KR20140105849A - Ferrite-austenite 2-phase stainless steel plate having low in-plane anisotropy and method for producing same - Google Patents

Abstract

The ferritic austenitic stainless steel sheet according to claim 1, wherein the ferritic austenite two-phase stainless steel sheet contains 0.001 to 0.10% of C, 0.01 to 1.0% of Si, 2 to 10% of Mn, 0.1 to 3.0% of Ni, , The balance being Fe and inevitable impurities, the austenite phase rate being 40 to 90% in terms of area ratio, the maximum strength of the crystal orientation of the ferrite phase being 10 or less, The hardness ratio of the austenite phase to the phase is 1.1 or more.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferrite-austenite two-phase stainless steel sheet having a small in-plane anisotropy and a method for producing the ferrite-

The present invention relates to a two-phase stainless steel sheet comprising a ferrite phase and an austenite phase with small anisotropy during processing, and a process for producing the same.

The present invention is based on Japanese Patent Application No. 2012-52876 filed on March 9, 2012, the contents of which are incorporated herein by reference.

The two-phase stainless steel sheet including a ferrite phase and an austenite phase is excellent in corrosion resistance and has a high strength because it is a microstructure and has excellent endothelial characteristics and is widely used in chemical plants and the like. However, since the duplex stainless steel sheet has lower ductility than the austenitic stainless steel, cracks may occur during press forming, and improvement in workability is desired.

A typical typical two-phase stainless steel was a high Ni and Mo content represented by SUS329J4L (25% Cr-7% Ni-3% Mo-0.1% N). However, in recent years, low-alloyed ferrite-austenite two-phase stainless steels having reduced Ni content or containing no Mo have been developed and applied to various fields (see, for example, Patent Document 1). In such low Ni and Mo containing steels, SUS304 (18% Cr-8% Ni) is added to the low Ni and Mo containing steels by adjusting the amount of austenite and ensuring corrosion resistance by adding Mn or N. [ , And SUS316 (18% Cr-10% Ni-2% Mo).

On the other hand, when the thin steel sheet is formed into various shapes and applied to various parts, the press formability becomes a problem. Of these press formability, there is an index called in-plane anisotropy. When the in-plane anisotropy is large, the shape of the remaining flange portion of the molded product is not fixed, or the portion called the earing of the end portion of the molded product is waved (so-called earing becomes large). When this problem occurs, the yield at the time of molding is markedly deteriorated, and the shape of the molded article is likely to be uneven, so that the in-plane anisotropy is desired to be small.

As described in Non-Patent Document 1, the ferrite austenite two-phase stainless steel sheet has an extreme in-plane anisotropy, which has a problem in the formability of a thin steel sheet. Here, the in-plane anisotropy is an in-plane anisotropy of r value, and? R represented by the following equation (1) is an index.

In the equation (1), r ₀ is an r value in the parallel direction 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 the direction of 45 ° with respect to the rolling direction. These r values are rank pod values (plastic deformation ratio), and are measured by a method complying with JIS Z2254. When? R is large, it means that the in-plane anisotropy is large. Therefore, in view of the above, the value of? R is desired to be small.

Patent Literature 1 discloses a method for producing a steel sheet in which molten steel of ferrite-austenite two-phase stainless steel is directly thin-plate cast, and there is no difference in mechanical properties between the rolling direction and the width direction and there is no anisotropy. This is a method of producing a thin plate directly from molten steel by omitting hot rolling and is different from a general manufacturing method in which hot rolling is performed as in the present invention. Also, Patent Document 1 is a technique for reducing the difference in strength and elongation in the rolling direction and the width direction, and is not related to the in-plane anisotropy of r value as in the present invention.

Japanese Unexamined Patent Application Publication No. 1-53705

Materials Transactions, Vol. 51, No. 4 (2010) pp. 644.

