JP7165202B2 - Austenitic stainless steel sheet and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to an austenitic stainless steel sheet and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2018-189321 filed in Japan on October 4, 2018, the contents of which are incorporated herein.

2. Description of the Related Art A member having a high surface glossiness is used for a housing or the like of an electronic device which is a precision machined part, and a member made of, for example, a stainless steel plate is often used. In recent years, in order to stably obtain members with high surface gloss, members are required to have better polishability than ever before.

In view of this situation, Patent Documents 1 to 4, for example, discuss improving the polishability of stainless steel plates.
Patent Literature 1 discloses a method for manufacturing a mirror-finished stainless steel sheet for curved mirrors, which is finished with lapping and has excellent surface gloss and image clarity.
Further, Patent Document 2 discloses an austenitic stainless steel for press molding with improved polishability for mirror finish.

Patent Literature 3 discloses a method for producing a stainless steel strip and a steel plate with excellent polishability.
Patent Document 4 discloses a method for producing a steel strip with few surface microdefects in the production of austenitic stainless steel, martensitic stainless steel, or ferrite + austenitic duplex stainless steel strip.

However, as a result of studies by the present inventors, it has been found that there are cases where sufficient polishability cannot be obtained with the above-described prior art, and that there is still room for further improvement.

In addition, the precision machined parts as described above are often manufactured by laminating stainless steel plates and performing diffusion bonding at a high temperature. For example, a manufacturing method is employed in which fine holes or patterns are formed on the surface by photoetching or precision processing using a laser, and then the same steel plates are laminated and diffusion-bonded. Demand for such precision machined parts and products is on the rise, and further expansion of application of diffusion bonding is expected in the future.

Steel sheets used for these applications are required to have good bondability.
For example, in Patent Documents 5 to 9, improvement of diffusion bondability is studied.
Patent Literature 5 proposes a method for manufacturing a diffusion-bonded product that utilizes the growth of crystal grains that accompanies phase transformation during diffusion bonding, and can be performed without applying special high-temperature heating or high surface pressure. .
Patent Document 6 describes a stainless steel with excellent joint reliability, which has a diffusion-bonded structure in which there are many places where crystal grains on the steel material side grow so as to penetrate into the mating side beyond the pre-bonding interface. Diffusion bonded products are disclosed.
Patent Literature 7 discloses a steel sheet whose diffusion bondability is improved by controlling the austenite phase fraction during diffusion bonding.
Patent Document 8 describes a stainless steel with an average crystal grain size in the foil thickness direction of 0.001 to 5 μm and an Al content of 0.5 to 8% as a stainless steel with excellent diffusion bondability. A steel foil is disclosed.
Patent Document 9 states that the etching surface becomes smoother by making the grains finer, thereby improving the diffusion bondability.

However, as a result of investigation by the present inventors, it was found that there are cases where sufficient diffusion bonding properties cannot be obtained with the above-described conventional technology, and that there is still room for further improvement.

Japanese Patent Laid-Open No. 3-169405 Japanese Patent Laid-Open No. 9-3605 Japanese Patent Application Laid-Open No. 62-253732 Japanese Patent Application Laid-Open No. 2000-273546 Japanese Patent Application Laid-Open No. 2013-103271 Japanese Patent Application Laid-Open No. 2013-173181 Japanese Patent Application Laid-Open No. 2016-89223 Japanese Patent Application Laid-Open No. 9-279310 WO2016/043125

The present invention has been made to solve the above problems, and an object of the present invention is to provide an austenitic stainless steel sheet having good polishability. In the present invention, "having good polishability" means that it can be easily smoothed by mechanical polishing. The austenitic stainless steel sheet preferably has good polishability and also good diffusion bondability.

(1) The austenitic stainless steel sheet according to one aspect of the present invention has a chemical composition in mass% of C: 0.005 to 0.150%, Si: 1.0% or less, Mn: 1.5% or less. , P: 0.10% or less, S: 0.010% or less, Al: 0.10% or less, Cr: 15.0 to 20.0%, Ni: 6.0 to 15.0%, N: 0 .005-0.150%, Mo: 0-2.0%, Cu: 0-1.5%, Nb: 0-0.500%, V: 0-0.150%, Ti: 0-0. 300%, B: 0 to 0.010%, total of Ca, Mg, Zr, Sn, Pb, W: 0 to 0.10%, balance: Fe and impurities, Md30 obtained by the following formula (i) is 60° C. or less, the surface layer portion has an area ratio of martensite of 5.0% or less, and an area ratio of austenite grains having a {110} plane orientation of 50% or more.
Md30 value = 497 - 462 x (C + N) - 9.2 x Si - 8.1 x Mn - 13.7 x Cr - 20 x (Ni + Cu) - 18.7 x Mo (i)
However, the element symbol in the above formula represents the content (% by mass) of each element in the steel, and 0 shall be substituted when it is not contained.
(2) In the austenitic stainless steel sheet according to (1) above, the chemical composition contains Nb: 0.010 to 0.500%, the Md30 value is 20 to 60 ° C., and the surface layer portion In, the average grain size of the austenite grains may be 5.0 μm or less, and the X-ray random intensity ratio of the {110}<112> orientation of the austenite grains may be 8.5 or more.
(3) The austenitic stainless steel sheet according to (1) or (2) above has the chemical composition, in mass %, of Mo: 0.1 to 2.0% and Cu: 0.1 to 1.5%. , Nb: 0.010 to 0.500%, V: 0.010 to 0.150%, Ti: 0.010 to 0.300%, and B: 0.001 to 0.010%. may contain one or more.
(4) A method for producing an austenitic stainless steel sheet according to another aspect of the present invention, wherein the austenitic stainless steel sheet according to any one of the above (1) to (3) is subjected to a rolling reduction of 50% or less. A step of performing temper rolling is provided.

