WO2013133259A1

WO2013133259A1 - Ferrite-austenite 2-phase stainless steel plate having low in-plane anisotropy and method for producing same

Info

Publication number: WO2013133259A1
Application number: PCT/JP2013/055945
Authority: WO
Inventors: 濱田　純一; 石丸　詠一朗
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2012-03-09
Filing date: 2013-03-05
Publication date: 2013-09-12
Also published as: TW201348465A; JP5869922B2; JP2013185231A; CN104471092A; KR20140105849A; CN104471092B; TWI519653B

Abstract

This ferrite austenite 2-phase stainless steel plate comprises, by mass%, C: 0.001-0.10%, Si: 0.01-1.0%, Mn: 2-10%, P < 0.05%, Ni: 0.1-3.0%, Cr: 15.0-30.0%, and N: 0.05-0.30%, with the remainder being Fe and inevitable impurities. The austenite phase percentage is 40-90% by surface area percentage, the maximum strength of the crystal orientation of the ferrite phase is 10 or less, and the hardness ratio of the austenite phase to the ferrite phase is 1.1 or greater.

Description

Ferrite-austenitic duplex stainless steel sheet with small in-plane anisotropy and method for producing the same

The present invention relates to a duplex stainless steel sheet composed of a ferrite phase and an austenite phase with small anisotropy during processing, and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2012-52876 filed in Japan on March 9, 2012, the contents of which are incorporated herein by reference.

A duplex stainless steel sheet composed of a ferrite phase and an austenite phase is excellent in corrosion resistance and has a fine structure, so it has high strength and excellent fatigue resistance, and is widely used in chemical plants and the like. However, since the duplex stainless steel sheet has lower ductility than austenitic stainless steel, cracks may occur during press forming, and improvement in workability is desired.

The conventional representative duplex stainless steel has a high Ni and Mo content represented by SUS329J4L (25% Cr-7% Ni-3% Mo-0.1% N). Recently, however, an alloy-saving ferritic / austenitic duplex stainless steel with reduced Ni content and no Mo has been developed and applied to various fields (see, for example, Patent Document 1). In such a reduced Ni and Mo-containing steel, the amount of austenite is adjusted and the corrosion resistance is secured by adding Mn and N. This reduced Ni and Mo-containing steel is SUS304 (18% Cr-8% Ni) and SUS316 (18% Cr-10% Ni-2% Mo) are also expected to be substituted.

On the other hand, when a thin steel sheet is formed into various shapes and applied to various parts, press formability becomes a problem. Among the press formability, there is an index called in-plane anisotropy. When the in-plane anisotropy is large, there is a problem that the shape of the flange remaining portion of the molded product is not constant, or a portion called an ear at the end of the molded product is wavy (so-called earring becomes large). If this problem occurs, the yield at the time of molding is remarkably deteriorated, and the non-uniformity of the shape of the molded product is likely to occur. Therefore, it is desired that the in-plane anisotropy is small.

As described in Non-Patent Document 1, the ferrite-austenitic duplex stainless steel sheet has a very large in-plane anisotropy and has a problem in the formability of the thin steel sheet. Here, the in-plane anisotropy is an r-value in-plane anisotropy, and Δr expressed by the following equation (1) is an index.
Δr = | (r ₀ + r ₉₀ ) / 2−r ₄₅ | Formula (1)
Here, r ₀ in the formula (1) is an r value in a direction parallel to the rolling direction, r ₉₀ is an r value in a direction perpendicular to the rolling direction, and r ₄₅ is an r value in a 45 ° direction with respect to the rolling direction. Value. These r values are Rankford values (plastic strain ratio), and are measured by a method according to JIS Z2254. When Δr is large, it means that the in-plane anisotropy is large. Therefore, a smaller Δr value is desired from the above viewpoint.

Patent Document 1 discloses a method in which a molten steel of ferrite / austenite duplex stainless steel is directly cast into a thin plate to produce a steel plate having no difference in mechanical properties between the rolling direction and the width direction and having no anisotropy. This is a method of manufacturing a thin plate directly from molten steel by omitting hot rolling, and is different from a general manufacturing method manufactured through hot rolling as in the present invention. Patent Document 1 is a technique for reducing the difference in strength and elongation between the rolling direction and the width direction, and is not a technique related to the in-plane anisotropy of the r value as in the present invention.

JP-A-1-53705

An object of the present invention is to provide a ferrite-austenitic duplex stainless steel sheet having a small r-value in-plane anisotropy and excellent press formability, and a method for producing the same, with particular attention to the crystal orientation strength of the ferrite phase. To do.

