JP4571662B2 - Stainless steel sheet manufacturing method - Google Patents

Description

  The present invention is a non-metallic inclusion that causes a reduction in the surface quality of a stainless steel sheet, and the maximum non-metallic inclusion in the slab stage is specified by making the size of the maximum non-metallic inclusion in the slab stage after making the steel sheet after rolling. The present invention relates to a method of manufacturing a stainless steel plate in which the size of metal inclusions is specified.

  In recent years, steel sheets having high surface quality and high internal quality have been demanded. For example, stainless steel plates used as injection needles are cold-rolled to about 0.3 mm to become products, and stainless steel plates used as electronic materials such as hard disk suspensions and gun parts are stamped and etched. Etc. to become a product. However, in these processing stages, if the steel sheet contains large non-metallic inclusions, the surface of the steel sheet is wrinkled during rolling, and the workability of the processing deteriorates. was there. For this reason, conventionally, the above-mentioned problems have been addressed by adjusting the composition of steel slab components to reduce the content of non-metallic inclusions itself, or reducing the content of large ones of them. .

  However, although such a countermeasure can suppress the formation of large non-metallic inclusions in the stainless steel plate, it cannot specifically identify the content and size of the inclusion and evaluate it. Usually, even if a stainless steel sheet contains large non-metallic inclusions, after finishing the steel sheet into a product, it is a product with poor surface quality, that is, a stainless steel sheet containing large non-metallic inclusions. However, there was a problem that it was impossible to prevent the occurrence of product defects.

  Therefore, conventionally, as a method for evaluating the non-metallic inclusions present in the above-described rolled sheet, for example, there has been a JIS cleanliness evaluation method (JIS G 0555). However, this evaluation method represents the proportion of non-metallic inclusions, and was not evaluated to that size.

  In addition, non-metallic inclusions are extracted from metal materials such as steel slabs by acid dissolution, and the particle size is evaluated with a microscope, and metal materials such as steel are dissolved by the EB method. A method of observing with a microscope has also been proposed (Patent Document 1, Patent Document 2). However, these methods have the problem that inclusions dissolve with acid, or the inclusions themselves dissolve and agglomerate. Also, it takes time to dissolve the acid, so it corresponds to the mass production process of products. There was a problem that it was difficult to do.

  Furthermore, as an evaluation method for nonmetallic inclusions, for example, the EB method of Patent Document 3 and the measurement of extraction residue by electrolysis of Patent Document 4 are known. In the method described in Patent Document 3, it is necessary to take out a test piece from a metal material such as a steel slab, and to irradiate an electron beam to float non-metallic inclusions on the surface of the specimen, and to dissolve the sample piece. There was a problem that this equipment was required. Moreover, in the method described in Patent Document 4, it is necessary to take out a test piece from a metal material such as a steel slab, electrolyze the test piece, elutriate the residue, and then magnetically sort to separate nonmetallic inclusions. Therefore, there is a problem that the test procedure is complicated.

JP-A-9-125199 JP 9-125200 A JP 2003-65980 A JP 2001-159627 A

  Therefore, the object of the present invention is to quickly and easily identify the size of the maximum non-metallic inclusions present in the stainless steel slab by observing the rolled plate before making it into a final product. The object is to propose a method for advantageously producing a stainless steel plate with guaranteed product quality with respect to metal inclusions.