According to the above aspect of the present invention, an austenitic stainless steel sheet having good polishability can be stably obtained industrially.
Further, according to a preferred embodiment of the present invention, it is possible to obtain an austenitic stainless steel sheet having good diffusion bondability in addition to good polishability.

It is a figure which shows ODF of a (phi)2=45 degree cross section.

Hereinafter, each requirement of the austenitic stainless steel sheet according to one embodiment of the present invention (the austenitic stainless steel sheet according to this embodiment) will be described in detail.

1. Chemical composition The reasons for limiting each element are as follows. "%" for content in the following description means "% by mass". In addition, a numerical range shown between "-" includes numerical values at both ends thereof. On the other hand, values indicated by "less than" and "more than" are not included in the range.

C: 0.005-0.150%
C is a powerful solid-solution strengthening element that increases the strength of steel sheets at low cost. However, if the C content is excessive, coarse carbides are formed, and random crystal rotation occurs around the carbides during rolling deformation during hot rolling or cold rolling, resulting in randomized crystal orientation. Therefore, the C content is made 0.150% or less. The C content is preferably 0.130% or less, more preferably 0.120% or less.
On the other hand, if the C content is less than 0.005%, it will only increase the manufacturing cost, and no particular effective effect will be obtained. Therefore, the C content should be 0.005% or more. In addition, C binds to Nb and precipitates as fine Nb compounds, and has the effect of suppressing recrystallization and grain growth. To obtain this effect, the C content is preferably 0.010% or more.

Si: 1.0% or less If the Si content is excessive, there is a high possibility that coarse oxides will be formed, and there is a concern that workability will deteriorate. Therefore, the Si content should be 1.0% or less. The Si content is preferably 0.6% or less.
On the other hand, Si is an element that is used as a deoxidizer during smelting and also contributes to the strengthening of steel. To obtain these effects, the Si content is preferably 0.1% or more.

Mn: 1.5% or less Mn is a strong austenite forming element. Therefore, if the Mn content is excessive, the amount of strain-induced martensite formed during cold rolling is reduced, thereby reducing the accumulation in the {110} plane orientation after final annealing. Also, fine crystal grains cannot be obtained. Therefore, the Mn content should be 1.5% or less. The Mn content is preferably 1.2% or less.
On the other hand, Mn is an element that contributes to prevention of brittle fracture during hot working and strengthening of steel. To obtain the above effects, the Mn content is preferably 0.1% or more.

P: 0.10% or less P is an impurity element. If the P content exceeds 0.10%, workability is significantly deteriorated. Therefore, the P content is limited to 0.10% or less. Since the P content is preferably as small as possible, it may be 0%. However, setting the P content to less than 0.005% is not preferable in terms of cost. Therefore, the lower limit of the P content may be 0.005%.

S: 0.010% or less S is an impurity element. If the S content exceeds 0.010%, it causes melt embrittlement during hot working. Therefore, the S content is limited to 0.010% or less. Since the S content is preferably as small as possible, it may be 0%. However, setting the S content to less than 0.001% is not preferable in terms of cost. Therefore, the lower limit of the S content may be 0.001%.

Al: 0.10% or less Al is an impurity element. If the Al content exceeds 0.10%, the workability is deteriorated, and oxides are generated during bonding, resulting in deterioration of the diffusion bondability. Therefore, the Al content is limited to 0.10% or less. Since the Al content is preferably as small as possible, it may be 0%. However, setting the Al content to less than 0.01% is not preferable in terms of cost. Therefore, the lower limit of the Al content may be 0.01%.

Cr: 15.0-20.0%
Cr is a basic element of stainless steel, and is an element that forms an oxide layer on the surface of the steel material to enhance corrosion resistance. In order to obtain this effect, the Cr content is made 15.0% or more. The Cr content is preferably 16.0% or more.
Cr, on the other hand, is a strong ferrite stabilizing element. Therefore, when the Cr content is excessive, delta ferrite is formed. This delta ferrite deteriorates the hot workability of the material. Therefore, the Cr content should be 20.0% or less. The Cr content is preferably 19.0% or less.

Ni: 6.0-15.0%
Ni is an austenite-generating element and an element that has the effect of stabilizing the austenite phase at room temperature. To obtain this effect, the Ni content is set to 6.0% or more. The Ni content is preferably 6.5% or more.
On the other hand, if the Ni content is excessive, the austenite phase is too stabilized, and strain-induced martensite transformation during cold rolling does not occur, thereby reducing the accumulation in the {110} plane orientation. Furthermore, Ni is an expensive element, and an excessive increase in the Ni content results in a significant increase in cost. Therefore, the Ni content should be 15.0% or less. The Ni content is preferably 11.0% or less, more preferably 9.0% or less.

N: 0.005-0.150%
N, like C, is a solid-solution strengthening element and an element that contributes to improving the strength of steel. Moreover, setting the N content to less than 0.005% is not preferable in terms of cost. Therefore, the N content is made 0.005% or more.
Also, N binds to Nb and precipitates as fine Nb compounds during hot rolling or annealing, and has the effect of suppressing recrystallization and grain growth. To obtain this effect, the N content is preferably 0.010% or more.
On the other hand, if the N content is excessive, a large number of coarse nitrides are generated during the steel sheet manufacturing process. These coarse nitrides act as starting points for fracture and markedly deteriorate the hot workability. In addition, N, like C, is also a strong austenite stabilizing element, and if the N content is excessive, the deformation-induced transformation necessary for grain refinement will not occur. Therefore, the N content should be 0.150% or less. The N content is preferably 0.130% or less, more preferably 0.120% or less.

