AUSTENITIC STAINLESS STEEL SHEET

- NIPPON STEEL CORPORATION

This austenitic stainless steel sheet contains, as a chemical composition, by mass %, C: 0.030% or less, Si: 1.0% or less, Mn: 1.5% or less, Cr: 15.0% or more and 20.0% or less, Ni: 6.5% or more and 9.0% or less, N: 0.030% or more and 0.150% or less, any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less, Mo: 0% or more and 2.0% or less, Cu: 0% or more and 1.5% or less, Co: 0% or more and 1.0% or less, P: 0.10% or less, S: 0.010% or less, and Al: 0.10% or less, in which a remainder includes Fe and impurities, the average grain size is 5.0 μm or less, and the non-recrystallization rate is more than 3% and 20% or less.

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Description
TECHNICAL FIELD OF THE INVENTION

The present invention relates to an austenitic stainless steel sheet.

RELATED ART

A metal mask is a plate mold that is used for printing of paste-like solder on a printed circuit board and is normally made of a stainless steel sheet having a thickness of about 0.10 to 0.25 mm as a material.

Conventionally, metal masks are often manufactured by a method called photoetching in which a metal sheet is chemically formed. The photoetching is a method in which a part of a metal sheet is masked by a photoresist method or the like, then, the metal sheet and an etchant are brought into contact with each other by spraying or immersion to melt an exposed metal sheet other than the mask surface, thereby forming an opening part. In addition, at the time of photoetching, there is also a case where a method for producing a metal mask that is uneven so as to correspond protrusions and recesses of a printed circuit board by a technique called half etching in which the sheet of the metal sheet is partially etched is used.

Incidentally, in recent years, there has been a demand for a higher-definition process in the manufacturing of metal masks in association with the densification of printed circuit boards. In order to meet such a demand, a laser process is increasingly used as a method for manufacturing a metal mask. The laser process is capable of dealing with a variety of shapes by irradiating a metal sheet, which is a workpiece material, with a laser to partially pierce the metal sheet or by forming an unevenness in a metal mask by welding metal sheets having different sheet thicknesses with a laser. Therefore, metal masks having a denser and complex structure can be produced by applying the laser process.

In a case where metal masks are manufactured by the laser process, for stainless steel sheets, which are the material for the metal masks, not only the flatness, strength high enough to withstand repeated squeegeeing, or the like of a material sheet, which are required as conventional metal masks, but also the smoothness of a laser-processed portion or suppression of the deterioration of the corrosion resistance of a laser-molten portion and a heat-affected zone are required.

For example, Patent Document 1 has proposed an austenitic stainless steel suitable for etching processes. However, there was a problem in that, in the stainless steel, the corrosion resistance of the material significantly deteriorates due to the precipitation of a Cr carbide in a molten portion or a heat-affected zone by the laser process.

In addition, Patent Document 2 has proposed a stainless steel sheet for photoetching in which the smoothness of an etched end surface is ensured by adjusting the chemical composition and manufacturing step conditions to obtain an average grain size of 15 μm or less.

As shown in Patent Document 2, it is known that, when the grain sizes are decreased, even in a case where laser cutting is performed, the processed surface is smoothened. However, in recent years, the fineness and precision at the time of processing a metal mask have been more strongly required than before, and the related art has not sufficiently satisfied these requirements.

Patent Document 3 has proposed an austenitic stainless steel sheet having all required characteristics at the time of a photoetching or laser process (a material is flat and has high hardness, and a processed surface can also be smoothened) by adjusting the chemical composition and manufacturing step conditions. However, in the technique of Patent Document 3, there has been a case where a coarse Cr carbide is formed in a heat-affected zone during a laser process, and the corrosion resistance deteriorates or the hardness extremely decreases.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Patent No. 2754225

[Patent Document 2] Japanese Patent No. 3562492

[Patent Document 3] Japanese Patent No. 5939370

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described above, in recent years, printed circuit boards have rapidly become denser and more complex, and there has been a demand for a higher-definition process on masks. However, at the moment, the characteristics of performance of stainless steel sheets, which are a material for masks, have not been improved in accordance to such a high-definition process. In the related art, as described above, a variety of studies have been conducted to improve the performance of stainless steel sheets, which are a material for masks, but it was extremely difficult to ensure the flatness and strength of the stainless steel sheets, which is the material, and the smoothness of a laser-processed portion and, furthermore, to suppress the deterioration of the corrosion resistance of a heat-affected zone or the like after the laser process.

In addition, in recent years, the laser process has been increasingly used as a processing method at the time of manufacturing, in addition to the metal masks described above, configurational members having a complex shape for a variety of precision instruments for which a stainless steel sheet is used. It is needless to say that such precision members also strongly require the smoothness of a laser-processed portion and there has been a demand for further improvement in the characteristics or performance of the stainless steel sheets which are the material.

In addition, in the manufacturing of the metal masks and the precision members, there are still many cases where the photoetching process is used due to easiness in workability. That is, at the moment, a processing method is selected depending on the complexity and precision of a member to be manufactured. Therefore, in recent years, there has been a desire for a high-performance stainless steel sheet capable of ensuring the smoothness or corrosion resistance of a processed portion regardless of whether the stainless steel sheet is processed by the photoetching process or the laser process.

In addition, for steel sheets that are used for precision instruments as described above, there is an increasing need for high-strengthening from the viewpoint of weight reduction of components.

The present invention has been made in view of the above-described current situation, and an object that the problem intends to achieve is to provide an austenitic stainless steel sheet in which the strength (YS) is high and the smoothness of a laser-processed portion or the corrosion resistance of a molten portion and a heat-affected zone are favorable.

Means for Solving the Problem

As described above, a laser process is a method in which the surface of a steel sheet is irradiated with a laser and the steel sheet is melted or joined by the heat. In addition, as described above, the refinement of crystal grains is effective for improving the smoothness of an etching-processed portion or a laser-processed portion. Therefore, there are cases where a heat treatment is performed on a base steel sheet for the purpose of this refinement.