In view of the above-mentioned problems of the prior art, the inventors conducted intensive research with the goal of overcoming them. As a result, the inventors found Cr: 5-30 wt%, Ni: 30 wt% or less, Si: 0.1-3 wt% , Mn: 0.3 to 3 wt%, Al: 0.0001 to 0.01 wt%, Ca: 0.00001 to 0.002 wt%, Mg: 0.00001 to 0.002 wt%, O: 0.001 to 0.007 wt%, the balance being Fe and inevitable impurities In the method of producing a stainless steel plate by hot rolling or hot and cold rolling a steel slab made of the material, a rolling reduction before final product: 90.00 to 99.83% was performed. Part of the cross section of the steel sheet cut at right angles or almost right angles to the rolling direction is used as the inspection reference area, and MnO within the inspection reference area: 1 to 45 wt%, CaO: 1 to 45 wt%, SiO ₂ : 10 _{~60wt%, Al 2 O 3:} 5~50wt%, MgO: 0.5~30wt%, Cr 2 O 3: 0.2~10wt%, FeO: width direction length of the non-metallic inclusions having 0.2-10% of the component composition Microscopic observation , From the most significant length of the value of the non-metallic inclusions of the steel sheet obtained by the observation, estimates the size} area _max of the maximum non-metallic inclusions in the slab stage of the steel sheet, its estimated maximum non-metallic size} area _max is Ru der less 300μm steel slab of inclusions, in producing the stainless steel plate using the estimated size} area _max of the estimated maximum non-metallic inclusion in the steel slab stage,
The width direction length (L) of the maximum non-metallic inclusion in an arbitrary inspection reference area S _{0 of} a cross section cut at right angles or almost at right angles to the rolling direction is measured, and this width direction length (L ) Is equal to the diameter of the maximum non-metallic inclusion existing in the area S ₀ ^* of the slab stage corresponding to the inspection reference area S ₀ , the maximum non-existence existing in the slab from the following equation (1) The procedure of calculating the √area of the metal inclusion area is repeated at a plurality of locations (n) of the cut surface, and the calculated n √areas are rearranged in ascending order, and the jth from the following equation (2) By obtaining a first standardized variable (ascending order) y _j corresponding to √area, and processing by extreme value statistics with this first standardized variable (ascending order) y _j and the √area, the following equation (3) The average normalization variable y shown in Fig. 2 is obtained, and then the particle size of the maximum nonmetallic inclusion is estimated. The recursion period T is calculated from the following equation (4) from the area S to be measured and the slab area S ₀ ^* corresponding to the inspection reference plane, and then the second from the recursion period T and the following equation (5). A standardized variable (area ratio) y _max is calculated, and from this second standardized variable (area ratio) y _max and the following equation (3), the square root √area _max of the cross-sectional area of the largest nonmetallic inclusion in the steel slab A method for producing a stainless steel sheet, characterized by calculating
Record
√area = √π (L / 2) ² (1)
y _j = −ln [−ln {j / (n + 1)}] (j = 1 to n) (2)
y = a√area + b (3)
T = (S + S ₀ ^* ) / S ₀ ^* (4)
y _max = -ln [-ln {(T-1) / T}] (5)
However,
n: Number of inspection
y _j : First normalization variable (ascending order)
y: Average normalization variable
a: Recursive coefficient
b: Constant
S: Area for estimating the size of the maximum nonmetallic inclusion
S ₀ ^* : Area of steel slab corresponding to inspection reference plane S ₀
T: Recursion period
y _max : Second normalization variable (area ratio)

The present invention preferably contains 50% by volume or less of MgO.Al ₂ O ₃ inclusions based on the total nonmetallic inclusions.

  According to the present invention, by observing the cross section of the stainless steel rolled plate and performing extreme value statistical processing, the size of the maximum non-metallic inclusions in the slab stage corresponding to the upstream process of the plate from the rolled plate is increased. In addition to being able to specify quickly and easily, the stainless steel plate can be properly classified according to the product application based on the size of the specified maximum nonmetallic inclusion. In addition, since the quality history of the stainless steel plate according to the present invention becomes clear, the occurrence of defective products can be eliminated even when used in products that require high-precision workability.

  Stainless steel slabs (hereinafter simply referred to as “steel slabs”) generally have a thickness of 100 to 300 mm. Therefore, when the state of non-metallic inclusions in the slab is to be observed with a microscope, its observation area Will be quite big. Further, the time required for the observation becomes enormous. Therefore, in the present invention, in order to reduce the observation area and simultaneously reduce the observation time, the slab is hot-rolled, or the cross-section of the rolled plate after the hot and cold-rolled steel plate is obtained. I decided to observe under a microscope. Note that the rolling reduction of the steel sheet is preferably about 90.00 to 99.83%. The reason is that if the rolling reduction is less than 90.00%, the plate thickness increases, and it takes a lot of time to observe the entire cross section in the thickness direction. On the other hand, if the rolling reduction is larger than 99.83%, nonmetallic inclusions are dispersed over a wide range in the rolling direction, and therefore, there is a possibility that nonmetallic inclusions may not be detected depending on the observed cross section.

  In general, in the process of rolling a steel slab by hot rolling, or hot and cold rolling, nonmetallic inclusions in the steel sheet are stretched together with the slab. However, the non-metallic inclusions are stretched in the rolling direction, but are hardly stretched in a direction perpendicular to or substantially perpendicular to the rolling direction. Accordingly, the grain size of the nonmetallic inclusions in the slab is substantially the same as the length in the direction perpendicular to or substantially perpendicular to the rolling direction of the nonmetallic inclusions in the drawn steel sheet, and appears in the width direction of the rolled steel sheet. The size, particularly the maximum length, of the nonmetallic inclusions (surface perpendicular to the rolling direction) can be considered as the maximum particle size of the nonmetallic inclusion particles in the slab.

  In addition, the angle with respect to the rolling direction of the cross section to be observed is a right angle or a substantially right angle, but is specifically an angle of 80 to 90 °, preferably 85 to 90 °. The reason is that if a cross section cut at an angle smaller than 80 ° with respect to the rolling direction is adopted as a cross section for observing the nonmetallic inclusion under a microscope, the nonmetallic inclusion will be cut obliquely. For this reason, the width of the inclusions to be measured is larger than the original value.

  In the present invention, the method for specifying the size of the nonmetallic inclusions in the steel slag is the microscopic observation of the length in the width direction of the maximum nonmetallic inclusions in the steel plate (procedure 1), the recursive line y by extreme value statistical processing. = Derivation of a√area + b (Procedure 2) and estimation of the size of the maximum nonmetallic inclusion in the steel slab stage (Procedure 3) are passed through.

