JP2020164956A - Ferritic stainless steel sheet and manufacturing method therefor - Google Patents

Abstract

To provide a ferritic stainless steel sheet having excellent formability and having excellent surface shape after being formed.SOLUTION: The ferritic stainless steel sheet comprises: Cr: 11.0-25.0%, C: 0.001-0.010%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.010-0.040%, S: 0.01% or less, N: 0.002-0.020%, Al: 0.003-1.0%, Ti: 0.05-0.30%, and Nb: 0.03-0.30% with balance comprising Fe and impurities; satisfies Formula (1); satisfies XTi: 0.035-0.060%, XNb: 0.010-0.025%, and XP: 0.001-0.010%, where the content of Ti, Nb, and P contained in the compounds in the steel are XTi, XNb, and XP, respectively; and has a crystal grain size number of 9.0 or more, an average plastic strain ratio rAve of 1.5 or less, and an in-plane anisotropy Δr of -0.4-0.1. Ti/48+Nb/93≥2×(C/12+N/14) (1).SELECTED DRAWING: None

Description

The present invention relates to a ferritic stainless steel sheet having excellent moldability and surface shape after molding.

Stainless steel is widely used in home appliances, kitchen products, building materials, etc. because it has excellent corrosion resistance, workability, and beauty. In particular, ferritic stainless steel does not contain or contains very little Ni, which is expensive and has a sharp price fluctuation, and therefore, its demand has been increasing in recent years as a material having excellent cost performance. However, when it is used for molding purposes, not only moldability but also surface shape after molding becomes an issue.

Ferritic stainless steel is formed into a product shape by utilizing the deep drawing property. To improve the deep drawing property, it is effective to reduce the solid solution carbon and the solid solution nitrogen. For this reason, a method of reducing carbon and nitrogen as much as possible in the smelting stage and then adding Ti and Nb to finely deposit as carbonitride is widely used, and so-called high-purity ferritic stainless steel has been developed. There is. As for the alloy cost, Ti is cheaper than Nb. In order to improve the deep drawing property, it is effective to utilize the anisotropy of the material, specifically, the plastic strain ratio (hereinafter, may be referred to as r value) which is an index thereof. If the anisotropy of the plastic strain ratio in the plate surface is too large, the problem of the surface shape described above occurs, and as shown in FIG. 1, the difference in height at the remainder of molding, so-called "ears", occurs. There is. The ears, which are unnecessary parts, will be cut off. Therefore, when the ears are large, the yield loss is large. As described above, the ferrite stainless steel sheet for forming is required to have excellent deep drawing property by adjusting the anisotropy of the plastic strain ratio.

Next, the surface shape after the molding process will be described. The surface shape in question here refers to fine irregularities (rough skin) that occur on the surface of the steel sheet after molding. Rough skin is generally proportional to the crystal grain size. High-purity ferritic stainless steels with reduced C and N to improve moldability tend to have larger crystal grains than ferrite stainless steels.

High-purity ferritic stainless steel typified by SUS430LX is often used in applications where relatively severe molding is performed and rough skin is noticeable, such as housings or utensils of home appliances. However, when the crystal grain size is large and the rough skin is large, the rough skin is generally removed by polishing. These are not only yield losses, but also environmental problems. It should be noted that the thickness of the steel sheet is generally required to be 0.5 mm or more in order to secure the strength reduced by the reduction of carbon and nitrogen which are solid solution strengthening elements.

As described above, there is a demand for a ferritic stainless steel having excellent deep drawing property and less rough skin. The related technologies are shown below.

Patent Document 1 discloses a steel in which high-purity ferritic stainless steel is used to control the size of precipitated particles as well as the crystal grain size to reduce rough skin and improve moldability, and a method for producing the same. However, there is no description of a method for controlling the deep drawing property and the anisotropy of the plastic strain ratio, and further improvement is required as described above.

In Patent Document 2, low-temperature hot rolling onto high-purity ferritic stainless steel containing Ti and Nb and cold rolling at a high processing rate are utilized to produce fine-grained steel with less rough skin. The technology is disclosed. Although a fine grain structure having a crystal grain size of 9.5 has been obtained for the stainless steel sheet, it is not sufficient to study the prevention of "ears" and the prevention of rough skin during deep drawing.

Patent Document 3 discloses a high-purity ferritic stainless steel containing Nb and / or Ti, which improves deep drawing property and rigging property by controlling the crystal grain size before final cold rolling and improves rough skin. ing. However, the crystal grain size is only 15 μm (the crystal grain size number is 8.8), and further improvement of rough skin is required.

Conventionally, in order to reduce rough skin of high-purity ferritic stainless steel, a method of making the grain size finer by utilizing strong pressure during steel sheet manufacturing has been studied, but when the manufacturing load is large and rough skin occurs on the contrary. Therefore, a technique capable of stably maintaining an excellent surface shape is desired.

Japanese Patent No. 4479888 JP-A-7-292417 Japanese Patent No. 3788311

The present invention has been made in view of the above, and an object of the present invention is to provide a ferritic stainless steel sheet having excellent moldability and surface shape after molding and a method for producing the same.