In order to solve the above-mentioned problems, the present inventors investigated in detail the r value of the duplex stainless steel sheet and the expression of its in-plane anisotropy. And as a result of repeating various examinations in order to achieve this purpose, the following knowledge was obtained.

The r value and the in-plane anisotropy of the duplex stainless steel in which the ferrite phase and the austenite phase are mixed depend on the crystal orientation strength (texture structure) of the ferrite phase. Here, the crystal orientation intensity is the diffraction intensity measured by the X-ray diffraction method, and specifically, is the ratio of the diffraction intensity to the diffraction intensity of the random sample. For this reason, the crystal orientation strength is the ratio of the diffraction intensity to the diffraction intensity when the crystal orientation is random (non-oriented crystal) and indicates the degree of orientation. The crystal orientation strength is also called an X-ray random intensity ratio of a specific crystal orientation. In the conventional duplex stainless steel, ferrite crystals develop significantly in a crystal orientation (rolling orientation) parallel to the rolling direction. For this reason, the maximum strength of the crystal orientation of the product plate (the maximum value of the crystal orientation strength) is increased. In this case, the r value in the specific direction (near 45 ° with respect to the rolling direction) is high, and the r value in the rolling direction and the width direction is low. On the other hand, by adjusting the components and the production method, the maximum strength of the crystal orientation of the cold-rolled sheet and product after cold rolling was weakened, and the in-plane anisotropy was reduced. Specifically, the amount of Ni is reduced, the amount of N or Mn is increased, the austenite phase as the second phase is hardened, and the fraction of the austenite phase is optimized. As a result, it has been found that the maximum strength of the crystal orientation of the ferrite phase is reduced during the cold rolling process. At that time, it has been found that it is effective to adjust the cold rolling reduction ratio and the annealing temperature, that is, it has been newly found that it is possible to weaken the maximum strength of the crystal orientation of the ferrite phase in the cold rolling process. In addition, it was possible to maintain the maximum strength of the crystal orientation at a small value in the subsequent annealing. As described above, it is possible to provide a product having a small in-plane anisotropy of r value as a material characteristic.

The present invention has been completed based on the above findings, and the gist of the invention is shown below.

(1) In mass%,
C: 0.001 to 0.10%,
Si: 0.01 to 1.0%,
Mn: 2 to 10%,
P ≦ 0.05%,
Ni: 0.1 to 3.0%,
Cr: 15.0 to 30.0%, and N: 0.05 to 0.30%,
The balance consists of Fe and inevitable impurities,
The in-plane anisotropy is characterized in that the austenite phase ratio is 40 to 90% in area ratio, the maximum strength of the crystal orientation of the ferrite phase is 10 or less, and the hardness ratio of the austenite phase to the ferrite phase is 1.1 or more. Small ferrite and austenitic duplex stainless steel sheet.

(2) Furthermore, in mass%,
Mo: 0.1 to 1.0%,
Cu: 0.1 to 3.0%,
B: 0.0005 to 0.0100%,
Al: 0.01 to 0.5%,
Ti: 0.005 to 0.30%,
Nb: 0.005 to 0.30%,
Zr: 0.005 to 0.30%,
Sn: 0.05 to 0.50%,
W: 0.1-2.0%,
The in-plane difference according to (1) or (2) above, which contains at least one selected from Mg: 0.0002 to 0.0100% and Ca: 0.0005 to 0.0100% Ferritic austenitic duplex stainless steel sheet with low isotropic properties.

(3) In-plane anisotropy according to (1) or (2) above, wherein Δr represented by the following formula (1), which is an index of in-plane anisotropy, is 0.5 or less: Ferritic / austenitic duplex stainless steel sheet with small size.
Δr = | (r ₀ + r ₉₀ ) / 2−r ₄₅ | Formula (1)
Here, r ₀ is an r value parallel to the rolling direction, r ₉₀ is an r value perpendicular to the rolling direction, and r ₄₅ is an r value 45 ° to the rolling direction.

(4) a step of cold-rolling the ferrite-austenitic duplex stainless steel having the component composition described in (1) or (2) above, and a subsequent annealing step;
In the cold rolling step, the rolling reduction is 90% or less,
In the annealing step, the annealing temperature is set to 1000 to 1100 ° C., the cooling rate to 500 ° C. is set to 5 ° C./sec or more, and the in-plane is maintained for 5 seconds or more in the temperature range of 400 to 500 ° C. in the cooling process. A method for producing a ferritic / austenitic duplex stainless steel sheet with low anisotropy.