In the procedure 1, first, an inspection as shown in FIG. 1 (a) is performed on a cross section perpendicular to or almost perpendicular to the rolling direction of a steel plate obtained by hot rolling or hot and cold rolling of a steel slab. A reference area S ₀ is defined. As shown in FIG. 1A, the inspection reference area S ₀ is a portion (area) surrounded by a length a provided in a part in the width direction of the steel plate and a length b corresponding to the plate thickness. Microscopic observation is performed using this inspection reference area S ₀ as one unit. The length a in the width direction of the inspection reference area S ₀ is preferably in the range of 10 to 40 mm. The reason is that if it is larger than 40 mm, the observation area becomes large and rapid inspection becomes difficult. On the other hand, if it is smaller than 10 mm, the detection sensitivity of non-metallic inclusions decreases. In addition, it is preferable that the board thickness b of this steel plate is 0.17-30 mm calculated from the said rolling reduction. More preferably, it is 0.5-10 mm.

Then, the inside of the inspection reference area S ₀ is observed with a microscope, and non-metallic inclusions having the largest width direction length (L) are searched for. And the width direction length (L) of the obtained maximum nonmetallic inclusion is the diameter of the maximum nonmetallic inclusion existing in a spherical shape in the area S ₀ ^* at the time of the steel slab corresponding to the inspection reference area S _0. Therefore, the square root √area of the area of the largest nonmetallic inclusion in the steel slab is obtained from the equation (1).
√area = √π (L / 2) ² (1)

As shown in FIG. 1B, the area S ₀ ^* of the steel slab corresponding to the inspection reference plane S ₀ is the thickness direction while keeping the width direction length a equal to the inspection reference area S _0. Is equal to b ′ (> b) corresponding to the thickness of the steel slab.

As shown in FIG. 1 (a), the inspection reference plane S ₀ is set so that the lengths a in the width direction are all equal to different n sites in the cross section in the thickness direction of the steel sheet. Repeat in

  The number n of inspection points is preferably 10 to 20 points. The reason for this is that, within this range, the burden of measurement work by microscopic observation is not so great, and measurement can be performed quickly, and statistically reliable guess data can be obtained. is there. More preferably, there are 15 locations. Moreover, it is preferable to select the inspection location from the vicinity of the center of the cross section perpendicular to the rolling direction of the steel sheet. The reason is to obtain an estimated sample of the maximum non-metallic inclusion size that is statistically reliable.

In procedure 2, first, the n √areas obtained in procedure 1 are rearranged in ascending order. Then, a first standardized variable (ascending order) y _j corresponding to √area corresponding to the j-th is obtained from equation (2).
y _j = −ln [−ln {j / (n + 1)}] (j = 1 to n) (2)
Finally, the following equation (3) is derived by performing processing based on extreme value statistics with the first standardized variable (ascending order) y _j .
y = a√area + b (3)

The procedure 3 is specifically to obtain √area _max . First, the recurrence period T is calculated from the area S ₀ ^* at the time of the steel slab corresponding to the inspection reference area S ₀ , the area S for estimating the size of the maximum non-metallic inclusion, and the following equation (4).
T = (S + S ₀ ^* ) / S ₀ ^* (4)

In addition, the area S for estimating the size of the maximum non-metallic inclusion is the area of the entire cross section in the thickness direction of the steel slab that has been screened in FIG. The area S for estimating the size of the maximum non-metallic inclusion is preferably about S = 325,000 mm ² . The reason is that this area corresponds to a surface of about 100 m with a thin plate having a thickness of about 1 mm, and this region can be sufficiently evaluated.

Next, the second normalization variable (area ratio) y _max is calculated from the recursion period T and the following equation (5).
y _max = -ln [-ln {(T-1) / T}] (5)
Finally, √area is calculated from this second normalization variable (area ratio) y _max and the above equation (3), and this is √area _max . This √area _max is an index for evaluating the surface quality of the steel sheet, and is a numerical value indicating the size of the maximum non-metallic inclusion in the steel slab.
In the present invention, the square root √area _max of the cross-sectional area of the maximum non-metallic inclusion in the steel slab is 300 μm or less, particularly in the case of the electronic material stainless steel plate or the stainless steel plate for medical equipment with the strictest quality requirements. It is preferable. The reason for this is that with steel plates exceeding 300 μm, rolling by hot rolling or hot and cold rolling causes many surface defects due to non-metallic inclusions, ensuring good surface quality. This is because it is not suitable as a steel plate for the above-mentioned use. In addition, Preferably it is 200 micrometers or less, More preferably, it is 150 micrometers or less.