However, there are cases where, due to the heat generated during laser irradiation or the above-described heat treatment, Cr, which is a main configurational element of a stainless steel bonds to C, a Cr carbide is formed in grain boundaries, and thus a chromium-depleted layer is formed near the grain boundaries. In this case, the corrosion resistance of a molten portion or a heat-affected zone significantly deteriorates. As a result of studies, the present inventors found that, when a chemical composition such as the C content in a material is strictly controlled and the formation of a Cr carbide is suppressed, it is possible to improve the corrosion resistance of a molten portion or a heat-affected zone after a laser process.

Furthermore, when the roughness of a laser-processed portion increases (the smoothness deteriorates), there is another problem in that the accuracy of a metal mask is insufficient or a gap is generated in a joint portion. As a result of studying the smoothness of an etching-processed portion or a laser-processed portion, the present inventors found that, when a microstructure including uniform fine crystal grains is provided to a base steel sheet by optimizing, in addition to the chemical composition of the stainless steel sheet, which is the material, the manufacturing process, the smoothness of a processed portion can be ensured.

In addition, regarding the strength, it is possible to increase the strength by further refining the crystal grains, but it is industrially difficult to refine crystal grains to a certain extent or higher from the viewpoint of the cost. As a result of conducting studies, the present inventors found that the strength can be increased by increasing the non-recrystallization rate.

Based on the above-described idea, the present inventors researched the chemical composition or microstructure of stainless steel sheets and the characteristics of processed portions in detail. As a result, the present inventors found that the object of the present invention is achieved by the following configuration and completed the present invention.

The gist of the present invention is as described below.

[1] An austenitic stainless steel sheet according to one aspect of the present invention contains, as a chemical composition, by mass %, C: 0.030% or less, Si: 1.0% or less, Mn: 1.5% or less, Cr: 15.0% or more and 20.0% or less, Ni: 6.5% or more and 9.0% or less, N: 0.030% or more and 0.150% or less, any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less, Mo: 0% or more and 2.0% or less, Cu: 0% or more and 1.5% or less, Co: 0% or more and 1.0% or less, Ca, Mg, Zr, Sn, Pb, and W in total: 0% or more and 0.10% or less, P: 0.10% or less, S: 0.010% or less, and Al: 0.10% or less, in which a remainder includes Fe and impurities, an average grain size is 5.0 μm or less, and a non-recrystallization rate is more than 3% and 20% or less.

[2] The austenitic stainless steel sheet according to the [1] may contain, in the chemical composition, by mass %, one or more of the group consisting of Cu: 0.1% to 1.5%, Mo: 0.1% to 2.0%, and Co: 0.1% to 1.0%.

Effects of the Invention

According to the above-described aspect of the present invention, it is possible to provide an austenitic stainless steel sheet in which the strength (YS) is high and the smoothness (roughness) of a laser-processed portion or the corrosion resistance of a molten portion and a heat-affected zone are favorable.

EMBODIMENTS OF THE INVENTION

Hereinafter, an austenitic stainless steel sheet according to an embodiment of the present invention (austenitic stainless steel sheet according to the present embodiment) will be described.

First, the reasons for limiting the chemical composition (composition) of the austenitic stainless steel sheet according to the present embodiment will be described below. Hereinafter, unless otherwise described, the mark “%” of the amount of each element means “mass %”.

C: 0.030% or less

C is an element that is precipitated in grain boundaries as a coarse Cr carbide due to heat generated during a laser process or during a heat treatment, significantly impairs the corrosion resistance of the steel sheet, and acts as a cause for a decrease in hardness. Therefore, the C content is preferably as small as possible. However, C is an element capable of increasing the strength of the steel sheet at a low cost and may thus be contained in a range of 0.030% or less where the corrosion resistance or the hardness is not adversely affected. From the viewpoint of the corrosion resistance after a laser process, the C content is desirably 0.020% or less.

As described above, the C content is preferably as small as possible, but an excessive reduction leads to an increase in the refining cost. Therefore, the lower limit of the C content is desirably set to 0.005%.

Si: 1.0% or less

Si is an element that is used as a deoxidation material during melting and also contributes to the strengthening of steel. However, when the Si content is excessively increased, the surface cleaning of the steel sheet deteriorates, and the cleaning efficiency and the like are adversely affected. Therefore, the Si content is set to 1.0% or less. The Si content is preferably set to 0.8% or less.

The Si content is preferably as small as possible, but an excessive reduction leads to an increase in the raw material cost. Therefore, the lower limit of the Si content is desirably set to 0.01%.

Mn: 1.5% or less

Mn is a useful element as an inexpensive austenite-forming element. However, when the Mn content is excessively increased, the amount of deformation-induced martensite that is formed during cold rolling decreases, and it is not possible to obtain fine crystal grains by subsequent annealing. In addition, when excessively contained, Mn also acts as a cause for the deterioration of the corrosion resistance. Therefore, the Mn content is set to 1.5% or less. The Mn content is preferably set to 1.2% or less.

An excessive reduction of the Mn content leads to an increase in the raw material cost. Therefore, the lower limit of the Mn content is desirably set to 0.01%.

Cr: 15.0% or more and 20.0% or less

Cr is a basic element of stainless steel and is an element necessary to form a metal oxide layer on the surface of a steel and enhance the corrosion resistance. At a Cr content of less than 15.0%, sufficient corrosion resistance cannot be obtained. Therefore, the Cr content is set to 15.0% or more. The Cr content is preferably 16.0% or more.

On the other hand, Cr is a strong ferrite-stabilizing element, and, when the Cr content is too large, δ ferrite is formed. This δ ferrite degrades the hot workability of the material. Therefore, the Cr content is set to 20.0% or less. The Cr content is preferably 19.0% or less.