The gist of the present invention is as follows.
[1] The chemical component is mass%,
Cr: 11.0% or more, 25.0% or less,
C: 0.001% or more, 0.010% or less,
Si: 0.01% or more, 1.0% or less,
Mn: 0.01% or more, 1.0% or less,
P: 0.010% or more and 0.040% or less,
S: 0.01% or less,
N: 0.002% or more, 0.020% or less,
Al: 0.003% or more, 1.0% or less,
Ti: 0.05% or more, 0.30% or less,
Nb: contains 0.03% or more and 0.30% or less, satisfies the following equation (1), and the balance is composed of Fe and impurities.
Ti, Nb, among the P, Ti contained in the compound in the steel, Nb, respectively P _{X Ti, X Nb,} when the _{X P,}
X _Ti : 0.035% or more, 0.060% or less,
X _Nb : 0.010% or more, 0.025% or less,
X _P: 0.001% or more, and satisfies the following 0.010%,
The crystal grain size number is 9.0 or higher,
The average plastic strain ratio r _Ave represented by (r ₀ + r ₉₀ + 2 × r ₄₅ ) / 4 satisfies the following equation (2).
_{(R 0 + r 90 -2 ×} r 45) / 2 plane anisotropy Δr is the following (3) represented by the ferritic stainless steel sheet which satisfies the equation.
Ti / 48 + Nb / 93 ≧ 2 × (C / 12 + N / 14)… (1)
1.5 ≤ r _Ave ... (2)
―0.4 ≦ Δr ≦ 0.1… (3)
However, r ₀ , r ₉₀ , and r ₄₅ are r ₀ : plastic strain ratio in the rolling direction, r ₄₅ : plastic strain ratio in the 45 ° direction with respect to the rolling direction, and r ₉₀ : plastic strain in the direction perpendicular to the rolling direction, respectively. The ratio.
[2] The chemical composition is further increased by mass%.
B: 0.0025% or less,
Sn: 0.50% or less,
Ni: 1.0% or less,
Cu: 1.0% or less,
Mo: 2.0% or less,
W: 1.0% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
Y: 0.10% or less,
Hf: 0.20% or less,
REM: 0.10% or less,
Sb: The ferritic stainless steel sheet according to [1], which contains 1 type or 2 or more types of 0.50% or less.
[3] The first step of hot rolling a steel piece having the chemical composition according to [1] or [2] into a steel sheet, and
The second step of heat-treating the steel sheet after the first step at a soaking temperature of T ₁ ° C. or higher and (T ₁ +35) ° C. or lower, and
A third step of subjecting the steel sheet after the second step to an aging heat treatment at a soaking temperature of 650 ° C. or higher and 750 ° C. or lower and a soaking time of 1 minute or more and 5 minutes or less.
The steel sheet after the third step is provided with a fourth step in which cold rolling and heat treatment are each performed once, or each is repeated two or more times.
The fourth step is at least subjected to final cold rolling at a rolling ratio of 75% or more, at least the final heat treatment (T 2 _-10) ℃ above, be carried out in (T 2 ₊₃₀₎ ℃ following soaking temperature The method for producing a ferritic stainless steel sheet according to [1] or [2].
Here, T ₁ and T ₂ are T ₁ : recrystallization temperature (° C.) after hot rolling and T ₂ : recrystallization temperature (° C.) after cold rolling, respectively.

According to the present invention, it is possible to industrially stably provide a ferritic stainless steel sheet having excellent moldability and surface shape after molding.

FIG. 1 is a schematic view showing a post-processed test piece in an example.

The present inventors have made a large number of high-purity ferritic stainless steels as prototypes, and have diligently studied their moldability (particularly deep drawing property), components affecting the surface shape after molding, and manufacturing methods. As a result, we have clarified a steel sheet and a manufacturing method that can satisfy all of deep drawing property, anisotropy, and rough skin by appropriately controlling the precipitates containing them in the component system in which P is adjusted together with Ti and Nb. This led to the present invention.

To state the point, the present invention utilizes a newly discovered compound of P. The P compound is finely precipitated by aging heat treatment on the material recrystallized after hot rolling, which will be described later, and the precipitate has a crystal orientation advantageous for deep drawing property at the start of recrystallization by heat treatment after cold rolling. It is considered to be preferentially generated, a desirable r value is obtained, and moldability is improved. At the same time, the grain growth is further suppressed by dispersing at high density together with the above-mentioned Ti and Nb carbonitrides, and the surface shape after molding is further improved by forming a fine grain structure. That is, even more excellent moldability and the surface shape after molding are compatible. The P compound is considered to have (Ti, Nb) FeP or a composition and structure similar to those of FeP. Therefore, it is considered that it can be applied to materials in which these compounds can be utilized, for example, high-purity ferritic stainless steel described in JIS G 4305.

Each requirement will be described in detail below.
First, the reasons for limiting the components will be described below. In addition, "%" display of the content of each element means "mass%".

Cr is an element that improves corrosion resistance, which is a basic property of stainless steel. If it is less than 11.0%, sufficient corrosion resistance cannot be obtained, so the lower limit is set to 11.0 or more. On the other hand, excessive Cr content promotes the formation of an intermetallic compound equivalent to the σ phase (Fe-Cr intermetallic compound) and promotes cracking during production. Therefore, the upper limit is 25.0% or less. From the viewpoint of stable manufacturability (yield, rolling defects, etc.), 14.0% or more and 22.0% or less are preferable. More preferably, it is 16.0% or more and 21.0% or less.

C is a strong solid solution strengthening element, and the moldability is lowered as the elongation is lowered. Therefore, it is necessary to avoid the excessive content of C. The upper limit is 0.010% or less. On the other hand, the lower limit is set to 0.001% or more because it is necessary to contain a certain amount of C for grain control and the refining cost increases for excessive reduction. Considering the refining cost and characteristics (moldability, crystal grain temperature), 0.002% or more and 0.008% or less are preferable.

Si is an element for improving oxidation resistance, but its moldability is lowered due to the excessive content of Si. Therefore, the upper limit is 1.0% or less. From the viewpoint of moldability, a lower value is preferable, but an excessive decrease causes an increase in raw material cost, so the lower limit is 0.01% or more. The preferred range is 0.05% or more and 0.60% or less, and more preferably 0.05% or more and 0.30% or less.

