JP7166878B2 - Ferritic stainless steel plate, manufacturing method thereof, and ferritic stainless steel member - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet, a method for manufacturing the same, and a ferritic stainless member.

Exhaust system members such as exhaust manifolds, converters, front pipes and mufflers of automobiles are required to have heat resistance such as high-temperature strength, thermal fatigue resistance, and oxidation resistance. Therefore, heat-resistant ferritic stainless steel containing Cr is used.

The ambient temperature in which the exhaust system members are used is increasing year by year, and it is necessary to increase the content of elements such as Cr, Mo, Nb, and Ti to improve high-temperature strength and thermal fatigue resistance. As a result, steel types such as SUS 429 series (14% Cr--Nb, Si-added steel) and SUS 444 series (18% Cr--Nb, Mo-added steel) are mainly used at present.

On the other hand, these members are press-formed from a steel plate, or formed into a desired shape after the steel plate is made into a steel pipe of a specified size (diameter). is required. For example, in steel sheets, deep drawability and hole expandability, which are closely related to press workability, are important characteristics.

By the way, among the alloying elements that are effective in improving heat resistance, Nb is particularly effective in improving high-temperature strength and thermal fatigue resistance, and is used in many heat-resistant ferritic stainless steels. On the other hand, Nb tends to form carbonitrides (NbC, NbN, etc.) or intermetallic compounds (Laves phase, etc.) during annealing, and these precipitates pin dislocations and grain boundaries of recrystallized grains, It is known to retard recrystallization.

Crystal grains having an orientation in which the {111} plane is parallel to the rolled surface contribute to improvement in workability. Therefore, it is preferable that the crystal grains having the above orientation are recrystallized during annealing or the like. However, as described above, if recrystallization is delayed by containing Nb, it becomes difficult for crystal grains having the above orientation to grow throughout the plate thickness. As a result, there is a problem that it becomes difficult to improve the workability of the steel sheet. In addition, when selecting exhaust system members for automobiles, cost is also considered important, and there is a demand for both further cost reduction and the above-mentioned material properties by omitting the number of steps.

Under these circumstances, various attempts have been made in the past to solve the problems related to the workability of heat-resistant ferritic stainless steel sheets.

In particular, for the deep drawability of steel sheets, the Lankford value (hereinafter referred to as "r value") is used as an index, and the characteristics are improved by controlling the crystal orientation. In addition, in order to reduce the waviness (so-called "ear" portion) that occurs in the lip portion during deep drawing, the in-plane anisotropy of the r value (hereinafter sometimes simply referred to as "Δr"). ) is also required to be as small as possible.

Furthermore, in the hole-expanding process of a steel plate, tensile stress is generated in all circumferential directions from the punched hole. Therefore, it is required not only to improve the r value, but also to secure the minimum r value (hereinafter sometimes simply referred to as “r _min ”).

In response to such demands, Patent Literature 1 discloses a steel in which conditions such as the annealing temperature, annealing time, and rolling reduction in the hot rolling process are controlled to improve workability. The steel disclosed in Patent Document 1 has a maximum r-value of about 1.6.

The above steel manufacturing method promotes recrystallization of the rough bar in the rough rolling in the hot rolling process, makes the recrystallization rate 30% or more, and further performs finish rolling and hot rolled sheet annealing at 750 to 1050 ° C. Thus, a steel sheet with high workability is obtained.

Further, Patent Document 2 discloses a steel pipe whose expansion rate exceeds 100% by performing two-stage annealing. For the above steel pipe, a 0.8 mm material with an r value of about 1.6 is disclosed.

In the method for producing the above steel, the hot-rolled steel strip is once cold-rolled during the process, then intermediate-annealed in a temperature range of 900 to 1050 ° C., cold-rolled again, and then subjected to a temperature range of 950 to 1100 ° C. A highly workable steel sheet is obtained by performing finish annealing at .

In Patent Document 3, a Cr-containing heat-resistant steel sheet containing 11 to 23% Cr, 0.8% or less Nb, or 1.0% or less Ti contains control of hot-rolling conditions, hot-rolled sheet annealing, and intermediate annealing. A steel having an average r-value of 2.0 or more by applying , and a method for producing the same are disclosed.

The method for producing the above steel includes hot-rolled sheet annealing at 800 to 1050 ° C., or hot-rolled sheet annealing, followed by intermediate cold rolling and intermediate annealing at 650 to 1050 ° C. to obtain the structure before finish cold rolling. And the precipitates are controlled to develop the {111}-oriented texture of the steel sheet, and at the same time, a removable lubricating coating film is applied to obtain a highly workable steel sheet. Further, Patent Document 4 discloses a heat-resistant steel for automotive exhaust system parts, which is cost-saving due to alloy saving.

JP-A-2005-325377 JP 2006-274419 A JP-A-2003-138349 JP 2014-162964 A

In the manufacturing processes in Patent Documents 1 to 3, heat treatment such as hot-rolled sheet annealing or intermediate annealing is performed to completely recrystallize the steel sheet before finish cold rolling, thereby improving the r value. However, by promoting recrystallization by heat treatment such as hot-rolled sheet annealing and intermediate annealing, the crystal orientation may not be uniform, and so-called randomization may progress.

Cold-rolling and annealing a steel sheet with such randomized crystal orientation may increase the in-plane anisotropy (Δr) of the r-value of the steel sheet. As a result, it is conceivable that defects such as cracks may occur during steel sheet forming such as hole expansion. In addition, the above method may increase the number of steps and greatly reduce productivity.

The steel for cost-saving exhaust system parts disclosed in Patent Document 4 has sufficient heat resistance, but there is no process for improving workability, and workability has not been sufficiently studied.

An object of the present invention is to solve the above problems and provide a ferritic stainless steel sheet for exhaust parts, a ferritic stainless steel member for exhaust system parts, and a ferritic To provide a method for manufacturing a stainless steel plate.

The present invention was made to solve the above problems, and the gist thereof is as follows.

(1) chemical composition, in mass %,
C: 0.001 to 0.020%,
Si: 0.010 to 1.50%,
Mn: 0.02-1.00%,
P: 0.01 to 0.050%,
S: 0.0001 to 0.010%,
Cr: 10.0 to 20.0%,
N: 0.001 to 0.030%,
Nb: 0.1 to 0.8%,
Ti: 0.01 to 0.30%,
Sn: 0.003 to 0.500%,
Mg: 0-0.0100%,
B: 0 to 0.0050%,
V: 0 to 1.0%,
Mo: 0-3.0%,
W: 0 to 3.0%,
Al: 0-0.3%,
Cu: 0-2.0%,
Zr: 0 to 0.30%,
Co: 0-0.50%,
Sb: 0 to 0.50%,
REM: 0-0.05%,
Ni: 0 to 2.0%,
Ca: 0 to 0.0030%,
Ta: 0 to 0.10%,
Ga: 0-0.1%,
balance: consisting of Fe and unavoidable impurities,
satisfying the following (i) formula and (ii) formula,
In the crystal orientation intensity of the ferrite phase at the center of the sheet thickness by X-ray diffraction, the {111} <112> crystal orientation intensity is 10.0 or more, and the {322} <236> crystal orientation intensity is 10.0 or more,
A ferritic stainless steel sheet having an average Lankford value (r _m ) calculated by the following formula (iii) of 1.10 or more and Δr calculated by the following formula (iv) of 0.2 or less.
Nb+Ti≧3(C+N) (i)
Nb+Sn≧0.2 (ii)
However, each element symbol in the above formulas (i) and (ii) indicates the content of each element contained in the steel sheet in mass %.
_rm =( _r0 + _2r45 + _r90 )/4 (iii)
Δr=(r ₀ +r ₉₀ )/2−r ₄₅ (iv)
However, each symbol in the above formula is defined as follows.
r ₀ : r value in the rolling direction r ₄₅ : r value in the direction of 45° to the rolling direction r ₉₀ : r value in the direction of 90° to the rolling direction

(2) The ferritic stainless steel sheet according to (1) above, which has a minimum Lankford value (r _min ) of 0.80 or more and satisfies the following formula (v).
n(1+r _min )≧0.40 (v)
However, each symbol in the above formula (v) is defined as follows.
n: work hardening index

(3) the chemical composition, in mass %,
Mg: 0.0002-0.0100%,
B: 0.0002 to 0.0050%,
V: 0.05 to 1.0%,
Mo: 0.2-3.0%, and W: 0.1-3.0%,
containing one or more selected from
The ferritic stainless steel sheet according to (1) or (2) above.