Hereinafter, although the typical manufacturing method of the stainless steel plate concerning this invention is demonstrated, this invention is not limited only to the method of illustration below.
First, raw materials are melted in an electric furnace, and then oxygen is blown and decarburized in AOD and / or VOD. As the raw material, iron scrap, stainless steel scrap, FeCr, FeNi or the like is used. Thereafter, Cr transferred into the slag is reduced with FeSi and / or metal Si, and further deoxidized and desulfurized. At this time, either or one of limestone and meteorite is added to form CaO—SiO ₂ slag together with the generated silica. Since magnesia-containing bricks (magcro, dolomite, magnesia carbon) are used as refractories for AOD or VOD, the slag is CaO-SiO ₂ -MgO system. For the purpose of protecting the refractory, refractory waste containing MgO may be added. Below, the component composition of the said stainless steel plate used by this invention is demonstrated.

Cr: 5-30wt%
Cr is an indispensable element for maintaining corrosion resistance and for forming a necessary passive film on the surface of the stainless steel plate. The presence of Cr improves acid resistance, pitting corrosion resistance, crevice corrosion resistance, and corresponding corrosion cracking resistance. Therefore, it is the most important element as a constituent component of the base material. If the Cr content is less than 5 wt%, the corrosion resistance will not be sufficient. On the other hand, if the Cr content is more than 30 wt%, a sigma phase is formed and the stainless steel plate is embrittled. For these reasons, the Cr content is preferably 5 to 30 wt%. Preferably, it is 6-28 wt%, More preferably, it is 6-25 wt%.

Ni: 30wt% or less
Ni improves the pitting corrosion resistance, crevice corrosion resistance, and stress cracking resistance in a solution environment containing chloride, and has the effect of making the structure austenitic. The present invention is effective for both austenitic stainless steel and ferritic stainless steel, so the Ni content is specified to be 30% or less. In addition, Preferably it is 28 wt% or less, More preferably, it is 25 wt%.

Si: 0.1-3wt%
Si is an element effective not only for acid resistance and pitting corrosion resistance but also for deoxidation. Further, the inclusion composition has a function of controlling the MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O ₃ —FeO-based nonmetallic inclusions. If the Si content is less than 0.1 wt%, deoxidation becomes insufficient and the amount of non-metallic inclusions increases. By increasing the number of non-metallic inclusions, the frequency of occurrence of large inclusions increases, and these large inclusions cause surface defects, making it impossible to satisfy the quality required for stainless steel sheets. On the other hand, if the Si content exceeds 3 wt%, the formation of a sigma phase composed of Fe, Cr, and Mo is promoted, and the stainless steel plate is embrittled and the weldability of the stainless steel plate is lowered. Furthermore, non-metallic inclusions are mainly composed of MgO / Al ₂ O ₃ spinel or MgO, and are easily clustered, causing surface defects.
Therefore, in the present invention, the Si content is determined to be 0.1 to 3 wt%. Within this range, it is preferably 0.2 to 2.5 wt%. More preferably, it is 0.3-2.3 wt%.
As the Si source, it is preferable to use a metal Si alloy and / or a FeSi alloy.

Mn: 0.3-3wt%
Mn is an element effective for deoxidation, and has a function of controlling the inclusion composition to MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O ₃ —FeO-based nonmetallic inclusions. However, the addition of an excessive amount of Mn (over 3 wt%) promotes the formation of the sigma phase and embrittles the stainless steel plate. Therefore, in the present invention, the Mn content is set to 0.3 to 3 wt%. Within this range, it is preferably 0.4 to 2.8 wt%. As the Mn source, metal Mn and / or SiMn is preferable.

Al: 0.0001 to 0.01wt%
Al has a function of controlling the inclusion composition to MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O ₃ —FeO-based nonmetallic inclusions. However, the addition of an excessive amount of Al (over 0.01 wt%) generates alumina inclusions, and the generated alumina inclusions are likely to cluster, and thus cause surface defects. Therefore, in the present invention, the Al content is determined to be 0.0001 to 0.01 wt%. In this range, it is preferably 0.0005 to 0.008 wt%.
In order to control the Al content within the above range, it is preferable to use metal Al or an FeSi alloy containing 0.1 to 2 wt% of Al.

Ca: 0.00001-0.002wt%
Ca has a function of controlling nonmetallic inclusions to MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O ₃ —FeO based nonmetallic inclusions. However, if the Ca content exceeds 0.002 wt%, the CaO concentration in the inclusions increases, which adversely affects corrosion resistance or etching processability. From such a viewpoint, the content of Ca is defined as 0.00001 to 0.002 wt%. More preferably, it is 0.00005-0.0015 wt%. In order to control Ca in this concentration range, it is preferable to use an FeSi alloy containing 0.1 to 2 wt% of Ca.

Mg: 0.00001-0.002wt%
Mg functions to control nonmetallic inclusions to MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O ₃ —FeO based nonmetallic inclusions. However, if the Mg content exceeds 0.002 wt%, nonmetallic inclusions are mainly composed of hard MgO / Al ₂ O ₃ spinel or MgO, which causes surface flaws and poor hole shape during etching. Or From such a viewpoint, the Mg content is defined as 0.00001 to 0.002 wt%. In this range, it is preferably 0.00005 to 0.0015 wt%. In order to control the Mg concentration within the above range, addition of metallic Mg is not preferable. Therefore, it is most preferable to use the following reaction.
2Al +3 (MgO) = (Al ₂ O ₃ ) +3 Mg ... Reaction in which Al reduces MgO in the slag () ... Components in the slag, ... Components in molten steel Specifically, metal Al is used, or FeSi alloy containing 0.1 to 2 wt% of Al is introduced to control the dissolved Al in molten steel to 0.0001 to 0.01 wt%. This Al reduces MgO in the slag, so that an appropriate amount of Mg can be supplied into the molten steel.