Conventionally, ferrite-austenite duplex stainless steel sheet has a large in-plane anisotropy and has a problem in press formability. On the other hand, according to one aspect of the present invention, a thin steel sheet of a ferrite-austenitic duplex stainless steel sheet with small in-plane anisotropy can be obtained. By applying the ferrite-austenitic duplex stainless steel sheet according to one embodiment of the present invention as a forming application in various fields such as home appliances, architecture, and automobiles, a great effect can be obtained for environmental measures and cost reduction of parts.

It is a figure which shows the texture and in-plane anisotropy ((DELTA) r) of this invention steel and a comparison steel. It is a figure which shows the relationship between the maximum intensity | strength of the crystal orientation of a ferrite phase, and in-plane anisotropy ((DELTA) r). It is a figure which shows the relationship between an austenite phase rate (gamma phase rate) and in-plane anisotropy ((DELTA) r). It is a figure which shows the relationship between cold rolling reduction and in-plane anisotropy ((DELTA) r).

Hereinafter, this embodiment will be described in detail.
First, the reasons for limiting the chemical components of the ferrite-austenitic duplex stainless steel sheet of this embodiment will be described. Here, the unit “%” of the content of the component means mass%.

If the C content exceeds 0.10%, the moldability and corrosion resistance are remarkably deteriorated, so the upper limit of the C content is 0.10%. C is an element necessary for stably generating an austenite phase, increasing the hardness difference between the austenite phase and the ferrite phase, and suppressing an increase in crystal orientation strength. If the amount of C is less than 0.001%, it is difficult to obtain a two-phase structure. For this reason, it is desirable that the lower limit of the C amount be 0.001%. Further, considering the refining cost and weldability, the C content is preferably 0.02 to 0.05%.

Si is an element useful as a deoxidizer, but if the Si content exceeds 1.0%, the hot workability deteriorates and it is difficult to manufacture. For this reason, the amount of Si shall be 1.0% or less. However, since 0.01% or more of Si is necessary for deoxidation, the lower limit of the Si amount is set to 0.01%. Further, considering the refining cost, oxidation resistance, and corrosion resistance, the Si content is preferably 0.3% to 0.8%.

Mn is an element added as a deoxidizer. Further, 2% or more of Mn is added in order to stably generate the austenite phase and increase the hardness difference between the austenite phase and the ferrite phase to suppress the increase in the maximum strength of the crystal orientation. If the Mn content exceeds 10%, the corrosion resistance is remarkably deteriorated, so the upper limit of the Mn content is 10%. Further, considering the oxidation resistance and pickling properties during production, the Mn content is preferably 3.0 to 6.0%. *

P is contained as an impurity, and deteriorates hot workability during production, so the upper limit of P content is 0.05%. However, excessively reducing the amount of P leads to an increase in refining costs, so the amount of P is preferably 0.02 to 0.04%.

Ni is an element that stably generates an austenite phase, and the lower limit of the Ni content is 0.1%. However, since the alloy cost is high, the upper limit of Ni content is set to 3.0% or less. However, excessively reducing the amount of Ni may lead to deterioration of corrosion resistance, so the amount of Ni is preferably 0.5 to 3.0%.

Add 15.0% or more of Cr to ensure corrosion resistance and oxidation resistance. On the other hand, since adding a large amount of Cr leads to an increase in alloy cost, the upper limit of the Cr amount is set to 30.0%. Further, considering the manufacturability, the Cr content is desirably 17.0 to 25.0%.

N improves the corrosion resistance of duplex stainless steel. Further, N stably generates the austenite phase, increases the hardness difference between the austenite phase and the ferrite phase, and suppresses the increase in the maximum strength of the crystal orientation. For this reason, 0.05% or more of N is added. On the other hand, if the amount of N exceeds 0.30%, it becomes extremely hard and castability and hot workability deteriorate. For this reason, the upper limit of the N amount is set to 0.30%. Further, considering the suppression of the weldability and the development of the ferrite phase texture to a specific crystal orientation, the N content is preferably 0.10 to 0.30%. The N amount is more preferably more than 0.15 to 0.30%.

Mo is an element that contributes to the improvement of corrosion resistance and high-temperature strength, and if necessary, 0.1% or more of Mo may be added. If the amount of Mo is less than 0.1%, the effect of improving the corrosion resistance and the high temperature strength cannot be obtained sufficiently. However, since Mo is an element that generates ferrite, an austenite phase is not sufficiently generated when the amount of Mo exceeds 1.0%. Therefore, the Mo amount is set to 0.1 to 1.0%. Considering the alloy cost and manufacturability, the Mo amount is preferably 0.1 to 0.5%.