The present invention particularly aims to provide a ferrite austenite two-phase stainless steel sheet having a low in-plane anisotropy of r value and excellent press formability, in consideration of the crystal orientation strength of a ferrite phase, and a method for producing the same.

In order to solve the above problems, the present inventors investigated in detail the r-value and the in-plane anisotropy of the two-phase stainless steel sheet. As a result of various studies to achieve these objects, 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 (aggregate structure) of the ferrite phase. Here, the crystal orientation intensity refers to the diffraction intensity measured by the X-ray diffraction method, specifically, the ratio of the diffraction intensity to the diffraction intensity of the random sample. For this reason, the crystal orientation strength is a ratio of the diffraction intensity to the diffraction intensity when the crystal orientation is random (crystals that are not aligned) and indicates the degree of orientation. The crystal orientation strength is also referred to as an X-ray random intensity ratio of a specific crystal orientation. In the conventional two-phase stainless steel, the crystal of ferrite develops remarkably in the crystal orientation (rolling orientation) parallel to the rolling direction. As a result, the maximum strength (the maximum value of the crystal orientation strength) of the crystal orientation of the product sheet becomes strong. In this case, the r value in a specific direction (in the vicinity of 45 占 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 manufacturing method, the maximum strength of the crystal orientation of the cold-rolled sheet after cold rolling and the product is weakened, and the in-plane anisotropy is reduced. Concretely, the amount of Ni is reduced and the amount of N or Mn is increased to lighten the austenite phase as the second phase and optimize the fraction of the austenite phase. As a result, it has been found that the maximum strength of the crystal orientation of the ferrite phase in the cold rolling process is reduced. At this time, it has been found that it is effective to adjust the cold rolling reduction and the annealing temperature, that is, it becomes possible to weaken the maximum strength of the crystal orientation of the ferrite phase in the cold rolling process. Also in the subsequent annealing, the maximum strength of the crystal orientation is maintained at a small value. As described above, it has been made possible to provide a product having an in-plane anisotropy of r value as a material property.

The present invention has been completed on the basis of the above knowledge, and the gist of the invention is described 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 being Fe and inevitable impurities,

Austenite phase-inversion ratio of 40 to 90% in terms of area ratio, a maximum strength of the crystal orientation of the ferrite phase is 10 or less, and a hardness ratio of the austenite phase to the ferrite phase is 1.1 or more. .

(2) 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 to 2.0%

Mg: 0.0002 to 0.0100% and

(1) or (2), further comprising at least one element selected from the group consisting of Ca: 0.0005 to 0.0100%.

(3) The ferrite-austenite two-phase stainless steel sheet according to the above (1) or (2), which has an in-plane anisotropy and has a small in-plane anisotropy, wherein?

[Equation 1]

Here, r ₀ is the r value in the parallel direction with respect to the rolling direction, r ₉₀ is the r value in the direction perpendicular to the rolling direction, and r ₄₅ is the r value in the direction of 45 ° with respect to the rolling direction.

(4) A method for producing a ferrite-austenite stainless steel comprising the steps of cold-annealing a ferrite austenite two-phase stainless steel having the composition described in the above (1) or (2)

In the cold rolling step, the rolling reduction is made 90% or less,

Wherein the annealing step is carried out at an annealing temperature of 1000 to 1100 캜, a cooling rate of up to 500 캜 at 5 캜 / sec or more, and a temperature of 400 캜 to 500 캜 of the cooling step for at least 5 seconds A method for producing a ferritic austenite two-phase stainless steel sheet having a small anisotropy.

Conventionally, a ferrite austenite two-phase 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 austenite two-phase stainless steel sheet having a small in-plane anisotropy is obtained. By applying the ferrite austenite two-phase stainless steel sheet according to one embodiment of the present invention to a molding application in various fields such as home appliances, construction, automobiles, etc., a great effect is obtained in environmental measures and in the cost reduction of parts.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the texture and the in-plane anisotropy (? R) of the inventive steel and comparative steel;
2 is a diagram showing the relationship between the maximum intensity of the crystal orientation of the ferrite phase and the in-plane anisotropy (? R).
Fig. 3 is a diagram showing the relationship between an austenite phase (? Phase) and an in-plane anisotropy (? R).
4 is a graph showing the relationship between the cold rolling reduction ratio and the in-plane anisotropy (? R).