O: 0.001 to 0.007 wt%
O in the stainless steel sheet includes dissolved oxygen (solid solution oxygen) and oxygen contained as an oxide as a nonmetallic inclusion. The sum of both is defined as the total oxygen concentration, and here, the total oxygen concentration (hereinafter referred to as “O concentration”) is defined. If the O concentration is lower than 0.001 wt%, the Mg concentration in the molten steel becomes higher than 0.002 wt%, and the nonmetallic inclusions in the stainless steel plate are mainly composed of hard MgO · Al ₂ O ₃ spinel or MgO. For this reason, it causes surface flaws and acid stains during etching. On the other hand, if the O concentration exceeds 0.007 wt%, deoxidation is insufficient and the amount of non-metallic inclusions increases. Increasing the number of non-metallic inclusions increases the frequency of large inclusions, which causes surface defects and makes it impossible to satisfy the quality required for stainless steel sheets. Therefore, in the present invention, the O content is set to 0.001 to 0.007 wt%. More preferably, it is 0.001 to 0.005 wt%.

In order to control the oxygen concentration within the above range, it is necessary to control the Si and Mn concentrations that contribute to deoxidation within the range defined in the present invention described above. Preferably, the basicity of slag (CaO / SiO ₂ : weight percent ratio of CaO and SiO _{2 in} slag) is controlled to 1.5-4.
Although not particularly limited, in addition to this, any one or both of Mo and Cu may be contained in an amount of 10 wt% or less depending on the application. Mo is effective for further improving the corrosion resistance, and Cu is effective for improving the deep drawing characteristics. In applications where corrosion resistance is required, N may be contained in an amount of 0.3 wt% or less.

Then, in the present invention, non-metallic inclusions contained in stainless steel sheet, basically its components, MnO: 1~45wt%, CaO: 1~45wt%, SiO 2: 10~60wt%, Al 2 MnO—CaO—SiO ₂ —Al ₂ O ₃ —MgO—Cr ₂ O having O ₃ : 5 to 50 wt%, MgO: 0.5 to 30 wt%, Cr ₂ O ₃ : 0.2 to 10%, FeO: 0.2 to 10 wt% _It consists of _3- FeO non-metallic inclusions. The reason for this is to reduce the size by stretching and splitting during hot rolling and cold rolling, and to prevent enlargement in the molten steel. Furthermore, MgO.Al ₂ O ₃ inclusions may be contained in an amount of 50% by volume or less of the entire nonmetallic inclusions contained in the stainless steel plate.
Hereinafter, the grounds for limiting the composition of the nonmetallic inclusions described above will be described.

The content of MnO in the nonmetallic inclusion is preferably 1 to 45 wt%. The reason is that when the content in the nonmetallic inclusion is less than 1 wt% or more than 45 wt%, the melting point of the nonmetallic inclusion is increased, and the stretchability is reduced. As a result, large inclusions remain in the rolled stainless steel plate. In addition, the large inclusions remaining in the stainless steel plate cause a decrease in the etching property of the stainless steel plate, so that the quality required for the stainless steel plate cannot be satisfied.
In order to control within this range, the Mn concentration may be controlled within the range defined in the present invention.

The content of CaO in the non-metallic inclusion is preferably 1 to 45 wt%. The reason for this is that if it is less than 1 wt% or more than 45 wt%, the melting point of the non-metallic inclusions is increased, so that the stretchability is reduced, and as a result, in the stainless steel sheet after rolling. This is because large inclusions remain. In addition, the large inclusions remaining on the stainless steel plate cause a decrease in the etching property of the stainless steel plate, so that the quality required for the stainless steel plate cannot be satisfied.
In order to control within this range, the Ca concentration may be controlled within the range defined in the present invention.

The content of SiO ₂ in the nonmetallic inclusion is preferably 10 to 60 wt%. The reason is that if it is less than 10 wt% or more than 60 wt%, the melting point of the non-metallic inclusions is increased, so that the stretchability is lowered, and as a result, in the stainless steel sheet after rolling. This is because large inclusions remain. In addition, the large inclusions remaining on the stainless steel plate cause a decrease in the etching property of the stainless steel plate, so that the quality required for the stainless steel plate cannot be satisfied.
In order to control within this range, the Si concentration may be controlled within the range defined in the present invention.