(4) the chemical composition, in mass %,
Al: 0.003-0.3%,
Cu: 0.1 to 2.0%,
Zr: 0.05 to 0.30%,
Co: 0.05-0.50%,
Sb: 0.01-0.50%, and REM: 0.001-0.05%,
containing one or more selected from
The ferritic stainless steel sheet according to any one of (1) to (3) above.

(5) the chemical composition, in mass %,
Ni: 0.1 to 2.0%,
Ca: 0.0001 to 0.0030%,
Ta: 0.01-0.10%, and Ga: 0.0002-0.1%,
containing one or more selected from
The ferritic stainless steel sheet according to any one of (1) to (4) above.

(6) The ferritic stainless steel sheet according to any one of (1) to (5) above, which has a grain size number of 5.0 or more.

(7) The ferritic stainless steel sheet according to any one of (1) to (6) above, which is used for exhaust system parts.

(8) A ferritic stainless steel member for exhaust system parts of automobiles or motorcycles, comprising the ferritic stainless steel sheet according to any one of (1) to (6) above.

(9) (a) A hot rolling step of heating a slab having the chemical composition according to any one of (1) to (6) above and hot rolling the slab to obtain a hot rolled steel sheet. ,
(b) a hot-rolled steel plate pickling step of pickling the hot-rolled steel plate to obtain a pickled steel plate without annealing the hot-rolled steel plate;
(c) cold-rolling the pickled steel sheet using a rolling mill having a roll diameter of 400 mm or more at a rolling reduction of 60% or more to obtain a cold-rolled steel sheet;
(d) an annealing step of annealing the cold-rolled steel sheet at an annealing temperature Tf (°C) that satisfies the following formula (vi);
A method for producing a ferritic stainless steel sheet, in which
850 + 300 Nb ≤ Tf (°C) ≤ 950 + 300 Nb (vi)
However, the symbols of the elements in the above formula indicate the contents of the elements in terms of % by mass.

(10) The method for producing a ferritic stainless steel sheet according to (9) above, wherein the equiaxed grain ratio of the slab in the step (a) is 30% or more.

(11) The average heating rate until reaching the annealing temperature Tf (° C.) in the step (d) is
15° C./s or more in the temperature range from the temperature rise start temperature to the recrystallization start temperature Ts (° C.) calculated by the following formula (vii),
10 ° C./s or less in the temperature range from the recrystallization start temperature Ts (° C.) to the annealing temperature Tf (° C.),
A method for producing a ferritic stainless steel sheet according to (9) or (10) above.
Ts (°C) = 750 + 300 Nb (vii)
However, the symbols of the elements in the above formula indicate the contents of the elements in terms of % by mass.

According to the present invention, there are provided a ferritic stainless steel sheet for exhaust parts, a ferritic stainless member for exhaust system parts, and a method for producing the ferritic stainless steel sheet, which are excellent in not only workability, particularly deep drawability but also hole expandability. can be done.

FIG. 1 is a diagram showing the relationship between the {111}<112> crystal orientation strength at the center of the plate thickness and the _rm of the steel plate. FIG. 2 is a diagram showing the relationship between the {322}<236> crystal orientation strength at the center of the sheet thickness and the Δr of the steel sheet. FIG. 3 is a diagram showing the relationship between hole expansibility and n value and r _min . FIG. 4 is a diagram showing the tube diameter with respect to the grain size number and the maximum roughness Rz.

In order to solve the above problems, the present inventors conducted detailed studies on the composition, structure in the manufacturing process, and formation of crystal orientation with respect to the workability of heat-resistant ferritic stainless steel sheets. As a result, the following findings were obtained.

(a) In a heat-resistant ferritic stainless steel sheet containing Nb, in order to obtain the desired workability, the average r value (r _m ), minimum r value (r _min ), and Δr indicating in-plane anisotropy , must be properly controlled. When the relationship between the above values and the deep drawability and hole expansibility of a given steel composition was investigated, it was found that when the steel plate had r _m of 1.10 or more, r _min of 0.80 or more, and Δr of 0.2 or less, In, a good high workability steel plate is obtained. Also, between the work hardening index n and r _min , n(1+r _min ) is preferably 0.40 or more.

(b) In order to obtain a steel sheet with r _m , r _min , and Δr within the above ranges, it is necessary to develop a texture with a specific orientation. As a specific orientation, when the texture of {111}<112> orientation develops, the r value in each direction improves with respect to the rolling direction. In addition to the above orientations, when the texture of the {322}<236> orientation is developed, the difference between the minimum r value and the maximum r value becomes small, and the above r _min and Δr values can be obtained.

(c) In order to develop the textures of the {111}<112> and {322}<236> orientations described above, it is preferable to appropriately control the composition and manufacturing process. For example, regarding the composition, it is effective to adjust the content of elements such as Nb and Sn within an appropriate range. This is because the above elements contribute to the formation of equiaxed crystals that are effective for the growth of the {111} oriented texture. In addition, regarding the manufacturing process, in order to preferentially recrystallize the crystal grains having the above orientation while suppressing the randomization of the crystal orientation, annealing after hot rolling is not performed, and annealing after cold rolling is performed. It is effective to appropriately control the heating rate in

(d) It is necessary to sufficiently grow {111}<112> crystal grains to improve the r value. However, when the crystal grain size is large, processed surface roughness such as ridging and orange peel becomes noticeable. As a result, the surface properties of the steel sheet deteriorate. Such surface roughening causes secondary work embrittlement and buckling during press working, resulting in work cracks and molding defects. Therefore, by setting the grain size number (hereinafter referred to as grain size number) of ferrite to 5.0 or more, it is possible to obtain a steel sheet having excellent resistance to work surface roughening.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.001 to 0.020%
C degrades toughness, corrosion resistance and oxidation resistance, and solid solution C in the matrix inhibits the development of {111}<112> and {322}<236> oriented textures. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the smaller the content, the better, and the C content is 0.020% or less, preferably 0.010% or less. However, excessive reduction of C leads to an increase in refining cost, so the C content is made 0.001% or more. Considering manufacturing cost and corrosion resistance, the C content is preferably 0.002% or more.

Si: 0.010-1.50%
Si is a deoxidizing element and also an element that improves oxidation resistance and high-temperature strength. In addition, the inclusion of Si reduces the amount of oxygen in the steel and facilitates the development of textures of {111}<112> orientation and {322}<236> orientation. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the Si content should be 0.010% or more. In order to significantly develop the texture, the Si content is preferably more than 0.300%, more preferably 0.800% or more.

On the other hand, if the Si content exceeds 1.50%, the steel sheet becomes significantly hardened, and the bendability deteriorates during steel pipe working. Therefore, the Si content is set to 1.50% or less. Considering toughness and pickling property during steel sheet production, the Si content is preferably 1.20% or less. The Si content is more preferably 1.00% or less.

Mn: 0.02-1.00%
Mn forms MnCr ₂ O ₄ or MnO at high temperatures and improves scale adhesion. Therefore, the Mn content is set to 0.02% or more. The Mn content is preferably more than 0.15%, more preferably 0.18% or more. On the other hand, if the content of Mn exceeds 1.00%, the amount of oxides increases and abnormal oxidation tends to occur. In addition, Mn forms compounds with S and inhibits the development of {111}<112> and {322}<236> oriented textures. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the Mn content is set to 1.00% or less. Also, considering the toughness and pickling property during steel sheet production, the Mn content is preferably 0.8% or less. Furthermore, considering flat cracks caused by oxides in steel pipe welds, the Mn content is more preferably 0.30% or less.