Hereinafter, this embodiment will be described in detail.

First, the reason for limiting the chemical composition of the ferrite austenite two-phase stainless steel sheet of the present embodiment will be described. Here, the unit of "%" of the content of the component means% by mass.

When the C content exceeds 0.10%, the upper limit of the C content is set to 0.10% in order to significantly deteriorate the moldability and corrosion resistance. C is an element necessary to stably produce an austenite phase and increase the hardness difference between an austenite phase and a ferrite phase to suppress an increase in crystal orientation strength. When the C content is less than 0.001%, it is difficult to obtain a two-phase structure. Therefore, the lower limit of the amount of C is preferably 0.001%. Further, in consideration of refining cost and weldability, the amount of C is preferably 0.02 to 0.05%.

Si is an element which is also useful as a deoxidizing agent, but when the amount of Si is more than 1.0%, the hot workability deteriorates, making it difficult to produce. Therefore, the amount of Si is set to 1.0% or less. However, since 0.01% or more of Si is required for deoxidation, the lower limit of the amount of Si is set to 0.01%. In view of 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. Mn is added in an amount of 2% or more in order to stably produce an austenite phase and increase the hardness difference between the austenite phase and the ferrite phase to suppress the increase of the maximum strength of the crystal orientation. When the amount of Mn exceeds 10%, the corrosion resistance remarkably deteriorates, so the upper limit of the amount of Mn is set to 10%. In consideration of oxidation resistance and pickling property at the time of production, the amount of Mn is preferably 3.0 to 6.0%.

P is contained as an impurity and deteriorates the hot workability at the time of production, so the upper limit of the amount of P is set to 0.05%. However, excessively decreasing the P amount leads to an increase in the refining cost, so that the P amount is preferably 0.02 to 0.04%.

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

Cr is added in an amount of 15.0% or more to ensure corrosion resistance and oxidation resistance. On the other hand, adding a large amount of Cr leads to an increase in alloy cost, so the upper limit of the amount of Cr is set to 30.0%. Further, considering the production of the composition, the amount of Cr is preferably 17.0 to 25.0%.

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

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

Cu may be added in an amount of from 0.1 to 3.0%, if necessary, in order to control the corrosion resistance and the austenite phase rate. When the amount of Cu is less than 0.1%, the effect of improving the corrosion resistance is not sufficiently obtained. When the amount of Cu exceeds 3.0%, the effect of improving the corrosion resistance is saturated, and the effect of controlling the phase rate of the austenite phase is also saturated. Considering the hot workability, the amount of Cu is preferably 0.1 to 2.0%.

B is an element segregated in grain boundaries to improve hot workability, and B may be added in an amount of 0.0005% or more, if necessary. When the B content is less than 0.0005%, the effect of improving the hot workability is not sufficiently obtained. However, since B is an element that generates ferrite, when the amount of B exceeds 0.0100%, austenite phase is not sufficiently generated. For this reason, the amount of B is set to 0.0005 to 0.0100%. Further, in consideration of intergranular corrosion resistance, the amount of B is preferably 0.0005 to 0.0030%.

Al can be utilized as a deoxidizer. Al also improves oxidation resistance and corrosion resistance. Therefore, 0.01 to 0.5% of Al may be added as needed. When the Al content is less than 0.01%, the effect of improving the oxidation resistance and corrosion resistance is not sufficiently obtained. When the Al content exceeds 0.5%, the effect of improving the oxidation resistance and corrosion resistance is saturated. Considering the toughness, the amount of Al is preferably 0.01 to 0.10%.