The content of Al ₂ O ₃ in the nonmetallic inclusion is preferably 5 to 50 wt%. The reason is that if the content of Al ₂ O ₃ is less than 5 wt%, the melting point of the non-metallic inclusions is increased, so that the stretchability is lowered, and as a result, the stainless steel sheet after rolling is large in size. This is because the inclusions remain. In addition, the large inclusions remaining on the stainless steel plate cause a decrease in the etching property of the stainless steel plate, so that the quality required for the stainless steel plate cannot be satisfied. On the other hand, when the content of Al ₂ O ₃ exceeds 50 wt%, nonmetallic inclusions are easily clustered. In addition, when this large-sized alumina type cluster-like inclusion generate | occur | produces, it becomes a surface defect of a stainless steel plate, and cannot satisfy the quality requested | required with respect to a stainless steel plate.
In order to control within this range, the Al concentration may be controlled within the range defined in the present invention.

It is preferable that the content of MgO in the nonmetallic inclusion is 0.5 to 30 wt%. If it is less than 0.5 wt% or more than 30 wt%, the melting point of the non-metallic inclusions is increased, so that the stretchability is lowered, and as a result, large interspersed in the stainless steel sheet after rolling. This is because things remain. In addition, the large inclusions remaining on the stainless steel plate cause a decrease in the etching property of the stainless steel plate, so that the quality required for the stainless steel plate cannot be satisfied. Further, when the MgO content exceeds 30 wt%, the non-metallic inclusions become water-soluble, which adversely affects the corrosion resistance and etching properties of the stainless steel plate.
In order to control within this range, the Mg concentration may be controlled within the range defined in the present invention.

The content of Cr ₂ O ₃ in the nonmetallic inclusion is preferably 0.2 to 10 wt%. The reason is as described below. When the content of Cr ₂ O ₃ is less than 0.2 wt%, the melting point of the non-metallic inclusions is raised, so that the stretchability is lowered, and as a result, the stainless steel plate after rolling This is because large inclusions remain inside and nonmetallic inclusions are easily clustered. In addition, when such cluster-like large inclusions are generated, surface defects of the stainless steel plate occur, and the required quality cannot be satisfied. On the other hand, if the content of Cr ₂ O ₃ exceeds 10 wt%, the absolute amount of non-metallic inclusions will increase, and the frequency of large inclusions will increase, causing surface defects in stainless steel sheets. is there.
In order to control within this range, the O concentration may be controlled within the range defined in the present invention.

The content of FeO in the nonmetallic inclusion is preferably 0.2 to 10 wt%. The reason is that when the FeO content is less than 0.2 wt%, the melting point of the non-metallic inclusions is increased, so that the stretchability is lowered, and as a result, the stainless steel after rolling is reduced. This is because large inclusions remain in the steel sheet and non-metallic inclusions are easily clustered. When such cluster-like large inclusions are generated, a surface defect of the stainless steel plate occurs, and the required quality cannot be satisfied. On the other hand, if the content of FeO exceeds 10 wt%, the absolute amount of non-metallic inclusions increases, so the frequency of large inclusions increases, and surface defects of stainless steel sheets are eliminated. Because it causes.
In order to control within this range, the O concentration may be controlled within the range defined in the present invention.

The MgO · Al ₂ O ₃ inclusion is preferably 50% by volume or less with respect to the nonmetallic inclusion. This is because, in addition to the non-metallic inclusions having the composition described above, a MgO · Al ₂ O ₃ inclusions, also contain less than 50% by volume of the total non-metallic inclusions contained in stainless steel, MgO · Al ₂ This is because the O ₃ inclusions do not cluster and increase in size, and therefore do not cause surface defects in the stainless steel plate. More preferably, it is 45 volume% or less. The MgO · Al ₂ O ₃ inclusion may contain 15% by volume or less of MnO and Cr ₂ O ₃ in total.

Hereinafter, examples will be described to clarify the effects of the present invention.
(Manufacture of stainless steel sheets)
A stainless steel plate having the metal composition shown in Table 1 was produced as follows.
First, a raw material 60 ton made of iron scrap, stainless steel scrap, FeCr, FeNi, or the like was adjusted to a predetermined composition of Fe—Cr—Ni or Fe—Cr stainless steel molten steel while melting in an electric furnace. Then, decarburization is performed by any of the three processes of AOD treatment, VOD treatment, or AOD → VOD treatment, and then in AOD or VOD, Si alloy iron such as FeSi alloy is added and transferred into slag. The Cr was reduced into molten steel. At this time, limestone and / or meteorite was added as a flux and adjusted to a predetermined basicity (CaO / SiO ₂ : weight ratio of CaO and SiO _{2 in} the slag). For protection purposes, MgO-containing refractory debris was added along with the flux.
Next, after adjusting a trace component and temperature, it was cast by a normal ingot casting method or by a continuous casting method. After that, for continuous cast slabs and for ordinary ingot slabs, after a forging step, hot rolling was performed to obtain a stainless steel hot rolled sheet having a thickness of 3.0 to 10.0 mm. Some of these hot-rolled sheets were further rolled to a thickness of 0.5 to 1.0 mm by cold rolling.

(Survey and evaluation)
The following investigations were performed on the stainless steel plates of Invention Examples 1 to 7 and Comparative Examples 1 to 6.

