KR20200100159A - Ferritic stainless steel sheet and its manufacturing method - Google Patents

Abstract

The ferritic stainless steel sheet of the present invention includes Cr: 11.0 to 30.0%, C: 0.001 to 0.030%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, P: 0.003 to 0.100%, S: 0.0100% or less, N: 0.030% or less, B: 0 to 0.0025%, Sn: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 2.00%, W: 0 to 1.00%, Al: 0 to 1.00%, Co: 0 to 0.50%, V: 0 to 0.50%, Zr: 0 to 0.50%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Y: 0 to 0.10%, Hf: 0 To 0.10%, REM: 0 to 0.10%, Sb: 0 to 0.50% and Ti: 0.40% or less, Nb: 0.50% or less, one or both of them, the remainder consisting of Fe and impurities, and present as a phosphide The amount of P is 0.003% by mass or more, and the crystal grain size number measured in JIS G 0551 is 9.0 or more.

Description

Ferritic stainless steel sheet and its manufacturing method

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet and a method of manufacturing the same, and particularly, to a ferritic stainless steel sheet having excellent formability during molding processing and a process surface roughness, and a manufacturing method thereof.

This application claims priority based on Japanese Patent Application No. 2018-069775 for which it applied to Japan on March 30, 2018, and uses the content here.

SUS304 (18Cr-8Ni), a representative steel grade of austenitic stainless steel, is widely used in home appliances, kitchenware, and building materials because of its excellent corrosion resistance, workability, and beauty. However, since SUS304 is expensive and contains a large amount of Ni, which has fluctuating prices, the price of the steel sheet is high. On the other hand, since ferritic stainless steel does not contain Ni or has a very small content, demand is increasing as a material excellent in cost performance. However, in the case of using ferritic stainless steel as a molding application, problems are the molding limitations and the deterioration of the surface roughness due to the formation of surface irregularities after molding.

First, compared with respect to the forming limit, in the case of austenitic stainless steel, stretch formability is excellent, but the stretch formability of ferritic stainless steel is low, and the shape cannot be changed significantly. However, since the deep drawability can be controlled by adjusting the crystal orientation (collective structure), when a ferritic stainless steel is used for molding purposes, a molding method mainly based on deep drawing is used.

Next, surface properties after molding processing, in particular, processing surface roughness (surface irregularities after molding) will be described. Here, "surface unevenness" refers to fine unevenness (surface roughness) generated on the surface of the steel sheet after processing or molding is performed, and since these fine unevenness correspond to crystal grains, the larger the crystal grain size, the more remarkable the surface unevenness.

In the case of austenitic stainless steel, since the work hardening property is excellent and the fine grain structure is relatively easy to produce, a steel sheet having a crystal grain size number of about 10 is manufactured. For this reason, surface irregularities (surface roughness) after molding processing are small, and there is hardly any problem. On the other hand, the grain size of ferritic stainless steel is about 9 in SUS430 and about 7 in SUS430LX, which is smaller than that of austenitic stainless steel. Here, the small particle size number indicates that the crystal grain size is large.

Ferritic stainless steel is a factor that tends to be granulated. In addition to the fact that ferritic stainless steel tends to have a large recrystallized grain size, high-purity ferritic stainless steel, such as SUS430LX, which reduces C and N to improve workability and formability. , Because it is easy to grow. In addition, in ferritic stainless steels, even if the number of cold rolling is increased to produce a product plate having a fine grain size, surface roughness is sometimes generated, and the cause is not necessarily clear.

When relatively strict formability is required, such as a housing or an article of a home appliance, a high-purity ferritic stainless steel such as SUS430LX is often used for ferritic stainless steel. In addition, in order to ensure the strength after molding, the thickness of the stainless steel sheet used is 0.6 mm or more in most cases, but as described above, the ferritic stainless steel has a large grain size, so the surface roughness after molding is large and The removal of surface irregularities by this is usually performed.

From the background described above, a method for reducing the surface roughness of high-purity ferritic stainless steel is disclosed.

Patent Document 1 discloses a ferritic stainless steel excellent in formability with little processing surface roughness by controlling the size and crystal grain size of precipitated particles using a high-purity ferritic stainless steel, and a method for producing the same. However, in Patent Document 1, although a steel sheet having a small crystal grain size is obtained, the deep drawability at the time of molding is not sufficient, and even though the crystal grain size is small, there is a problem that the surface roughness after molding is likely to occur.

In Patent Document 2, a ferritic stainless steel containing Ti and Nb is hot-rolled at a low temperature, and a stainless steel having excellent inner surface roughness at the time of forming is made fine by taking a high cold-rolling rate. The manufacturing technology is disclosed. By such a technique, the stainless steel of Patent Document 2 has a crystal grain size number of 9.5 and a fine structure is obtained, but the surface roughness after cup drawing molding is not necessarily sufficient.

Patent Document 3 discloses a ferritic stainless steel excellent in deep drawability, ridging property, and inner surface roughness by controlling the crystal grain size before final cold rolling of a steel having a component composition containing Nb and/or Ti. However, in Patent Document 3, the crystal grain size of the final product is 15 µm (crystal grain size number 9.1), and the surface roughness is insufficient.

As described above, when the molding process of ferritic stainless steel is considered, it is possible to mold into a predetermined shape, and it is very difficult to satisfy the surface characteristics after molding. For this reason, when ferritic stainless steel is used for molding, it is necessary to perform a polishing step in order to remove surface irregularities generated after molding. However, in this polishing process, it takes a polishing time and increases the manufacturing cost. In addition, there is a problem such as a lot of dust generated by polishing.

Japanese Patent No. 4749888 Japanese Patent Application Publication No. Hei 7-292417 Japanese Patent No. 3788311 Publication

The present invention has been made in view of the above problems, and provides a ferritic stainless steel sheet having excellent molding processability and post-molding surface roughness, and a method for producing the same.

As a factor affecting the processing surface roughness of ferritic stainless steel, the grain size and deformation amount are known. However, as described above, even if the crystal grain size or the amount of deformation is increased by controlling the cold rolling conditions, there is a case where the processing surface roughness occurs, and in recent years, a steel capable of more stably suppressing the occurrence of the processing surface roughness is desired. there was.