Ti forms N and TiN, and is an element effective for refining the texture of a welded portion and a casting structure. Ti is an element for improving corrosion resistance. Therefore, Ti may be added in an amount of 0.005 to 0.30%, if necessary. If the amount of Ti is less than 0.005%, the effect of making the texture of the welded portion and the casting structure finer is not fully manifested. When the amount of Ti is more than 0.30%, the effect becomes saturated and causes surface flaws in the manufacturing process of the steel sheet. Considering the alloy cost and toughness, the amount of Ti is preferably 0.005 to 0.15%.

Nb has an action similar to that of Ti and improves the strength, and Nb may be added in an amount of 0.005 to 0.30%, if necessary. When the amount of Nb is less than 0.005%, the effect of making the texture of the welded portion and the casting structure finer is not fully manifested. If the amount of Nb exceeds 0.30%, the effect is saturated. Considering the alloy cost and toughness, the amount of Nb is preferably 0.005 to 0.15%.

Zr is an element which has an action similar to that of Ti or Nb and improves oxidation resistance. Zr of 0.005 to 0.30% may be added if necessary. If the amount of Zr is less than 0.005%, the effect of making the texture of the welded portion and the casting structure finer is not sufficiently manifested, and the effect of improving the oxidation resistance is not fully manifested. When the amount of Zr exceeds 0.30%, the effect is saturated. Considering the alloy cost and toughness, the amount of Zr is preferably 0.005 to 0.15%. On the other hand, if the amount of Zr exceeds 0.15%, the toughness tends to decrease.

Sn is an element for improving corrosion resistance, and may be added in an amount of 0.05 to 0.50% Sn, if necessary. When the amount of Sn is less than 0.05%, the effect of improving the corrosion resistance is not fully manifested. If the amount of Sn exceeds 0.50%, the effect is saturated. Considering the hot workability and weldability, the amount of Sn is preferably 0.05 to 0.20%.

W is an element that improves corrosion resistance and heat resistance, and may contain 0.1 to 2.0% of W, if necessary. When the W content is less than 0.1%, the effect of improving the corrosion resistance and heat resistance is not sufficiently exhibited. When the amount of W exceeds 2.0%, the effect is saturated. Considering the alloy cost and toughness, the amount of W is preferably 0.1 to 1.0%.

Mg is an element utilized as a deoxidizing agent. Mg is an element effective for refining the texture of a welded portion and a casting structure. For this reason, 0.0002 to 0.0100% of Mg may be added, if necessary. If the amount of Mg is less than 0.0002%, the effect of refining the texture of the welded part and the casted structure is not fully manifested. If the amount of Mg exceeds 0.0100%, the effect is saturated. Considering the production, Mg content is preferably 0.0002 to 0.0020%.

Ca is combined with S to improve hot workability, so Ca may be added in an amount of 0.0005 to 0.0100%, if necessary. When the amount of Ca is less than 0.0005%, the effect of improving the hot workability is not fully manifested. When the amount of Ca exceeds 0.0100%, the effect is saturated. Considering the corrosion resistance, the amount of Ca is preferably 0.0005 to 0.0010%.

Next, the crystal orientation strength of the ferrite phase as a point of the present embodiment will be described.

By the rolling and the heat treatment, the ferrite phase and the austenite phase crystal develop to a specific crystal orientation. Crystals in which specific crystal orientations are developed influence the properties of the steel sheet. The degree of development (degree of orientation) to a specific crystal orientation is proportional to the crystal orientation intensity measured by the X-ray diffraction method and the neutron diffraction method. Here, the crystal orientation intensity is a ratio of the diffraction intensity to the diffraction intensity of the 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, but in this embodiment, the crystal orientation strength obtained by the X-ray diffraction method is specified. Fig. 1 shows a ferrite-like texture of a two-phase stainless steel plate having different in-plane anisotropy (inventive steel and comparative steel). These two-phase stainless steel sheets were cold-rolled annealing plates having a thickness of 1.0 mm, and were manufactured under the conditions that the cold rolling reduction rate was 78% and the annealing temperature was 1050 ° C. The texture was measured by the following method. First, the steel sheet was subjected to mechanical polishing and electrolytic polishing to develop the central region of the sheet thickness. (200), (310) and (211) of the central region of the plate thickness were measured using an X-ray diffraction apparatus (manufactured by Rigaku Denki Kogyo K.K.) and using Mo-K? Three dimensional crystal orientation density function was obtained from these positive electrode viscosity using spherical harmonic function method.