P: 0.01 to 0.050%
Like Si, P is a solid-solution strengthening element, so from the viewpoint of material quality and toughness, the smaller the content, the better. In addition, P dissolved in the parent phase inhibits the development of textures of {111}<112> orientation and {322}<236> orientation. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the P content should be 0.050% or less. On the other hand, excessive reduction of P leads to an increase in refining cost, so the P content is made 0.01% or more. Considering production cost and oxidation resistance, the P content is preferably 0.015% or more and preferably 0.030% or less.

S: 0.0001 to 0.010%
From the viewpoint of material quality, corrosion resistance, and oxidation resistance, the smaller the amount of S, the better. In particular, an excessive content of S forms a compound with Ti or Mn, which causes cracks originating from inclusions during steel pipe bending. In addition, the presence of these compounds inhibits the development of {111}<112> and {322}<236> oriented textures. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the S content should be 0.010% or less. On the other hand, excessive reduction of S leads to an increase in refining cost, so the S content is made 0.0001% or more. Furthermore, considering manufacturing cost and corrosion resistance, the S content is preferably 0.0005% or more and preferably 0.0050% or less.

Cr: 10.0-20.0%
Cr is a necessary element to ensure the most important properties in exhaust components such as high temperature strength and oxidation resistance. Therefore, the Cr content is set to 10.0% or more. On the other hand, if the Cr content exceeds 20.0%, the toughness deteriorates, the manufacturability deteriorates, and brittle cracking or poor bendability particularly occurs in steel pipe welds.

In addition, excessive solute Cr inhibits the development of {111}<112> and {322}<236> oriented textures. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the Cr content is set to 20.0% or less. From the viewpoint of the toughness of the hot-rolled sheet during steel sheet production, the Cr content is preferably 18.0% or less. From the viewpoint of manufacturing cost, the Cr content is preferably less than 14.0%.

N: 0.001 to 0.030%
N, like C, degrades low-temperature toughness, workability, and oxidation resistance. In addition, since N dissolved in the matrix phase inhibits the development of textures of {111}<112> orientation and {322}<236> orientation, the smaller the content, the better. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the N content is set to 0.030% or less. On the other hand, an excessive reduction in N leads to an increase in refining costs. Therefore, the N content is set to 0.001% or more. Considering manufacturing cost and toughness, the N content is preferably 0.005% or more, more preferably 0.008% or more. For the above reasons, the N content is preferably 0.020% or less.

Nb: 0.1-0.8%
Nb combines with C or N to form carbonitrides. In addition, Nb causes compositional supercooling during casting, and is effective in forming a {111}<112> oriented texture. ) is significantly increased. In this way, Nb develops the {111}<112> oriented texture of the product sheet and promotes an improvement in the r-value and the expandability of the steel pipe.

In addition, Nb has high solid-solution strengthening ability and precipitation strengthening ability in a high temperature range, and improves high-temperature strength and thermal fatigue properties. Furthermore, when Nb is not contained, recrystallization occurs during hot rolling, inhibiting the development of {322}<236>. The development of {111}<112> and {322}<236> orientations improves the minimum r value and also improves hole expansibility. Therefore, the Nb content is set to 0.1% or more.

On the other hand, excessive Nb content significantly deteriorates the toughness in the steel sheet manufacturing stage. In addition, it precipitates coarse intermetallic compounds called carbonitrides or Laves phases during annealing. Such precipitates retard recrystallization by pinning grain boundaries. As a result, the development of the texture of the target orientation is suppressed, and the r value is lowered. Therefore, the Nb content is set to 0.8% or less. Considering the intergranular corrosion resistance of the weld zone, manufacturing cost and manufacturability, the Nb content is preferably 0.15% or more and preferably 0.55% or less.

Ti: 0.01-0.30%
Ti combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. In addition, Ti nitride increases the equiaxed grain ratio by becoming the nucleus of crystal grains during slab casting. As a result, the texture of the {111}<112> orientation of the steel sheet is developed and the r value is improved. Thus, the effect of binding with C and N to fix these elements is exhibited by containing 0.01% or more of Ti, so the Ti content is set to 0.01% or more and 0.11%. % or more. On the other hand, if the Ti content exceeds 0.30%, the steel becomes hard due to solid solution Ti, and the toughness deteriorates. Therefore, the Ti content should be 0.30% or less. Considering the production cost and the like, the Ti content is preferably 0.05% or more and preferably 0.25% or less.

Here, if the total content of Ti and Nb is less than 3 (C+N), C and N cannot be sufficiently fixed, and excessive C and N dissolve in the steel and harden it, reducing workability. Therefore, it is necessary to satisfy the following formula (i).
Nb+Ti≧3(C+N) (i)
However, each element symbol in the above formula (i) indicates the content of each element contained in the steel sheet in mass%.

The left-hand side value in the above formula (i) is preferably 0.1 or more, more preferably 0.15 or more, for the effect of increasing the equiaxed crystal ratio. From the viewpoint of hardening the material and manufacturing cost, the left side value in the above formula (i) is preferably 1.0 or less.

Sn: 0.003-0.500%
Sn, like Nb, causes compositional supercooling during slab casting and increases the equiaxed grain ratio. As a result, the {111}<112> oriented texture of the steel sheet is developed, promoting an improvement in the r value and the expandability of the steel pipe. In addition, the development of the {111}<112> orientation also improves the minimum r-value, thus improving the hole expansibility. Sn is more likely than Nb to cause a compositionally supercooled region, and these effects are produced by containing 0.003% or more of Sn. Therefore, the Sn content is set to 0.003% or more. However, when Sn is contained in an amount exceeding 0.500%, it causes excessive segregation and lowers the low temperature toughness of the steel pipe weld zone. Therefore, the Sn content is set to 0.500% or less. Considering high-temperature properties, manufacturing costs and toughness, the Sn content is preferably 0.300% or less.

As mentioned above, Nb and Sn significantly increase the equiaxed fraction of the slab and contribute to the formation of the {111}<112> oriented texture. However, Nb may delay recrystallization by pinning grain boundaries in the annealing process after cold rolling. Therefore, it is effective to increase the equiaxed grain ratio of the slab by adding Nb and Sn in combination. Therefore, the total content of Nb and Sn must satisfy the following formula (ii).
Nb+Sn≧0.2 (ii)
However, each element symbol in the above formula (ii) indicates the content of each element contained in the steel sheet in mass %.

The left side value in the above formula (ii) is preferably 0.2 or more, more preferably 0.25 or more, from the viewpoint of increasing the equiaxed grain size. Moreover, from the viewpoint of deterioration in toughness due to hardening of the material, the left-hand side value in the above formula (ii) is preferably 0.6 or less.

In addition to the components described above, the present invention preferably contains one or more groups selected from the following Group A, Group B, and Group C components as necessary. Elements classified into the A group are elements that affect the texture of the {111}<112> orientation. Elements classified into the B group are elements that improve high-temperature properties such as high-temperature strength and oxidation resistance. Elements classified into the C group are elements that improve toughness, corrosion resistance, and the like.

Group A element Mg: 0 to 0.0100%
Mg, like Al, forms Mg oxide in molten steel and acts as a deoxidizing agent. In addition, Mg increases the equiaxed crystal ratio of the slab with finely crystallized Mg oxide serving as nuclei. Then, in subsequent steps, precipitation of Nb and Ti-based fine precipitates is promoted. Specifically, when the above-mentioned precipitates are finely precipitated in the hot-rolling process, they become recrystallization nuclei in the hot-rolling process and subsequent annealing process of the hot-rolled sheet. As a result, a very fine recrystallized structure is obtained. This recrystallized structure also contributes to the development of a {111}<112> oriented texture and an improvement in toughness. Therefore, it may be contained as necessary.

However, an excessive Mg content results in deterioration of oxidation resistance, deterioration of weldability, and the like. Therefore, the Mg content should be 0.0100% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably 0.0002% or more. Considering the refining cost, the Mg content is preferably 0.0003% or more and preferably 0.0020% or less.