Therefore, the present inventors investigated the relationship between the working surface roughness and the metal structure in ferritic stainless steel. It was found for the first time that not only the crystal grain size and the amount of deformation known in the prior art, but also the amount of precipitation of precipitates in the steel affects the surface roughness. In addition, in order to control the amount of precipitation in an appropriate range, it is necessary to control the heat treatment temperature before and after cold rolling, and it was found that rapid heating is required in the heat treatment after cold rolling.

The summary of one embodiment of the present invention is as follows.

[1] In mass%,

Cr: 11.0% or more and 30.0% or less,

C: 0.001% or more and 0.030% or less,

Si: 0.01% or more and 2.00% or less,

Mn: 0.01% or more and 2.00% or less,

P: 0.003% or more and 0.100% or less,

S: 0.0100% or less,

N: 0.030% or less,

B: 0% or more and 0.0025% or less,

Sn: 0% or more and 0.50% or less,

Ni: 0% or more and 1.00% or less,

Cu: 0% or more and 1.00% or less,

Mo: 0% or more and 2.00% or less,

W: 0% or more and 1.00% or less,

Al: 0% or more and 1.00% or less,

Co: 0% or more and 0.50% or less,

V: 0% or more and 0.50% or less,

Zr: 0% or more and 0.50% or less,

Ca: 0% or more and 0.0050% or less,

Mg: 0% or more and 0.0050% or less,

Y: 0% or more and 0.10% or less,

Hf: 0% or more and 0.10% or less,

REM: 0% or more and 0.10% or less,

Sb: 0% or more and 0.50% or less are included, and

Ti: 0.40% or less, Nb: 0.50% or less, one or both are included, and the remainder is composed of Fe and impurities,

The amount of P present as a phosphor is 0.003% by mass or more,

A ferritic stainless steel sheet having a crystal grain size number of 9.0 or higher as measured in JIS G 0551.

[2] in mass%,

B: 0.0001% or more and 0.0025% or less,

Sn: 0.005% or more and 0.50% or less,

Ni: 0.05% or more and 1.00% or less,

Cu: 0.05% or more and 1.00% or less,

Mo: 0.05% or more and 2.00% or less,

W: 0.05% or more and 1.00% or less,

Al: 0.05% or more and 1.00% or less,

Co: 0.05% or more and 0.50% or less,

V: 0.05% or more and 0.50% or less,

Zr: 0.05% or more and 0.50% or less,

Ca: 0.0001% or more and 0.0050% or less,

Mg: 0.0001% or more and 0.0050% or less,

Y: 0.001% or more and 0.10% or less,

Hf: 0.001% or more and 0.10% or less,

REM: 0.001% or more and 0.10% or less,

Sb: The ferritic stainless steel sheet according to the above [1], further containing 0.005% or more and 0.50% or less of one or two or more.

[3] A hot-rolling step of hot rolling the steel having the components described in [1] or [2] above, and a hot-rolled sheet annealing step of performing heat treatment at a temperature of 850°C to 900°C after the hot rolling step. And, after the hot-rolled sheet annealing step, a cold-rolling step of rolling with a rolling rate of 75% or more and 90% or less, and a cold-rolled sheet annealing step performed following the cold-rolling step, and in the cold-rolled sheet annealing step, During the heating process, the average temperature rise rate in the temperature range of 400°C to 800°C is 80°C/s or more, and the maximum attainment temperature of the plate temperature is 880°C or more and 980°C or less, and cooling is started within 5 seconds after reaching the maximum temperature. And, the method for producing a ferritic stainless steel sheet according to [1] or [2], characterized in that cooling is performed at an average cooling rate of 50°C/s or more in a temperature range from the highest attainable temperature to 700°C. .

[4] A hot rolling step of hot rolling the steel having the components described in [1] or [2] above, and after the hot rolling step, heat treatment is performed at a temperature of 850°C or more and 900°C or less, and exist as a print material. A hot-rolled sheet annealing process in which the amount of P is 0.003% by mass or more; a cold-rolling process in which a rolling rate is 75% or more and 90% or less after the hot-rolled sheet annealing process; and a cold-rolled sheet performed following the cold-rolling process. An annealing process is provided, and in the cold-rolled sheet annealing process, the average temperature increase rate in the temperature range of 400°C to 800°C during the heating process is 80°C/s or more, and the maximum attainment temperature of the plate temperature is 880°C or more and 980°C. The above [1], characterized in that: below, and starting cooling within 5 sec after reaching the highest reaching temperature, and cooling with an average cooling rate in the temperature range from the highest reaching temperature to 700°C being 50°C/s or more. ] Or the method for producing a ferritic stainless steel sheet according to [2].

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet having excellent molding processability and surface roughness after molding.

1 is a TEM observation result (TEM photograph) of a recrystallized structure of a ferritic stainless steel sheet according to the present embodiment.
2 is a diagram showing the relationship between the crystal grain size number and the precipitation amount Pp of P according to the present embodiment.

Hereinafter, each requirement of the ferritic stainless steel sheet according to an embodiment of the present invention will be described in detail. In addition, the indication of "%" of the content of each element means "mass%".

The reasons for limiting the component (I) will be described below.

Cr is an element that improves corrosion resistance, which is a basic characteristic of stainless steel. If it is less than 11.0%, since sufficient corrosion resistance cannot be obtained, the lower limit is 11.0% or more. On the other hand, when an excessive amount of Cr is contained, the formation of an intermetallic compound corresponding to the sigma phase (Fe-Cr intermetallic compound) is promoted to promote cracking during manufacture, so the upper limit is set to 30.0% or less. From the viewpoint of stable manufacturability (yield, rolling marks, etc.), 14.0% or more and 25.0% or less are preferable. More preferably, 16.0% or more and 20.0% or less are good.

Since C is an element that lowers the moldability important in the present embodiment, the smaller one is preferable, and the upper limit is set to 0.030% or less. However, since excessive reduction causes an increase in refining cost, the lower limit is set to 0.001% or more. When both refining cost and moldability are considered, 0.002% or more and 0.020% or less are preferable.

Si is an element that improves oxidation resistance, but containing an excessive amount of Si causes a decrease in formability, so the upper limit is set to 2.00% or less. From the viewpoint of formability, it is preferable that the amount of Si is low, but excessive reduction causes an increase in raw material cost, so the lower limit is set to 0.01% or more. From the viewpoint of manufacturability, a preferable range is 0.05% or more and 1.00% or less, and more preferably 0.05% or more and 0.30% or less.

