[0085] In each of the examples, the molten steel thus obtained above was continuously cast and subjected to hot rolling, so as to provide a hot rolled steel sheet having a sheet thickness of 3.8 mm. The heating temperature in the hot rolling was 1,230 C. For the resulting hot rolled steel sheet, the L cross section was observed with an SEM, and according to the aforementioned section Measurement Method for Number Density of Inclusion in Hot Rolled Steel Sheet, the total number density of the TiN based inclusion and the spinel based inclusion containing one or more of Al and Mg each having a circle equivalent diameter of 6.0 m or more was measured. As a result, the total number density was from 0.20 to 0.45 per square millimeter for Conventional Examples Nos. 1 to 4, and 0.02 per square millimeter for Invention Example No. 5. It was understood that the formation of a coarse hard inclusion was significantly suppressed by the manufacturing method according to the invention.

[0086] FIG. 3 exemplifies the SEM micrograph of the typical TiN based inclusion observed on the L cross section of the hot rolled steel sheet of Conventional Example No. 4, and FIG. 4 exemplifies the SEM micrograph of the typical TiN based inclusion observed on the L cross section of the hot rolled steel sheet of Invention Example No. 5. In all the micrographs, the horizontal direction agrees with the rolling direction. The cross-hair cursor appearing in the micrographs shows the beam irradiation position in the EDX analysis.

[0087] Subsequently, a specimen collected from the hot rolled steel sheet was subjected to hot rolled sheet annealing at 1,050 C. for 60 seconds, cold rolling, intermediate annealing at 1,050 C. for 60 seconds, cold rolling, and an aging treatment at 500 C. for 30 minutes, so as to produce a thin sheet material having a sheet thickness of 120 m having a mixed structure of a deformation induced martensite phase and an austenite phase as the matrix (metal basis material). In all the resulting thin sheet materials, the tensile strength in the rolling direction exceeded 2,000 N/mm.sup.2.

[0088] The L cross section of the thin sheet material was measured for the number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more according to the aforementioned section Measurement Method for Number Density of Non-metallic Inclusion having Length in Rolling Direction of 40 m or more. As described above, a group of the non-metallic inclusion particles that lined up with an interparticle distance in the rolling direction of 20 m or less and an interparticle distance in the sheet thickness direction of 10 m was assumed to be one non-metallic inclusion. As a result of the measurement, the number density of the non-metallic inclusion having a length in the rolling direction of 40 m or more on the L cross section was from 8.2 to 33.2 per square millimeter for Conventional Examples Nos. 1 to 4, and 2.4 per square millimeter for Invention Example No. 5. As a result of the EDX analysis, all the non-metallic inclusions counted were constituted by TiN based inclusion particles or spinel based inclusion particles containing one or more of Al and Mg.

[0089] In Invention Example, the number of the hard non-metallic inclusion having a length in the rolling direction of 40 m or more, which was a factor deteriorating the fatigue resistance characteristics, was largely decreased as compared to Conventional Examples.

[0090] For reference, a production example for the case where SUS 304 (having a Si content of 0.55%) is subjected to deoxidation with an FeSi alloy is shown. A ladle that had been used for manufacturing a Ti-containing stainless steel as the previous batch was used as the container housing the molten steel, and the final decarburization process by blowing oxygen into the Cr-containing molten iron was completed in the VOD equipment, thereby providing a molten steel having a C content of approximately 0.05% having a Cr oxide-containing slag on the molten steel surface. The FeSi alloy corresponding to the ferrosilicon No. 2 was added to the molten steel, so as to control the Si content. Industrial quick lime (CaO) was added as a slag forming flux. Thereafter, chromium nitride was added to complete the component control. As a result of the analysis of the final slag collected, the slag basicity was 1.65. The molten steel was continuously cast and subjected to hot rolling in the ordinary method, so as to provide a hot rolled steel sheet having a sheet thickness of 3.5 mm. The hot rolled steel sheet was investigated for the state of the presence of the non-metallic inclusion in the same manner as in Nos. 1 to 5. As a result, a TiN based inclusion and a spinel based inclusion containing one or more of Al and Mg each having a circle equivalent diameter of 6.0 m or more were not found. It is understood from the comparison between the example of SUS 304 and Conventional Examples Nos. 1 to 4 that it is considerably difficult to suppress the formation of a hard inclusion in a stainless steel species having a large Si content.