Similar to Si, the upper limit of Mn is set to 1.0% or less because the content of a large amount of Mn causes a decrease in moldability. A lower value is preferable from the viewpoint of moldability, but an excessive decrease causes an increase in raw material cost, so the lower limit is 0.01% or more. From the viewpoint of manufacturability, the preferable ranges are 0.05% or more and 0.40% or less, and more preferably 0.05% or more and 0.30% or less.

P is a constituent element of the P compound which is considered to form a crystal orientation advantageous for deep drawing by recrystallization by heat treatment after cold rolling, and the lower limit is 0.010% or more. However, if P is excessively contained, the moldability deteriorates and the processing of the material becomes difficult, so the upper limit is set to 0.040% or less. Considering both moldability and manufacturing cost, it is preferably 0.012% or more and 0.038% or less, and more preferably 0.015% or more and 0.035% or less.

S is preferably low because it promotes cracking during manufacturing, and the upper limit is 0.01% or less. On the other hand, an excessive decrease causes an increase in refining cost, so the lower limit is set to 0.0003% or more. From the viewpoint of manufacturability and cost, it is preferably 0.0004% or more and 0.0030% or less.

Like C, N is a strong solid solution strengthening element, and as the elongation decreases, the moldability decreases. Therefore, the upper limit is set to 0.020% or less. However, since it is necessary to contain a certain amount of N for crystal grain control and long-term refining is required for excessive reduction, the lower limit is 0.002% or more. Preferably, it is 0.005% or more and 0.015% or less.

Al is used as a deoxidizing element. It also contributes to the improvement of moldability. In order to exert these effects, 0.003% or more is required. On the other hand, if a large amount of Al is contained, the material is cured and the ductility is lowered, so the upper limit is set to 1.0% or less. It is preferably 0.10% or less, more preferably 0.01% or more and 0.07% or less.

Ti is an element that forms a carbonitride that suppresses the growth of crystal grains. Therefore, the lower limit is set to 0.05% or more. On the other hand, excessive Ti content increases the alloy cost, resulting in a decrease in the manufacturability of the steel sheet and a decrease in elongation due to an increase in the recrystallization temperature. Therefore, the upper limit of Ti is 0.30% or less. From the viewpoint of moldability and manufacturability, it is preferably 0.08% or more and 0.25% or less. More preferably, it is 0.10% or more and 0.20% or less.

Nb is an element that forms a carbonitride that suppresses grain growth like Ti. Since the size of the compound containing Nb is also relatively small, the possibility of becoming a starting point of cracking during molding of the steel sheet is small. Therefore, the lower limit is set to 0.03% or more. On the other hand, excessive Nb content lowers the ductility of the steel sheet, leading to an increase in alloy cost and a decrease in manufacturability due to an increase in recrystallization temperature. The upper limit is 0.30% or less. Preferably, it is 0.05% or more and 0.15% or less.

Regarding the contents of Ti, Nb, C, and N, the following equation (1) is satisfied.

Ti / 48 + Nb / 93 ≧ 2 × (C / 12 + N / 14) ・ ・ ・ (1)

When the above formula (1) is not satisfied, the precipitation of carbonitride is small, the solid solution amount of C or N is large, the deep drawing property of the steel sheet is lowered, and cracks are likely to occur during molding. In addition, when welded, the corrosion resistance of the same part is reduced.

Also, in ferritic stainless steel sheet according to the present embodiment, among Ti, Nb, P contained in the steel sheet, Ti contained in the compound in the steel sheet, Nb, _X the amount of P each _Ti, X Nb, and _{X P} When you do
X _Ti : 0.035% or more and 0.060% or less,
X _Nb : 0.010% or more and 0.025% or less,
X _P: it is necessary to satisfy 0.001% to 0.010% or less. The compound in the steel sheet contains Ti and / or Nb, and is a P compound typified by (Ti, Nb) carbonitride and (Ti, Nb) FeP. These contribute to the formation of fine-grained structures by suppressing grain growth due to the pinning effect of dispersion of a large number of fine precipitates. Further, the P compound is considered to form a crystal orientation advantageous for deep drawing property when recrystallized by heat treatment after cold rolling, and a desirable r value can be obtained. To obtain such _effects, X _Ti, X _Nb, respectively the lower limit of the _{X P is,} X Ti: 0.035% or _more, X Nb: 0.010% or _more, X P: 0.001% or more It is preferable to have.

On the other hand, excessive dispersion of the precipitates lowers the moldability due to the lower ductility of the steel sheet, and also lowers the r value. _Thus, each upper limit of the _{X Ti,} X _Nb, X _{P is,} X Ti: 0.060% or _less, X Nb: 0.025% or _less, X P: is preferably 0.010% or less. By satisfying these values at the same time, a crystal orientation advantageous for deep drawing property is generated in recrystallization by heat treatment, and a crystal orientation advantageous for fine grain organization and deep drawing property is maintained by suppressing grain growth by the compound. It is considered that the surface shape after molding is compatible with the excellent moldability.

X _{Ti, X Nb,} measurement of the X _P is the electrowinning residue method to extract the compound in the steel as a residue, Ti in the compound, Nb, determined by quantifying the amount of P. The electrolytic solution used in the electrolytic extraction residue method is a 10% acetylacetone-1% tetramethylammonium-methanol solution, and a 0.2 μm mesh organic filter is used to recover the residue. By setting the amount of residue to 1 g or more, variation in analysis accuracy is prevented. The analysis of Ti, Nb, P in the residue is performed by ICP emission spectroscopy.

In the ferritic stainless steel of the present embodiment, in addition to the above basic composition, B: 0.0025% or less, Sn: 0.50% or less, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 1.0% or less, Co: 0.50% or less, V: 0.50% or less, Zr: 0.50% or less, Ca: 0.0050% or less, Mg: One or more of 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, Sb: 0.50% or less may be contained. .. These elements may not be contained, and the lower limit in that case is 0% or more.