Like Si, when Mn contains a large amount of Mn, the moldability decreases, so the upper limit is set to 2.00% or less. From the viewpoint of moldability, it is preferable that the amount of Mn is lower, but excessive reduction causes an increase in the cost of raw materials, so the lower limit is set to 0.01% or more. From the viewpoint of manufacturability, a preferable range is 0.05% or more and 1.00% or less, and more preferably 0.05% or more and 0.30% or less.

P is an important element that contributes to the improvement of the surface roughness by precipitation as a phosphorus in the steel sheet of the present embodiment. The amount of P is made 0.003% or more because the amount of precipitation of the prints is secured and the surface roughness is improved. However, since P is an element that lowers the formability, the upper limit is made 0.100% or less. In addition, excessive reduction in the amount of P causes an increase in the raw material cost, and when both the formability and the surface roughness are considered, the preferable range is 0.010% or more, 0.050% or less, more preferably 0.020% or more, It is 0.040% or less.

S is an impurity element, and since it promotes cracking during production, the lower one is preferable, and the upper limit is set to 0.0100% or less. The lower the amount of S is, the more preferable it is, and 0.0030% or less is preferable. On the other hand, since excessive reduction causes an increase in refining cost, the lower limit is preferably 0.0003% or more. From the viewpoint of manufacturability and cost, the preferable range is 0.0004% or more and 0.0020% or less.

Like C, N is an element that lowers the moldability, and the upper limit is set to 0.030% or less. However, since excessive reduction leads to an increase in refining cost, the lower limit is preferably 0.002% or more. From the viewpoint of moldability and manufacturability, the preferable range is 0.005% or more and 0.015% or less.

Any one or both of Ti and Nb is contained as follows.

Ti combines with C and N to fix C and N as precipitates such as TiC and TiN, and improves the r value and product extensibility through high purity. In order to obtain these effects, when containing Ti, it is preferable to make the lower limit 0.03% or more. On the other hand, when the content is excessive, an increase in the alloy cost and a decrease in manufacturability accompanying an increase in the recrystallization temperature are caused, so the upper limit is set to 0.40% or less. From the viewpoint of moldability and manufacturability, the preferable range is 0.05% or more and 0.30% or less. In addition, a suitable range in which the above effect of Ti is actively utilized is 0.10% or more and 0.20% or less.

Like Ti, Nb is also a stabilizing element that fixes C and N, and increases the r value and product extensibility through high purity of steel by this action. In order to obtain these effects, when Nb is contained, the lower limit is preferably set to 0.03% or more. On the other hand, excessive inclusion leads to an increase in alloy cost and a decrease in manufacturability accompanied by an increase in recrystallization temperature, so the upper limit is set to 0.50% or less. From the viewpoint of alloy cost and manufacturability, the preferable range is 0.03% or more and 0.30% or less. In addition, a suitable range in which the above effect of Nb is actively utilized is 0.04% or more and 0.15% or less. More preferably, it is 0.06 to 0.10%.

The ferritic stainless steel sheet of this embodiment is composed of Fe and impurities other than the above-described elements (the balance), but in this embodiment, in addition to the above-described basic composition, one or two or more of the following element groups You may selectively contain. That is, the lower limit of the content of B, Sn, Ni, Cu, Mo, W, Al, Co, V, Zr, Ca, Mg, Y, Hf, REM, and Sb is 0% or more.

In addition, the "impurity" in this embodiment is a component mixed by various factors of the manufacturing process, including raw materials such as ore and scrap when industrially manufacturing steel, and includes a component that is inevitably mixed.

B is an element that improves secondary workability. Since 0.0001% or more is required in order to exhibit the effect, this is set as the lower limit. On the other hand, since excessive inclusion causes deterioration in manufacturability, particularly castability, the upper limit is 0.0025% or less. A preferable range is 0.0003% or more and 0.0012% or less.

Since Sn is an element having an effect of improving corrosion resistance, it may be contained depending on the corrosive environment at room temperature. Since the effect is exhibited at 0.005% or more, this is set as the lower limit. On the other hand, when it is contained in a large amount, deterioration in manufacturability is caused, so 0.50% or less is used as the upper limit. In consideration of manufacturability, the preferable range is 0.02% or more and 0.10% or less.

Ni, Cu, Mo, Al, W, Co, V, and Zr are elements effective to increase corrosion resistance or oxidation resistance, and may be contained as necessary. The effect is expressed by making each content of Ni, Cu, Mo, Al, W, Co, V, and Zr 0.05% or more. However, excessive inclusion causes not only a decrease in formability, but also leads to an increase in alloy cost and impairing manufacturability. Therefore, the upper limit of Ni, Cu, Al, and W is 1.00% or less. The upper limits of Ni, Cu, Al, and W are preferably 0.50% or less. Since Mo causes a decrease in manufacturability, the upper limit is set to 2.00% or less. The upper limit of Mo is preferably 1.00% or less. The upper limit of Co, V, and Zr is 0.50% or less in consideration of the effect of improving corrosion resistance or oxidation resistance. The lower limit of the more preferable content of any element of Ni, Cu, Mo, Al, W, Co, V, and Zr is 0.10% or more.

Ca and Mg are elements which improve hot workability and secondary workability, and may be contained as necessary. However, excessive contention leads to impairing the manufacturability, so the upper limit of Ca and Mg is 0.0050% or less. All preferred lower limits are 0.0001% or more. When manufacturability and hot workability are considered, the preferable range is 0.0002% or more and 0.0010% or less for both Ca and Mg.

Y, Hf, and REM are elements effective for improving hot workability, steel cleanliness, and oxidation resistance, and may be contained as necessary. In the case of containing, the upper limit is respectively 0.10% or less. A preferable lower limit is 0.001% or more in all of Y, Hf, and REM. Here, "REM" in this embodiment is composed of one or more selected from the group of elements (lanthanoids) belonging to atomic numbers 57 to 71, for example, La, Ce, Pr, Nd, etc. . In addition, the content of "REM" in this embodiment is the total amount of lanthanoids.

Like Sn, Sb is an element having an effect of improving corrosion resistance, and may be contained as necessary. However, when it is contained in a large amount, the deterioration of the manufacturability is caused, so 0.50% or less is the upper limit. On the other hand, since the effect of improving corrosion resistance is exhibited at 0.005% or more, this is set as the lower limit.

