JPWO2015105045A1 - Ferritic stainless steel and manufacturing method thereof - Google Patents

Abstract

Provided is a ferritic stainless steel having sufficient corrosion resistance and formability (elongation and average r value is large, | Δr | is small) and excellent in surface properties with little occurrence of linear flaws, and a method for producing the same. Ferritic stainless steel of the present invention is in mass%, C: 0.005-0.05%, Si: 0.02-0.50%, Mn: 0.05-1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5- Contains 18.0%, Al: 0.001 to 0.10%, N: 0.01 to 0.06%, V: 0.01 to 0.25%, Ti: 0.001 to 0.020%, Nb: 0.001 to 0.030%, with the balance consisting of Fe and inevitable impurities And V / (Ti + Nb) ≧ 2.0.

Description

  The present invention relates to a ferritic stainless steel having sufficient corrosion resistance and formability and excellent surface properties free from the occurrence of linear flaws due to hot rolling or annealing, and a method for producing the same.

  Since ferritic stainless steel is inexpensive and excellent in corrosion resistance, it is used in various applications such as building materials, transportation equipment, home appliances, kitchen appliances, and automobile parts, and its application range is expanding further in recent years. In order to be applied to these applications, not only corrosion resistance but also sufficient formability that can be processed into a predetermined shape (large elongation (hereinafter, elongation is sufficiently large) The average rankford value (hereinafter sometimes referred to as the average r value) is large, and the absolute value of the in-plane anisotropy of the r value (hereinafter sometimes referred to as | Δr |) is small. ) Is required. In addition, when applied to applications that require surface aesthetics, it is also necessary to have excellent surface properties.

On the other hand, in Patent Document 1, in mass%, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less Ti: 0.005% or less, Cr: 11-30%, Ni: 0.7% or less, and 0.06 ≦ (C + N) ≦ 0.12, 1 ≦ N / C and 1.5 × 10 ⁻³ ≦ (V × N) ≦ 1.5 A ferritic stainless steel excellent in formability and ridging resistance, characterized by satisfying × 10 ⁻² (C, N, and V each represents mass% of each element) is disclosed. However, Patent Document 1 does not mention any anisotropy. Further, it is necessary to perform so-called box annealing (for example, annealing at 860 ° C. for 8 hours) after hot rolling. Such a box annealing process has a problem of low productivity because it takes about one week to complete when heating and cooling processes are included.

  In Patent Document 2, in mass%, C: 0.01 to 0.10%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, Ni: 0.01 to 0.50%, Cr: 10 to 20%, Mo: 0.005 to 0.50% , Cu: 0.01-0.50%, V: 0.001-0.50%, Ti: 0.001-0.50%, Al: 0.01-0.20%, Nb: 0.001-0.50%, N: 0.005-0.050% and B: 0.00010-0.00500% Hot rolling the contained steel, followed by hot-rolled sheet annealing in the ferrite single-phase temperature range using a box furnace or AP line (annealing and pickling line) continuous furnace, followed by cold rolling and finish annealing. Ferritic stainless steels with excellent workability and surface properties characterized by the above are disclosed. However, when a box furnace is used, there is a problem that productivity is low as in Patent Document 1 described above. Patent Document 2 does not mention any elongation at all. However, when hot-rolled sheet annealing is performed in a ferrite single-phase temperature range in a continuous annealing furnace, recrystallization becomes insufficient because the annealing temperature is low, and ferrite single Elongation is lower than when box annealing is performed in the phase temperature range. In general, ferritic stainless steel as in Patent Document 2 has a problem that a crystal grain group (colony) having a similar crystal orientation is formed during casting or hot rolling, and | Δr | becomes large.

Japanese Patent No. 3588281 (Republication WO00 / 60134) Japanese Patent No. 3582001 (Japanese Patent Laid-Open No. 2001-3134)

  The present invention solves such problems, and provides a ferritic stainless steel having sufficient corrosion resistance and formability, and excellent surface properties free from the occurrence of linear flaws due to hot rolling or annealing, and a method for producing the same. The purpose is to provide.

  In the present invention, sufficient corrosion resistance refers to a salt spray cycle test ((salt spray (35 ° C., 5 ° C.) specified in JIS H 8502) on a steel plate whose surface is polished and polished with # 600 emery paper. (Mass% NaCl, spray 2h) → Drying (60 ° C, relative humidity 40%, 4h) → Wet (50 ° C, relative humidity ≥ 95%, 2h)))) It means that the rusting area ratio on the surface (= rusting area / total area of steel plate × 100 [%]) is 25% or less.