B: 0 to 0.0050%
B is an element that improves grain boundary strength by segregating at grain boundaries, and improves secondary workability and low temperature toughness. In addition, B improves the high temperature strength in the medium temperature range. Therefore, it may be contained as necessary. However, when the content of B exceeds 0.0050%, B compounds such as Cr ₂ B are generated, deteriorating intergranular corrosion resistance and fatigue properties. In addition, it inhibits the development of {111}<112> oriented texture, leading to a decrease in the r value. Therefore, the B content should be 0.0050% or less.

On the other hand, in order to obtain the above effects, the B content is preferably 0.0002% or more. Considering weldability and manufacturability, the B content is preferably 0.0003% or more and preferably 0.0010% or less.

V: 0-1.0%
V combines with C or N to improve corrosion resistance and heat resistance. Therefore, it may be contained as necessary. However, if the V content exceeds 1.0%, coarse carbonitrides are formed, which lowers the toughness and, in addition, inhibits the development of the {111}<112> oriented texture. Therefore, the V content is set to 1.0% or less. Furthermore, considering the production cost and manufacturability, the V content is preferably 0.2% or less. On the other hand, in order to obtain the above effects, the V content is preferably 0.05% or more.

Mo: 0-3.0%
Mo is an element that improves corrosion resistance, and is an element that suppresses crevice corrosion particularly in a pipe material having a crevice structure. Therefore, it may be contained as necessary. However, when the Mo content exceeds 3.0%, the moldability is significantly deteriorated, and the manufacturability is deteriorated. In addition, since Mo inhibits the development of the {111}<112> oriented texture, the Mo content should be 3.0% or less. On the other hand, in order to obtain the above effects, the Mo content is preferably 0.2% or more. Considering the sharp development of the {111} <112> orientation texture, alloy cost, and productivity, the Mo content is preferably 0.4% or more and 2.0% or less. is preferred.

W: 0-3.0%
W may be contained as necessary in order to increase high-temperature strength. However, an excessive W content results in deterioration of toughness and reduction in elongation. In addition, the formation of the Laves phase, which is an intermetallic compound phase, is increased, inhibiting the development of the texture of the {111}<112> orientation and lowering the r value. Therefore, the W content should be 3.0% or less. Considering manufacturing cost and manufacturability, the W content is preferably 2.0% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.1% or more.

Group B element Al: 0 to 0.3%
Al is used as a deoxidizing element. Also, Al improves high-temperature strength and oxidation resistance. In addition, Al serves as precipitation sites for TiN and Laves phases, contributes to fine precipitation of precipitates, and has the effect of improving low-temperature toughness. Therefore, it may be contained as necessary. However, an Al content of more than 0.3% results in reduced elongation, poor weldability and poor surface quality. In addition, the formation of coarse Al oxides reduces the low temperature toughness. Therefore, the Al content is set to 0.3% or less. On the other hand, in order to obtain the above effects, the Al content is preferably 0.003% or more. Considering the refining cost, the Al content is preferably 0.01% or more and preferably 0.1% or less.

Cu: 0-2.0%
Cu is an element that improves corrosion resistance and improves high-temperature strength in a medium temperature range by precipitation of Cu dissolved in the matrix phase, that is, precipitation of so-called ε-Cu. Therefore, it may be contained as necessary. However, an excessive Cu content causes a decrease in toughness and ductility due to hardening of the steel sheet. Therefore, the Cu content is set to 2.0% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.1% or more. Considering oxidation resistance and manufacturability, the Cu content is preferably less than 1.5%.

Zr: 0-0.30%
Zr is an element that improves oxidation resistance and may be contained as necessary. However, containing more than 0.30% of Zr significantly deteriorates manufacturability such as toughness and pickling property. It also coarsens compounds of Zr and carbon and nitrogen. As a result, the steel sheet structure at the time of hot rolling annealing is coarsened and the r value is lowered. Therefore, the Zr content should be 0.30% or less. Considering the production cost, the Zr content is preferably 0.20% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.05% or more.

Co: 0-0.50%
Co is an element that improves high-temperature strength, and may be contained as necessary. However, excessive content deteriorates toughness and workability. Therefore, the Co content is set to 0.50% or less. Furthermore, considering the production cost, the Co content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.05% or more.

Sb: 0-0.50%
Sb segregates at grain boundaries to increase high-temperature strength, so it may be contained as necessary. However, if the Sb content exceeds 0.50%, excessive segregation occurs and the low temperature toughness of the steel pipe weld zone is lowered. Therefore, the Sb content is set to 0.50% or less. Considering high-temperature properties, manufacturing costs and toughness, the Sb content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Sb content is preferably 0.01% or more.

REM: 0-0.05%
REMs (rare earth elements) refine various precipitates and improve toughness and oxidation resistance. Therefore, it may be contained as necessary. However, when the REM content exceeds 0.05%, the castability is significantly deteriorated. Therefore, the REM content is set to 0.05% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.001% or more. Considering refining cost and manufacturability, the REM content is more preferably 0.003% or more, and preferably 0.01% or less.

REM (rare earth elements) refers to a total of 17 elements, including two elements, scandium (Sc) and yttrium (Y), and fifteen elements (lanthanides) from lanthanum (La) to lutetium (Lu). The above REM content means the total content of these elements, and may be added singly or as a mixture.

C group element Ni: 0 to 2.0%
Since Ni is an element that improves toughness and corrosion resistance, it may be contained as necessary. However, if the Ni content exceeds 2.0%, an austenitic phase is generated, inhibiting the development of the {111}<112> oriented texture, lowering the r-value, and significantly deteriorating the steel pipe bendability. Therefore, the Ni content is set to 2.0% or less. Considering manufacturing costs, the Ni content is preferably 0.5% or less. On the other hand, the contribution of Ni to the toughness is manifested at 0.1% or more, so the Ni content is preferably 0.1% or more.

Ca: 0-0.0030%
Since Ca is an element effective as a desulfurization element, it may be contained as necessary. However, when the Ca content exceeds 0.0030%, coarse CaS is generated, degrading toughness and corrosion resistance. Therefore, the Ca content should be 0.0030% or less. On the other hand, in order to obtain the above effects, the Ca content is preferably 0.0001% or more. Considering the refining cost and manufacturability, the Ca content is more preferably 0.0003% or more, and preferably 0.0020% or less.

Ta: 0-0.10%
Since Ta combines with C and N and contributes to improvement of toughness, it may be contained as necessary. However, when the Ta content exceeds 0.10%, the manufacturing cost increases and the manufacturability remarkably deteriorates. Therefore, the Ta content is set to 0.10% or less. On the other hand, in order to obtain the above effects, the Ta content is preferably 0.01% or more. In consideration of refining cost and manufacturability, the Ta content is more preferably 0.02% or more, and preferably 0.08% or less.

Ga: 0-0.1%
Ga is contained as necessary in order to improve corrosion resistance and suppress hydrogen embrittlement. Ga content is 0.1% or less. On the other hand, in order to obtain the above effects, the Ga content is preferably 0.0002% or more in view of the formation of sulfides and hydrides. From the viewpoints of manufacturing cost, manufacturability, ductility and toughness, the Ga content is more preferably 0.0005% or more, and preferably 0.020% or less.

In the chemical composition of the present invention, the balance is Fe and unavoidable impurities. Here, the "inevitable impurities" are components that are mixed in by various factors in raw materials such as ores, scraps, and manufacturing processes when steel is industrially manufactured, and are within the range that does not adversely affect the present invention. means acceptable.

2. r-value, crystal orientation strength, and grain size of steel sheet 2-1. Calculation method of r _m and Δr The r value of the steel sheet in the present invention was measured according to JIS Z 2254 by the method described below, and the test piece was measured parallel to the rolling direction, in the 45 ° direction, and in the 90 ° direction. After determining the values, the average r-value was calculated.