Fig. 1 is a cross-sectional view (φ2 = 45 ° cross-section) in which the crystal orientation intensity can be seen as a contour line. Here, the crystal orientation intensity is the ratio of the diffraction intensity to the diffraction intensity of the random sample. 1, the crystal orientations (rolling orientations) of the ferrite phase parallel to the rolling direction are {100} <011> and {211} <011>. In the texture structure of the comparative steel of FIG. 1, crystals are remarkably developed in the orientations of {100} < 011 > and {211} < 011 > in the rolling directions of the ferrite phase and the maximum strength of the crystal orientation ) Is high as 18. In addition,? R indicating an in-plane anisotropy of r-value is as high as 1.34, and press formability is poor.

On the other hand, in the texture structure of the steel of the present invention shown in Fig. 1, the development of the crystal in the rolling direction is suppressed, and the maximum strength of the crystal orientation (maximum value of the crystal orientation strength) is 8 and lower than that of the comparative steel. Also,? R is 0.38, which means that the anisotropy is small. From the above results, it can be seen that the anisotropy can be effectively reduced by suppressing the in-plane anisotropy of the r value depending on the aggregate structure of the ferrite phase and the development of a specific aggregate structure. Fig. 2 shows the relationship between the maximum intensity of the crystal orientation of the ferrite phase and? R. When the maximum strength is 10 or less,? R becomes 0.5 or less. For this reason, in the present 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. DELTA r is preferably as low as possible, but if DELTA r is 0.5 or less, there is no problem with the shape at the time of pressing. For this reason, in the present embodiment,? R is specified to be 0.5 or less. The? R is more preferably 0.4 or less.

The maximum strength of the crystal orientation is the maximum of the crystal orientation strength of all the crystal orientations. When the three-dimensional crystal orientation density function is obtained from these three positive electrode viscosity measurements, information on the crystal orientation intensity of all the crystal orientations is obtained by measuring the positive electrode viscosity of each of the positive electrode active material layers 200, 310, and 211.

The in-plane anisotropy is an in-plane anisotropy of the r-value, and? R, which is represented by the following known formula (1), becomes the index.

[Equation 1]

In the equation (1), r ₀ is an r value in the parallel direction 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 the direction of 45 ° with respect to the rolling direction. These r values are rank pod values (plastic deformation ratio), and are measured by a method complying with JIS Z2254. When? R is large, it means that the in-plane anisotropy is large. Therefore, it is desired that the value of? R is small in view of the above.

In this embodiment, the austenite phase ratio (area ratio) of the ferrite-austenite two-phase stainless steel also becomes an element for reducing the in-plane anisotropy. The austenite phase is precipitated in the hot rolling step as the second phase, and the amount of precipitation thereof is changed by the temperature. In the present embodiment, a new technical idea that low in-plane anisotropy is expressed by controlling the crystal orientation strength (aggregate structure) of the ferrite phase to cold rolling and maintaining the crystal orientation characteristic (aggregate structure) even after cold rolling and annealing, . When there is no austenite phase or when the difference in hardness between the austenite phase and the ferrite phase is small, the specific crystal orientation on the ferrite phase is rapidly developed due to the rolling strain (development of rolling aggregate structure). In this case, the maximum strength of the crystal orientation becomes strong even after the subsequent heat treatment (development of recrystallized texture).