In order to control the corrosion resistance and the phase ratio of the austenite phase, 0.1 to 3.0% of Cu may be added as necessary. If the amount of Cu is less than 0.1%, the effect of improving the corrosion resistance cannot be obtained sufficiently. If the amount of Cu exceeds 3.0%, the effect of improving the corrosion resistance is saturated and the effect of controlling the phase ratio of the austenite phase is also saturated. In consideration of hot workability, the amount of Cu is preferably 0.1 to 2.0%.

B is an element that segregates at the grain boundaries and improves hot workability, and 0.0005% or more of B may be added as necessary. If the amount of B is less than 0.0005%, the effect of improving hot workability cannot be sufficiently obtained. However, since B is an element that generates ferrite, if the amount of B exceeds 0.0100%, an austenite phase is not sufficiently generated. Therefore, the B content is set to 0.0005 to 0.0100%. Further, considering the intergranular corrosion, the B content is preferably 0.0005 to 0.0030%.

Al can be used as a deoxidizer. Moreover, Al improves oxidation resistance and corrosion resistance. For this reason, 0.01 to 0.5% Al may be added as necessary. If the amount of Al is less than 0.01%, the effect of improving oxidation resistance and corrosion resistance cannot be obtained sufficiently. If the Al content exceeds 0.5%, the effect of improving oxidation resistance and corrosion resistance is saturated. In consideration of toughness, the Al content is preferably 0.01 to 0.10%.

Ti is an element that forms N and TiN and is effective in refining the structure of the weld and cast structure. Ti is an element that improves the corrosion resistance. Therefore, 0.005 to 0.30% Ti may be added as necessary. If the amount of Ti is less than 0.005%, the effect of refining the welded portion and the structure of the cast structure is not sufficiently exhibited. If the amount of Ti exceeds 0.30%, the effect is saturated and it causes surface flaws in the manufacturing process of the steel sheet. Considering the alloy cost and toughness, the Ti amount is preferably 0.005 to 0.15%.

Nb is an element having an effect similar to that of Ti and improving the strength, and 0.005 to 0.30% Nb may be added as necessary. If the amount of Nb is less than 0.005%, the effect of refining the welded portion and the structure of the cast structure is not sufficiently exhibited. If the amount of Nb exceeds 0.30%, the effect is saturated. Considering the alloy cost and toughness, the Nb content is preferably 0.005 to 0.15%.

Zr is also an element that has a similar effect to Ti and Nb and improves the oxidation resistance, and 0.005 to 0.30% Zr may be added if necessary. If the amount of Zr is less than 0.005%, the effect of refining the welded portion and the structure of the cast structure is not sufficiently exhibited, and the effect of improving the oxidation resistance is not sufficiently exhibited. If the amount of Zr exceeds 0.30%, the effect is saturated. Considering the alloy cost and toughness, the Zr content is preferably 0.005 to 0.15%. In addition, when the amount of Zr exceeds 0.15%, the toughness tends to decrease.

Sn is an element that improves corrosion resistance, and 0.05 to 0.50% Sn may be added as necessary. If the amount of Sn is less than 0.05%, the effect of improving the corrosion resistance is not sufficiently exhibited. If the amount of Sn exceeds 0.50%, the effect is saturated. In consideration of hot workability and weldability, the Sn content is preferably 0.05 to 0.20%.

W is an element that improves corrosion resistance and heat resistance, and 0.1 to 2.0% of W may be added as necessary. When the amount of W is less than 0.1%, the effect of improving the corrosion resistance and heat resistance is not sufficiently exhibited. If the amount of W exceeds 2.0%, the effect is saturated. In consideration of alloy costs and toughness, the W content is preferably 0.1 to 1.0%.

Mg is an element used as a deoxidizer. Mg is an element effective for refinement of the welded part and the cast structure. Therefore, 0.0002 to 0.0100% Mg may be added as necessary. If the amount of Mg is less than 0.0002%, the effect of refining the welded portion and the structure of the cast structure is not sufficiently exhibited. If the amount of Mg exceeds 0.0100%, the effect is saturated. Considering manufacturability, the amount of Mg is preferably 0.0002 to 0.0020%.