Ni: 6.5% or more and 9.0% or less

Ni is an austenite-forming element and is an element necessary to stably obtain austenite at room temperature. In order to obtain this effect, the Ni content is set to 6.5% or more. The Ni content is preferably 7.0% or more.

On the other hand, when the Ni content is too large, austenite is excessively stabilized, process-induced martensitic transformation during cold rolling is suppressed, and it is not possible to obtain fine crystal grains by subsequent annealing. Furthermore, Ni is an expensive element, and an increase in the Ni content leads to a significant increase in the cost. Therefore, the Ni content is set to 9.0% or less. The Ni content is preferably 8.5% or less.

N: 0.030% or more and 0.150% or less

Similar to C, N is a solid solution strengthening element and is an element that contributes to improving the strength of steel. In addition, N is an element that bonds to Nb, V, and Ti to be precipitated as a fine nitride during annealing and has an effect of suppressing grain growth by an austenite pinning effect. In order to obtain these effects, the N content is set to 0.030% or more. The N content is preferably 0.050% or more.

On the other hand, when the N content is excessively increased, a large number of coarse nitrides are formed in the manufacturing process of the steel sheet. The austenitic stainless steel sheet fractures from coarse nitrides as base points, the hot workability significantly deteriorates, and it is difficult to manufacture the steel sheet. Therefore, the N content is set to 0.150% or less. The N content is preferably 0.130% or less.

Any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less

Nb, V, and Ti are all elements that form a fine carbide, nitride, or composite carbonitride and suppress the grain growth of crystals by the austenite pinning effect. That is, Nb, V, and Ti are effective elements for the refinement of crystal grains. Therefore, the total amount of any one of Nb, V, and Ti or two or more thereof is 0.030% or more. The total amount of Nb, V, and Ti is preferably 0.050% or more.

On the other hand, when the total amount of these elements is excessively increased, these elements crystallize as a coarse inclusion during melting, which consequently creates an adverse influence of the significant deterioration of manufacturability or the excessive remaining of a non-recrystallized portion after annealing due to the suppression of recrystallization. In addition, containing a large amount of these elements directly leads to an increase in the cost of the material. Therefore, the total amount of these elements is set to 0.300% or less. The total amount of these elements is preferably 0.200% or less.

The austenitic stainless steel sheet according to the present embodiment may selectively contain one or more from the following element group in addition to the above-described basic composition. That is, the following elements are arbitrary elements that are contained as necessary, and the lower limit thereof is 0%.

Mo: 0% or more and 2.0% or less

Mo is an element that improves the corrosion resistance of the stainless steel sheet. In a case where this effect is obtained, the Mo content is desirably set to 0.1% or more. The Mo content is more desirably 0.5% or more.

On the other hand, when the Mo content is excessively increased, a brittle phase called Laves phase is likely to be precipitated. In addition, Mo is an expensive element and leads to an increase in the cost. Therefore, even in a case where Mo is contained, the Mo content is set to 2.0% or less. The Mo content is preferably 1.0% or less.

Cu: 0% or more and 1.5% or less

Similar to Ni, Cu is an austenite-forming element and is an element capable of adjusting the stability of austenite at a lower cost than Ni. Therefore, Cu may be contained. In a case where the above-described effect is obtained, the Cu content is desirably set to 0.1% or more.

On the other hand, when the Cu content is excessively increased, segregation occurs in the grain boundaries in the manufacturing process. This boundary segregation significantly degrades hot workability, which makes manufacturing difficult. Therefore, even in a case where Cu is contained, the Cu content is set to 1.5% or less. The Mo content is preferably 1.0% or less.

Co: 0% or more and 1.0% or less

Co is an austenite-forming element and is an element having an effect of suppressing δ ferrite, which is a brittle phase that is formed during a laser process. Therefore, Co may be contained. In a case where the above-described effect is obtained, the Co content is desirably set to 0.1% or more.

Incidentally, Co is an expensive element. Therefore, even in a case where Co is contained, the Co content is set to 1.0% or less.

The austenitic stainless steel sheet according to the present embodiment is basically made up of Fe and impurities (including unavoidable impurities) in addition to the above-described elements, but may contain, in addition to the individual elements described above, other elements (for example, Ca, Mg, Zr, Sn, Pb, and W) as long as the effect of the austenitic stainless steel sheet according to the present embodiment is not impaired. When the total amount of Ca, Mg, Zr, Sn, Pb, and W is 0.10% or less, the effect of the austenitic stainless steel sheet according to the present embodiment is not impaired.

In addition, in the manufacturing of stainless steel, a scrap raw material is often used. Therefore, stainless steel inevitably contains a variety of impurity elements. It is difficult to generally determine the amount of the impurity elements. Therefore, the impurities in the present embodiment mean elements that are contained in an amount in which the action and effect of the austenitic stainless steel sheet according to the present embodiment are not impaired. As such impurities, for example, P, S, O and the like are exemplary examples. The P content is preferably 0.10% or less and more preferably 0.05% or less. The S content is preferably 0.01% or less. The O content is 20 to 30 ppm when the austenitic stainless steel sheet is manufactured by normal steelmaking steps and rarely exceeds 50 ppm. When the O content is 50 ppm or less, the effect of the austenitic stainless steel sheet according to the present embodiment is not impaired.

Next, the microstructure will be described.

The austenitic stainless steel sheet according to the present embodiment has an average grain size of 5.0 μm or less and a non-recrystallization rate of more than 3% and 20% or less.

Average Grain Size

As the average grain size decreases, the roughness of a laser-processed surface decreases. This effect significantly appears particularly when the average grain size is 5.0 μm or less. In addition, when the average grain size is decreased, YS improves. Therefore, the average grain size is set to 5.0 μm or less. In order to further exhibit the above-described effect, the average grain size is desirably 3.0 μm or less.

Incidentally, in order to set the average grain size to less than 1.0 μm, the cost significantly improves. Therefore, the average grain size is preferably set to 1.0 μm or more.

The average grain size is calculated using only crystal grains that meet the definition of a recrystallized grain to be described below.