B is an element that improves the secondary processability. It is considered that the expression is 0.0001% or more in order to exert the effect. However, since excessive content of B causes deterioration of manufacturability, especially castability, the upper limit is set to 0.0025% or less. It is preferably 0.0020% or less, and more preferably 0.0012% or less.

Sn may be contained depending on the corrosive environment in order to improve the corrosion resistance. It is considered that this effect is exhibited at 0.005% or more. However, since the content of a large amount of Sn causes deterioration of manufacturability, the upper limit is set to 0.50% or less. In consideration of manufacturability, it is preferably 0.20% or less, and more preferably 0.10% or less.

Ni, Cu, Mo, W, Co, V, and Zr are elements effective for enhancing corrosion resistance or oxidation resistance, and these elements are contained as necessary. It is considered that the effects of these elements are expressed at 0.1% or more. However, excessive inclusion of these elements may not only reduce moldability, but also increase alloy cost and hinder manufacturability. Therefore, the upper limit of Ni, Cu, and W is 1.0% or less, the upper limit of Mo is 2.0% or less, and the upper limit of Co, V, and Zr is 0.50% or less.

Ca and Mg are elements that improve hot workability and secondary workability, and are contained as necessary. It is considered that this effect is exhibited at 0.0001% or more. However, since excessive content of these elements hinders the manufacturability of the steel sheet, the upper limit is set to 0.0050% or less. It is preferably 0020% or less, and more preferably 0.0010% or less.

Y, Hf, and REM are elements effective for improving hot workability and cleanliness of steel and improving oxidation resistance, and are contained as necessary. It is considered that these effects are exhibited at 0.001% or more. However, since excessive content of these elements hinders the manufacturability of the steel sheet, the upper limit is set to 0.10% or less for Y and REM and 0.20% or less for Hf. In addition, "REM" in this embodiment refers to an element (lanthanoid) belonging to atomic numbers 57 to 71, and is, for example, La, Ce, Pr, Nd and the like.

Sb is an element having an effect of improving corrosion resistance like Sn, and is contained as necessary. This effect is considered to be expressed at 0.005% or more. However, since excessive Sb content causes deterioration in manufacturability, the upper limit is set to 0.50% or less.

The ferritic stainless steel plate of the present embodiment is composed of Fe and impurities (including unavoidable impurities) other than the elements described above, but can contain the following as long as the effects of the present invention are not impaired. In the present embodiment, for example, Bi, Pb, Se, H, Ta and the like may be contained, but in that case, it is preferable to reduce as much as possible. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and if necessary, Bi ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, H ≦ 100 ppm, Ta ≦ 500 ppm. It may contain more than a seed.

Next, the metal structure will be described.
The ferrite-based stainless steel plate of the present embodiment has a crystal grain size number of 9.0 or more. The above-mentioned rough skin is less likely to occur as the crystal grain size number is larger, that is, the smaller the grain size is. In order to obtain an excellent surface shape after molding, the crystal grain size number is set to 9.0 or more. It is preferably more than 9.0, more preferably 9.5 or more, and even more preferably 10.0 or more.

The method for measuring the crystal grain size number can be obtained by the line segment method of JIS G 0551 (2013). As for the numbers, for example, 9 corresponds to an average line segment length of 14.1 μm per crystal grain crossing the inside of the crystal grain, and 10 corresponds to the same line segment length of 10.0 μm. The investigation is carried out by an optical microscope microstructure photograph of the cross section of the test piece after etching, and the number of crystal grains per sample is measured to be 500 or more. The etching solution is preferably aqua regia or reverse aqua regia, but other solutions may be used as long as the grain boundaries can be determined. Since the grain boundaries may not be clearly visible due to the orientation of adjacent crystal grains, it is preferable to perform deep etching. Twins will not be measured during the survey.

The plastic strain ratio (r value) is a value indicating the contribution of the decrease in the width of the test piece to the elongation (increase in the length), and the ferritic stainless steel improves the deep drawing property. The average plastic strain ratio _rAve is calculated by the following formula (2), and the in-plane anisotropy Δr is calculated by the following formula (3). The larger the average plastic strain ratio _rAve, the better the moldability, so it is set to 1.5 or more. It is preferably 1.7 or more. The smaller the absolute value of the in-plane anisotropy Δr, the better the moldability, and when it is less than -0.4 and more than 0.1, the in-plane anisotropy becomes large, and the generation of ears becomes remarkable during deep drawing. In addition, cracks are likely to occur during molding. Preferably, it is −0.3 or more and 0.1 or less.

The method for measuring the plastic strain ratio is in accordance with JIS Z 2254 (2008), and the test piece collected in parallel with each direction of the steel sheet is subjected to a tensile strain of 15% or more. Here, r ₀ : the plastic strain ratio measured by collecting the test piece parallel to the rolling direction of the plate surface, and r ₄₅ : the plasticity measured by collecting the test piece in the 45 ° direction with respect to the rolling direction of the plate surface. Strain ratio, r ₉₀ : The plastic strain ratio measured by collecting the test piece in the 90 ° direction with respect to the rolling direction of the plate surface.

1.5 ≤ r _Ave ... (2)
―0.4 ≦ Δr ≦ 0.1… (3)
However, a _{r Ave} = the _{r Ave (r 0 + r 90} + 2 × r 45) / 4 of the formula (2), [Delta] r of formula (3) is _{Δr = (r 0 + r 90} -2 × r 45) / 2 Is.