In addition, sufficient formability means that the elongation at break in a tensile test based on JIS Z 2241 is 25% or more when using a specimen taken in a direction perpendicular to the rolling direction, and a tensile test based on JIS Z2241 The average r value calculated by the following equation (1) when applying a strain of 15% in the above is 0.65 or more, and the in-plane anisotropy of the r value calculated by the following equation (2) (hereinafter, This means that the absolute value (| Δr |) of Δr) is 0.30 or less.
Average r value = (r _L + 2 × r _D + r _C ) / 4 (1)
Δr = (r _L −2 × r _D + r _C ) / 2 (2)
Here, r _L is an r value when a tensile test is performed in a direction parallel to the rolling direction, r _D is an r value when a tensile test is performed in a direction of 45 ° with respect to the rolling direction, and r _C is a direction perpendicular to the rolling direction. The r value when a tensile test is performed.

  As a result of studying to solve the problem, before cold-rolling the steel sheet after hot rolling on ferritic stainless steel with an appropriate component, annealing was performed in a temperature range in which the ferrite phase and the austenite phase become two phases. It has been found that a ferritic stainless steel having sufficient corrosion resistance and formability can be obtained. In addition, wire rods on the steel sheet surface are suppressed by further defining V, Ti, and Nb within the above-mentioned range of appropriate steel components so as not to precipitate coarse Cr carbonitride during hot rolling. As a result, it was found that not only the corrosion resistance and moldability but also the surface properties were excellent.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] By mass%, C: 0.005 to 0.05%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: 0.001 ~ 0.10%, N: 0.01 ~ 0.06%, V: 0.01 ~ 0.25%, Ti: 0.001 ~ 0.020%, Nb: 0.001 ~ 0.030%, the balance is Fe and inevitable impurities, and V / (Ti + Nb ) Ferritic stainless steel that satisfies ≧ 2.0.
[2] By mass%, C: 0.01 to 0.05%, Si: 0.02 to 0.50%, Mn: 0.2 to 1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 16.0 to 18.0%, Al: 0.001 ~ 0.10%, N: 0.01 ~ 0.06%, V: 0.01 ~ 0.25%, Ti: 0.001 ~ 0.015%, Nb: 0.001 ~ 0.025%, the balance is Fe and inevitable impurities, and V / (Ti + Nb ) Ferritic stainless steel that satisfies ≧ 2.0.
[3] In the mass%, the composition further includes one or more selected from Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 0.5%, and Co: 0.01 to 0.5%. The ferritic stainless steel according to [1] or [2].
[4] In the mass%, the composition further includes one or more selected from Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050%, REM: 0.01 to 0.10%, Ca: 0.0002 to 0.0020% The ferritic stainless steel according to any one of [1] to [3].
[5] The steel slab having the composition according to any one of [1] to [4] is hot-rolled and then annealed at a temperature range of 880 to 1000 ° C. for 5 seconds to 15 minutes. A method for producing a ferritic stainless steel, in which a hot-rolled annealed sheet is used, followed by cold rolling, followed by cold-rolled sheet annealing in a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.
In addition, in this specification, all% which shows the component of steel is the mass%.

  According to the present invention, a ferritic stainless steel having a sufficient corrosion resistance and formability (elongation and average r value is large, | Δr | is small) and excellent in surface properties with little generation of linear defects is obtained. It is done.

Hereinafter, the present invention will be described in detail.
Ferritic stainless steel is mass%, C: 0.005-0.05%, Si: 0.02-0.50%, Mn: 0.05-1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5-18.0%, Al: 0.001 to 0.10%, N: 0.01 to 0.06%, V: 0.01 to 0.25%, Ti: 0.001 to 0.020%, Nb: 0.001 to 0.030%, the balance is Fe and inevitable impurities, and V /(Ti+Nb)≧2.0. In the present invention, the balance of the component composition is important, and in particular, the balance of V, Ti, and Nb is important. V: 0.01 to 0.25%, Ti: 0.001 to 0.020%, Nb: 0.001 to 0.030%, and satisfying V / (Ti + Nb) ≧ 2.0 are important requirements. By using such a combination of component compositions, it is possible to obtain a ferritic stainless steel having sufficient corrosion resistance and sufficient formability, and having excellent surface properties with little generation of linear flaws.