The austenitic stainless steel sheet according to the present embodiment basically has a chemical composition containing the above elements with the balance being Fe and impurities. However, in order to improve various properties, one or more selected from Mo, Cu, Nb, V, Ti, and B may be contained within the range described later. However, since these elements do not necessarily have to be contained, the lower limit is 0%.
Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially, and has an adverse effect on the austenitic stainless steel sheet according to the present embodiment. It means what is allowed as long as it does not give

Mo: 0-2.0%
Mo is an element that improves the corrosion resistance of the material. Therefore, it may be contained as necessary. To obtain the above effects, the Mo content is preferably 0.1% or more.
On the other hand, Mo is an extremely expensive element, and an excessive increase in the content causes a significant increase in cost. Therefore, even if Mo is contained, the Mo content should be 2.0% or less. Mo content is preferably 1.0% or less.

Cu: 0-1.5%
Cu is an austenite-generating element and is an element effective in adjusting the stability of the austenite phase. Therefore, it may be contained as necessary. In order to obtain the above effects, the Cu content is preferably 0.1% or more.
On the other hand, when the Cu content is excessive, Cu segregates at grain boundaries during the manufacturing process. Such grain boundary segregation causes a significant deterioration in hot workability, so if Cu segregates at grain boundaries, production becomes difficult. Therefore, even if Cu is contained, the content of Cu should be 1.5% or less. The Cu content is preferably 1.0% or less.

Nb: 0-0.500%
Nb is an element that forms fine carbides or nitrides during annealing. These fine carbides or nitrides suppress the growth of crystal grains due to the pinning effect, so Nb is an element effective in refining the crystal grains of the material. Further, Nb is an element that develops the working texture of austenite by forming a solid solution or suppressing recrystallization during hot working as a carbonitride. Therefore, it may be contained.
When the average grain size of the austenite grains is 5.0 μm or less and the X-ray random intensity ratio of the {110} <112> orientation of the austenite grains is 8.5 or more, the Nb content can be 0.010% or more. preferable. The Nb content is more preferably 0.030% or more, more preferably 0.040% or more.
On the other hand, if the Nb content is excessive, recrystallization is suppressed, and a large amount of non-recrystallized portion remains after annealing, and hot workability deteriorates. Moreover, Nb is an extremely expensive element, and an excessive increase in the content causes a significant increase in cost.
Therefore, even if Nb is contained, the Nb content should be 0.500% or less. The Nb content is preferably 0.300% or less, more preferably 0.200% or less.

V: 0-0.150%
Ti: 0-0.300%
Both V and Ti are elements that have the effect of suppressing recrystallization, strengthening a desirable texture, and refining crystal grains. Therefore, one or more selected from these may be contained as necessary. In order to obtain the above effect, it is preferable to contain one or more selected from V: 0.010% or more and Ti: 0.010% or more.
On the other hand, if the above elements are contained excessively, workability deteriorates. Therefore, even if they are contained, the V content is 0.150% or less, and the Ti content is 0.300% or less.

B: 0-0.010%
B is an element that strengthens grain boundaries and is an element that contributes to the improvement of hot workability. Therefore, it may be contained as necessary. In order to obtain the above effects, the B content is preferably 0.001% or more.
On the other hand, an excessive B content deteriorates workability. Therefore, even when B is contained, the B content is set to 0.010% or less.

As described above, the chemical composition of the austenitic stainless steel sheet according to the present embodiment contains essential elements, the balance being Fe and impurities, or the essential elements, one or more optional elements, and the balance being Fe. and impurities. Examples of "impurities" include Ca, Mg, Zr, Sn, Pb, W, etc., in addition to P, S, and Al described above. The total amount of impurity elements such as Ca, Mg, Zr, Sn, Pb and W excluding P, S and Al is preferably 0.10% or less.

Md30 value: 60° C. or less The Md30 value is an index indicating the stability of austenite in the austenitic stainless steel sheet or the like according to the present embodiment, and is calculated from the chemical composition when rolling is performed with a rolling reduction of 30%. This value is considered to correspond to the temperature at which 50% by volume of deformation-induced martensite is formed at . If the Md30 value exceeds 60°C, the austenite reversely transformed during the heat treatment may become martensite again during the cooling process or temper rolling. In this case, the amount of austenite is reduced, and as a result, the area ratio of grains having {110} plane orientation is also reduced.
Therefore, the Md30 value obtained by the following formula (i) should be 60° C. or less. The Md30 value is preferably 55°C or less, more preferably 50°C or less.
Md30 value (° C.)=497−462×(C+N)−9.2×Si−8.1×Mn−13.7×Cr−20×(Ni+Cu)−18.7×Mo (i)
However, the element symbol in the above formula represents the content (% by mass) of each element in the steel, and 0 shall be substituted when it is not contained.

If the Md30 value is 20 ° C. or more, by utilizing the transformation from austenite to deformation-induced martensite (martensite) during cold rolling and the reverse transformation from deformation-induced martensite to austenite in the subsequent heat treatment, Fine crystal grains are obtained. It is also advantageous for the development of the {100} plane orientation, particularly the {110}<112> orientation.
Therefore, when the average grain size of the austenite grains is 5.0 μm or less and the X-ray random intensity ratio of the {110} <112> orientation of the austenite grains is 8.5 or more, the Md30 value is 20 to 60 ° C. It is preferable to The Md30 value is more preferably 25°C or higher, more preferably 30°C or higher.

2. Metallographic structure In order to obtain good polishability, it is important to control the metallographic structure in the surface layer of the steel sheet. Specifically, it is necessary to adjust the area ratio of martensite and austenite grains having a {110} plane orientation in the surface layer portion of the steel sheet to the range shown below. Each rule will be explained in detail. In the present embodiment, the surface layer portion of the steel plate means a region from the surface to a position of 1/10 of the plate thickness in the plate thickness direction.