The metal structure of the ferritic stainless steel sheet of the present embodiment has a ferrite single-phase structure. In the case of a multi-phase structure containing an austenite phase or a martensite phase, it is relatively easy to make the crystal grain size finer, and in addition, the austenite phase exhibits high moldability due to the TRIP effect. However, in general, the raw material cost increases as the alloying elements increase, and ear cracks and the like occur during manufacturing, resulting in a decrease in yield. If the ear cracks occur significantly, it becomes difficult to manufacture. Therefore, 98% or more by volume is a ferrite phase. It is preferably 99% or more, and 100% is naturally the most preferable from the viewpoint of manufacturing and processability of the steel sheet. The residual structure other than the ferrite phase is 0 to 2%. Although carbon dioxide compounds, P compounds, etc. are also present in steel, these are extremely small in total and are ignored here.

According to the regulations of these metal structures, excellent moldability and post-molding surface shape can be obtained as described later in Examples. The reason is that the investigation is ongoing, but at the moment, it is speculated as follows as mentioned above.

In order to improve the deep drawing property, it is effective to reduce C and N that dissolve in solid solution. It is already known that it is fixed as a short nitride by containing Ti and Nb. The present invention further utilizes a compound of P. The P compound is finely precipitated by aging heat treatment on the material recrystallized after hot rolling, which will be described later. However, priority is given to the crystal orientation that is advantageous for deep drawing property at the start of recrystallization in the heat treatment after cold rolling. The desired r value is obtained, and the moldability is improved. At the same time, it is dispersed at high density together with the carbonitrides of Ti and Nb to further suppress the grain growth, and the fine grain structure is formed, so that the surface shape after molding is further improved. That is, the present invention achieves both excellent moldability and a surface shape after molding when the amount of the Ti, Nb, P compound is a predetermined amount. The P compound is considered to have (Ti, Nb) FeP or a composition and structure similar to those of FeP.

The thickness of the ferritic stainless steel sheet of the present embodiment is not particularly limited, but it is preferably 0.5 mm or more, preferably 0.6 mm or more from the viewpoint of ensuring strength. This is because if the plate thickness is thin, the strength of the molded part may be insufficient. However, it is necessary to design the plate thickness in consideration of the size and shape of the product to be manufactured, the load capacity, and the like.

Next, a method for manufacturing a ferritic stainless steel sheet will be described.
In the method for producing a ferritic stainless steel sheet of the present embodiment, the steel is melted and cast by a conventional method to obtain a steel piece to be subjected to hot rolling. This steel piece may be a steel ingot forged or rolled, but from the viewpoint of productivity, it is preferable to manufacture the steel piece by continuous casting. Further, it may be manufactured by using a thin slab caster or the like.

Next, a first step of hot rolling the steel pieces into a steel plate, a second step of performing a heat treatment for the purpose of recrystallization, and a third step of performing an aging heat treatment for the purpose of compound precipitation. The ferrite-based stainless steel sheet of the present embodiment is produced by performing a fourth step of repeating cold rolling and heat treatment for the purpose of recrystallization once or twice or more.

In the first step, the steel pieces are heated to a predetermined temperature by hot rolling and then reduced to a predetermined plate thickness. The temperature and pass schedule are not particularly specified, and the temperature is heated and maintained at a uniform temperature to the inside above the recrystallization temperature, and the thickness is reduced efficiently.

In the second step, the steel sheet after hot rolling is heat-treated for the purpose of recrystallization. In the heat treatment of the second step, it is necessary to control the reached temperature as the soaking temperature in the range of T ₁ ° C. to (T ₁ +35) ° C. in order to finely precipitate the P compound in the aging heat treatment of the next step. .. T ₁ is the recrystallization temperature (° C.) after hot rolling. The recrystallization temperature T ₁ changes depending on the influence of the steel composition, the plate thickness, the temperature of hot rolling, and the rolling reduction rate. By controlling the recrystallization temperature T ₁ , the P compound can be deposited finely and at high density. Further, when the ultimate temperature is less than T ₁ ° C., unrecrystallized grains remain and insufficient softening makes subsequent cold rolling difficult. In addition, the rigging property and formability of the steel sheet are also poor. On the other hand, when the reached temperature exceeds (T ₁ +35) ° C., grain growth progresses and the effect remains even after the post-process, and the grain size of the steel sheet after repeated cold rolling and heat treatment becomes coarse and varies. Also becomes large, and the surface roughness after molding becomes poor. The soaking time of the heat treatment in the second step is preferably 5 minutes or less.

Next, the aging heat treatment carried out in the third step is held at a soaking temperature of 650 ° C. or higher and 750 ° C. or lower for a soaking time of 1 minute (60 seconds) or more and 5 minutes or less based on the experimental results. It is considered that the P compound does not precipitate when the soaking temperature is less than 650 ° C or higher than 750 ° C. If the holding time is less than 1 minute (60 seconds), the amount of the compound precipitated will be insufficient. Conversely, if it exceeds 5 minutes, increasing the amount of precipitation of the compound, X _{Ti, X Nb,} the value of X _P is more likely to exceed the above-mentioned range, excellent moldability and molded after surface shape I can't get it. In addition, industrially, processing in a short time is desirable. Preferably, the conditions are 660 ° C. or higher and 740 ° C. or lower for 2 minutes or longer and 4 minutes or shorter.

In the fourth step, the heat treatment is performed once after one cold rolling, or the cold rolling is performed twice or more and the heat treatment is appropriately performed during the cold rolling, and the final heat treatment is performed after the final cold rolling. Do. In the fourth step, at least the cold rolling ratio in the final cold rolling needs to be 70% or more. When the cold rolling is performed only once in the fourth step, the one cold rolling becomes the final cold rolling. By cold rolling, a crystal orientation effective for deep drawing is formed together with the subsequent heat treatment. The rolling ratio of the final cold rolling is preferably 75% or more, more preferably 80% or more. The upper limit of the rolling ratio is not particularly limited, but it is realistic to set it to 97% or less from the viewpoint of the capacity and cost of the rolling mill. Further, the cold rolling ratio in cold rolling other than the final may be the same as the rolling ratio in the final cold rolling, or may be changed as appropriate. The soaking time of the final heat treatment in the fourth step is preferably 5 minutes or less.