Since Ca combines with S to improve hot workability, 0.0005 to 0.0100% Ca may be added as necessary. When the Ca content is less than 0.0005%, the effect of improving hot workability is not sufficiently exhibited. If the Ca content exceeds 0.0100%, the effect is saturated. In consideration of corrosion resistance, the Ca content is preferably 0.0005 to 0.0010%.

Next, the crystal orientation strength of the ferrite phase that is the point of this embodiment will be described.

By rolling and heat treatment, the crystals of the ferrite phase and austenite phase develop in a specific crystal orientation. Crystals having developed a specific crystal orientation affect the properties of the steel sheet. The degree of development to a specific crystal orientation (degree of orientation) is proportional to the crystal orientation strength measured by X-ray diffraction method, neutron diffraction method or the like. Here, the crystal orientation intensity is a ratio of the diffraction intensity to the diffraction intensity of a random sample, and is also referred to as an X-ray random intensity ratio of a specific crystal orientation. There are various methods for measuring the crystal orientation strength. In this embodiment, the crystal orientation strength obtained by the X-ray diffraction method is defined. FIG. 1 shows the texture of the ferrite phase of duplex stainless steel sheets (invention steel and comparative steel) having different in-plane anisotropies. These duplex stainless steel sheets were 1.0 mm thick cold-rolled / annealed plates, manufactured under conditions of a cold-rolling reduction ratio of 78% and an annealing temperature of 1050 ° C. Texture was measured by the following method. First, the steel plate was mechanically polished and electrolytically polished to reveal the central region of the plate thickness. Using an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.), positive electrode dot diagrams of (200), (310), and (211) in the central region of the plate thickness were measured using Mo-Kα rays. A three-dimensional crystal orientation density function was obtained from these positive dot diagrams using the spherical harmonic function method.

FIG. 1 is a three-dimensional texture represented by the Bunge method, and is a cross section (φ2 = 45 ° cross section) in which the crystal orientation strength can be seen with contour lines. Here, the crystal orientation intensity is the ratio of the diffraction intensity to the diffraction intensity of the random sample. In FIG. 1, the crystal orientation (rolling orientation) of the ferrite phase parallel to the rolling direction is {100} <011>, {211} <011>. In the texture of the comparative steel of FIG. 1, crystals are remarkably developed in the {100} <011> and {211} <011> orientations, which are the rolling orientations of the ferrite phase, and the maximum strength of the crystal orientation (crystal orientation The maximum intensity) is as high as 18. Further, Δr indicating the in-plane anisotropy of the r value is as high as 1.34, and the press formability is inferior.

On the other hand, in the texture of the steel of the present invention shown in FIG. 1, the growth of crystals in the rolling orientation is suppressed, and the maximum strength of crystal orientation (maximum value of crystal orientation strength) is 8, which is lower than that of the comparative steel. . Further, Δr is 0.38, which shows that anisotropy is small. From the above results, the in-plane anisotropy of the r value depends on the texture of the ferrite phase that is the parent phase, and the anisotropy can be effectively reduced by suppressing the development of a specific texture. I understand. FIG. 2 shows the relationship between the maximum strength of the crystal orientation of the ferrite phase and Δr. When the maximum intensity is 10 or less, Δr is 0.5 or less. For this reason, in this embodiment, the maximum strength of the crystal orientation of the ferrite phase is defined as 10 or less. The lower limit value of the maximum strength of the crystal orientation of the ferrite phase is 1 in the random state. Δr is preferably low, but if Δr is 0.5 or less, there is no problem with the shape during pressing. For this reason, in this embodiment, Δr is defined as 0.5 or less. Δr is more preferably 0.4 or less.
The maximum intensity of crystal orientation is the maximum value among the crystal orientation strengths of all crystal orientations. By measuring the positive dot diagrams of (200), (310), and (211) and obtaining a three-dimensional crystal orientation density function from these three positive electrode dot diagrams, information on the crystal orientation strength of all crystal orientations can be obtained. .

The in-plane anisotropy is the r value in-plane anisotropy, and Δr represented by the following formula (1) is used as an index.
Δr = | (r ₀ + r ₉₀ ) / 2−r ₄₅ | Formula (1)
Here, r ₀ in the formula (1) is an r value in a direction parallel to the rolling direction, r ₉₀ is an r value in a direction perpendicular to the rolling direction, and r ₄₅ is an r value in a 45 ° direction with respect to the rolling direction. Value. These r values are Rankford values (plastic strain ratio), and are measured by a method according to JIS Z2254. When Δr is large, it means that the in-plane anisotropy is large. Therefore, a smaller Δr value is desired from the above viewpoint.