On the other hand, in the case of the steel composition of the present embodiment, the parent phase ferrite phase is soft compared to the austenite phase phase phase of the second phase. As a result, when the steel sheet is deformed in the state of being restrained by the roll in the cold rolling process, the ferrite phase undergoes extremely nonuniform deformation from the hard austenite phase. The present inventors have 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 is reduced when the hardness of the austenite phase is 1.1 times or more the hardness of the ferrite phase. Since a large amount of heterogeneous strain is introduced from the hard austenite phase to the parent ferrite phase during the transformation process, the crystal orientation rotation is localized and nonuniform. As a result, it is considered that the development of a specific crystal orientation is suppressed, thereby reducing anisotropy. In order to stabilize the small in-plane anisotropy, the hardness ratio of the austenite phase to the ferrite phase is preferably 1.2 or more. When the hardness ratio exceeds 2.0, the austenite phase is remarkably cured and cracks are generated at the interface between the ferrite phase and the austenite phase during the molding process. Therefore, the upper limit of the hardness ratio is preferably 2.0.

Further, the present inventors also investigated austenite phase ratio (fraction of austenite phase (area fraction)). A cold-rolled sheet having the same composition as that of the inventive steel of FIG. 1 was prepared and the annealing temperature was adjusted at 950 ° C to 1150 ° C to prepare samples having various austenite phase rates. Further, in order to change the austenite phase-to-phase ratio, the annealing temperature of the cold-rolled sheet was changed from 950 캜 to 1150 캜. The austenite phase rate and? R of the obtained sample were measured. Here, the austenite phase rate is measured by a ferrite meter, but it may be obtained by an image analyzer or an EBSP analyzer. The austenite phase rate of the sample prepared at an annealing temperature of 1100 ° C was 40%. The austenite phase rate of the sample prepared at an annealing temperature of 1000 ° C was 90%.

3 shows the relationship between the austenite phase-to-plane anisotropy (? R). As shown in Fig. 3, when the austenite phase ratio is 40% or more and 90% or less,? R becomes 0.5 or less. For this reason, the lower limit of the austenite phase rate is set to 40%, and the upper limit is set to 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 is excessively increased, excessive uneven deformation is caused from the austenite phase during the cold rolling process, and the aggregate structure of the ferrite phase after cold annealing is developed. As a result, it is considered that the anisotropy is increased. Therefore, the austenite phase ratio is 40 to 90%. The austenite phase ratio is preferably 50 to 80%, more preferably 60 to 80%, in view of stably reducing in-plane anisotropy stably and also considering strength and ductility.

Next, the manufacturing method will be described.

The method of manufacturing a steel sheet according to the present embodiment includes each step of steel-making, hot-rolling, pickling, cold rolling, annealing and pickling. In steelmaking, a method in which a steel containing the above-mentioned essential components and components added as needed is dissolved in a converter or an electric furnace and subsequently subjected to secondary refining is suitably applied. The molten steel to be melted 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 thickness in continuous rolling. In the hot rolling, the slab is rolled by a hot rolling mill including a plurality of stands and is wound up continuously. In the present embodiment, casting and hot rolling conditions are not specifically defined, but may be appropriately selected depending on the components.

After the hot rolling, hot-rolled sheet annealing may be omitted or pickled, and then cold rolling is performed. In the cold rolling, the reduction ratio of the cold rolling is set to 90% or less. Fig. 4 shows the relationship between the reduction rate and? R. If the reduction rate exceeds 90%,? R exceeds 0.5 and the in-plane anisotropy becomes large. When the deformation due to cold rolling becomes excessively large, the maximum strength of the crystal orientation of the ferrite phase is drastically increased (crystals remarkably develop in the rolling direction). As a result, it is considered that the in-plane anisotropy is increased. In consideration of ductility and productivity, the reduction ratio of cold rolling is preferably 30 to 80%. Other conditions (roll diameter, number of passes, rolling temperature, etc.) in cold rolling are not particularly specified, and may be suitably selected according to productivity.