In the fourth step of repeating the heat treatment and once or twice or more cold rolling, heat treatment after at least the final cold rolling, the temperature reached the soaking temperature _{(T 2 -10) ℃ ~ (} T 2 +30 ) It is necessary to control in the temperature range. T ₂ is the recrystallization temperature (° C.) after the final cold rolling. The recrystallization temperature T ₂ also changes due to the influence of the steel component, the plate thickness, and the cold reduction rate, similarly to the recrystallization temperature T ₁ after hot spreading. If the ultimate temperature is lower than (T 2 _-10) ℃, recrystallization, becomes softened both insufficient, the formability of the steel sheet is remarkably deteriorated, and defective molding. On the other hand, when the temperature exceeds (T ₂ +30) ° C., the recrystallized grains grow, the desired r value cannot be obtained, and the shape after molding deteriorates. Further, if the crystal grain size exceeds a predetermined value, the surface shape after molding becomes defective. The heat treatment conditions other than the final heat treatment may be the same as the final heat treatment conditions, or may be changed as appropriate.

The recrystallization temperatures T ₁ and T ₂ can be determined by observing the metallographic structure after heat treatment using a hot-rolled plate or a cold-rolled plate. That is, the steel sheet is cut, the temperature is raised at a heating rate of 10 ° C./s, the temperature is maintained at the ultimate temperature T ° C. for 30 seconds, and then the steel sheet is rapidly cooled. The quenching is water cooling or He quench. The ultimate temperature T in the heat treatment is 10 levels at a pitch of 20 ° C. The cross section (L cross section) of the heat-treated steel sheet viewed from the rolling width direction is polished and etched to reveal grain boundaries. Recrystallized / unrecrystallized is judged from the crystal grain shape and the presence or absence of strain in the grain by an optical microscope. Recrystallization proceeds when the ultimate temperature T is high, but the lowest temperature at which the metal structure becomes completely recrystallized is set as the recrystallization temperature.

Further, the heat treatment after hot rolling in the second step and the final heat treatment after cold rolling in the fourth step are aimed at recrystallization, which is considered to proceed rapidly, and suppress grain growth and take a short time from the industrial aspect. It is preferable to carry out with. Specifically, it is 5 minutes or less. Further, it is preferably 1 second or more and 3 minutes or less. The heat treatment may be carried out in either a batch type furnace or a continuous type furnace, and may be generally used in the production of stainless steel. The heat treatment atmosphere may be a non-oxidizing atmosphere containing hydrogen gas, nitrogen gas, or the like, or may be an atmosphere. When the surface scale is formed, it may be descaled. However, since the final heat treatment after cold rolling affects the surface shape after product processing, it is more preferable to carry out in a non-oxidizing atmosphere.

By going through the above manufacturing steps, the ferritic stainless steel sheet of the present embodiment can be obtained.

The ferrite-based stainless steel sheet of the present embodiment is excellent in moldability and surface shape after molding. In particular, the ferrite-based stainless steel sheet of the present embodiment has improved deep drawability among moldability. Further, since the ferritic stainless steel sheet of the present embodiment shows an excellent value of the in-plane anisotropy Δr, the in-plane anisotropy of the steel sheet becomes small, and the ear of the molded product after deep drawing is made smaller. be able to.

Next, an example of the present invention will be shown. The conditions of the examples are one condition example adopted for confirming the feasibility and effect of the present invention. Therefore, the present invention is not limited to the conditions used in the following examples. Various conditions can be adopted as long as the object is achieved without departing from the gist of the present invention.
The underlined lines in the table below indicate those outside the scope of the present invention.

Stainless steel with the composition shown in Table 1 was melted in a small laboratory-level melting furnace and hot-rolled to a thickness of about 5 mm as the first step. Next, after investigating the recrystallization temperature (T ₁ ° C.) of the hot-rolled plate, as a second step, heat treatment was carried out at a predetermined temperature mainly at (T ₁ + 10) ° C. for 3 minutes. Next, as a third step, a predetermined aging heat treatment for the purpose of compound precipitation was carried out except for some steel sheets. Then, as a fourth step, cold rolling was carried out. For cold rolling, a small single-stand rolling mill at the laboratory level was used, and the processing ratio was changed to reduce the thickness to 0.6 mm. Further, the recrystallization temperature (T ₂ ° C.) of the cold rolled plate was investigated, and a heat treatment was carried out mainly at T ₂ ° C. at a predetermined temperature for 30 seconds. In this way, the steel plate No. 1-No. 26 was manufactured. The survey results at T ₁ and T ₂ ° C are also shown in Table 2.

The obtained stainless steel plate No. 1-No. Twenty-six characteristics were investigated.
The amounts of Ti, Nb, and P contained in the compound after the aging heat treatment, the crystal particle size number (GSNo.) In the L cross section, and r _Ave and Δr as the deep drawing property were measured. The method for measuring each characteristic is as described above.

Next, a sample having a diameter of 110 mm was cut out from the obtained steel sheet, and the formability was investigated by a cylindrical deep drawing test. The evaluation was 0 when it was moldable and x when it cracked in the middle. FIG. 1 shows a schematic view of the post-processed test piece that was capable of deep drawing. Next, with respect to the test piece that could be deep-drawn, the difference between the maximum height and the minimum height of the ear shown in FIG. 1 was measured as the ear height. Those with an ear height of 5 mm or less were evaluated as acceptable, and those with an ear height of more than 5 mm were evaluated as rejected.