The ferritic stainless steel sheet of this embodiment is composed of Fe and impurities (including inevitable impurities) other than the above-described elements, but contains in addition to each of the above-described elements to the extent that the effect of this embodiment is not impaired. I can make it. In this embodiment, for example, Bi, Pb, Se, H, Ta, etc. may be contained, but in that case, it is preferable to reduce it as much as possible. On the other hand, in the limit of solving the problem of this embodiment, the content ratio of these elements is controlled, and if necessary, 1 of Bi≤100ppm, Pb≤100ppm, Se≤100ppm, H≤100ppm, Ta≤500ppm You may contain more than a species.

(II) Next, the metal structure will be described.

The ferritic stainless steel sheet of this embodiment is made of a single-phase ferrite structure having a crystal grain size number of 9.0 or more.

The crystal grain size number should be 9.0 or more. The processing surface roughness after molding is less likely to occur as the grain size number increases, that is, as the grain size of ferrite grains decreases, so this is the lower limit. In order to further suppress the surface roughness, it is preferably more than 9.5, and more preferably more than 10.0. However, when the grain size of the crystal grains becomes excessively small, there is a fear that the strength increases and the press formability decreases. For this reason, it is preferable that the crystal grain size number is 12 or less.

The crystal grain size number can be obtained by the line segment method of JIS G 0551 (2013). In addition, "particle size number: 9" corresponds to that the average line segment length per crystal grain crossing the inside of the crystal grain is 14.1 µm, and "particle size number: 10" means that the average line segment length per crystal grain crossing the inside of the crystal grains is 10.0 It corresponds to that of µm. In the measurement of the crystal grain size, the number of crystal grains transverse to one sample is set to 500 or more from an optical microscopic structure photograph of a cross section of a test piece. The etching solution is good for aqua regia or inverse aqua regia, but other solutions may be used as long as the grain boundary can be determined. Further, depending on the orientation relationship of adjacent crystal grains, since grain boundaries may not be clearly visible, it is preferable to perform thick etching. In addition, in the measurement of the grain boundary, it is assumed that the twin grain boundary is not measured.

In addition, the metal structure of the ferritic stainless steel sheet of the present embodiment is composed of a single-phase ferrite structure, and a precipitate (printed product) of P described later is generated. This means that it does not contain an austenitic or martensitic structure. This is because it is relatively easy to make the crystal grain size fine in the case of containing an austenite phase or martensite structure. In addition, the austenite phase exhibits high moldability due to the TRIP effect. However, in addition to an increase in the cost of raw materials, a decrease in yield such as edge cracking tends to occur during manufacturing, so that the metal structure is a single-phase ferrite structure. In addition, there are cases in which precipitates such as carbonitrides are present in addition to phosphides in the steel, but these do not greatly influence the effect of the present embodiment, and the structure of the columnar phase is described above.

(III) Next, the amount of P precipitated will be described.

In general, it is considered that the content of P in the ferritic stainless steel sheet should be reduced from the viewpoint of lowering the formability (r value and product elongation). However, as a result of the study of the present inventors, it was found for the first time that the amount of precipitation of prints in the steel affects the roughness of the working surface. From this, in this embodiment, by controlling the amount of P present as a print, that is, the amount of precipitation Pp of P in addition to the control of the crystal grain size, it is clarified that the processing surface roughness can be further suppressed stably, P It is characterized in that it defines the precipitation amount Pp of.

In this way, since the prints in the steel greatly contribute to suppressing the processing surface roughness, it is necessary to ensure the amount of P precipitated. From this, in the present embodiment, the amount of P (precipitated amount Pp of P) present as a print is made 0.003% by mass or more. Preferably it is 0.004 mass% or more, More preferably, it is 0.005 mass% or more. The upper limit of the precipitation amount Pp of P is not particularly limited, but since the upper limit of the P content of the steel sheet is 0.100% or less, the upper limit of the precipitation amount Pp of P may be 0.100% or less so as to be the same. Incidentally, examples of the prints mentioned in the present embodiment include Fe phosphides, Mn phosphides, Ti phosphides, Nb phosphides, Al phosphides, and the like, but the type or composition is not particularly limited. That is, in the present embodiment, it is important that the amount of P present as a print (precipitate Pp of P) is within the above range, regardless of the specific composition and form of the print.

The details of the method of controlling the precipitation amount Pp of P within the above range will be described later, but the treatment temperature of the heat treatment (hot rolled sheet annealing and finish annealing) performed before and after the cold rolling process is controlled, and the heating in the heat treatment after cold rolling It can be controlled by performing the process rapidly.

The cause of the precipitated prints contributing to the suppression of processing surface roughness is under intensive investigation, but at present, it is judged as follows.

In general, since the precipitate is easily precipitated in the grain boundary phase, it is considered that most of the prints precipitated by hot-rolled sheet annealing are also precipitated in the grain boundary phase. Thereafter, it is considered that the metal structure is crushed by cold rolling and, with the elongation in the rolling direction, the prints deposited in the grain boundary phase are arranged substantially parallel to each other in the rolling direction. From that state, when recrystallization is attempted by performing finish annealing with rapid heating, short-term maintenance, and rapid cooling, a recrystallized structure of a metal structure is obtained without changing the above-described precipitation state of the print material. That is, by performing the finish annealing by rapid heating, holding for a short period of time, and rapid cooling, a recrystallized structure is obtained in which the prints are kept parallel to the rolling direction.

In fact, the inventors of the present invention can confirm that the prints in the crystal grains of the recrystallized structure are arranged parallel in the rolling direction in the thin film TEM observation of the product plate manufactured by this manufacturing method (within the manufacturing method range of this embodiment described later). have. 1 shows a result of TEM observation of a recrystallized structure in a steel sheet manufactured under conditions satisfying the present embodiment described later. As also clear from Fig. 1, it can be confirmed that P-products are precipitated so as to follow the rolling direction in the crystal grains of the recrystallized structure. In addition, whether or not the precipitate precipitated in the crystal grains was a P product was identified by EDS analysis and electron diffraction pattern analysis.