Specifically, the average grain size is obtained from an orientation map measured in the center portion of the sheet thickness by an electron backscatter diffraction (EBSD) method using a scanning electron microscope. More specifically, from the surface to the vicinity of the position of the center of the sheet thickness of a cross section perpendicular to the rolling direction of the steel sheet, a 20 μm×20 μm region is defined as one visual field, and measurement is performed on at least five visual fields or more with a measurement pitch set to 0.1 μm. A boundary where the orientation difference between adjacent measurement points is 10° or more is defined as a grain boundary, and a region that satisfies all conditions of (i) the region is surrounded by a grain boundary, (ii) the aspect ratio is 0.8 to 1.2, and (iii) the number of measurement points that are included in the grain boundary is 5 or more is regulated as a recrystallized grain. At this time, a twin grain boundary (Σ3 grain boundary) is not regarded as the grain boundary.

The average grain size is obtained from the diameter of a circle having the same area as the average grain area of the recrystallized grains calculated by the quadrature method from this orientation map.

The measurement software and analysis software for EBSD are not specified, but are, for example, TSL OIM Data Collection and OIM Analysis.

Non-Recrystallization Rate

In the austenitic stainless steel sheet according to the present embodiment, the non-recrystallization rate (proportion of a non-recrystallized portion) is set to more than 3% and equal to or less than 20% in order to increase YS.

When the average grain size is controlled to be finer, it is possible to increase YS, but it is industrially difficult to stably refine crystal grains less than 1.0 μm from the viewpoint of the cost. Therefore, in order to ensure sufficient YS (for example, 800 MPa or more) with the average grain size of 5.0 μm or less (preferably 1.0 μm or more), the non-recrystallization rate is set to more than 3%. The non-recrystallization rate is preferably 5% or more.

On the other hand, when the non-recrystallization rate exceeds 20%, coarse non-recrystallized grains elongated in the rolling direction become visible on the sample surface, and the smoothness of a processed portion deteriorates. Therefore, the non-recrystallization rate is set to 20% or less.

In the present embodiment, crystal grains that are not determined as the recrystallized grain by the above-described method are determined as non-recrystallized grains, and the area ratio of the non-recrystallized grains to the measured area is defined as the non-recrystallization rate.

Residual Amount of Martensite (Residual Amount of α′)

Martensite is a full hard structure. Therefore, when martensite is excessively present before polishing, the laser workability or polishing property deteriorates. Therefore, the residual amount of martensite is desirably set to 5% or less. The residual amount is obtained by measurement with a ferrite scope.

In a case where an austenitic stainless steel sheet has been manufactured under conditions where the above-described average grain size and non-recrystallization rate can be obtained within the range of the chemical composition of the austenitic stainless steel sheet according to the present embodiment described above, the average diameter of Nb, V, and Ti-based carbonitrides is 30 nm or less, and the number density of the Nb, V, and Ti-based carbonitrides per unit area becomes 10 to 50 carbonitrides/μm2.

The Nb, V, and Ti-based carbonitrides are effective for suppressing grain growth by the austenite pinning effect in grain boundaries and obtaining a fine grain structure. The average diameter of the Nb, V, and Ti-based carbonitrides of 30 nm or less is effective for the austenite pinning effect. However, even when the average diameter is 30 nm or less, if the Nb, V, and Ti-based carbonitrides are unevenly distributed and precipitated, the austenite pinning effect is not evenly developed, fine crystal grains and coarse crystal grains are present in a mixed form, and the smoothness of a processed surface becomes uneven. Therefore, the Nb, V, and Ti-based carbonitrides need to be finely and uniformly dispersed and precipitated.

In addition, when the average diameter of the Nb, V, and Ti-based carbonitrides is 30 nm or less, at the time of melting the austenitic stainless steel sheet by a laser process or the like, a region where the amounts of nitrogen and carbon are significantly high locally (high concentration region) is not generated, and a Cr carbonitride that degrades the corrosion resistance is less likely to be formed. However, if the number density of the Nb, V, and Ti-based carbonitrides is too high, when the region melts or receives heat due to the laser process, the precipitates of the Nb, V, and Ti-based carbonitrides melt, and the region where C is high (high concentration region) is likely to be formed. As a result, this C bonds to Cr during the subsequent cooling to precipitate a Cr carbide, and the corrosion resistance deteriorates.

Therefore, it is preferable that the average diameter of the Nb, V, and Ti-based carbonitrides is 30 nm or less and, regarding the precipitation state (number density), when one visual field is defined as one square micrometer, and the number density (carbonitrides/μm2) of the Nb, V, and Ti-based carbonitrides is measured at a total of 10 visual fields, both the minimum number density and the maximum number density of the measured number densities of the total 10 visual fields are within a range of 10 to 50 carbonitrides/μm2. This makes it possible to suppress the unevenness in the development of the austenite pinning effect and to ensure uniform smoothness in the sheet surface.

In the present embodiment, the Nb, V, and Ti-based carbonitrides are precipitated in a final annealing step to be described below. When annealing is performed under conditions where the above-described average grain size and non-recrystallization rate can be obtained, it is possible to precipitate the Nb, V, and Ti-based carbonitrides while suppressing the coarsening, and thus the average diameter of the Nb, V, and Ti-based carbonitrides becomes 30 nm or less, and the number density is within a range of 10 to 50 carbonitrides/μm2.

The average diameter of the Nb, V, and Ti-based carbonitrides is the average value of the diameters of 50 to 100 precipitates observed in each sample when a sample collected from the steel sheet by the replica method is observed with a TEM, the area of each observed precipitate is calculated, and the diameter of a circle having the same area as this area is defined as the diameter of the precipitate.

The “Nb, V, and Ti-based carbonitride” mentioned in the present embodiment includes a carbide or nitride containing Nb, V, and/or Ti and, furthermore, a composite carbonitride thereof. In addition, the Nb, V, and Ti-based carbonitrides may be observed with a transmission electron microscope (TEM). The configurational elements of the precipitate can be identified by an EDS analysis.