In this embodiment, the austenite phase ratio (area ratio) of the ferrite-austenite duplex stainless steel is also an element that reduces the in-plane anisotropy. The austenite phase precipitates as a second phase in the hot rolling step, and the amount of precipitation changes depending on the temperature. In this embodiment, the crystal orientation strength (texture) of the ferrite phase is controlled by cold rolling, and the crystal orientation characteristics (texture) are maintained even after cold rolling and annealing. I found a new technical idea to develop anisotropy. When there is no austenite phase or when the hardness difference between the austenite phase and the ferrite phase is small, a specific crystal orientation of the ferrite phase develops rapidly due to rolling deformation (development of the rolling texture). In this case, the maximum strength of the crystal orientation becomes stronger by the subsequent heat treatment (development of recrystallized texture).

On the other hand, in the case of the steel composition of the present embodiment, the ferrite phase of the parent phase is softer than the austenite phase of the second phase. For this reason, when receiving deformation in a state of being constrained by a roll in the cold rolling process, the ferrite phase is subjected to extremely nonuniform deformation from the hard austenite phase. The present inventors measured the hardness of the austenite phase and the ferrite phase in detail by the nanoindentation method. As a result, it was found that the anisotropy becomes small when the hardness of the austenite phase is 1.1 times or more the hardness of the ferrite phase. Since a large amount of non-uniform strain is introduced from the hard austenite phase to the ferrite phase of the parent phase during the deformation process, crystal orientation rotation occurs locally and non-uniformly. For this reason, it is considered that the development of a specific crystal orientation is suppressed, thereby reducing the anisotropy. In order to stabilize the small in-plane anisotropy, the hardness ratio of the austenite phase to the ferrite phase is desirably 1.2 or more. When the hardness ratio exceeds 2.0, the austenite phase is markedly hardened, and cracks occur at the interface between the ferrite phase and the austenite phase during molding. For this reason, the upper limit of the hardness ratio is desirably 2.0.

In addition, the present inventors also investigated the austenite phase ratio (austenite phase fraction (area fraction)). Cold-rolled sheets having the same composition as the steel of the present invention shown in FIG. 1 were prepared, and the annealing temperature was adjusted at 950 ° C. to 1150 ° C. to prepare samples having various austenite phase ratios. In order to change the austenite phase ratio, the annealing temperature of the cold-rolled sheet was changed from 950 ° C. to 1150 ° C. The austenite phase rate and Δr of the obtained sample were measured. Here, although the austenite phase ratio was measured with a ferrite meter, it may be obtained by an image analysis device, an EBSP analysis device, or the like. The sample prepared at an annealing temperature of 1100 ° C. had an austenite phase ratio of 40%. The sample prepared at an annealing temperature of 1000 ° C. had an austenite phase ratio of 90%.

FIG. 3 shows the relationship between the austenite phase ratio and the in-plane anisotropy (Δr). As shown in FIG. 3, when the austenite phase ratio is 40% or more and 90% or less, Δr is 0.5 or less. For this reason, the minimum of an austenite phase rate shall be 40%, and an upper limit shall be 90%. Thus, it has been found that the effect of reducing the in-plane anisotropy and stabilizing the small in-plane anisotropy is also affected by the austenite phase ratio (area fraction). When the austenite phase ratio increases excessively, it undergoes excessive nonuniform deformation from the austenite phase during the cold rolling process, and the texture of the ferrite phase after cold rolling annealing develops. For this reason, it is thought that anisotropy becomes large. Therefore, the austenite phase ratio is 40 to 90%. Further, when the in-plane anisotropy is stably reduced and the strength and ductility are taken into consideration, the austenite phase ratio is preferably 50 to 80%, and more preferably 60 to 80%.

Next, a manufacturing method will be described.
The method for producing a steel sheet according to the present embodiment includes steps of steelmaking, hot rolling, pickling, cold rolling, annealing and pickling. In steelmaking, a method in which steel containing the essential components and components added as necessary is melted in a converter or an electric furnace, followed by secondary refining is suitably applied. The molten steel is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. In hot rolling, a slab is rolled by a hot rolling mill composed of a plurality of stands and then wound. In the present embodiment, the casting and hot rolling conditions are not particularly defined, and may be appropriately selected according to the components.