A. Implementation of Extreme Value Statistical Processing Here, an example 1 of the invention will be described as an example.
In the cross section at 80 to 90 ° with respect to the rolling direction of the stainless steel plate, 15 thicknesses (n = 15) are selected as the plate thickness × 30 mm width as one inspection reference area S ₀ and the maximum in the width direction existing in the cross section. The maximum width (L) of the non-metallic inclusion having a width was observed and measured using an optical microscope, and the square root (√area) of the cross-sectional area of the largest non-metallic inclusion in the stainless steel slab was obtained. Here, the value of √area is √area = √π (L / 2) ² . Note that the width of the non-metallic inclusions was measured with a 200 × field of view using an optical microscope.
The square roots (√area) of the cross-sectional areas of the largest non-metallic inclusions in the stainless steel slab obtained from the 15 observation points obtained above are arranged in order from the smallest value, and (√area) _{1 in} ascending order. (√area) ₂ , ... (√area) ₁₅ Then, a first normalized variable (ascending order) y _j corresponding to the value of √area ((√area) _j ) of the _jth nonmetallic inclusion corresponding to the _jth was obtained from equation (3).
Table 1 summarizes j, (√area) _j and y _j .

  In addition, the results of Table 1 show the maximum non-metallic inclusions in the stainless steel slab, with the value of the square root of the cross-sectional area of the largest non-metallic inclusion in the stainless steel slab as the horizontal axis and the corresponding normalized variable as the vertical axis. FIG. 1 is a plot in which the value of the square root of the cross-sectional area of an object is plotted in ascending order. Then, the plotted points were recursed linearly to obtain a straight line going up to the right as shown in FIG. This straight line is the equation (1).

Next, from the value of the area S (325,000 mm ² ) for estimating the size of the maximum non-metallic inclusion, the area S ₀ ^* corresponding to the stainless steel slab state of the inspection reference area S ₀ , and the formula (2) ′ A recursion period T was obtained, and a second standardized variable (area ratio) y _max was obtained from this T and the expression (3) ′. Then, from this y _max and the equation (1) ′, the square root √ area _max of the cross-sectional area of the estimated maximum non-metallic inclusion in the stainless steel slab was obtained.

B. Analysis of metal composition Quantitative analysis was performed by fluorescent X-ray analysis.

C. Analysis of non-metallic inclusion composition
The inclusions in the stainless steel plate were quantitatively analyzed by EDS (energy dispersive analyzer) at 10 locations, and the average value was taken as the composition of the nonmetallic inclusions in the stainless steel plate. This value is an average value including spinel inclusions.

D. Analysis of the proportion of spinel inclusions
When a 10g stainless steel plate is dissolved in a bromine methanol solution, only the metal part is dissolved. When this is filtered, non-metallic inclusions are obtained in the residue. The residue of this nonmetallic inclusion was observed by SEM, and the volume% was measured. The spinel inclusions are angular and can be distinguished.

E. Analysis of surface defects In the appearance inspection line, the length of 100 m (the width was in the range of 0.7 to 1.5 m) was visually observed to determine the presence or absence of defects.

(result)
Table 2 summarizes the component composition (wt%) of the stainless steel slab subjected to the analysis of B to E and the average composition (wt%) of nonmetallic inclusions which are inevitable impurities of the alloy. And the said analysis result performed about the stainless steel slab of Table 2 was put together in Table 3.

As shown in Invention Examples 1 to 7 in Table 3, √area _max is 300 μm or less, and the nonmetallic inclusions are MnO—SiO ₂ —Al ₂ O ₃ —CaO—MgO—FeO system, and MnO: 1 to 45wt%, CaO: 1~45wt%, SiO 2: 10~60wt%, Al 2 O 3: 5~50wt%, MgO: 0.5~30wt%, Cr 2 O 3: 0.2~10wt%, FeO: 0.2~10wt %, And the ratio of spinel inclusions is 50% by volume or less with respect to the entire nonmetallic inclusions, the surface is rolled even to the product thickness (0.01 to 0.5 mm). The occurrence of defects was not confirmed.

On the other hand, as shown in Comparative Examples 1 to 6, when the value of √area _max exceeded 300 μm, generation of surface defects was confirmed when rolling to a product thickness (0.01 to 0.5 mm). In particular, for stainless steel sheets with a √area _max value exceeding 300 μm, surface defects are confirmed when rolling to product thickness (0.01 to 0.5 mm) even if the proportion of spinel inclusions is 50% by volume or less with respect to the total metal inclusions. (Comparative Examples 1, 2, and 4).
In FIG. 2, the result of implementation of the extreme value statistical processing of Invention Examples 2 and 4 and Comparative Examples 2 and 3 is shown.