The sheet thickness of the austenitic stainless steel sheet according to the present embodiment is not particularly limited and may be appropriately designed in consideration of the size, shape, load, or the like of a product to be manufactured. From the viewpoint of ensuring the strength, the sheet thickness is 0.05 mm or more, preferably 0.08 mm or more, and more preferably 0.10 mm or more. This is because, in a case where the sheet thickness is thin, the strength becomes insufficient after processing. The sheet thickness may be 0.2 mm or less.

Next, a method for manufacturing the austenitic stainless steel sheet according to the present embodiment will be described.

The austenitic stainless steel sheet according to the present embodiment is capable of obtaining the effect as long as the austenitic stainless steel sheet has the above-described characteristics, and the manufacturing method is not limited. However, the austenitic stainless steel sheet can be obtained by a manufacturing method in which hot rolling, cold rolling, and individual heat treatments (annealing) are combined and pickling is appropriately performed as necessary. That is, as an example of the manufacturing method, for example, a manufacturing method including each step of steelmaking, hot rolling, hot-rolled sheet annealing, cold rolling (including intermediate annealing), and cold-rolled sheet annealing (final annealing) can be adopted. Operating conditions for the final annealing as a final finishing treatment and the cold rolling that is performed prior to the final annealing are preferably conditions to be described below in order to satisfy each of the above-described requirements for the steel sheet, and the other steps and conditions are not particularly limited.

In the austenitic stainless steel sheet according to the present embodiment, as described above, it is important to set the average grain size to 5.0 μm or less, desirably 3.0 μm or less, in order to obtain the smoothness of a processed portion. Crystal grain nuclei generates grain boundaries in the primary phase or defects such as dislocations as sites. Therefore, for the refinement of the average grain sizes, it is effective to form a large amount of deformation-induced martensite (α′) including a large number of dislocations by the cold rolling compared with austenite. The amount of α′ that is formed during the cold rolling increases as the rolling reduction (sheet thickness reduction rate) increases. Therefore, in the cold rolling before the final annealing, the rolling reduction is desirably set to 65% or more, more desirably 67% or more, and still more desirably 70% or more. The upper limit of the rolling reduction is not particularly limited and may be set to 95% or less from the viewpoint of the capacity of a rolling mill.

In the middle of the cold rolling, intermediate annealing is preferably performed once or more. That is, the cold rolling of the present embodiment is preferably two or more rounds of rolling including intermediate annealing therebetween. The intermediate annealing may be batch-type annealing or continuous annealing.

In a case where the intermediate annealing is performed, in the intermediate annealing, the steel sheet is heated to a temperature of 1050° C. or higher and cooled to 500° C. or lower at an average cooling rate of 10° C./sec or faster. According to the above-described intermediate annealing, no carbonitrides are formed in the middle.

When the temperature of the intermediate annealing is low, the Nb, V, and Ti-based carbonitrides do not form a solid solution again, recrystallization during the final annealing cannot be suppressed, and thus the non-recrystallization rate becomes low. In addition, when the average cooling rate is slow, the carbonitrides are coarsely precipitated during the cooling, recrystallization during the subsequent final annealing cannot be suppressed, and thus the non-recrystallization rate becomes low.

On the cold-rolled steel sheet, cold-rolled sheet annealing (final annealing) is performed.

In the heat treatment (final annealing) after the cold rolling, it is preferable that a temperature rising rate S1 from 500° C. and 800° C. is set to 40° C./sec or faster, a maximum attainment temperature T1 is set to 800° C. or higher and 950° C. or lower, a time t1 for the steel sheet to be exposed to 800° C. or higher is set to 15 seconds or shorter, and a cooling rate S2 from 800° C. to 500° C. is set to 30° C./sec or faster.

When α′ is reverse-transformed to austenite (γ) at the time of the final annealing, it is possible to suppress the recovery of dislocations and to form a γ from the α′ including a large number of dislocations by increasing the temperature rising rate. This means that, that is, α′ is reverse-transformed to a γ in a state where a large amount of nucleation sites remain, and a fine γ grain structure can be obtained. Additionally, the dislocations in α′ also function as nucleation sites of the Nb, V, and Ti-based carbonitrides. That is, it is indispensable to increase the temperature rising rate and precipitate these carbonitrides in a state where the recovery of dislocations is suppressed in order to decrease the average diameter of the Nb, V, and Ti-based carbonitrides. More specifically, since a number of dislocations remain at the time of the reverse transformation from α′ to γ or the precipitation of the Nb, V, and Ti-based carbonitrides, the temperature rising rate S1 from 500° C. to 800° C. where the dislocations move and recover actively is set to 40° C./sec or faster. The temperature rising rate S1 is preferably 45° C./sec or faster. The temperature rising rate S1 is desirable as fast as possible, the upper limit thereof is not particularly limited, may be set depending on the capacity or scale of an annealing facility or the like, and may be set to, for example, 100° C./sec or slower.

When the maximum attainment temperature T1 at the time of the final annealing is too high, γ grains finely generated at the time of the temperature rise or the Nb, V, and Ti-based carbonitrides become coarse. Therefore, the maximum attainment temperature T1 at the time of the final annealing is set to 950° C. or lower. The maximum attainment temperature T1 is preferably set to 920° C. or lower.

On the other hand, when the maximum attainment temperature T1 is too low, the reverse transformation to the γ becomes insufficient, and the α′ remains or non-recrystallized γ, which is not recrystallized, excessively remains. Since the α′ or non-recrystallized γ impairs the smoothness of a processed portion, the maximum attainment temperature T1 is set to 800° C. or higher. The maximum attainment temperature T1 is preferably set to 820° C. or higher.