When a stainless steel sheet having such precipitated prints is processed and deformed, the prints arranged parallel to each other may hinder the movement of dislocations. As a result, it is thought that this printed matter exhibited the same effect as the grain boundary and contributed to suppression of the processing surface roughness.

The precipitation amount Pp of P is measured by the following electrolytic extraction residue method.

From the center of the stainless steel sheet in the width direction, a test piece of about a square size of 30 mm on one side is cut out, and the entire surface of the test piece corresponding to the surface of the steel sheet is wet-polished with a water resistant polishing paper of number #600. After polishing, the test specimen base material (stainless steel base material) is dissolved by electrolysis in a methanol solution containing 10% maleic anhydride and 2% tetramethylammonium chloride at an electrostatic potential of -100 mV. After electrolysis, the residue (precipitate) remaining in the solution without being dissolved is captured using a 200 nm mesh filter. The captured precipitate is washed with pure water and dried. Subsequently, the precipitate is dissolved with aqua regia and perchloric acid, and elemental analysis is performed using ICP emission spectroscopy in accordance with JIS G 1258 to determine the mass of P in the precipitate. The amount of P obtained was divided by the amount of change in the mass of the test piece by electrolysis ("the mass of the test piece before electrolysis"-"the mass of the test piece after electrolysis") and expressed as a percentage as "precipitated amount Pp of P" (mass%). do.

(IV) Next, the manufacturing method of the ferritic stainless steel sheet of this embodiment is demonstrated.

In the method for producing a ferritic stainless steel sheet according to the present embodiment, hot rolling, hot-rolled sheet annealing, cold-rolling, and cold-rolled sheet annealing (finish annealing) are combined, and if necessary, pickling is performed appropriately. That is, as an example of the manufacturing method, for example, a manufacturing method comprising each step of steel making-hot rolling-hot rolled sheet annealing-cold rolling-cold rolled sheet annealing (finish annealing) can be adopted.

In this embodiment, the conditions to be controlled in order to satisfy both the crystal grain size and the precipitated state of the phosphorus as described above are the conditions of the heat treatment after hot rolling (hot rolled sheet annealing), the cold rolling rate, and the heat treatment after cold rolling (cold rolling). Plate annealing), and other processes and conditions are not particularly limited.

After hot rolling, heat treatment (hot-rolled sheet annealing) is performed at a temperature of 850°C or more and 900°C or less to ensure the amount of precipitate Pp of the printed matter after the heat treatment. If the heat treatment temperature is less than 850°C, recrystallization defects occur in the center of the sheet thickness, and there is a concern that a decrease in formability due to a decrease in r value or a deterioration in polishing characteristics after processing due to occurrence of leasing may occur. For this reason, the lower limit of the heat treatment temperature for hot-rolled sheet annealing is set to 850°C or higher. It is preferably 860°C or higher. In addition, when the heat treatment temperature exceeds 900°C, the amount of precipitated product is insufficient, and the above-described precipitation amount Pp cannot be secured. Therefore, the upper limit of the heat treatment temperature for hot-rolled sheet annealing is set to 900°C or less. It is preferably 880°C or less, and more preferably 870°C or less. Moreover, since the precipitation state hardly changes in the annealing after cold rolling (finish annealing), it is important to control the precipitation amount Pp of P in this step. By annealing the hot-rolled sheet, in the step after the annealing of the hot-rolled sheet, the amount of P (precipitated amount Pp of P) present as a phosphor is preferably 0.003% by mass or more.

The rolling rate in subsequent cold rolling is set to 75% or more and 90% or less.

In order to make the recrystallized grain size fine by the heat treatment performed after cold rolling, it is necessary to increase the amount of introduced deformation. Recrystallization starts from the part where a lot of deformation is introduced. That is, the larger the amount of processing (the larger the rolling rate) is, the larger the portion (nucleus) becomes the starting point for recrystallization, so the recrystallized grain size becomes smaller. From this point of view, in order to increase the crystal grain size number (to decrease the crystal grain size), the higher the rolling rate is better. If the rolling rate is less than 75%, these effects cannot be obtained, and the r value is lowered, and there is a fear that the formability may decrease. For this reason, in this embodiment, the rolling rate is set to 75% or more. Moreover, since the r value is improved as the rolling rate is higher, the rolling rate is preferably 80% or more. On the other hand, when the rolling rate is more than 90%, the r value conversely decreases, and there is a fear that a decrease in formability may occur. Therefore, the rolling rate is made into a range of 90% or less.

After cold rolling, heat treatment (cold-rolled sheet annealing) is continuously performed, but the present embodiment is characterized in that this heat treatment is performed rapidly. Specifically, in the cold-rolled sheet annealing, the average temperature increase rate in the temperature range of 400°C to 800°C is 80°C/s or more during the heating process. The maximum reached temperature is 880°C or more and 980°C or less. Cooling is started within 5 sec after reaching the maximum reached temperature, and the average cooling rate in the temperature range from the maximum reached temperature to 700°C is set to 50°C/s or more to cool.

In addition, the "average temperature increase rate in a temperature range of 400°C to 800°C" in the present embodiment is the increase width (400°C) of the steel sheet temperature in the temperature range divided by the time required for temperature increase in the temperature range. Make it a value. "Average cooling rate in the temperature range from the maximum reached temperature to 700°C" means the width of the temperature drop of the steel sheet from the maximum reached temperature to 700°C, from the point when the maximum temperature reached to 700°C. It is a value divided by the required time. In addition, the temperature (°C) in the following description all indicates the steel sheet temperature.

As described above, in the present embodiment, the printed matter deposited by hot-rolled sheet annealing is crushed by cold rolling to obtain a precipitation state arranged in parallel in the cold-rolling direction, and recrystallization is performed while maintaining this precipitation state, thereby forming a product sheet. Get Further, even if the product plate provided with the prints in the above-described precipitated state is subjected to molding and deformation, the movement of dislocations may be hindered by the prints, so that it becomes possible to suppress the processing surface roughness.

From this, it becomes important to perform cold-rolled sheet annealing under conditions capable of recrystallization while maintaining the precipitation state after cold rolling.