A specific method for calculating the r value will be described below. JIS No. 13B tensile test pieces were taken from the cold-rolled annealed sheet from parallel, 45°, and 90° directions with respect to the rolling direction, and after applying a strain of 10 to 20% in a tensile test, the following (a) formula is calculated by substituting each value into Also, the average r value (r _m ) is calculated using the following formula (iii). Similarly, the in-plane anisotropy (Δr) of the r value is calculated using the following formula (iv).

r=ln( _W0 /W)/ln( _t0 /t) (a)
However, each symbol in the above formula is defined as follows.
_W0 : Width of test piece before tensile deformation W: Width of test piece after tensile deformation _t0 : Thickness of test piece before tensile deformation t: Thickness of test piece after tensile deformation

_rm =( _r0 + _2r45 + _r90 )/4 (iii)
Δr=(r ₀ +r ₉₀ )/2−r ₄₅ (iv)
However, each symbol in the above formula is defined as follows.
r ₀ : r value in the rolling direction r ₄₅ : r value in the direction of 45° to the rolling direction r ₉₀ : r value in the direction of 90° to the rolling direction

2-2. Relationship Between rm and _Δr and Crystal Orientation Strength The relationship between the {111}<112> crystal orientation strength at the center of the sheet thickness and the _rm of the steel sheet will be described with reference to FIG. FIG. 1 is a diagram showing the relationship between the {111}<112> crystal orientation strength at the thickness center of the material and the _rm of the steel sheet.

Here, the above texture is measured using an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.), using Mo-Kα rays, the central region of the plate thickness (combination of mechanical polishing and electrolytic polishing) (200), (110), and (211) pole figures of (200), (110), and (211) were obtained, from which the ODF (Orientation Distribution Function) was obtained using spherical harmonic functions. Based on these measurement results, the {111}<112> crystal orientation intensity and the {322}<236> crystal orientation intensity were calculated. In addition, in the present invention, the crystal orientation strength of the ferrite phase, which is the mother phase, is measured.

From FIG. 1, when the strength of the {111}<112> crystal orientation, which is a crystal orientation causing sliding deformation with greater strain in the plate width direction than the strain in the thickness direction, increases, the reduction in plate thickness is suppressed. As a result, _rm is improved as shown in FIG. Therefore, r _m can be set to 1.10 or more by setting the {111}<112> crystal orientation intensity at the thickness center to 10.0 or more. In the steel sheet according to the present invention, r _m is set to 1.10 or more, preferably 1.20 or more.

Based on the above, in the steel sheet according to the present invention, the {111}<112> crystal orientation strength of the ferrite phase at the center of the plate thickness by X-ray diffraction is set to 10.0 or more. Further, in the steel sheet according to the present invention, r _m is set to 1.10 or more.

Next, the relationship between the {322}<236> crystal orientation strength at the thickness center and the Δr of the steel sheet will be described with reference to FIG. FIG. 2 is a diagram showing the relationship between the strength of the {322}<236> crystal orientation at the thickness center of the material and the Δr of the steel sheet. An increase in the {322}<236> crystal orientation changes the anisotropy of the r-value of the steel sheet, making Δr smaller than usual. From FIG. 2, when the {322}<236> crystal orientation strength of the material is 10.0 or more, Δr can be 0.2 or less. Therefore, in the steel sheet according to the present invention, Δr is set to 0.2 or less, preferably 0.0 or less. In addition, when Δr becomes small, r ₀ and r ₉₀ become too much smaller than r ₄₅ as shown in formula (iv), and the anisotropic balance becomes too extreme. is preferred.

In the steel sheet according to the present invention, the {322}<236> crystal orientation strength of the ferrite phase at the center of the sheet thickness by X-ray diffraction is set to 10.0 or more. Also, in the steel sheet according to the present invention, Δr is set to 0.2 or less.

2-3. Calculation method of the minimum r value (r _min ) In the present invention, the minimum r value (r _min ) is the smallest r value among r ₀ , r ₄₅ and r ₉₀ , and is obtained from the calculated r values in each direction. be able to.

2-4. Relationship Between Minimum r and n Values and Hole Expandability In order to provide good hole expandability in the present invention, it is preferred that the r _min and n values satisfy the following formula (v). It is preferable that the hole expandability, which is an index thereof, be 100% or more for a steel material for automobile structural parts.

FIG. 3 is a diagram showing the relationship between hole expansibility and n value and r _min . From FIG. 3, when the following formula (v) is satisfied, it is possible to provide a highly workable stainless steel sheet with a hole expansion rate of 100% or more. In addition, since it is difficult to improve the n value by the manufacturing method, and it is necessary to ensure the minimum r _min in order to satisfy the formula (v), r _min is preferably 0.80 or more, and 0 0.90 or more is more preferable.
n(1+r _min )≧0.40 (v)
However, each symbol in the above formula (v) is defined as follows.
n: work hardening index r _min : minimum r value

Based on the above, the steel sheet according to the present invention preferably has a minimum Lankford value (r _min ) of 0.80 or more and satisfies the above formula (v).

In order to obtain even better hole expansibility, it is more preferable to set the left side value of the above formula (v) to 0.45 or more.

Here, the n-value indicates a work hardening index and is one of indices indicating deep drawability or stretch formability. The n-value can be calculated, for example, according to JIS Z 2253, by extracting a JIS No. 13B tensile test piece from a steel plate and obtaining a stress-strain curve. Specifically, it can be calculated by obtaining the value of n in the following formula (b).
σ=Cε ⁿ (b)
However, each symbol in the above formula is defined as follows.
σ: true stress ε: true strain

The hole expansion ratio, which is an index for evaluating the hole expandability, is calculated by the method described below. Specifically, according to JIS Z 2256, a 60° conical punch is pushed into a punched hole of φ10mm to widen the hole little by little. The expansion rate λ is calculated using the following formula (c).
λ=100×(D−D ₀ )/D ₀ (c)
However, each symbol in the above formula is defined as follows.
D: hole diameter after hole expansion test D ₀ : hole diameter before hole expansion test

2-5. Relationship Between Grain Size Number of Steel Plate and Work Surface Roughness Steel materials for automobile structural parts are formed mainly by multiple press workings. At this time, if the crystal grain size is large, the surface properties of the steel sheet deteriorate due to processing surface roughness such as ridging and orange peel. Such surface roughening becomes a starting point of secondary work embrittlement and buckling during press working, which causes work cracks and molding defects. In order to prevent this, it is preferable to set the maximum height roughness (Rz) to 20 μm or less when measuring the processed surface roughness, which will be described later, by the following method. As a result, a stainless steel sheet having work roughening resistance suitable for press working can be obtained. Rz is preferably 15 μm or less.

FIG. 4 is a diagram showing the relationship between the grain size number and Rz. From FIG. 4, in order for Rz to be 20 μm or less, it is necessary to satisfy the crystal grain size number of 5.0 or more. When the crystal grain size is 5.0 or more, it is possible to provide a stainless steel sheet suitable for press working in which the roughening of the work surface is suppressed.

Based on the above results, the grain size number is preferably 5.0 or more, preferably 6.0 or more, in order to provide a steel material for automobile structural parts with good resistance to roughening by machining (Rz: 20 μm or less). is more preferred. However, in order to obtain the above workability, it is necessary to sufficiently develop the {111}<112> crystal orientation strength by crystal grain growth, so the crystal grain size number is preferably 10.0 or less, and 8.0. More preferably: The grain size number is the grain size number of ferrite as described above, and is measured by observing with an optical microscope according to JIS G 0551. Specifically, the grain size number is measured by a comparative method.

In addition, the roughened surface was measured by collecting a JIS No. 5 test piece, applying an elongation strain of 16% in the rolling direction, and measuring the surface profile of the roughened surface generated on the surface of the test piece with a two-dimensional roughness measuring instrument. evaluate. Rz is measured using a two-dimensional roughness meter in accordance with JIS B 0601.