First, the technical contents of the present invention will be described in detail.
The inventors examined a technique for obtaining a predetermined formability by short-time hot-rolled sheet annealing using a high-productivity continuous annealing furnace, instead of long-time hot-rolled sheet annealing such as box annealing (batch annealing). did. The problem with the prior art using a continuous annealing furnace is that annealing is performed in the ferrite single-phase temperature range, so that sufficient recrystallization does not occur and sufficient elongation cannot be obtained, and after the colony is cold-rolled sheet annealed In other words, | Δr | is large. Therefore, the inventors performed hot rolling sheet annealing in a two-phase region of a ferrite phase and an austenite phase, and then cold-rolled and cold-rolled sheet annealing by a conventional method, and finally made a ferrite single-phase structure again. I devised that.

  That is, recrystallization of the ferrite phase is promoted by performing the hot-rolled sheet annealing in the two-phase region of the ferrite phase and austenite that are higher in temperature than the ferrite single-phase temperature region. As a result, it is avoided that the ferrite crystal grains introduced with work strain by hot rolling remain until after cold-rolled sheet annealing, and the elongation after cold-rolled sheet annealing is improved. Also, when the austenite phase is generated from the ferrite phase by hot-rolled sheet annealing, the austenite phase is generated with a crystal orientation different from that of the ferrite phase before annealing, which effectively destroys the ferrite phase colony. Is done. Therefore, in the metal structure of the cold-rolled annealed sheet after cold rolling and cold-rolled sheet annealing, a γ-fiber texture that improves the r value develops, the colonies are divided, and the anisotropy of the metal structure Is relaxed and an excellent characteristic that | Δr | becomes small is obtained.

  Further, when hot-rolled sheet annealing is performed in a two-phase region of a ferrite phase and an austenite phase, after the hot-rolled sheet annealing, a two-phase structure of a ferrite phase and a martensite phase transformed from the austenite phase is obtained. However, by cold rolling this hot-rolled annealed sheet containing the martensite phase, the martensite phase is harder than the ferrite phase, so the ferrite phase near the martensite phase is preferentially deformed and rolled. The strain concentrates and the recrystallization sites during cold-rolled sheet annealing further increase. Thereby, recrystallization at the time of cold-rolled sheet annealing is further promoted, and the anisotropy of the metal structure after the cold-rolled sheet annealing is further relaxed.

  However, when hot-rolled sheet annealing is performed on the steel of the conventional component in the two-phase region of the ferrite phase and austenite phase, linear wrinkles (hereinafter referred to as linear wrinkles) along the rolling direction after cold-rolled sheet annealing are performed. It has become clear that a new problem arises that surface properties are significantly reduced.

  Therefore, the inventors investigated the cause of the occurrence of linear flaws by performing hot-rolled sheet annealing in a two-phase region of a ferrite phase and an austenite phase in order to achieve both formability and surface properties. As a result, it was found that the linear wrinkles were caused by a remarkably hard martensite phase present in the surface layer portion of the steel sheet after hot-rolled sheet annealing. That is, if there is a remarkably hard martensite phase in the surface layer of the steel sheet after hot-rolled sheet annealing, strains concentrate at the interface between the remarkably hard martensite phase and the ferrite phase in the subsequent cold rolling, and microcracks are generated. It has been found that after cold-rolled sheet annealing, it becomes a linear wrinkle. The martensite phase is formed by transformation of the austenite phase formed during hot-rolled sheet annealing in the two-phase region of the ferrite phase and the austenite phase during the cooling process. When the hardness of each martensite crystal grain in the metal structure was investigated, many martensite phases had a Vickers hardness (HV) of about 300 to 400, whereas some martensite phases markedly exceeded HV500. It was found that microcracks in the cold rolling occurred at the interface between the martensite phase and the ferrite phase, which is extremely hard, exceeding HV500.

  Thus, the inventors have clarified the cause of locally forming a significantly hard martensite phase exceeding HV500 after hot-rolled sheet annealing, and have intensively studied the countermeasure technique. As a result, it was found that an extremely hard martensite phase is formed when coarse Cr carbonitride is present before hot-rolled sheet annealing. This mechanism is considered as follows. In hot-rolled sheet annealing, the austenite phase is formed by the solid solution of Cr carbonitride precipitated by hot rolling. When the Cr carbonitride before hot-rolled sheet annealing is coarse, the amount of C supplied to the austenite phase increases. Therefore, in the periphery where coarse Cr carbonitride is dissolved, the C concentration is locally higher than the portion where coarse Cr carbonitride is not dissolved. From this austenite phase having a high C concentration, a remarkably hard martensite phase is produced after hot-rolled sheet annealing.