Area ratio of martensite in surface layer: 5.0% or less Martensite is a hard structure. Therefore, if martensite is excessively present in the surface layer of the steel sheet in the manufacturing stage before polishing, the polishability deteriorates. Further, when the area ratio of martensite increases, the area ratio of grains having the {110} plane orientation of austenite relatively decreases. Therefore, the area ratio of martensite in the surface layer of the steel sheet is set to 5.0% or less. The area ratio is preferably 4.0% or less, more preferably 3.0% or less.
In addition, if there is a lot of martensite in the surface layer of the steel sheet, the martensite transforms to the austenite phase when heat is applied for diffusion bonding or laser processing, and the flatness of the steel sheet decreases, resulting in poor diffusion bondability. do. In addition, when the surface layer portion of the steel sheet contains a large amount of martensite, the area ratio of the austenite phase decreases, so the proportion of grains having {110}<112> orientation in the entire structure also decreases. Therefore, also from the viewpoint of diffusion bondability, the area ratio of martensite in the surface layer portion is preferably 5.0% or less.
In the austenitic stainless steel sheet according to this embodiment, the structure other than martensite is substantially austenite.

The area ratio of martensite in the surface layer portion is obtained by the following procedure.
First, an fcc structure and a bcc structure are selected as crystal structures for a surface parallel to the surface of a steel plate having an area of 100 μm×100 μm or more, which is electropolished or chemically polished, and measured by EBSD. Then, a region that is not determined as an fcc structure, that is, has a bcc crystal structure, or a region that cannot be measured due to high strain (however, neither includes a linear region such as a grain boundary) is regarded as martensite, and the area find the rate.

If finish polishing with an abrasive such as colloidal silica is performed during sample preparation, the austenite phase of the surface layer may undergo deformation-induced martensite transformation. Therefore, samples are always prepared by electropolishing or chemical polishing. In addition, since the surface layer portion, which is an area from the surface to a position of 1/10th of the plate thickness in the plate thickness direction, is observed, the amount of polishing is limited to 1/10th of the plate thickness.

Area ratio of austenite grains having {110} plane orientation in the surface layer: 50% or more The {110} plane orientation is a typical main orientation of rolling texture of austenite. By setting the area ratio of the austenite grains having the above plane orientation to 50% or more in the surface layer portion of the austenitic stainless steel sheet according to the present embodiment, good polishability is ensured. The area ratio of the austenite grains having the above plane orientation is preferably 52% or more, more preferably 55% or more. Although no particular upper limit is set, if the area ratio of the austenite grains having the plane orientation described above exceeds 85%, the toughness is lowered, so it is desirable to set the upper limit to 85%.

The area ratio of the austenite grains having the {110} plane orientation in the surface layer portion can be determined by the following method.
First, a region having an area of 500 μm×500 μm or more is measured by EBSD in the surface layer portion of the material produced by the method described above. A region having a crystal structure of fcc and surrounded by grain boundaries of 15° or more is regarded as an austenite grain. Strictly speaking, the {110} plane orientation is a crystal orientation in which the <110> axis is parallel to the vector perpendicular to the surface of the steel sheet (= 0 angle difference with the vector perpendicular to the surface). The form shall allow an angular difference of 0 to 15°. The area ratio (%) of the austenite grains having the {110} orientation is obtained by dividing the total area of the grains having the {110} orientation among the austenite grains by the measured area and multiplying the result by 100.

Average grain size of austenite grains in the surface layer: 5.0 μm or less By setting the average grain size of the austenite grains in the surface layer to 5.0 μm or less, the number of crystal grains per unit area increases, {110}<112> It is thought that diffusion bondability is improved because the existence frequency of grains having orientation is averaged. Further, when the average grain size of the austenite grains is 5.0 μm or less, the processed surface becomes smooth when etching processing or the like is performed.
Therefore, in order to improve the diffusion bondability, it is preferable to set the average grain size of the austenite grains in the surface layer to 5.0 μm or less.

The average grain size of austenite grains is calculated by the following procedure.
First, in the surface layer portion of the material produced by the above method, an area of 100 μm × 100 μm or more was measured by EBSD. The average area S per crystal grain is calculated from the number of crystal grains included in a predetermined area.
Then, from the average area, the average grain size D of the austenite grains is calculated by the following formula (iii).
D=(2S/π) ^0.5 (iii)

X-ray random intensity ratio of {110} <112> orientation: 8.5 or more The {110} <112> orientation is a typical main orientation of rolling texture of austenite. By setting the concentration in the same direction to 8.5 or more at the surface layer portion (joint surface) of the steel plate, high diffusion bondability is ensured. Therefore, when improving the diffusion bondability, it is preferable to set the X-ray random intensity ratio of the {110}<112> orientation to 8.5 or more. The X-ray random intensity ratio of the {110}<112> orientation is more preferably 9.0 or more, more preferably 10.0 or more. Although the upper limit of the X-ray random intensity ratio of the {110}<112> orientation is not particularly set, if the X-ray random intensity ratio exceeds 20.0, the misorientation of 15° or more between adjacent crystal grains cannot be satisfied, It is desirable to set this value as the upper limit because it will not act as an effective grain boundary.

The X-ray random intensity ratio of the {110} <112> orientation is calculated by the series expansion method based on multiple pole figures among the {200}, {311}, and {220} pole figures measured by X-ray diffraction. It can be obtained from the crystal orientation distribution function (referred to as Orientation Distribution Function, ODF) representing the three-dimensional texture. The X-ray random intensity ratio in this embodiment means that the X-ray intensity of a standard sample and a test material that do not have accumulation in a specific direction is measured by an X-ray diffraction method or the like under the same conditions, and the obtained test It is a numerical value obtained by dividing the X-ray intensity of the material by the X-ray intensity of the standard sample. A standard sample that does not have a specific accumulation in any of the {200}, {311}, and {220} plane measurements is used. Although the method for preparing the standard sample is not specified, it is preferable to prepare by compressing and sintering Fe-based metal powder such as Fe--C, Fe--Ni, and Fe--Cr, which has an fcc crystal structure stably at room temperature. Common.
FIG. 1 shows an ODF of a φ2=45° cross-section where the crystal orientations described above are displayed. The {110}<112> orientation strictly refers to the orientation represented by φ1=55° and Φ=90°. However, since measurement errors may occur due to test piece processing and sample setting, the maximum value in the range of φ1=50-60° and Φ=85-90° is representative as the intensity ratio of this orientation.