In addition, when the time for the steel sheet to be exposed to high temperatures is long or the cooling rate after the temperature rise is slow at the time of the final annealing, the fine γ crystal grains grow and become coarse. Therefore, the time t1 for the steel sheet to be exposed to 800° C. or higher is set to 15 seconds or shorter, and the cooling rate S2 from 800° C. to 500° C. is set to 30° C./sec or faster. It is preferable that the time t1 for the steel sheet to be exposed to 800° C. or higher is set to 10 seconds or shorter and the cooling rate S2 is set to 35° C./sec or faster.

“Time t1 for the steel sheet to be exposed to 800° C. or higher” is measured from a point in time where the steel sheet has reached 800° C., and the point in time may be in the middle of the temperature rise stage. In addition, even when the cooling has begun, if the temperature of the steel sheet is 800° C. or higher, the time for the steel sheet to be exposed to such temperatures is also included in the measurement.

In the present embodiment, the temperature rising rate S1 is an average rate obtained by dividing the amount of temperatures raised (300° C.) from 500° C. to 800° C. by a temperature rise time required for the temperature rise. Similarly, the cooling rate S2 is also an average rate obtained by dividing the amount of temperatures lowered (300° C.) from 800° C. to 500° C. by the cooling time required for cooling.

After the maximum attainment temperature T1 is reached, the steel sheet may be held for a certain period of time or may begin to be cooled as it is. That is, as long as the maximum attainment temperature T1 and the time t1 for the steel sheet to be exposed to 800° C. or higher are within the above-described ranges, the temperature of the steel sheet may undergo any temperature history.

After a fine γ-grain structure and a uniform precipitate distribution are obtained by the final annealing, temper rolling, correction with a tension leveler, and stress relieving annealing may be performed to adjust performance such as hardness. In the temper rolling, the steel sheet is strongly rolled at a rolling reduction of rolling reduction 30% to 50% (cumulative rolling reduction in a case where a plurality of passes of rolling is performed) in order to obtain predetermined hardness, and deformation-induced martensite is formed. Since this martensite is formed using the original γ grain or a region smaller than the original γ grain as a unit, an α′ that is formed from the fine γ grains is finely dispersed. After that, in the correction with a tension leveler, the steel sheet is bent and bent back along a leveler roll while being supplied with tension, whereby the flatness can be improved.

In the stress relieving annealing, not only is residual stress inside the material removed, but deformation-induced martensite formed by the temper rolling or the correction with a tension leveler is also reverse-transformed to fine austenite, whereby, similar to before the temper rolling, the laser-processed surface becomes smooth. The stress relieving annealing (stress relief annealing) may be performed at 700° C. to 800° C. where stress relieving annealing is ordinarily performed.

The austenitic stainless steel sheet according to the present embodiment can be manufactured with the above-described steps.

As described above, the austenitic stainless steel sheet according to the present embodiment has uniform fine crystal grains and a predetermined non-recrystallized portion in the microstructure, it is possible to improve the smoothness of a processed portion and the corrosion resistance of a molten portion and a heat-affected zone while increasing YS. Therefore, the austenitic stainless steel sheet can be preferably adopted as a base steel sheet for photoetching processes or laser processes. In addition, in the austenitic stainless steel sheet according to the present embodiment, since the smoothness of a processed portion can be sufficiently ensured, the austenitic stainless steel sheet can be suitably used as a material for metal masks that are manufactured using a photoetching process or a laser process (including laser cutting or laser welding).

EXAMPLES

Next, examples of the present invention will be shown, but conditions in the examples are one example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention is capable of adopting a variety of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

The chemical compositions of steels that were materials under test are shown in Table 1. Among individual components, for any component that was contained in an amount outside the scope of the present invention, the numerical value for the amount of the component is underlined. In Table 1, A to H are chemical compositions that satisfy the regulations of the present invention, and I to R are chemical compositions of comparative steels that do not satisfy the regulations. In Table 1, the unit of each content is “mass %”, and the remainder is Fe and impurities.

Small ingots (25 kg) having the chemical compositions A to R in Table 1 were melted, and a cutting process, hot rolling, hot-rolled sheet annealing, and descaling were performed. After that, intermediate annealing and cold rolling were repeated one to three times under the conditions summarized in Tables 2 to 3, and then final annealing was performed. Furthermore, on Invention Examples 33 to 36, temper rolling, correction with a tension leveler, and stress relief annealing were performed as shown in Table 3.

Test pieces were collected from the obtained steel sheets having a thickness of 0.2 mm, and a variety of characteristics were investigated in the following manner.

Average grain size: In the above-described manner, a grain boundary was defined as a boundary where the angle of adjacent grains is 10° or more, and the average grain size was obtained from the diameter of a circle having the same area as the average grain area calculated by the quadrature method from an EBSD orientation map in the center portion of the sheet thickness.

Residual amount of martensite (residual amount of α′): The amount of martensite was measured with a ferrite scope using the steel sheet after the final annealing. The amount of martensite was measured five times in each steel sheet, and the average value thereof was regarded as the residual amount of martensite.

Non-recrystallization rate: The non-recrystallization rate was measured in the above-described manner.

Roughness of laser-processed portion: Steel sheets in Table 2 and Table 3 were cut with a laser, and the line roughness (arithmetic average roughness: Ra) of the cut surfaces was measured with a contact-type roughness meter. The line roughness was measured in a 90-degree direction with respect to the sheet thickness direction. Based on the current market needs, steel sheets with an arithmetic average roughness Ra of 1.00 μm or less were regarded as pass, and steel sheets with an arithmetic average roughness Ra of more than 1.00 μm were regarded as fail.

YS: YS was obtained by performing a tensile test under conditions where a tensile load was applied such that the amount of the distance between gauge points changed became 3 mm/min using the No. 13B tensile test that is regulated in JIS Z 2241:2011 that was collected such that the longitudinal direction of the test piece became parallel to the rolling direction.

In a case where YS was 800 MPa or more, steel sheets were determined to have a sufficient strength.