After hot rolling, it may be subjected to hot-rolled sheet annealing or may be omitted, and pickled, and then cold-rolled. In cold rolling, the rolling reduction of cold rolling is 90% or less. FIG. 4 shows the relationship between the rolling reduction and Δr. When the rolling reduction exceeds 90%, Δr exceeds 0.5 and the in-plane anisotropy increases. When the strain in the cold rolling becomes excessively large, the maximum strength of the crystal orientation of the ferrite phase rapidly increases (crystals develop significantly in the rolling orientation). This is thought to increase the in-plane anisotropy. Further, considering the ductility and productivity, the rolling reduction of cold rolling is desirably 30 to 80%. Other conditions in the cold rolling (roll diameter, number of passes, rolling temperature, etc.) are not particularly defined, and may be appropriately selected according to productivity.

Annealing after cold rolling is performed to adjust the austenite phase ratio. In order to set the austenite phase ratio to 40% or more, the heating temperature for annealing is set to 1100 ° C. or less. In order to set the austenite phase ratio to 90% or less, the heating temperature for annealing is set to 1000 ° C. or higher. However, annealing at an excessively high temperature conversely decreases the austenite phase ratio and coarsens the crystal grains. For this reason, the maximum strength of the crystal orientation of the ferrite phase is increased. Therefore, the heating temperature (annealing temperature) for annealing is set to 1000 to 1100 ° C. Further, from the viewpoint of ductility and toughness, the annealing temperature is desirably 1020 to 1075 ° C. On the other hand, if the cooling rate after heating is too slow, Cr carbonitride precipitates during the cooling process, and toughness and corrosion resistance deteriorate. For this reason, the cooling rate to 500 degreeC shall be 5 degrees C / sec or more. When the cooling rate exceeds 500 ° C./sec, the shape of the steel sheet is remarkably deteriorated, so the upper limit of the cooling rate is set to 500 ° C./sec. In consideration of productivity and pickling properties, the cooling rate is preferably 10 to 50 ° C./sec, and the cooling method may be appropriately selected such as air-water cooling or water cooling.

In order to increase the hardness of the austenite phase to 1.1 times the hardness of the ferrite phase, it is necessary to harden the austenite phase by concentrating N in the austenite. In this embodiment, the temperature is maintained for 5 seconds or longer in the temperature range of 400 to 500 ° C. during the cooling process. Thereby, N is concentrated in the austenite phase. However, when the holding time exceeds 500 seconds, the productivity is remarkably deteriorated, so the upper limit of the holding time is set to 500 seconds. Furthermore, in consideration of productivity, the holding time is desirably 60 sec or less.

The manufacturing method in other processes is not particularly defined, but the thickness of the hot rolled sheet, the annealing atmosphere of the cold rolled sheet, etc. may be selected as appropriate. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Furthermore, the thickness of the product may be selected according to the required thickness of the member (processed member).

Steel with the component composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to form a hot-rolled coil having a thickness of 3.5 mm. Thereafter, the hot-rolled coil was annealed and pickled, and cold-rolled at a reduction rate of 78% to obtain a cold-rolled sheet. The cold rolled sheet was then annealed. In the annealing step, the cold-rolled sheet was heated to 1050 ° C., and then cooled at a cooling rate of 10 ° C./sec to 500 ° C. After annealing, pickling was performed to obtain a product plate. The product plate thus obtained was measured for Δr, maximum crystal orientation strength, and austenite phase ratio by the methods described above.

Steel No. 1 to 10 ferritic-austenitic duplex stainless steel sheets have steel components in the range specified in the present embodiment, and the austenite phase rate and the maximum strength of the crystal orientation of the ferrite phase are in the range specified in the present embodiment. Satisfied. Further, Δr which is an anisotropy index is 0.5 or less, and the in-plane anisotropy is small.
On the other hand, Steel No. 11 is steel corresponding to SUS329J4L, and the amounts of Ni and Mo are out of the range defined in the present embodiment. Further, the austenite phase ratio is low, and the maximum strength of the crystal orientation of the ferrite phase is remarkably high. For this reason, Δr is more than 0.5 and the anisotropy is large.

Steel No. No. 12 C, steel no. 14 Mn, and steel No. The N amount of 17 is less than the lower limit of the range defined in the present embodiment. Since C, Mn, and N are austenite-forming elements, The austenite phase ratios of 12, 14, and 17 and the maximum strength of the crystal orientation of the ferrite phase were out of the range defined in this embodiment. For this reason, Δr is large.