Annealing after cold rolling is carried out to adjust the austenite phase ratio. The heating temperature of the annealing is set to 1100 DEG C or lower in order to set the austenite phase rate to 40% or more. The heating temperature of the annealing is set to 1000 占 폚 or more so as to make the austenite phase rate 90% or less. However, annealing at an excessively high temperature, conversely, reduces the austenite phase ratio and coarsens the crystal grains. This results in an increase in the maximum strength of the crystal orientation of the ferrite phase. Therefore, the heating temperature (annealing temperature) of the annealing is set at 1000 to 1100 캜. From the viewpoint of ductility and toughness, the annealing temperature is preferably 1020 to 1075 占 폚. Further, if the cooling rate after heating is too low, Cr carbonitride precipitates in the cooling process, and toughness and corrosion resistance are deteriorated. Therefore, the cooling rate up to 500 占 폚 is set to 5 占 폚 / sec or more. When the cooling rate exceeds 500 deg. C / sec, the steel sheet shape significantly deteriorates, so the upper limit of the cooling rate is set to 500 deg. C / sec. In consideration of productivity and acidity, the cooling rate is preferably 10 to 50 占 폚 / sec, and the cooling method may be suitably selected from radial cooling and water cooling.

In order to make the hardness of the austenite phase 1.1 times the hardness of the ferrite phase, it is necessary to harden the austenite phase by concentrating N in the austenite. In the present embodiment, the cooling process is performed for 5 seconds or longer in a temperature range of 400 to 500 캜. Thereby, N is concentrated on the austenite phase. However, when the holding time exceeds 500 seconds, the productivity is significantly deteriorated, so the upper limit of the holding time is set to 500 seconds. In consideration of productivity, the holding time is preferably 60 seconds or less.

The manufacturing method of the other steps is not specifically defined, but the thickness of the hot-rolled sheet and the annealing atmosphere of the cold-rolled sheet may be appropriately selected. In addition, temper rolling or tension leveler may be applied after cold rolling and annealing. In addition, the plate thickness of the product may be selected in accordance with the thickness of the required member (member after processing).

Example

The steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to obtain 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 ratio 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 占 폚 and then cooled at a cooling rate of 10 占 폚 / sec to 500 占 폚. After annealing, pickling was performed to obtain a product plate. The product plate thus obtained was measured for? R, the maximum strength of the crystal orientation, and the austenite phase rate by the above-described method.

The ferritic austenite two-phase stainless steel sheet of steels Nos. 1 to 10 has a steel component within the range specified in the present embodiment, and the austenite phase ratio and the maximum strength of the crystal orientation of the ferrite phase are within the range defined in the present embodiment It satisfies. Also,? R, which is an index of anisotropy, is 0.5 or less and in-plane anisotropy is small.

On the other hand, Steel No. 11 is a steel equivalent to SUS329J4L, and the amounts of Ni and Mo are out of the range specified in the present embodiment. In addition, the austenite phase ratio is low and the maximum strength of the crystal orientation of the ferrite phase is remarkably high. As a result,? R is more than 0.5 and the anisotropy is large.

The C amount of steel No. 12, the Mn amount of steel No. 14, and the N amount of steel No. 17 are smaller than the lower limit of the range specified in the present embodiment. Since C, Mn, and N are austenite generating elements, the austenite phase rate of the steels No. 12, 14, and 17 and the maximum strength of the crystal orientation of the ferrite phase were outside the ranges specified in this embodiment. As a result,? R is large.

The amount of Si of steel No. 13, the amount of Cr of steel No. 16, the amount of Mo of steel No. 18, the amount of B of steel No. 20, the amount of Al of steel No. 21, The amount of W of .26 is larger than the upper limit of the range defined in the present embodiment. As steels No. 13, 16, 18, 20, 21, 25 and 26, the ferrite phase ratios were increased because Si, Cr, Mo, B, Al, Sn and W were ferrite generating elements. As a result, the ferrite phase remarkably develops in the rolling direction, and Δr is large.