3. Manufacturing Method Next, the manufacturing method will be described. The steel sheet manufacturing method according to the present invention includes, for example, the steps of steelmaking-hot rolling-pickling-cold rolling-annealing. A preferred method for manufacturing a steel sheet according to the present invention will be described below.

3-1. Slab Casting Process In steelmaking, a method of smelting steel containing the above-mentioned essential elements and optionally selected elements in a converter, followed by secondary refining, is suitable. Subsequently, the melted molten steel is made into a slab according to a known casting method (continuous casting). In addition, the casting conditions shall follow normal continuous casting conditions.

In the present invention, in the step of producing a slab by casting, it is preferable to form equiaxed grains that serve as nucleation sites for recrystallized grains having {111}<112> orientation. In the present invention, equiaxed crystals are defined as crystal grains having an aspect ratio of 1/2 or less in a slab, having a polycrystalline structure in which the shape and orientation of crystal grains are isotropic.

The grain boundaries of these equiaxed grains become nucleation sites for recrystallized grains having {111}<112> orientation in the cold-rolled sheet annealing process, which will be described later. Therefore, the equiaxed crystal ratio in the slab is preferably 30% or more, more preferably 50% or more. The equiaxed crystal ratio of the slab can be measured from the macro-etched structure obtained according to JIS G 0553. Specifically, the equiaxed crystal ratio of the slab is a value indicating the ratio of the area occupied by the equiaxed crystals in an arbitrary section of the slab in percentage.

In order to form equiaxed grains in the slab, it is preferable to adjust the contents of Nb and Sn as described above. Since Nb and Sn tend to segregate, when the molten steel solidifies, Sn and Nb distributed from the solid phase to the liquid phase segregate at the tips of the columnar crystals. Such segregation induces compositional supercooling in the liquid phase, producing equiaxed solidification nuclei. Then, equiaxed crystals are formed from the solidification nuclei described above, and the solidified structure has a high equiaxed crystal ratio.

It is also known that the equiaxed grain ratio of continuously cast ferritic stainless steel slabs is affected by casting conditions, and it is generally said that the equiaxed grain ratio increases when the casting temperature is low. Therefore, in order to obtain the equiaxed grain ratio within the preferred range described above, the degree of superheat ΔT (ΔT = molten steel temperature-solidification temperature) is preferably 100°C or less. On the other hand, if ΔT is 0° C. or less, the viscosity of the molten steel increases, which is not preferable because problems such as nozzle clogging tend to occur during operation. From the above, ΔT is preferably in the range of 0° C. or higher and 100° C. or lower. ΔT is more preferably 20° C. or higher and 80° C. or lower.

3-2. Hot Rolling Step Subsequently, the slab is preferably heated and hot rolled to a predetermined plate thickness by continuous rolling to form a hot rolled steel plate (hot rolling step). If the heating temperature of the slab during hot rolling is less than 1100° C., Nb does not completely dissolve into a solid state and precipitates are formed, which may adversely affect subsequent steps. On the other hand, if the heating temperature of the slab exceeds 1250° C., the slab may sag due to high temperature deformation due to its own weight, which is not preferable. Therefore, the heating temperature of the slab during hot rolling is preferably 1100 to 1250°C. Furthermore, considering the productivity and the occurrence of surface flaws, it is more preferable to set the slab heating temperature to 1150 to 1200°C. In the present invention, the slab heating temperature and the hot rolling start temperature are synonymous.

In the hot rolling step, the heated slab is preferably subjected to multiple passes of rough rolling, followed by unidirectional finish rolling in multiple stands. As a result, the slab becomes a hot-rolled sheet and is wound into a coil. The finish rolling temperature is preferably 1150 to 950° C., and the coiling temperature is preferably 600° C. or less in order to avoid the formation of Nb-based precipitates during coiling.

In addition, when a method of precipitating Nb is taken in order to promote recrystallization during annealing after cold rolling described below, the coiling temperature is set to a temperature range of 700 ° C. or higher, which is the peak of the generation of Nb-based precipitates. preferably. However, if the coiling temperature is excessively high, excessive generation of Nb-based precipitates, reduction in hot-rolled sheet toughness due to coarsening, and remelting of Nb are caused. Therefore, it is preferable to set the winding temperature to a temperature range of 900° C. or less. More preferably, the winding temperature is 750° C. or higher and 850° C. or lower.

By setting the coiling temperature to 700° C. or higher as described above, Nb dissolved in the steel can be precipitated in advance and the precipitate can be grown. Then, in the annealing step after cold rolling described below, it is possible to suppress the formation of fine precipitates simultaneously with recrystallization during annealing. Such fine Nb precipitates excessively enhance the pinning effect of recrystallized grains during annealing, making it difficult to complete recrystallization, so the annealing temperature needs to be higher.

On the other hand, by setting the coiling temperature within the above range and precipitating Nb, the recrystallization can be performed at a lower annealing temperature from the start to the completion thereof. As a result, it is possible to develop the {111}<112> crystal orientation of the steel sheet and to obtain a structure having a crystal grain size number of 5.0 or more, and to obtain a steel sheet having both an average r value and resistance to work surface roughening. can. Precipitating Nb in this way is particularly effective in steel types containing 0.3% or more of Nb.

However, when the coiling is performed at a temperature of 700° C. or higher, the hot-rolling strain is recovered. The development of crystal orientation is lower than for steel sheets coiled at temperatures below 600°C. Therefore, it is desirable to appropriately select the winding temperature according to the required properties.

3-3. Hot-Rolled Steel Plate Pickling Step Subsequently, it is preferable to pickle the hot-rolled steel plate to obtain a pickled steel plate without performing hot-rolled steel annealing on the obtained hot-rolled steel plate (hot-rolled steel plate pickling step). This pickled steel sheet is preferably provided as a cold-rolled material in the cold-rolling process, which will be described later. This is different from the general manufacturing method in which a hot-rolled steel sheet is usually subjected to hot-rolled steel annealing to obtain a regular-grain recrystallized structure. In general, hot-rolled sheet annealing facilitates structure control, but the hot-rolling strain disappears, hindering the development of the {111}<112> crystal orientation and improving the _r90 {322}<236>. The crystal orientation is difficult to develop in the subsequent annealing process after cold rolling.

By the way, the {322}<236> crystal orientation develops more strongly when a steel plate with a developed texture called α-fiber ({011}//RD ({100} to {111}<011>)) is annealed. do. This α-fiber develops in ferritic stainless steel sheets by hot rolling and cold rolling. However, when the hot-rolled sheet is annealed, the α-fiber developed during hot rolling disappears before cold rolling, and randomization of crystal orientation proceeds. For this reason, it is preferable not to carry out hot-rolled sheet annealing in the present invention.

In addition, when hot-rolled sheet annealing is not performed, randomization of the hot-rolled structure does not occur, so the structure of the slab remains as it is even in the cold rolling described later, and the recrystallization behavior after cold rolling is greatly affected. It will happen. As described above, the nucleation sites of recrystallized grains with {111}<112> orientation are equiaxed grain boundaries of the slab. Therefore, it is preferable to set the equiaxed crystal ratio of the slab within the above range.

3-4. Cold rolling step Subsequently, the pickled steel sheet is preferably cold-rolled at a rolling reduction of 60% or more using a rolling mill having a roll diameter of 400 mm or more to obtain a cold-rolled steel sheet (cold rolling step). . Here, shear strain during cold rolling can be suppressed by setting the roll diameter to 400 mm or more. In addition, shear deformation introduced by shear strain becomes a nucleation site for randomly oriented grains, so it is necessary to suppress it. As a result, in the subsequent annealing step, the formation of {111}<112> crystal grains, which improves the r-value, is promoted.

In addition, when the rolling reduction increases, the stored energy that serves as the driving force for recrystallization increases. As a result, the {111}<112> crystal orientation facilitates preferential nucleation and selective growth. Therefore, it is preferable that the rolling reduction of cold rolling is 60% or more. Moreover, it is more preferable that the rolling reduction of the cold rolling is 70% or more.