In order to maintain the precipitation state after cold rolling and obtain the effect of surface roughening, the average temperature increase rate in the temperature range of 400°C to 800°C during the heating process is 80°C/s or more, and 5 after reaching the maximum temperature. Start cooling within seconds. That is, the temperature range of 400°C to 800°C is rapidly heated to an average temperature increase rate of 80°C/s or more, and is heated to the highest temperature reached (880°C or more and 980°C or less), and the holding time at the maximum reached temperature is 5 seconds. Cooling is started within the range. Further, in this embodiment, when maintaining at the highest reaching temperature, the temperature may be kept constant, but the holding temperature fluctuates within the range of the highest reaching temperature ±10°C (maximum reaching temperature -10°C to maximum reaching temperature + 10°C). It is also allowed. However, when the holding temperature fluctuates within the above range, it is necessary to control so as not to deviate from the appropriate range (880°C or more and 980°C or less) of the maximum reached temperature.

When the average temperature increase rate in the temperature range of 400°C to 800°C is less than 80°C/s or the holding time is more than 5 seconds, the prints may become solid solution and the amount of precipitation as a product may not be secured. In addition, rapid temperature increase in the temperature range of 400°C to 800°C has the effect of miniaturizing the recrystallized grain size, and is effective in suppressing the roughness of the processed surface. In addition, when the temperature is rapidly increased in the presence of the precipitate, grain growth is suppressed by the pinning effect of the precipitate, so that the particle diameter of the product is further refined, and the processing surface roughness is further suppressed. From this point of view, the average temperature increase rate in the temperature range of preferably 400°C to 800°C is 150°C/s or more.

In addition, from the viewpoint of maintaining the precipitated state of the printed matter, the holding time at the maximum reached temperature is preferably 2 seconds or less. You may start cooling immediately after reaching the holding time of 0 second, that is, the highest temperature reached.

In the present embodiment, since the temperature raising process is performed by rapid heating, the time required for temperature increase is short. In order to complete the recrystallization in this short time, the maximum reached temperature is set to 880°C or higher. If the maximum reached temperature is less than 880°C, recrystallization is insufficient, and there is a concern that workability may deteriorate due to a decrease in elongation. Therefore, in this embodiment, the maximum reached temperature is 880°C or higher, preferably 900°C or higher. On the other hand, when crystal grain growth after completion of recrystallization proceeds, the surface roughness may be deteriorated due to insufficient precipitation due to coarsening of crystal grains or solid solution of prints, so the maximum reached temperature is 980°C or less as the upper limit. . It is preferably 950°C or less.

If crystal grain growth or solid solution of prints proceeds during the cooling process, the surface roughness of the hole is deteriorated. Therefore, the lower limit of the average cooling rate in the temperature range from the maximum reached temperature to 700°C is 50°C/s or more. . It is preferably 100°C/s or more. The upper limit of the average cooling rate in the temperature range from the highest reaching temperature to 700°C is preferably 500°C/s or less.

In addition, in cold-rolled sheet annealing, it is possible to secure a print material to obtain a recrystallized structure by heat treatment in a region at a lower temperature than the above-described conditions for a long time, but the crystal grain size is increased and the inner surface roughness characteristic is deteriorated. In addition, the state of precipitation of the prints in the particles is arranged in parallel in the rolling direction, and for the first time, the effect of suppressing the surface roughness is exhibited. For this reason, even if a phosphor is deposited in the process of annealing the cold-rolled sheet, it does not exhibit the effect. That is, it is important to perform cold-rolled sheet annealing under the above-described conditions in which the precipitation state of the prints is controlled by cold rolling and the precipitation state can be maintained.

The ferritic stainless steel sheet according to the present embodiment can be manufactured by the manufacturing method described above.

In addition, in this embodiment, the hot-rolled sheet annealing and the cold-rolled sheet annealing may be batch annealing or continuous annealing. In addition, each annealing may be bright annealing performed in an oxidation-free atmosphere such as hydrogen gas or nitrogen gas, if necessary, or may be annealing in the air.

In addition, the plate thickness applied to the ferritic stainless steel sheet of the present embodiment is not particularly limited, but from the viewpoint of securing strength, it is preferably 0.5 mm or more, and preferably 0.6 mm or more. This is because, when the plate thickness is thin, the strength may become insufficient in parts after molding. It is necessary to design in consideration of the size, shape, and load-bearing load of the parts to be manufactured.

As described above, according to the present embodiment, it is possible to provide a ferritic stainless steel sheet having excellent molding processability and post-molding surface roughness. Further, since the ferritic stainless steel sheet of the present embodiment is excellent in surface roughness, it is particularly suitable for applications requiring polishing to remove surface irregularities (surface roughness) after molding processing.

Example

Next, examples of the present invention are shown. The conditions in this example are one example of conditions employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as the object of the present invention is achieved without deviating from the requirements of the present invention.

In addition, in the table shown below, the underline indicates out of the scope of the present embodiment.

Stainless steel having the component composition shown in Table 1 was melted and cast into a slab, and the slab was rolled to a predetermined plate thickness by hot rolling. Then, hot-rolled sheet annealing, cold-rolling, and cold-rolled sheet annealing were performed to manufacture stainless steel sheets (product sheets) No. 1 to 44 having a thickness of 0.6 mm. Heat treatment temperature (annealing temperature) of hot-rolled sheet annealing, cold-rolling rate, average rate of temperature increase between 400 and 800°C in cold-rolled sheet annealing, maximum temperature reached, time required to start cooling (holding time), and maximum temperature reached The average cooling rate in the temperature range from to 700°C was changed as shown in Tables 2 to 4. In addition, the annealing time (holding time) in the hot-rolled sheet annealing was in the range of 40 to 60 seconds.

Next, the test piece was cut out from the vicinity of the width center of the obtained stainless steel sheets No. 1 to No. 44, and the crystal grain size number (GSN) was measured by a line segment method in accordance with JIS G 0551 (2013). In addition, when measuring the crystal grain size, the number of crystal grains transverse to one sample was set to 500 or more, rather than the optical microscopic structure photograph of the cross section of the test piece.

Further, samples of φ 110 mm were cut out from stainless steel sheets No. 1 to No. 44, and a cup forming test with a shrinkage ratio of 2.2 was performed with a hydraulic forming tester. It can be seen that the shrinkage ratio greatly affects the surface roughness after cup forming, but other forming conditions do not. In addition, the cup forming test conditions carried out this time were: a punch diameter of 50 mm, a punch shoulder R of 5 mm, a die diameter of 52 mm, a die shoulder R of 5 mm, a wrinkle pressing pressure of 1 ton, and a clearance of 1.67 t (t Is the plate thickness). In addition, as a lubricant between the sample and the punch, Idemitsu Kosan Co., Ltd. anti-rust oil "Daphne Oil Coat Z3 (registered trademark)" was applied. After that, in order to protect the surface of the steel sheet after molding, a lubricating sheet "Nichias Co., Ltd. Naflon Tape TOMBO9001" was affixed.