Table 4 shows the influence of the rolling reduction on the inspection accuracy and the inspection time. Here, in each example, the same specimen (stainless steel slab of Comparative Example 3 in Table 2) in which surface defects were finally recognized at the stage of appearance inspection was used. As shown in Invention Examples 8 and 9 in Table 4, when the rolling reduction ratio of the material to be inspected is 90.00 to 99.83%, since √area _max exceeds 300 μm, it can be properly evaluated that surface defects occur. . Also, the time required for the inspection is 3 hours or less, which shows that it is sufficiently practical.

When the rolling reduction shown in the comparative example exceeded 99.83% (Comparative Example 7), the measurement result was 300 μm or less although the surface defect was finally confirmed. This is because the degree of dispersion of inclusions has increased, and accurate measurement has been impossible.
On the other hand, when the rolling reduction is less than 90.00% (Comparative Examples 8 and 9), it is possible to estimate the surface defect with high accuracy, but the time required for the inspection is 3 days or 7 days. Long and inconvenient for conducting a practical test.

  The technology of the present invention is used in the field of stainless steel manufacturing, particularly in fields that require cold rolling to about 0.01 to 0.5 mm and then require high precision processing such as punching and etching, such as hard disks. It is also used in the field of manufacturing electronic materials such as suspension materials and gun parts.

Sectional drawing of the thickness direction of a stainless steel plate and a stainless steel slab. A recursive line with extreme statistics. A recursive line with extreme statistics.

Explanation of symbols

1 Inspection standard area (S ₀ )
2 Area for estimating the size of the largest non-metallic inclusion (S)
3 Area at the time of steel slab corresponding to inspection reference plane S ₀ (S ₀ ^* )

Claims (2)

Cr: 5-30 wt%, Ni: 30 wt% or less, Si: 0.1-3 wt%, Mn: 0.3-3 wt%, Al: 0.0001-0.01 wt%, Ca: 0.00001-0.002 wt%, Mg: 0.00001-0.002 wt% , O: 0.001 to 0.007 wt%, and the balance of the steel slab consisting of Fe and inevitable impurities, hot rolled or hot and cold rolled to produce a stainless steel plate,
Rolling reduction prior to final product: the steel plate subjected to from 90.00 to 99.83% of the rolling, to not perpendicular to the rolling direction, and the inspection standard area a part of a cross section cut at approximately a right angle, the inspection criteria is within the area MnO: 1~45wt%, CaO: 1~45wt %, SiO 2: 10~60wt%, Al 2 O 3: 5~50wt%, MgO: 0.5~30wt%, Cr 2 O 3: 0.2~ 10wt%, FeO: 0.2~10wt% of microscopically observing a widthwise length of the non-metallic inclusions having the component composition, the maximum width direction length values non-metallic inclusions of the steel sheet obtained by the observation the maximum estimates the size} area _max of nonmetallic inclusions, size} area _max is Ru der less 300μm steel slab of the estimated maximum non-metallic inclusions in the slab stage of the steel sheet, a stainless steel plate using in manufacturing,
Estimating the size √area _max of the estimated maximum non-metallic inclusions in the steel slab stage ,
The width direction length (L) of the maximum non-metallic inclusion in an arbitrary inspection reference area S _{0 of} a cross section cut at right angles or almost at right angles to the rolling direction is measured, and this width direction length (L ) Is equal to the diameter of the maximum non-metallic inclusion existing in the area S ₀ ^* of the slab stage corresponding to the inspection reference area S ₀ , the maximum non-existence existing in the slab from the following equation (1) The procedure of calculating the √area of the metal inclusion area is repeated at a plurality of locations (n) of the cut surface, and the calculated n √areas are rearranged in ascending order, and the jth from the following equation (2) By obtaining a first standardized variable (ascending order) y _j corresponding to √area, and processing by extreme value statistics with this first standardized variable (ascending order) y _j and the √area, the following equation (3) The average normalization variable y shown in Fig. 2 is obtained, and then the particle size of the maximum nonmetallic inclusion is estimated. The recursion period T is calculated from the following equation (4) from the area S to be measured and the slab area S ₀ ^* corresponding to the inspection reference surface . A standardized variable (area ratio) y _max is calculated, and from this second standardized variable (area ratio) y _max and the following equation (3), the square root √area _max of the cross-sectional area of the largest nonmetallic inclusion in the steel slab A method for producing a stainless steel sheet, characterized by calculating
Record
√area = √π (L / 2) ² (1)
y _j = −ln [−ln {j / (n + 1)}] (j = 1 to n) (2)
y = a√area + b (3)
T = (S + S ₀ ^* ) / S ₀ ^* (4)
y _max = -ln [-ln {(T-1) / T}] (5)
However,
n: Number of inspection
y _j : First normalization variable (ascending order)
y: Average normalization variable
a: Recursive coefficient
b: Constant
S: Area for estimating the size of the maximum nonmetallic inclusion
S ₀ ^* : Area of steel slab corresponding to inspection reference plane S ₀
T: Recursion period
y _max : Second normalization variable (area ratio) _{2. The} method for producing a stainless steel plate according to claim 1, wherein MgO · Al ₂ O ₃ inclusions are contained in an amount of 50% by volume or less of the whole nonmetallic inclusions.

Images