Here, the crystal orientation is usually indicated by (hkl) or {hkl} for the orientation perpendicular to the sheet surface, and by [uvw] or <uvw> for the orientation parallel to the rolling direction. {hkl}, <uvw> is a generic term for equivalent planes, and (hkl), [uvw] refers to individual crystal planes. That is, since this embodiment targets the fcc structure, for example, (111), (-111), (1-11), (11-1), (-1-11), (-11-1 ), (1-1-1) and (-1-1-1) planes are equivalent and indistinguishable.
In such cases, these orientations are collectively referred to as {111}.

Since ODF is also used to indicate the orientation of a crystal structure with low symmetry, it is generally expressed as φ1 = 0 to 360°, φ = 0 to 180°, and φ2 = 0 to 360°, and each orientation is (hkl), [uvw]. However, in the present invention, since the fcc crystal structure with high symmetry is targeted, Φ and φ2 are expressed in the range of 0 to 90°. The range of φ1 varies depending on whether or not symmetry due to deformation is taken into account in the calculation. That is, a method is selected in which the average value of the same direction at φ1=0 to 360° is expressed on the ODF from 0 to 90°. In this case, (hkl), [uvw] and {hkl}<uvw> are synonymous.
Therefore, for example, the (110)[1-12] X-ray random intensity ratio of the ODF in the φ2=45° cross section shown in FIG. 1 is synonymous with the X-ray random intensity ratio of {110}<112> orientation. be.

A sample for X-ray diffraction is prepared as follows.
In order to improve the diffusion bondability, the X-ray random intensity ratio of the surface layer which becomes the bonding surface is important. For X-ray measurement, some degree of mechanical polishing, chemical polishing, or electropolishing is required to obtain flatness of the measurement surface or to remove strain. Therefore, adjustment is made so that the surface layer portion from the surface of the steel sheet to the position of 1/10 of the thickness of the steel sheet serves as the measurement surface.
If X-ray diffraction measurement is difficult, a statistically sufficient number of measurements may be performed by EBSD (Electron Back Scattering Pattern) method or ECP (Electron Channeling Pattern) method.

The plate thickness of the austenitic stainless steel plate according to the present embodiment is not limited, but is, for example, 0.5 mm or less.

3. Manufacturing Method The method for manufacturing the austenitic stainless steel sheet according to the present embodiment is not particularly limited, but it can be manufactured by the method shown below. In the method for producing an austenitic stainless steel sheet according to the present embodiment, the steel is melted and cast by a conventional method to obtain a billet to be subjected to hot rolling. The steel slab may be obtained by forging or rolling a steel ingot, but from the viewpoint of productivity, it is preferable to manufacture the steel slab by continuous casting. Moreover, you may manufacture using a thin slab caster etc.
The austenitic stainless steel sheet according to the present embodiment can be manufactured by applying a manufacturing method including the steps described below to the obtained steel slab.

(a) Heating Step Normally, after casting, the steel slab is cooled and heated again for hot rolling. In the method for manufacturing an austenitic stainless steel sheet according to the present embodiment, the heating temperature of the steel slab during hot rolling is set to 1150° C. or higher. This is because if the heating temperature is less than 1150 ° C., coarse carbonitrides remain undissolved and may become the starting point of cracks during hot working, and randomization of the texture during hot rolling is promoted (preferably This is because the formation of texture is suppressed). The heating temperature is desirably 1170° C. or higher. Although the upper limit of the heating temperature is not particularly specified, heating to more than 1400° C. may lead to a decrease in productivity and cause growth of orientations that do not develop in normal rolling. Therefore, it is desirable to set the upper limit to 1400°C.
Processes such as continuous casting-direct rolling (CC-DR) where the molten steel is cast and then hot rolled (without reheating) before it has cooled below 1150°C may be employed. .

(b) Hot Rolling Step In the method for manufacturing an austenitic stainless steel sheet according to the present embodiment, hot rolling is performed on the heated steel slab. At that time, hot rolling is completed in a temperature range of 880 to 1000°C. If the hot-rolling finish temperature is lower than 880°C, the deformation resistance becomes too high, which significantly impairs productivity and promotes the development of a shear layer on the surface layer of the hot-rolled sheet. It is desirable that the end temperature be 900° C. or higher.
On the other hand, when the end temperature of hot rolling exceeds 1000 ° C., recrystallization occurs in all rolling passes, so that the degree of accumulation of the texture in the steel sheet after hot rolling decreases (the structure is randomized), and the { The area ratio of austenite grains having a 110} plane orientation is reduced. In addition, the X-ray random intensity ratio of the {110}<112> orientation in the surface layer also decreases. Therefore, the hot rolling finish temperature is set to 1000° C. or lower. The ending temperature is desirably 980° C. or lower, more desirably 950° C. or lower.

Further, in the hot rolling process, the shape ratio L determined by the following formula (ii) is set to 4.5 or less in each of the final two passes. When at least one of the shape ratios of the final two passes exceeds 4.5, a layer called a shear layer having a crystal orientation different from that of the thickness center layer is formed on the surface layer of the hot-rolled sheet due to friction between the steel sheet and the rolling rolls. be done. Since the shear layer does not include the {110} plane orientation, the area ratio of the austenite grains having the {110} plane orientation decreases when the shear layer develops in the hot rolling stage. In addition, since the shear layer does not include the {110}<112> orientation, the {110}<112> of the surface layer portion is also reduced. The shape ratio L is desirably 4.2 or less, more desirably less than 4.0. Although there is no particular lower limit to the shape ratio L, if it is less than 2.5, the thickness of the hot-rolled sheet increases, and the cold rolling load increases. Therefore, it is desirable to set the shape ratio to 2.5 or more in both of the final two passes. The shape ratio L is more preferably 2.8 or more, and more preferably 3.0 or more.