In addition, rough skin was measured on the post-processed test piece that could be deep-drawn. For rough skin, a two-dimensional contact type surface roughness measuring machine was used to measure the surface roughness at the center of the height of the vertical wall portion for a length of 5 mm parallel to the height direction. Comparing the arithmetic mean roughness Ra described in JIS B 0031 (2003), the standard is 2.0 μm, which is the same as that of ordinary cold-rolled steel sheet, and the case of less than that is ○, and the case of more than 2.0 μm is ×. And said. When Ra exceeds 2.0 μm, rough skin becomes remarkable.

In the deep drawing test, the punch diameter is 50 mm, the punch shoulder R is 5 mm, the die diameter is 53 mm, the die shoulder R is 8 mm, the wrinkle pressing pressure is 10 tons, and the lubrication between the sample and the punch is manufactured by Idemitsu Kosan Co., Ltd. After applying the rust preventive oil "Daphne Oil Coat Z3 (registered trademark)", the lubricating sheet "Nachias Corporation Naflon Tape TOMBO9001" was attached.

Table 2 shows the stainless steel plate No. 1-No. The results of 26 surveys are shown. It was separately confirmed that all the steel sheets of the examples of the present invention were ferrite single phases (excluding the austenite phase and the martensite structure).

No. of the example of the present invention. 1, 5, 7, 9, 11, 13, 15, and 19 have the contents of Ti, Nb and P in the compound and the crystal grain size number within the range of the present invention, and are excellent because they can be deep drawn. It showed moldability, was suppressed from rough skin after molding, and had an excellent surface shape after molding. Further, the ear height of the molded product after deep drawing molding was also reduced, the in-plane anisotropy was small, and the moldability was excellent.

On the other hand, in the comparative example, as shown below, either the compound content or the crystal particle size number is not achieved, and the deep drawing property is not achieved, so that molding is not possible, or even if molding is possible, rough skin is caused. Occurred.

No. No. 2 has a high heat treatment temperature after hot rolling, and No. In No. 3, the aging heat treatment temperature was high, _XP and the particle size number were not reached, and rough skin occurred. In addition, Δr was not reached, and the ear height was also poor.

No. In No. 4, the heat treatment temperature after cold rolling was low, both softening and recrystallization were insufficient, and molding was difficult. It was difficult to measure the crystal grain size and r-value.

No. 6 has a short aging _{time, X Nb,} X P, not achieved in grain size both, caused the rough skin. Δr was not reached, and the ear height was also poor.

No. In No. 8, it was considered that the cold rolling ratio was low, the crystal grain size was not reached, and the desired crystal orientation was not obtained, _rAve was not reached, and molding was difficult.

No. 10 does not perform the _aging, X P, the grain size is not achieved, rough skin has occurred. In addition, Δr was not reached, and the ear height was also poor.

No. In No. 12, the aging temperature was low, _XP and the particle size number were not reached, and rough skin occurred. Δr was not reached, and the ear height was also poor.

No. 14 has a long time of _{aging, X Nb,} X _P, not achieved in both the grain size number, rough skin has occurred. Δr was not reached, and the ear height was also poor.

No. 16, lower the heat treatment temperature after hot rolling _is, X _P, is _{r Ave} not achieved, molding was difficult.

No. 17 without aging, since lower cold rolling reduction, X _{Ti, X P,} is not reached in both the grain size number, the molding was difficult.

No. In No. 18, the heat treatment temperature after cold rolling was high, and the crystal grain size, _rAve , and Δr were not reached, and molding was difficult.

No. 20 has a low aging _{temperature, X Ti,} X _P, not achieved in both the grain size number, rough skin has occurred. In addition, Δr was not reached, and the ear height was also poor.

Comparative Example No. using steel I whose Ti content is not applicable. In 21 and 22, X _Ti was not reached. Further, in Comparative Examples Nos. 23 and 24 using steel J whose Nb content was not the target, X _Nb was not reached. Furthermore, Comparative Example No.25,26 the P content using a steel K outside object _{X P} becomes not achieved. In addition, No. In most of 21 to 26, the particle size number was not reached, and molding defects or rough skin occurred. In addition, No. In most of 21 to 26, Δr was not reached and the ear height was also poor.

According to the present invention, it is possible to industrially stably provide a ferritic stainless steel sheet having excellent moldability and surface shape after molding. Further, since the steel sheet of the present invention does not cause rough skin after molding, it is possible to omit the polishing step after molding for the purpose of removing the rough skin, which is effective in terms of environment and manufacturing cost. You can fully enjoy it.