Corrosion resistance: Samples obtained by melting a steel sheet in Table 2 and Table 3 with a laser were immersed in 6% FeCl3+1% HCl at a liquid temperature of 5° C. for 24 hours, and steel sheets in which pitting corrosion did not occur were regarded as having favorable corrosion resistance, and steel sheets in which pitting corrosion occurred were regarded as having poor corrosion resistance.

Steel sheet Nos. 1 to 11 and 33 to 36 in Table 2 satisfied the regulations of the present invention, the roughness of the laser-processed portion was small, YS was high, and the corrosion resistance had no problems. In these examples, the average diameters of Nb, V, and Ti-based carbonitrides were 30 nm or less, and the number densities of the Nb, V, and Ti-based carbonitrides per unit area were 10 to 50 carbonitrides/μm2.

Steel sheets 12 to 32 were steel sheet of the comparative examples and had poor roughness in the laser-processed portion, had a low strength (YS), had corrosion resistance degraded due to the precipitation of a Cr carbide, or did not satisfy one or more target characteristics.

Steel sheets 12 to 16 and 21 to 26 were examples in which the chemical compositions satisfied the regulation of the present invention, but the manufacturing methods were not within the preferable range, and the average grain sizes or the non-recrystallization rates were not within the scope of the present invention.

Steel sheets 17 to 20 and 27 to 32 had a chemical composition outside the scope of the present invention and had an average grain size or non-recrystallization rate outside the scope of the present invention.

TABLE 1 C Si Mn P S Al Cr Ni Cu Co Mo N Nb Ti V Others Nb + Ti + V A 0.018 0.5 0.8 0.01 0.004 0.02 18.2  8.1 0.2 0.1 0.3 0.045 0.080 0.010 0.010 — 0.100 B 0.020 0.4 1.2 0.02 0.005 0.01 17.2  6.7 0.3 — — 0.121 0.050 0.010 0.010 — 0.070 C 0.012 0.5 1.1 0.01 0.002 0.02 17.2  7.1 0.3 0.1 0.3 0.061 0.015 0.010 0.020 Mg: 0.005 0.045 Zr: 0.003 D 0.020 0.5 0.8 0.03 0.003 0.05 18.1  8.2 0.3 0.1 0.3 0.043 0.080 — — — 0.080 E 0.020 0.4 1.2 0.01 0.001 0.02 17.2  6.7 0.3 0.1 0.3 0.122 — — 0.290 Ca: 0.0003 0.290 Pb: 0.0008 F 0.011 0.5 0.9 0.02 0.002 0.01 17.2  7.2 0.3 0.1 0.5 0.060 — 0.060 — — 0.060 G 0.021 0.5 0.7 0.02 0.002 0.03 18.1  7.1 1.5 0.5 0.2 0.034 0.060 0.050 0.150 Sn: 0.008 0.260 H 0.015 0.4 0.8 0.01 0.003 0.02 17.6  8.2 — 0.2 0.3 0.050 0.040 0.050 0.120 W: 0.01 0.210 I 0.010 0.4 1.2 0.02 0.006 0.02 17.8  6.7 0.3 0.1 0.3 0.010 0.002 0.002 — — 0.004 J 0.033 0.5 1.6 0.05 0.004 0.01 18.2 10.2 0.2 0.1 0.3 0.129 0.050 0.012 — — 0.062 K 0.081 0.5 0.4 0.01 0.002 0.03 18.5  8.2 0.3 0.2 0.1 0.030 0.020 0.002 — Sn: 0.005 0.022 L 0.049 0.5 0.4 0.02 0.002 0.02 18.6  8.5 0.5 0.1 0.2 0.030 0.050 0.350 — — 0.400 M 0.020 0.4 2.1 0.01 0.003 0.02 17.6  7.6 0.2 — 0.3 0.045 0.020 0.080 0.020 Mg: 0.008 0.120 N 0.015 0.5 0.8 0.02 0.002 0.05 18.6  6.7 0.3 0.1 0.2 0.060 0.080 0.200 0.150 — 0.430 0 0.018 0.6 1.2 0.03 0.004 0.03 17.2 12.5 0.3 0.2 0.3 0.035 0.015 0.070 0.000 — 0.085 P 0.016 0.5 0.7 0.02 0.002 0.04 18.9  6.8 0.2 0.1 — 0.006 0.050 0.080 0.080 — 0.210 Q 0.012 0.4 0.9 0.02 0.001 0.03 16.8  7.2 0.3 0.2 0.2 0.075 0.020 0.420 0.050 W: 0.008 0.490 R 0.091 0.4 0.8 0.01 0.003 0.02 17.6  8.5 — 0.1 0.3 0.045 0.000 0.050 0.000 — 6.050