L=(√(R×(t _in −t _out )))/((2t _out +t _in )/3) (ii)
However, the meanings of the symbols in the formula are as follows.
L: Shape ratio in the relevant pass R: Roll radius in the relevant pass (mm)
t _in : Entry side plate thickness in the relevant pass (mm)
t _out : Outlet plate thickness in the pass (mm)

(c) Winding process The steel sheet (hot-rolled sheet) that has been hot-rolled under the above conditions is coiled in a temperature range of 900°C or less. If the coiling temperature exceeds 900° C., recrystallization proceeds during coiling, weakening the desired texture. The winding temperature is desirably 880° C. or lower, more desirably 850° C. or lower.
Although the lower limit of the coiling temperature is not specified, even if the coiling temperature is less than 550° C., no particular effect is obtained, and the strength of the coil increases, making unwinding difficult. Therefore, it is desirable to set the winding temperature to 550° C. or higher.

After completion of the above hot rolling, cold rolling and annealing are repeated one or more times to produce a steel sheet in the same manner as a general process. At that time, only the respective final steps are limited as described later. Except for the final step, there is no particular limitation, but the general temperature for annealing other than the final step (intermediate annealing) is 900 to 1100°C.

(d) Final Cold Rolling Step When the rolling reduction (rolling reduction) of the final cold rolling is less than 40%, no deformed texture is formed and the {110} plane orientation is not developed. Therefore, the rolling rate of the final cold rolling is set to 40% or more. Further, if the rolling reduction is less than 40%, the martensitic transformation during cold rolling does not sufficiently occur, and grain refinement due to reverse transformation during subsequent annealing does not occur. Therefore, from the viewpoint of reducing the austenite grain size, the rolling ratio is set to 40% or more. The rolling reduction is desirably 45% or more, more desirably 50% or more.
On the other hand, if the rolling reduction exceeds 90%, an orientation different from that of normal rolling develops, the area ratio of austenite grains having {110} plane orientation decreases, and the load on the equipment becomes extremely high. Therefore, the rolling reduction is set to 90% or less. The rolling reduction is desirably 85% or less, more desirably 80% or less.

Furthermore, when the roll diameter of the rolling rolls in the final cold rolling process is small, a shear layer is formed on the surface layer of the steel sheet due to friction between the steel sheet and the rolling rolls, and an orientation different from the {110} plane orientation develops. do. Therefore, the roll diameter of the rolling rolls in the final cold rolling step is set to 80 mm or more. The roll diameter is desirably 90 mm or more, more desirably 100 mm or more.

(e) Final Annealing Step When the final annealing temperature is lower than 600°C, the processed α structure remains and the proportion of austenite grains having {110} plane orientation decreases. Therefore, polishing property cannot be ensured. If the final annealing temperature is less than 600°C, reverse transformation does not occur and the average austenite grain size exceeds 5.0 µm. Therefore, the final annealing temperature should be 600° C. or higher. The final annealing temperature is desirably 650° C. or higher, more desirably 700° C. or higher.
On the other hand, if the final annealing temperature exceeds 1000° C., grain growth is accelerated, the grain size is coarsened, the toughness is lowered, and orientations other than the {110} plane orientation are developed. In addition, the X-ray random intensity ratio of the {110}<112> orientation also decreases. Therefore, the final annealing temperature should be 1000° C. or lower. The final annealing temperature is desirably 980° C. or lower, more desirably 970° C. or lower.
The holding time at the annealing temperature (ultimate temperature) shall be 60 seconds or less. Holding for more than 60 seconds causes randomization of texture and coarsening of grain size. From this point of view, the retention time is preferably 30 seconds or less, more preferably 10 seconds or less.

In addition to the above manufacturing conditions, the hot-rolled sheet may be annealed (intermediate annealing) before cold rolling. The annealing temperature before cold rolling is desirably 600 to 1000°C. This is because the hot-rolled sheet is not sufficiently softened at less than 600°C, and the working load during cold rolling increases. This is because the tissue randomizes as the

Cold rolling (temper rolling) for the purpose of adjusting the mechanical properties of the steel sheet after the final annealing, followed by the reduction of residual stress (strain relief) that causes changes in the shape of the sheet and the γ parent phase. A heat treatment for the purpose of reverse transformation may be performed. Through these steps, the mechanical properties of the austenitic stainless steel sheet can be adjusted within a preferred range.
When temper rolling is performed, it is desirable that the rolling reduction is 50% or less. This is because if the rolling reduction is 50% or less, it is possible to adjust the mechanical properties to the required ones specified in JIS (G4305) or the like. When heat treatment is performed, the heat treatment temperature is preferably 600.degree. C. to 900.degree. C., more preferably 650 to 850.degree. This is because if the temperature is less than 600° C., the effect of strain relief cannot be obtained and reverse transformation does not occur. Also, at a temperature exceeding 900° C., the effect of performance adjustment in cold rolling disappears.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

<Example 1>
Steel having the chemical composition shown in Table 1 is melted to produce a steel slab, the steel slab is heated, and subjected to rough hot rolling, followed by Tables 2-1 and 2-2. Finish rolling was performed under the conditions shown in . In Tables 2-1 and 2-2, SRT (°C) is the billet heating temperature, L1 is the shape ratio in the pass before the final pass, L2 is the shape ratio in the final pass, FT (°C) indicates the temperature after the final pass of finish rolling, that is, the temperature at the finish delivery side. CT (°C) indicates the winding temperature.
After hot rolling, pickling is performed, intermediate cold rolling is performed with a rolling reduction of 60% (intermediate cold rolling), intermediate annealing is performed at 1050 ° C. for 20 minutes, and final cold rolling is performed to obtain a thickness of 0.2 mm. of steel sheets were obtained. CR (%) indicates the rolling rate of the final cold rolling. After that, annealing was performed to raise the temperature to the attained temperature indicated by AT (°C).