Steel No. No. 13 Si amount, steel no. No. 16 Cr, steel no. No. 18 Mo amount, steel no. B amount of 20 steel No. No. 21 Al amount, steel no. Sn amount of 25, and steel No. The W amount of 26 is larger than the upper limit of the range defined in the present embodiment. Since Si, Cr, Mo, B, Al, Sn, and W are ferrite-forming elements, steel Nos. In 13, 16, 18, 20, 21, 25, and 26, the ferrite phase ratio increased. For this reason, a ferrite phase develops notably in the rolling direction and Δr is large.

Steel No. 15 Ni and steel No. 15 The amount of 19 Cu is larger than the upper limit of the range defined in the present embodiment. Since Ni and Cu are austenite-forming elements, steel No. In 15 and 19, the austenite phase ratio increased excessively, and the maximum strength of the crystal orientation of the ferrite phase was out of the range defined in the present embodiment. For this reason, Δr is large.
Steel No. 22 Ti amount, steel No. Nb amount of 23 and steel No. The amount of 24 Zr is larger than the upper limit of the range defined in the present embodiment. For this reason, steel no. In Nos. 22 to 24, Ti, Nb, and Zr were combined with C and N, which are austenite generating elements, and austenite generation was suppressed, and the austenite phase ratio was lowered. For this reason, Δr is large.

Steel No. of the present invention example Steel samples having the same steel composition as 1 to 4 were used to produce steel samples by changing the cold rolling reduction ratio and the annealing conditions of the cold rolled sheet, and Δr, crystal orientation strength, and austenite phase ratio were determined by the methods described above. It was measured. Table 4 shows the obtained results.

As shown in Table 4, the steel sample No. of the present invention example. 101 to 104 were manufactured under the conditions defined in this embodiment. These steel sample Nos. 101 to 104 have small Δr and small in-plane anisotropy. For this reason, the press formability was good. In contrast, the steel sample No. Nos. 105 to 110 were manufactured under conditions where the cold rolling reduction ratio, the cold rolled sheet annealing temperature, and the cooling rate deviated from the ranges defined in the present embodiment. Steel sample No. of these comparative examples. 105 to 110 have large Δr and large in-plane anisotropy. For this reason, there was a problem in press formability.

The ferrite-austenitic duplex stainless steel sheet of this embodiment has a small in-plane anisotropy of r value and excellent press formability. For this reason, the ferrite-austenitic duplex stainless steel sheet of this embodiment is suitably applied to a press-formed product that requires excellent corrosion resistance.

Claims

In mass%
C: 0.001 to 0.10%,
Si: 0.01 to 1.0%,
Mn: 2 to 10%,
P ≦ 0.05%,
Ni: 0.1 to 3.0%,
Cr: 15.0 to 30.0%, and N: 0.05 to 0.30%,
The balance consists of Fe and inevitable impurities,
The in-plane anisotropy is characterized in that the austenite phase ratio is 40 to 90% in area ratio, the maximum strength of the crystal orientation of the ferrite phase is 10 or less, and the hardness ratio of the austenite phase to the ferrite phase is 1.1 or more. Small ferrite and austenitic duplex stainless steel sheet.
Furthermore, in mass%,
Mo: 0.1 to 1.0%,
Cu: 0.1 to 3.0%,
B: 0.0005 to 0.0100%,
Al: 0.01 to 0.5%,
Ti: 0.005 to 0.30%,
Nb: 0.005 to 0.30%,
Zr: 0.005 to 0.30%,
Sn: 0.05 to 0.50%,
W: 0.1-2.0%,
The in-plane anisotropy according to claim 1, containing at least one selected from Mg: 0.0002 to 0.0100% and Ca: 0.0005 to 0.0100% Ferritic / austenitic duplex stainless steel sheet.
The ferrite austenite 2 having small in-plane anisotropy according to claim 1 or 2, wherein Δr represented by the following formula (1), which is an index of in-plane anisotropy, is 0.5 or less. Phase stainless steel sheet.
Δr = | (r 0 + r 90 ) / 2−r 45 | (1)
Here, r 0 is an r value parallel to the rolling direction, r 90 is an r value perpendicular to the rolling direction, and r 45 is an r value 45 ° to the rolling direction.
A step of cold-rolling a ferrite-austenitic duplex stainless steel having the component composition according to claim 1 or 2, and a subsequent annealing step,
In the cold rolling step, the rolling reduction is 90% or less,
In the annealing step, the annealing temperature is set to 1000 to 1100 ° C., the cooling rate to 500 ° C. is set to 5 ° C./sec or more, and the in-plane is maintained for 5 seconds or more in the temperature range of 400 to 500 ° C. in the cooling process. A method for producing a ferritic / austenitic duplex stainless steel sheet with low anisotropy.