The Ni amount of the steel No. 15 and the Cu amount of the steel No. 19 are larger than the upper limit of the range defined in the present embodiment. Since Ni and Cu are the austenite generating elements, in the steels No. 15 and No. 19, the austenite phase rate is excessively increased and the maximum strength of the crystal orientation of the ferrite phase is out of the range specified in the present embodiment. As a result,? R is large.

The amount of Ti in steel No. 22, the amount of Nb in steel No. 23, and the amount of Zr in steel No. 24 are larger than the upper limit of the range specified in this embodiment. As a result, in the steel Nos. 22 to 24, Ti, Nb and Zr were combined with C and N which are the austenite generating elements, the formation of austenite was inhibited, and the austenite phase rate was decreased. As a result,? R is large.

By using the steel having the same steel composition as that of the steels Nos. 1 to 4 of the present invention and changing the cold rolling reduction ratio and the annealing condition of the cold-rolled steel sheet, a steel sample was prepared and the Δr, crystal orientation strength and austenite phase ratio Respectively. The obtained results are shown in Table 4.

As shown in Table 4, the steel samples Nos. 101 to 104 of the present invention were produced under the conditions specified in the present embodiment. In the steel samples Nos. 101 to 104 of the present invention examples,? R is small and in-plane anisotropy is small. As a result, the press formability was good. On the other hand, the steel samples No. 105 to 110 of the comparative examples were produced under the conditions that the cold rolling reduction rate, cold rolling plate annealing temperature, and cooling rate were out of the range specified in the present embodiment. The steel samples Nos. 105 to 110 of these comparative examples had large? R and large in-plane anisotropy. As a result, there was a problem in press formability.

The ferrite austenite two-phase stainless steel sheet of the present embodiment has an in-plane anisotropy of r value and is excellent in press formability. As a result, the ferrite austenite two-phase stainless steel sheet of the present embodiment is suitably applied to a press-molded article requiring excellent corrosion resistance.

Claims (4)

In terms of% by 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 being Fe and inevitable impurities,
Austenite phase-stable ferrite austenite having a low in-plane anisotropy, characterized by an austenite phase-to-area ratio of 40 to 90%, a maximum strength of a crystal orientation of a ferrite phase of 10 or less, and a hardness ratio of an austenite phase to ferrite phase of 1.1 or more. Steel plate. The method according to claim 1,
In terms of% by 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 to 2.0%
Mg: 0.0002 to 0.0100% and
And Ca: 0.0005 to 0.0100%. The ferrite-austenite two-phase stainless steel sheet having a low in-plane anisotropy. 3. The method according to claim 1 or 2,
A ferrite-austenite two-phase stainless steel sheet having an in-plane anisotropy of not more than 0.5, which is an index of in-plane anisotropy and expressed by the following formula (1).
[Equation 1]

Here, r ₀ is the r value in the parallel direction with respect to the rolling direction, r ₉₀ is the r value in the direction perpendicular to the rolling direction, and r ₄₅ is the r value in the direction of 45 ° with respect to the rolling direction. A process for producing a ferritic stainless steel comprising the steps of cold-rolling a ferritic austenite two-phase stainless steel having the compositional formula as set forth in claim 1 or 2 and an annealing step thereafter,
In the cold rolling step, the rolling reduction is made 90% or less,
Characterized in that the annealing temperature is 1000 to 1100 占 폚 and the cooling rate to 500 占 폚 is 5 占 폚 / sec or more and the temperature is maintained at 400 to 500 占 폚 for 5 seconds or more during the cooling process. A method for producing a ferrite austenite two-phase stainless steel sheet having a small in-plane anisotropy.

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