The gist of the present invention is as follows.
[1] The chemical component is mass%,
Cr: 11.0% or more, 25.0% or less,
C: 0.001% or more, 0.010% or less,
Si: 0.01% or more, 1.0% or less,
Mn: 0.01% or more, 1.0% or less,
P: 0.010% or more and 0.040% or less,
S: 0.01% or less,
N: 0.002% or more, 0.020% or less,
Al: 0.003% or more, 1.0% or less,
Ti: 0.05% or more, 0.30% or less,
Nb: contains 0.03% or more and 0.30% or less, satisfies the following equation (1), and the balance is composed of Fe and impurities.
Ti, Nb, among the P, Ti contained in the compounds in extracted steels by electrowinning residue method, Nb, respectively P _{X Ti,} X Nb, when the _{X P,} in mass%,
X _Ti : 0.035% or more, 0.060% or less,
X _Nb : 0.010% or more, 0.025% or less,
X _P: 0.001% or more, and satisfies the following 0.010%,
The crystal grain size number is 9.0 or higher,
The average plastic strain ratio r _Ave represented by (r ₀ + r ₉₀ + 2 × r ₄₅ ) / 4 satisfies the following equation (2).
_{(R 0 + r 90 -2 ×} r 45) / 2 plane anisotropy Δr is the following (3) represented by the ferritic stainless steel sheet which satisfies the equation.
Ti / 48 + Nb / 93 ≧ 2 × (C / 12 + N / 14)… (1)
1.5 ≤ r _Ave ... (2)
―0.4 ≦ Δr ≦ 0.1… (3)
However, r ₀ , r ₉₀ , and r ₄₅ are r ₀ : plastic strain ratio in the rolling direction, r ₄₅ : plastic strain ratio in the 45 ° direction with respect to the rolling direction, and r ₉₀ : plastic strain in the direction perpendicular to the rolling direction, respectively. the ratio der Ri, Ti in the formula (1), Nb, C and N are the contents of the elements in the steel sheet (mass%).
[2] The chemical composition is further increased by mass%.
B: 0.0025% or less,
Sn: 0.50% or less,
Ni: 1.0% or less,
Cu: 1.0% or less,
Mo: 2.0% or less,
W: 1.0% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
Y: 0.10% or less,
Hf: 0.20% or less,
REM: 0.10% or less,
Sb: The ferritic stainless steel sheet according to [1], which contains 1 type or 2 or more types of 0.50% or less.
[3] The first step of hot rolling a steel piece having the chemical composition according to [1] or [2] into a steel sheet, and
The second step of heat-treating the steel sheet after the first step at a soaking temperature of T ₁ ° C. or higher and (T ₁ +35) ° C. or lower, and
A third step of subjecting the steel sheet after the second step to an aging heat treatment at a soaking temperature of 650 ° C. or higher and 750 ° C. or lower and a soaking time of 1 minute or more and 5 minutes or less.
The steel sheet after the third step is provided with a fourth step in which cold rolling and heat treatment are each performed once, or each is repeated two or more times.
The fourth step is at least subjected to final cold rolling at a rolling ratio of 75% or more, at least the final heat treatment (T 2 _-10) ℃ above, be carried out in (T 2 ₊₃₀₎ ℃ following soaking temperature The method for producing a ferritic stainless steel sheet according to [1] or [2].
Here, T ₁ and T ₂ are T ₁ : recrystallization temperature (° C.) after hot rolling and T ₂ : recrystallization temperature (° C.) after cold rolling, respectively.

Claims (3)

The chemical composition is mass%,
Cr: 11.0% or more, 25.0% or less,
C: 0.001% or more, 0.010% or less,
Si: 0.01% or more, 1.0% or less,
Mn: 0.01% or more, 1.0% or less,
P: 0.010% or more and 0.040% or less,
S: 0.01% or less,
N: 0.002% or more, 0.020% or less,
Al: 0.003% or more, 1.0% or less,
Ti: 0.05% or more, 0.30% or less,
Nb: contains 0.03% or more and 0.30% or less, satisfies the following equation (1), and the balance is composed of Fe and impurities.
Ti, Nb, among the P, Ti contained in the compound in the steel, Nb, respectively P _{X Ti, X Nb,} when the _{X P,}
X _Ti : 0.035% or more, 0.060% or less,
X _Nb : 0.010% or more, 0.025% or less,
X _P: 0.001% or more, and satisfies the following 0.010%,
The crystal grain size number is 9.0 or higher,
The average plastic strain ratio r _Ave represented by (r ₀ + r ₉₀ + 2 × r ₄₅ ) / 4 satisfies the following equation (2).
_{(R 0 + r 90 -2 ×} r 45) / 2 plane anisotropy Δr is the following (3) represented by the ferritic stainless steel sheet which satisfies the equation.
Ti / 48 + Nb / 93 ≧ 2 × (C / 12 + N / 14)… (1)
1.5 ≤ r _Ave ... (2)
―0.4 ≦ Δr ≦ 0.1… (3)
However, r ₀ , r ₉₀ , and r ₄₅ are r ₀ : plastic strain ratio in the rolling direction, r ₄₅ : plastic strain ratio in the 45 ° direction with respect to the rolling direction, and r ₉₀ : plastic strain in the direction perpendicular to the rolling direction, respectively. The ratio. The chemical composition is further increased by mass%.
B: 0.0025% or less,
Sn: 0.50% or less,
Ni: 1.0% or less,
Cu: 1.0% or less,
Mo: 2.0% or less,
W: 1.0% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
Y: 0.10% or less,
Hf: 0.20% or less,
REM: 0.10% or less,
Sb: The ferrite-based stainless steel sheet according to claim 1, which contains one or more of 0.50% or less. The first step of hot rolling a steel piece having the chemical composition according to claim 1 or 2 into a steel sheet, and
The second step of heat-treating the steel sheet after the first step at a soaking temperature of T ₁ ° C. or higher and (T ₁ +35) ° C. or lower, and
A third step of subjecting the steel sheet after the second step to an aging heat treatment at a soaking temperature of 650 ° C. or higher and 750 ° C. or lower and a soaking time of 1 minute or more and 5 minutes or less.
The steel sheet after the third step is provided with a fourth step in which cold rolling and heat treatment are each performed once, or each is repeated two or more times.
The fourth step is at least subjected to final cold rolling at a rolling ratio of 75% or more, at least the final heat treatment (T 2 _-10) ℃ above, be carried out in (T 2 ₊₃₀₎ ℃ following soaking temperature The method for producing a ferritic stainless steel sheet according to claim 1 or 2, wherein the ferritic stainless steel sheet is produced.
Here, T ₁ and T ₂ are T ₁ : recrystallization temperature (° C.) after hot rolling and T ₂ : recrystallization temperature (° C.) after final cold rolling, respectively.

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