3-5. Annealing process after cold rolling 3-5-1. Annealing Temperature Subsequently, the cold-rolled steel sheet is preferably annealed at an annealing temperature Tf (°C) that satisfies the following formula (vi) (annealing step). For annealing after cold rolling, the annealing temperature and the heating rate during annealing are controlled in consideration of the delay in recrystallization due to the addition of Nb in order to develop recrystallized grains in the {111}<112> orientation. is preferred. As for the annealing temperature, an excessively high temperature invites coarsening of crystal grains and causes rough skin such as orange peel. For this reason, the annealing temperature (Tf (°C)) after cold rolling is preferably within the temperature range calculated by the following formula (vi), as described above.

850 + 300 Nb ≤ Tf (°C) ≤ 950 + 300 Nb (vi)
However, the symbols of the elements in the above formula indicate the contents of the elements in terms of % by mass.

3-5-2. Heating Rate from Temperature Rise Start Temperature to Recrystallization Start Temperature Ts (° C.) In the annealing step, it is preferable to control the average heating rate within the following range. Specifically, the average heating rate is preferably 15° C./s or more in the temperature range from the heating start temperature to the recrystallization start temperature Ts (° C.) calculated by the following formula (vii). Further, the average heating rate is more preferably 20° C./s or more in the temperature range from the heating start temperature to the recrystallization start temperature Ts (° C.) calculated by the following formula (vii). In addition, in the present invention, the average heating rate is controlled within the preferable value range by controlling the time from the heating start temperature to the target temperature.

This is to suppress the formation of precipitates when the temperature is raised from the annealing start temperature to the recrystallization start temperature Ts (°C). The formation of fine precipitates during annealing retards recrystallization and relatively inhibits the formation of {111}<112> oriented recrystallized grains formed in the early stages of recrystallization. As a result, crystal orientations other than the {111}<112> orientation also grow, resulting in randomization of the crystal orientations. Therefore, the average heating rate in the temperature range from the start of annealing to the recrystallization start temperature Ts (°C) is preferably within the above range.

The recrystallization start temperature Ts (° C.) is obtained by the following formula (vii) taking into consideration the delay in recrystallization due to the addition of Nb.
Ts (°C) = 750 + 300 Nb (vii)
However, the symbols for the elements in the above formula (vii) indicate the contents of the elements in terms of % by mass.

3-5-3. Heating rate from recrystallization start temperature Ts (°C) to annealing temperature Tf (°C)

Among the recrystallized grains, crystal grains of {111}<112> orientation are more likely to occur in the initial stage of recrystallization than crystal grains of other orientations. Also, crystal grains of {111}<112> orientation grow during annealing and develop by eroding crystal grains of other orientations. If the temperature rises to a temperature at which grains of other orientations recrystallize before the {111} recrystallization starts, the development of the {111} <112> orientation texture is suppressed and workability is reduced. will decline. Therefore, before the fine precipitates are formed during annealing, the recrystallization starting temperature Ts is reached, and then the recrystallization is slowly progressed in order to grow crystal grains of {111}<112> orientation. is desirable. Therefore, in the steel sheet according to the present invention, the average heating rate from the recrystallization start temperature Ts (° C.) to the annealing temperature Tf (° C.) described later is preferably 10° C./s or less, and 5° C./s More preferably: By controlling the heating rate in this way, the {111}<112> orientation is strongly developed in the steel sheet.

3-6. Production Method and Crystal Grains As described above, since the grain boundaries of the equiaxed grains of the slab serve as nucleation sites for recrystallized grains having the {111}<112> orientation, the By omitting the sheet annealing, the {111}<112> orientation is strongly developed in the steel sheet.

In the cold rolling, introduction of shear deformation is suppressed more than usual by large-diameter roll rolling with a diameter of 400 mm or more. As a result, generation of randomly oriented grains due to shear deformation is suppressed. Furthermore, the combined effect of early {111}<112> oriented grain nucleation results in a lower {111}<112 > oriented recrystallization texture develops strongly.

In this way, the {111}<112> orientation texture develops, and at the same time, the {322}<236> orientation texture also develops. As a result, a ferritic stainless steel sheet having the properties desired in the present invention can be obtained.

3-7. Other manufacturing conditions The thickness of the slab, the thickness of the hot-rolled sheet, etc. may be appropriately designed. In cold rolling, roll roughness, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected. Annealing may be performed by bright annealing in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, if necessary. Moreover, you may carry out annealing in air|atmosphere. Furthermore, after annealing, a tension leveler step for temper rolling or shape correction may be performed, and the sheet may be threaded.

3-8. Method for producing ferritic stainless steel member The steel plate produced by the above method is used as an exhaust part member for automobiles, motorcycles, or the like using the following method. These members are pressed from a steel plate, or formed into a desired shape after forming a steel pipe of a predetermined size (diameter) from the steel plate.

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

A steel having the chemical composition and equiaxed grain ratio shown in Table 1 was melted, cast into a slab, and hot rolled under the conditions of a slab heating temperature of 1150 ° C. and a coiling temperature of 500 ° C. to a thickness of 3.8 mm. A hot-rolled steel sheet was obtained. After that, the pickled hot-rolled steel sheet was cold-rolled to a thickness of 1.0 mm using rolls with a diameter of 400 mm, and subjected to continuous annealing and pickling. The specific annealing conditions are shown in Table 2, and the annealing time was 100 seconds.

The thus obtained cold-rolled steel sheets were subjected to r-value measurement, hole expansion test, and crystal orientation strength measurement.

Regarding the r-value, according to JIS Z 2254, the method described above was performed, and test pieces were taken from parallel, 45°, and 90° directions to the rolling direction, and after obtaining the r-value, the average r-value (r _m ) was calculated. Specifically, a JIS No. 13B tensile test piece was taken, and after applying a strain of 10 to 20% in parallel, 45 ° direction, and 90 ° direction in the tensile test with respect to the rolling direction, the above was performed by a predetermined method. value was calculated.

The evaluation of hole expansibility conforms to JIS Z 2256. Push a 60° conical punch into a punched hole of φ10 mm to gradually expand the hole, stop the punch when a crack appears in the hole, and change the hole diameter. was calculated using the following formula (c).
λ=100×(D−D ₀ )/D ₀ (c)
However, each symbol in the above formula is defined as follows.
D: hole diameter after hole expansion test D ₀ : hole diameter before hole expansion test

For crystal orientation intensity measurement, an X-ray diffraction device (manufactured by Rigaku Denki Kogyo Co., Ltd.) is used to measure the center region of the plate thickness using Mo-Kα rays (the center region is revealed by a combination of mechanical polishing and electrolytic polishing). (200), (110), and (211) positive pole figures were obtained, from which ODF (Orientation Distribution Function) was obtained using spherical harmonic functions. Based on these measurement results, the {111}<112> crystal orientation intensity and the {322}<236> crystal orientation intensity were calculated. In addition, in the present invention, the crystal orientation strength of the ferrite phase, which is the mother phase, is measured.

The results are summarized in Table 2 below.

All of the examples shown in Table 2 were produced under the preferred production conditions of the present invention. Codes B1 to B28 satisfy the composition specified in the present invention, the crystal orientation strength (texture) of the ferrite phase, each r value correlated with deep drawability are good, the hole expandability is satisfied, and the workability Excellent for On the other hand, the symbols b1 to b10 do not satisfy the composition specified in the present invention, the crystal orientation strength (texture) of the ferrite phase, each r value correlated with deep drawability, and hole expandability are low, Poor workability.

Table 3 summarizes the results of manufacturing the steel types listed in Table 1 under the manufacturing conditions shown in Tables 3 and 4. Also in Tables 3 and 4, the steel sheets were cast into slabs and hot-rolled under the conditions of a slab heating temperature of 1150°C and a coiling temperature of 500°C to obtain hot-rolled steel sheets with a thickness of 3.8 mm. After that, the pickled hot-rolled steel sheet was cold-rolled at the rolling reduction shown in the table using rolls having the roll diameter shown in the table, and subjected to continuous annealing and pickling. The annealing time was 100 seconds.