About the sample which could be molded at the shrinkage ratio of 2.2, the surface roughness after cup molding was measured, and the processed surface roughness was evaluated.

Here, when the degree and fluctuation of the surface roughness for each portion of the sample (molded product) after cup forming was investigated, it was found that there were fluctuations inside and outside the vertical wall portion. The investigation result will be described in detail.

The present inventors examined the surface roughness of each portion of the sample after cup forming. The processing surface roughness after cup forming is not simply proportional to the crystal grain size and the amount of deformation, as is generally known, and the formation of irregularities on the surface of the molded product is suppressed by contact with the mold during molding. I noticed that the illuminance was getting smaller. Particularly, in the outer wall of the vertical wall of the molded product, the force pressed against the mold during molding is strong, and the generation of irregularities during molding and suppression of the irregularities due to contact with the mold are competing. I could see that it was getting bigger. Therefore, it was considered inappropriate to evaluate the processing surface roughness after cup forming on the outer wall of the vertical wall portion.

Therefore, the surface roughness of the inner wall of the vertical wall portion where the force pressed by the mold was relatively small was measured. As a result, it was found that the surface roughness after cup forming could be measured with high precision. Further, since the inner wall has a larger surface roughness than the outer wall, the inner wall having a large roughness takes the most polishing time in the polishing step after molding. Therefore, it is considered that it is appropriate to perform the measurement of the surface roughness (evaluation of the processed surface roughness) assuming polishing after molding on the inner wall of the vertical wall portion of the molded article. If the evaluation of the processed surface roughness is good at the inner wall of the vertical wall portion of the molded article, it can be judged to be good also at the outer wall.

At the center of the height inside the vertical wall portion of the sample after cup forming, for a length of 5 mm in parallel to the height direction, the surface roughness measurement described in JIS B 0601 was performed using a two-dimensional contact type surface roughness meter, and the arithmetic average roughness Ra was calculated. Based on the arithmetic mean roughness Ra 1.00 μm, when Ra is less than 1.00 μm, it is judged that the processed surface roughness evaluation is good (“○”), and when Ra is 1.00 μm or more, the processed surface roughness evaluation is poor (“×” ).

In the same manner as above, the precipitation amount Pp of P in the product plate was measured by the electrolytic extraction residue method.

First, from the center of the width direction of the stainless steel sheet, a test piece having a size of about a square with a side of 30 mm was cut out, and the entire surface of the test piece corresponding to the surface of the steel sheet was wet-polished with a water resistant polishing paper having a number #600. After polishing, the test piece base material (stainless steel base material) was dissolved by electrolysis in a methanol solution containing 10% maleic anhydride and 2% tetramethylammonium chloride at an electrostatic potential of -100 mV. After electrolysis, the residue (precipitate) that remained in the solution without dissolving was captured using a 200 nm mesh filter. The captured precipitate was washed with pure water and dried. Subsequently, the precipitate was dissolved with aqua regia and perchloric acid, and elemental analysis was performed using ICP emission spectroscopy in accordance with JIS G 1258 to determine the mass of P in the precipitate. The amount of P obtained was divided by the amount of change in the mass of the test piece by electrolysis ("the mass of the test piece before electrolysis"-"the mass of the test piece after electrolysis") and expressed as a percentage as "precipitated amount Pp of P" (mass%). did.

In addition, the precipitation amount Pp of P in the hot-rolled annealing plate before cold rolling was also measured by the same method.

As described above, the measurement results and evaluation results are shown in Tables 5 to 7.

As shown in Tables 2 to 7, according to the present embodiment, ferritic stainless steel having excellent surface roughness after processing and excellent formability by controlling the precipitation amount of prints by appropriate annealing conditions and rolling conditions. It was found that the steel plate could be obtained.

In the example of the present invention, Ra<1.00 µm and processing surface roughness was suppressed.

On the other hand, Nos. 25 and 26 in Tables 2 to 7 are examples in which the component composition is out of the range, but both the precipitation amount Pp and the crystal grain size number of P are within the range of the embodiment, but the moldability is deteriorated and thus completely drawing. I couldn't. In addition, Nos. 27 and 28 are examples of using steel L in which Ti and Nb are not added, but the immobilization of P is insufficient, so that the amount of P precipitated Pp is less than 0.001%, and the moldability deteriorated, and it was not possible to draw completely. .

In Nos. 3 and 22, since the average temperature increase rate at the time of annealing the cold-rolled sheet was too low, the solid solution of the prints proceeded and the precipitated amount Pp of P was insufficient. Moreover, the crystal grain size number became small, and the processing surface roughness deteriorated.

In Nos. 5, 10, 12, and 24, since the holding time was too long, the solid solution of the printed matter proceeded and the amount of P precipitated Pp was insufficient. Moreover, the crystal grain size number also became small, and the processing surface roughness deteriorated.

In Nos. 6 and 15, since the annealing temperature at the time of annealing the hot-rolled sheet was low and the average temperature increase rate was too low, the crystal grain size number became small and the processed surface roughness deteriorated.

In No. 7, since the cold rolling rate was small and the maximum reached temperature was too high, grain growth proceeded, the crystal grain size number became small, and the processing surface roughness deteriorated.

In No. 9, since the annealing temperature at the time of annealing the hot-rolled sheet was too high, the precipitation amount Pp of P could not be ensured, and the processing surface roughness deteriorated.

In No. 16, since the maximum reached temperature was too high, the crystal grain size number became small, and the processing surface roughness deteriorated.

In No. 19, since the average temperature increase rate at the time of annealing the cold-rolled sheet was low and the holding time was too long, the solid solution of the printed matter proceeded and the precipitated amount Pp of P was insufficient. Further, the crystal grain size number was also small, and the processing surface roughness deteriorated.

In No. 20, since the cold rolling rate was too small, the crystal grain size number became small. As a result, the processing surface roughness deteriorated.