TABLE 2 Cooling Final Time Intermediate rate after Final cold annealing Maximum t1 at Chemical annealing intermediate rolling temperature attainment 800° C. composition temperature annealing reduction rise rate S1 temperature or higher No. (Table 1) (° C.) (° C./s) (%) (° C./s) T1 (° C.) (s) Present  1 A 1060 30 67 45  870 10 Invention  2 A 1080 40 67 40  920 12 Example  4 B 1070 38 70 45  870 10  5 C 1060 40 67 40  890 10  6 D 1070 25 80 40  890 10  7 E 1080 30 67 40  920 12  8 F 1100 35 70 45  870 10  9 G 1090 38 67 40  890 10 10 H 1120 40 67 45  870 12 11 H 1100 38 67 45  870 12 Comparative 12 A 1050 21 67 45  970 20 Example 13 A 1060 26 67 30  950 10 14 A 1070 31 67 40  790  0 15 B 1050 18 67 40  900 20 16 D 1100 40 55 40  920 10 17 I 1090 42 67 40  920 10 18 J 1080 50 67 40  900 10 19 K 1120 35 67 40  940 10 20 L 1100 28 67 40  890 10 21 E  950 20 85 40  850 10 22 E 1080 35 85 40  780  0 23 C 1050 40 67 40 1000 14 24 C 1100 35 67 45  940 20 25 F 1060 30 70 40  900 10 26 F 1080  7 70 40  920 12 27 M 1100 29 67 45  900 10 28 N 1050 35 72 40  870 12 29 O 1080 35 70 50  880 10 30 P 1100 40 76 50  900 10 31 Q 1080 48 68 50  920  8 32 R 1060 38 70 50  870 10 Non- Amount Roughness Cooling recrystallization Average of of laser- rate S2 rate grain size α' left processed YS Corrosion No. (° C./s) (%) (μm) (%) portion (μm) (MPa) resistance Present  1 35 10 1.6 0.0 0.91  865 Favorable Invention  2 42  9 3.5 0.0 0.94  875 Favorable Example  4 35 13 1.2 0.0 0.75  905 Favorable  5 40 13 4.0 0.0 0.95  896 Favorable  6 40  8 1.2 0.2 0.88  861 Favorable  7 42  7 3.9 0.0 0.96  817 Favorable  8 35 16 3.5 0.0 0.85  971 Favorable  9 40 15 1.5 0.0 0.98  933 Favorable 10 35 18 1.5 0.0 0.90  972 Favorable 11 85 15 1.2 0.0 0.85  924 Favorable Comparative 12 45  0 7.2 0.0 1.35  753 Poor Example 13 45  2 4.9 0.0 0.96  772 Poor 14 40 80 3.5 5.1 1.70 1341 Favorable 15 20  0 5.5 0.0 1.10  752 Poor 16 40  4 7.0 0.0 1.22  813 Poor 17 40  0 6.2 0.0 1.20  745 Favorable 18 40  0 8.5 0.0 1.31  762 Favorable 19 40  6 5.5 0.0 1.15  812 Poor 20 40  2 1.5 0.0 0.90  789 Poor 21 40  1 5.0 3.0 0.98  782 Poor 22 40 90 2.4 6.2 1.55 1253 Poor 23 45  0 5.7 0.0 1.21  753 Poor 24 40  0 5.3 0.0 1.15  762 Poor 25 15  2 4.8 0.0 0.98  790 Poor 26 40  0 5.5 0.0 1.17  762 Favorable 27 35  5 5.8 1.2 1.16  812 Poor 28 45 90 3.8 3.2 1.85 1351 Poor 29 35 15 6.8 0.0 1.27  826 Favorable 30 40  0 5.8 0.0 1.17  789 Favorable 31 40 60 5.1 0.0 1.42  856 Favorable 32 42  5 4.7 0.6 0.98  802 Poor

TABLE 3 Cooling Final Final Maximum Intermediate rate after cold annealing attainment Time t1 Chemical annealing intermediate rolling temperature temperature at 800° C. Cooling Temper composition temperature annealing reduction rise rate S1 T1 or higher rate S2 rolling No. (Table 1) (° C.) (° C./s) (%) (° C./s) (° C.) (s) (° C./s) (%) Present 33 A 1060 38 67 45 870 10 40 45 Invention 34 B 1070 40 67 45 870 10 40 50 Example 35 C 1060 45 67 40 890 10 40 45 36 D 1070 38 67 40 890 10 40 50 Correction Stress relief Roughness with annealing Non- Average Amount of laser- tension temperature recrystallization grain size of α' left processed YS Corrosion No. leveler (° C.) rate (%) (μm) (%) portion (MPa) resistance Present 33 Yes 750  5 1.5 0.2 0.89 920 Favorable Invention 34 Yes 800 13 1.2 0.0 0.74 913 Favorable Example 35 Yes 750 15 2.5 0.5 0.92 894 Favorable 36 Yes 780 13 1.2 0.0 0.90 900 Favorable

INDUSTRIAL APPLICABILITY

It is possible to provide an austenitic stainless steel sheet having a high strength (YS) and capable of improving the smoothness (roughness) of a laser-processed portion and the corrosion resistance of a molten portion and a heat-affected zone. The austenitic stainless steel sheet of the present invention is suitable for being used as a material for metal masks that are manufactured using a photoetching process or a laser process (including laser cutting or laser welding) and is of highly industrial applicability.

Claims

1. An austenitic stainless steel sheet comprising,

as a chemical composition, by mass %:
C: 0.030% or less;
Si: 1.0% or less;
Mn: 1.5% or less;
Cr: 15.0% or more and 20.0% or less;
Ni: 6.5% or more and 9.0% or less;
N: 0.030% or more and 0.150% or less;
any one of Nb, V, and Ti or two or more thereof in total: 0.030% or more and 0.300% or less;
Mo: 0% or more and 2.0% or less;
Cu: 0% or more and 1.5% or less;
Co: 0% or more and 1.0% or less;
Ca, Mg, Zr, Sn, Pb, and W in total: 0% or more and 0.10% or less;
P: 0.10% or less;
S: 0.010% or less; and
Al: 0.10% or less,
wherein a remainder includes Fe and impurities,
an average grain size is 5.0 μm or less, and
a non-recrystallization rate is more than 3% and 20% or less.

2. The austenitic stainless steel sheet according to claim 1, comprising, in the chemical composition, by mass %, one or more of:

Cu: 0.1% to 1.5%;
Mo: 0.1% to 2.0%; and
Co: 0.1% to 1.0%.
Patent History
Publication number: 20220372602
Type: Application
Filed: Oct 17, 2019
Publication Date: Nov 24, 2022
Applicant: NIPPON STEEL CORPORATION (Tokyo)
Inventors: Masayoshi SAWADA (Tokyo), Hiroshi KAMIO (Tokyo), Masayuki SHIBUYA (Tokyo), Atsushi KIKUCHI (Tokyo)
Application Number: 17/641,764
Classifications
International Classification: C22C 38/58 (20060101); C22C 38/52 (20060101); C22C 38/50 (20060101); C22C 38/48 (20060101); C22C 38/46 (20060101); C22C 38/44 (20060101); C22C 38/42 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101); C22C 38/00 (20060101);