Regarding the obtained austenitic stainless steel sheet, the martensite (α') area ratio, the average grain size of austenite grains (γ), the area ratio of austenite grains having {110} plane orientation, and {110}<112 in the surface layer portion. > azimuth X-ray random intensity ratio was measured.

The average grain size of austenite grains and the area ratio of martensite in the surface layer portion were measured by the following methods. First, a plane parallel to the steel plate surface at a position of 1/10 of the plate thickness in the plate thickness direction from the steel plate surface and having an area of 500 μm×500 μm was measured by EBSD. Then, among the regions determined to have the fcc structure, a region surrounded by boundaries with a misorientation of 15° or more is regarded as one crystal grain. An average area S was calculated. From the average area, the average grain size D of the austenite grains was calculated by the above formula (iii).
In addition, a region that cannot be determined as an fcc structure, that is, a region that has a bcc crystal structure, or a region that cannot be measured due to high strain is regarded as martensite (however, linear regions such as grain boundaries are excluded). The area ratio was obtained.
The area ratio of martensite (α' area ratio) and the average grain size of austenite grains (γ grain size) are the average values after the final annealing.

The area ratio of the austenite grains having the {110} plane orientation in the surface layer portion ({110} plane γ area ratio) was measured as follows.
First, a region having an area of 500 μm×500 μm was measured by EBSD on a plane parallel to the surface of the steel plate similar to the above. Then, a region with a crystal structure of fcc and surrounded by grain boundaries of 15 ° or more is regarded as an austenite grain, of which the <110> axis is oriented at 0 to 15 ° with respect to the vector perpendicular to the surface of the steel plate Grains having a crystal orientation were taken as austenite grains having a {110} plane orientation. Then, the area ratio of the austenite grains having the {110} plane orientation was obtained by dividing the total area of the grains having the {110} plane orientation by the measured area and multiplying by 100.

The X-ray random intensity ratio of the {110}<112> orientation ({110}<112>X-ray random intensity ratio) of the surface layer portion of the steel sheet was measured as follows.
First, the steel plate was mechanically polished and buffed, and then electropolished to remove distortion. X-ray diffraction was performed using An X-ray diffraction of a standard sample without accumulation in a specific direction was also performed under the same conditions.
Based on the {200}, {311}, and {220} pole figures obtained by X-ray diffraction, an ODF was obtained by a series expansion method. Then, the X-ray random intensity ratio was determined from this ODF. The X-ray diffraction of the surface layer portion was measured on the front side of the steel plate.

Polishability was evaluated with respect to the above austenitic stainless steel plate.
Abrasiveness was evaluated as follows.
After extracting a test piece having a length of 100 mm, a width of 150 mm, and a thickness of 0.2 mm from the above austenitic stainless steel plate, a surface pressure of 8.0 N/cm ² is applied to the sampled test piece, and alumina of abrasive grain #400 is applied. , a rotation speed of 300 rpm, and a polishing time of 10 seconds. Then, the roughness Ra after polishing was measured according to JIS B 0601:2013. In this example, when the roughness Ra after polishing was 0.050 μm or less, it was determined that the austenitic stainless steel sheet had good polishability.

Furthermore, the diffusion bondability of the austenitic stainless steel sheets was evaluated as follows.
After stacking two steel plates of 50 mm×50 mm (×thickness) obtained from the above austenitic stainless steel plates, a stress of 30 MPa was applied and diffusion bonding was performed by holding at 900° C. for 30 seconds. After that, ultrasonic flaw detection was used to evaluate the voids in the diffusion bonding portion.
The diffusion bonding portion was evaluated by the transmission method, and the position where the transmission pulse height was 25% or more was determined as the diffusion bonding portion, and the position where the transmission pulse height was less than 25% was determined as the void portion, and the area ratio of the diffusion bonding portion was calculated.
In this example, it was determined that the diffusion bondability of the austenitic stainless steel plate was good when the area ratio of the diffusion bonding portion was 70% or more.

In the transmission method, ultrasonic waves emitted from the transmitting probe pass through the object to be measured and are received by the receiving probe due to factors such as scattering caused by defects in the object. This is a method of grasping the size and degree of defects inside the measurement object from the degree of attenuation. The height of the transmitted pulse received after passing through the measurement object is measured compared to the transmitted ultrasonic pulse. It is evaluated that the closer the received transmission pulse height is to 100%, the fewer the defects in the measurement object and the better the diffusion bonding, and the lower the received transmission pulse height, the poorer the bonding.
In this embodiment, tap water is used as the contact medium, and an austenitic stainless steel plate with a thickness of 0.4 mm, preferably an austenitic stainless steel plate with the chemical composition range of the present application, is used as a test piece for calibration to vibrate the ultrasonic probe. After adjusting the particle diameter to be 0.5 mm, transmitted pulses were measured at a pitch of 0.2 mm for each of the length and width of the object to be measured.

As is clear from the results shown in Tables 2-1 and 2-2, examples of the present invention (Test Nos. 1 to 3, 5, 7, 9, 11, 13, 15, 17, 22, 24, 26, 28 , 31, 33, 35, 37, 39, and 44), the area ratio of martensite was reduced and the {110} plane orientation was developed, resulting in excellent polishability.