Codes C1 to C5 shown in Table 3 satisfy the composition range defined by the present invention, and the manufacturing conditions are preferable manufacturing conditions in the present invention. Therefore, the crystal orientation strength of the ferrite phase was within the specified range of the present invention, and each r value and hole expansibility, which correlate with deep drawability, were also good. On the other hand, in the case of the symbols c1 to c4, which satisfy the composition specified in the present invention but deviate from the preferable manufacturing conditions, the crystal orientation strength of the steel sheet is outside the specified range of the present invention, and the desired average r value, rmin, and Δr, At least one of these was unnecessary, and the deep drawability was poor, or sufficient hole expandability could not be obtained, and the workability was poor.

Among the invention examples shown in Table 4, D1 to D3 did not satisfy the heating rate during annealing, which is the more preferable production condition of the invention. For this reason, compared with D4 to D9, which satisfied the heating rate conditions during annealing, the crystal orientation strength, average r value, minimum r value, and hole expansibility were inferior. However, in any of D1 to D3, the crystal orientation strength of the ferrite phase, each r value correlated with deep drawability, and hole expandability within the specified range of the present invention are satisfied, and the workability is excellent. result.

In Table 5, the steel grades listed in Table 1 were cast into slabs and hot-rolled at a slab heating temperature of 1150° C. to obtain hot-rolled steel sheets with a thickness of 3.8 mm. Thereafter, the pickled hot-rolled steel sheet was cold-rolled at a rolling reduction of 70% using rolls with a diameter of 400 mm, and subjected to continuous annealing and pickling. The coiling temperature after hot rolling was 800°C for E1 to E3, and 500°C for E4. The results are summarized in Table 5 below.

The grain size number was measured by observing with an optical microscope in accordance with JIS G 0551. Specifically, the grain size number is the grain size number of ferrite and was measured by a comparative method.

For the roughened surface, a JIS No. 5 test piece was sampled, 16% elongation strain was applied in the rolling direction, and the surface profile of the roughened surface generated on the surface of the test piece was measured with a two-dimensional roughness measuring instrument. Roughness (Rz) was evaluated. The maximum height roughness Rz was measured using a two-dimensional roughness meter according to JIS B0601. In addition, when Rz was 20 μm or less, it was judged that the work roughening resistance was good.

In the present invention examples E1 to E4 shown in Table 5, the content of ingredients and the manufacturing method were all within the scope of the present invention, and good results were obtained regarding the quality of the steel sheets. Furthermore, the present invention examples E1 to E3 satisfied the preferred grain size number in the present invention, and thus had good work roughening resistance. On the other hand, E4 had a high Nb content, but the coiling temperature was 700° C. or less, so it did not satisfy the preferred crystal grain size number in the present invention, and the work roughening resistance was poor.

Claims (11)

The chemical composition, in mass %,
C: 0.001 to 0.020%,
Si: 0.010 to 1.50%,
Mn: 0.02-1.00%,
P: 0.01 to 0.050%,
S: 0.0001 to 0.010%,
Cr: 10.0 to 20.0%,
N: 0.001 to 0.030%,
Nb: 0.1 to 0.8%,
Ti: 0.01 to 0.30%,
Sn: 0.003 to 0.500%,
Mg: 0-0.0100%,
B: 0 to 0.0050%,
V: 0 to 1.0%,
Mo: 0-3.0%,
W: 0 to 3.0%,
Al: 0-0.3%,
Cu: 0-2.0%,
Zr: 0 to 0.30%,
Co: 0-0.50%,
Sb: 0 to 0.50%,
REM: 0-0.05%,
Ni: 0 to 2.0%,
Ca: 0 to 0.0030%,
Ta: 0 to 0.10%,
Ga: 0-0.1%,
balance: consisting of Fe and unavoidable impurities,
satisfying the following (i) formula and (ii) formula,
In the crystal orientation intensity of the ferrite phase at the center of the sheet thickness by X-ray diffraction, the {111} <112> crystal orientation intensity is 10.0 or more, and the {322} <236> crystal orientation intensity is 10.0 or more,
A ferritic stainless steel sheet having an average Lankford value (r _m ) calculated by the following formula (iii) of 1.10 or more and Δr calculated by the following formula (iv) of 0.2 or less.
Nb+Ti≧3(C+N) (i)
Nb+Sn≧0.2 (ii)
However, each element symbol in the above formulas (i) and (ii) indicates the content of each element contained in the steel sheet in mass %.
_rm =( _r0 + _2r45 + _r90 )/4 (iii)
Δr=(r ₀ +r ₉₀ )/2−r ₄₅ (iv)
However, each symbol in the above formula is defined as follows.
r ₀ : r value in the rolling direction r ₄₅ : r value in the direction of 45° to the rolling direction r ₉₀ : r value in the direction of 90° to the rolling direction 2. The ferritic stainless steel sheet according to claim 1, which has a minimum Lankford value (r _min ) of 0.80 or more and satisfies the following formula (v).
n(1+r _min )≧0.40 (v)
However, each symbol in the above formula (v) is defined as follows.
n: work hardening index The chemical composition, in mass %,
Mg: 0.0002-0.0100%,
B: 0.0002 to 0.0050%,
V: 0.05 to 1.0%,
Mo: 0.2-3.0%, and W: 0.1-3.0%,
containing one or more selected from
The ferritic stainless steel sheet according to claim 1 or 2. The chemical composition, in mass %,
Al: 0.003-0.3%,
Cu: 0.1 to 2.0%,
Zr: 0.05 to 0.30%,
Co: 0.05-0.50%,
Sb: 0.01-0.50%, and REM: 0.001-0.05%,
containing one or more selected from
A ferritic stainless steel sheet according to any one of claims 1 to 3. The chemical composition, in mass %,
Ni: 0.1 to 2.0%,
Ca: 0.0001 to 0.0030%,
Ta: 0.01-0.10%, and Ga: 0.0002-0.1%,
containing one or more selected from
A ferritic stainless steel sheet according to any one of claims 1 to 4. The ferritic stainless steel sheet according to any one of claims 1 to 5, which has a grain size number of 5.0 or more. The ferritic stainless steel sheet according to any one of claims 1 to 6, which is used for exhaust system parts. A ferritic stainless steel member for exhaust system parts of automobiles or motorcycles, comprising the ferritic stainless steel sheet according to any one of claims 1 to 6. A method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 6,
(a) a hot rolling step of heating a slab having the chemical composition according to any one of claims 1 to 6 and hot rolling the slab to obtain a hot rolled steel sheet;
(b) a hot-rolled steel plate pickling step of pickling the hot-rolled steel plate to obtain a pickled steel plate without annealing the hot-rolled steel plate;
(c) cold-rolling the pickled steel sheet using a rolling mill having a roll diameter of 400 mm or more at a rolling reduction of 60% or more to obtain a cold-rolled steel sheet;
(d) an annealing step of annealing the cold-rolled steel sheet at an annealing temperature Tf (°C) that satisfies the following formula (vi);
A method for producing a ferritic stainless steel sheet, in which
850 + 300 Nb ≤ Tf (°C) ≤ 950 + 300 Nb (vi)
However, the symbols of the elements in the above formula indicate the contents of the elements in terms of % by mass. 10. The method for producing a ferritic stainless steel sheet according to claim 9, wherein the equiaxed grain ratio of the slab in the step (a) is 30% or more. The average heating rate until reaching the annealing temperature Tf (° C.) in the step (d) is
15° C./s or more in the temperature range from the temperature rise start temperature to the recrystallization start temperature Ts (° C.) calculated by the following formula (vii),
10 ° C./s or less in the temperature range from the recrystallization start temperature Ts (° C.) to the annealing temperature Tf (° C.),
A method for producing a ferritic stainless steel sheet according to claim 9 or 10.
Ts (°C) = 750 + 300 Nb (vii)
However, the symbols of the elements in the above formula indicate the contents of the elements in terms of % by mass.

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