In No. 21, since the annealing temperature at the time of annealing the hot-rolled sheet was too high, the precipitation amount Pp of P could not be ensured, and the processing surface roughness deteriorated.

In No. 14, since the maximum reached temperature was too high, grain growth proceeded, the crystal grain size number became small, and the processing surface roughness deteriorated.

In No. 31, since the average cooling rate at the time of annealing the cold-rolled sheet was low, the solid solution of the printed matter proceeded, the precipitated amount Pp of P was insufficient, and the crystal grain size number also decreased, and the processing surface roughness deteriorated.

In No. 32, since the average cooling rate at the time of annealing the cold-rolled sheet was low, the solid solution of the prints proceeded, the precipitation amount Pp of P was insufficient, and the processed surface roughness deteriorated.

In No. 36, since the annealing temperature at the time of annealing the hot-rolled sheet was too high, the amount of precipitation Pp of P could not be ensured, and the processing surface roughness deteriorated.

In No. 38, since the average temperature increase rate at the time of annealing the cold-rolled sheet was low and the maximum reached temperature was too high, grain growth proceeded, the crystal grain size number decreased, and the processing surface roughness deteriorated.

In addition, in the region in which the particle size number is not less than 9.0 and the amount of precipitated P is less than 0.003% in FIG. 2, since it is relatively fine, the processing surface roughness may be desired to slightly decrease, but there is no effect of suppressing the processing surface roughness due to the P product. , Compared to the examples of the present invention having a large amount of precipitation P at the same degree of particle size number, the surface roughness is poor.

In addition, about the steel component whose P is less than 0.003%, when it produced similarly to No. 4 of Tables 2-7, the amount of precipitation P was 0.003% or less, and Ra after a molding test was 1.00 micrometer or more. The steel composition in which P exceeded 0.1% was prepared in the same manner as in No. 4 in Tables 2 to 7, but the moldability was poor and molding could not be performed.

According to this embodiment, it is possible to provide a ferritic stainless steel sheet excellent in molding processability and surface roughness after molding, and a method for producing the same. For this reason, the ferritic stainless steel sheet of this embodiment is suitably applied for a molding application.

Claims (4)

In mass%,
Cr: 11.0% or more and 30.0% or less,
C: 0.001% or more and 0.030% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.01% or more and 2.00% or less,
P: 0.003% or more and 0.100% or less,
S: 0.0100% or less,
N: 0.030% or less,
B: 0% or more and 0.0025% or less,
Sn: 0% or more and 0.50% or less,
Ni: 0% or more and 1.00% or less,
Cu: 0% or more and 1.00% or less,
Mo: 0% or more and 2.00% or less,
W: 0% or more and 1.00% or less,
Al: 0% or more and 1.00% or less,
Co: 0% or more and 0.50% or less,
V: 0% or more and 0.50% or less,
Zr: 0% or more and 0.50% or less,
Ca: 0% or more and 0.0050% or less,
Mg: 0% or more and 0.0050% or less,
Y: 0% or more and 0.10% or less,
Hf: 0% or more and 0.10% or less,
REM: 0% or more and 0.10% or less,
Sb: 0% or more and 0.50% or less are included, and
Ti: 0.40% or less, Nb: 0.50% or less, one or both are included, and the remainder is composed of Fe and impurities,
The amount of P present as a phosphor is 0.003% by mass or more,
A ferritic stainless steel sheet having a crystal grain size number of 9.0 or higher as measured in JIS G 0551. The method according to claim 1, by mass%,
B: 0.0001% or more and 0.0025% or less,
Sn: 0.005% or more and 0.50% or less,
Ni: 0.05% or more and 1.00% or less,
Cu: 0.05% or more and 1.00% or less,
Mo: 0.05% or more and 2.00% or less,
W: 0.05% or more and 1.00% or less,
Al: 0.05% or more and 1.00% or less,
Co: 0.05% or more and 0.50% or less,
V: 0.05% or more and 0.50% or less,
Zr: 0.05% or more and 0.50% or less,
Ca: 0.0001% or more and 0.0050% or less,
Mg: 0.0001% or more and 0.0050% or less,
Y: 0.001% or more and 0.10% or less,
Hf: 0.001% or more and 0.10% or less,
REM: 0.001% or more and 0.10% or less,
Sb: A ferritic stainless steel sheet further containing 0.005% or more and 0.50% or less of one or two or more. It is a manufacturing method of the ferritic stainless steel sheet according to claim 1 or 2,
A hot rolling step of hot rolling the steel having the component according to claim 1 or 2,
After the hot rolling process, a hot-rolled sheet annealing process for performing heat treatment at a temperature of 850°C or more and 900°C or less,
After the hot-rolled sheet annealing step, a cold rolling step of rolling with a rolling rate of 75% or more and 90% or less, and
A cold-rolled sheet annealing process performed subsequent to the cold-rolling process is provided,
In the cold-rolled sheet annealing process, the average temperature increase rate in the temperature range of 400°C to 800°C during the heating process is 80°C/s or more, and the maximum attainment temperature of the plate temperature is 880°C or more and 980°C or less, and the maximum temperature reached A method for producing a ferritic stainless steel sheet, characterized in that cooling is started within 5 sec after reaching the temperature, and the average cooling rate in the temperature range from the highest reached temperature to 700°C is 50°C/s or more. It is a manufacturing method of the ferritic stainless steel sheet according to claim 1 or 2,
A hot rolling step of hot rolling the steel having the component according to claim 1 or 2,
After the hot rolling step, heat treatment is performed at a temperature of 850° C. or higher and 900° C. or lower, and a hot-rolled sheet annealing step in which the amount of P present as a print is 0.003 mass% or more;
After the hot-rolled sheet annealing step, a cold rolling step of rolling with a rolling rate of 75% or more and 90% or less, and
A cold-rolled sheet annealing process performed subsequent to the cold-rolling process is provided,
In the cold-rolled sheet annealing process, the average temperature increase rate in the temperature range of 400°C to 800°C during the heating process is 80°C/s or more, and the maximum attainment temperature of the plate temperature is 880°C or more and 980°C or less, and the maximum temperature reached A method for producing a ferritic stainless steel sheet, characterized in that cooling is started within 5 sec after reaching the temperature, and the average cooling rate in the temperature range from the highest reached temperature to 700°C is 50°C/s or more.

Images