Moreover, test no. For 3, 9, 13, 22, 24, 26, 28, 31, 33, 35, 37, 39, and 44, the average grain size of the austenite grains is smaller, and the {110}<112> orientation of the austenite grains is X Since the linear random intensity ratio was high, the diffusion bondability was also excellent.

On the other hand, Test No. 18-21, 41 and 43 are comparative examples using steels whose chemical composition is outside the scope of the invention. Test no. In No. 18, the C content was too large, so the texture was randomized and the {110} plane orientation was not sufficiently developed. Test no. In No. 19, the Cr content was too high, so cracks occurred during hot rolling and the test was stopped. Test no. In Nos. 20, 21 and 43, the Md30 value was too high and the amount of martensite was excessive, resulting in poor polishability.
Test no. In No. 41, since the Nb content was too high, the hot workability deteriorated and cracks occurred at the edges of the hot-rolled sheet, so the test was stopped.

Test no. 4, 6, 8, 10, 12, 14, 16, 23, 25, 27, 29, 30, 32, 34, 36, and 38 all satisfy the chemical composition of the present invention, but the production This is a comparative example in which the desired texture was not obtained as a result of conditions outside the preferred range of the present invention.

Test no. In all of Nos. 4, 6, 25 and 27, the shape ratio of both or any one of the final two stages of hot rolling exceeded 4.5. Therefore, a shear texture developed in the surface layer of the hot-rolled sheet. As a result, the development of the {110} plane orientation after cold-rolling annealing was finally suppressed, and the polishability was lowered. Test no. In Nos. 8 and 29, the temperature at which hot rolling was finished was too high. Also, test no. 8 also had a high winding temperature. Therefore, recrystallization progressed and the desired texture did not develop.

Test no. In Nos. 10 and 32, since the rolling reduction in cold rolling was too low, the texture did not develop. Test no. In No. 12, since the roll diameter of the rolling rolls in the final cold rolling was too small, a shear texture developed in the surface layer of the steel sheet. Test no. In Nos. 14 and 36, the final annealing temperature was too low, so reverse transformation did not occur and the martensite fraction increased. Test no. In Nos. 16 and 38, since the annealing temperature was too high, recrystallization progressed and the desired texture did not develop sufficiently. Therefore, these examples were inferior in polishability.
Test no. In No. 23, the heating temperature before hot rolling was low. Therefore, randomization of the texture during hot rolling was promoted. As a result, the desired texture was not fully developed.
Test no. No. 30 had a low finish rolling completion temperature. Therefore, the development of the surface shear layer was promoted. As a result, the desired texture was not fully developed.
Test no. No. 34 had a high winding temperature. Recrystallization progressed during winding. As a result, the desired texture was not fully developed.

<Example 2>
A steel having the chemical composition (A, I, F2, I2) shown in Table 1 is melted to produce a steel billet, the steel billet is heated and subjected to hot rough rolling, and then, Finish rolling was performed under the conditions shown in Table 3. After hot rolling, pickling was performed, intermediate cold rolling was performed with a rolling reduction of 55%, intermediate annealing was performed at 1120°C for 20 minutes, and final cold rolling was performed. After that, annealing was performed to raise the temperature to the attained temperature indicated by AT (°C). Furthermore, after annealing, temper rolling was performed at the rolling reduction shown in Table 3, and strain relief annealing was performed.

As can be seen from Table 3, good polishability was obtained even when temper rolling and stress relief annealing were performed. In addition, all of these examples had the necessary mechanical properties specified in the JIS standard (G4305) and the like.

ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel plate which has favorable polishability can be obtained industrially stably. Therefore, the austenitic stainless steel sheet according to the present invention is suitable as a material for members that require high surface gloss, such as housings of electronic devices.

Claims (4)

The chemical composition, in mass %,
C: 0.005 to 0.150%,
Si: 1.0% or less,
Mn: 1.5% or less,
P: 0.10% or less,
S: 0.010% or less,
Al: 0.10% or less,
Cr: 15.0 to 20.0%,
Ni: 6.0 to 15.0%,
N: 0.005 to 0.150%,
Mo: 0-2.0%,
Cu: 0-1.5%,
Nb: 0 to 0.500%,
V: 0 to 0.150%,
Ti: 0 to 0.300%,
B: 0 to 0.010%,
Total of Ca, Mg, Zr, Sn, Pb and W: 0 to 0.10%,
balance: Fe and impurities,
Md30 value obtained by the following formula (i) is 60 ° C. or less,
In the surface layer, the area ratio of martensite is 5.0% or less, and the area ratio of austenite grains having a {110} plane orientation is 50% or more.
Austenitic stainless steel plate.
Md30 value = 497 - 462 x (C + N) - 9.2 x Si - 8.1 x Mn - 13.7 x Cr - 20 x (Ni + Cu) - 18.7 x Mo (i)
However, the element symbol in the above formula represents the content of each element in mass % in the steel, and 0 shall be substituted when it is not contained. The chemical composition contains Nb: 0.010 to 0.500%,
The Md30 value is 20 to 60° C.,
The average grain size of the austenite grains in the surface layer portion is 5.0 μm or less, and the X-ray random intensity ratio of the {110} <112> orientation of the austenite grains is 8.5 or more.
The austenitic stainless steel sheet according to claim 1. The chemical composition, in mass %,
Mo: 0.1-2.0%,
Cu: 0.1-1.5%,
Nb: 0.010 to 0.500%,
V: 0.010 to 0.150%,
Ti: 0.010 to 0.300%, and
B: 0.001 to 0.010%,
containing one or more selected from
The austenitic stainless steel sheet according to claim 1 or 2. A step of subjecting the austenitic stainless steel sheet according to any one of claims 1 to 3 to temper rolling under conditions where the rolling reduction is 50% or less,
A method for producing an austenitic stainless steel sheet.

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