  Therefore, the inventors examined a technique that does not precipitate coarse Cr carbonitride during hot rolling. As a result, V, Ti, and Nb are included in the steel component so that V: 0.01 to 0.25%, Ti: 0.001 to 0.020%, Nb: 0.001 to 0.030%, and V / (Ti + Nb) ≧ 2.0. Thus, it was found that precipitation of coarse Cr carbonitride during hot rolling can be avoided.

  In other words, by applying these elements, Cr carbonitride that precipitates during hot rolling becomes composite carbonitride (Cr, V, Ti, Nb) (C, N) containing V, Ti, and Nb. As a result, the present inventors have found that finer and more uniform precipitation occurs compared to Cr carbonitride, and that the formation of coarse Cr carbonitride is suppressed.

  Such an effect is manifested by containing an appropriate amount of V. Ti and Nb have a stronger affinity for C and N than Cr, and form carbonitrides more easily than Cr. Therefore, when Ti or Nb is contained alone, it precipitates as Ti (C, N) or Nb (C, N) different from Cr carbonitride and suppresses the formation of coarse Cr carbonitride. The effect to do is not obtained.

  On the other hand, V is also an element having a strong affinity for C and N. However, since V has a tendency to form (Cr, V, Ti, Nb) (C, N), which is a composite carbonitride with Cr, Ti and Nb, an appropriate amount of V is contained in addition to Ti and Nb. In this case, Cr carbonitride precipitates as (Cr, V, Ti, Nb) (C, N). This (Cr, V, Ti, Nb) (C, N) is a precipitate containing V, Ti, and Nb, which has a lower diffusion rate than Cr, so that the growth or coarsening after precipitation is V, Ti, and Nb. The rate of precipitates is limited, and the precipitate size becomes finer than that of conventional Cr carbonitrides, and the formation of coarse carbonitrides in hot rolling can be effectively suppressed.

  Due to these effects, when hot-rolled sheet annealing is performed in the two-phase region of the ferrite phase and austenite phase, the formation of a remarkably hard martensite phase due to the solid solution of coarse Cr carbonitride is suppressed, and the cold It has been clarified that the occurrence of linear wrinkles after sheet annealing is greatly reduced.

  That is, in order to obtain a predetermined formability without reducing surface properties by short-time hot-rolled sheet annealing using a continuous annealing furnace, rather than long-time hot-rolled sheet annealing such as box annealing (batch annealing). In addition, it is necessary not only to perform hot-rolled sheet annealing in a two-phase region of a ferrite phase and an austenite phase, but also to make a steel component containing V, Ti and Nb in an appropriate composition.

Next, the component composition of the ferritic stainless steel of the present invention will be described.
Hereinafter, unless otherwise specified,% means mass%.

C: 0.005-0.05%
C promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range where the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.005% or more needs to be contained. However, if the C content exceeds 0.05%, the steel sheet becomes hard and the ductility decreases. Moreover, even if it has this invention, a remarkably hard martensite phase will produce | generate after hot-rolled sheet annealing, and will induce the linear wrinkles after cold-rolled sheet annealing. Therefore, the C content is in the range of 0.005 to 0.05%. The lower limit is preferably 0.01%, more preferably 0.015%. The upper limit is preferably 0.035%, more preferably 0.03%, and even more preferably 0.025%.

Si: 0.02 ~ 0.50%
Si is an element that acts as a deoxidizer during steel melting. In order to obtain this effect, a content of 0.02% or more is necessary. However, if the Si content exceeds 0.50%, the steel sheet becomes hard and the rolling load during hot rolling increases. Moreover, the ductility after cold-rolled sheet annealing decreases. Therefore, the Si content is in the range of 0.02 to 0.50%. Preferably it is 0.10 to 0.35% of range. More preferably, it is 0.25 to 0.30% of range.

Mn: 0.05-1.0%
Mn, like C, promotes the formation of an austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.05% or more needs to be contained. However, if the amount of Mn exceeds 1.0%, the amount of MnS produced increases and the corrosion resistance decreases. Therefore, the amount of Mn is made 0.05 to 1.0% in range. The lower limit is preferably 0.1%, more preferably 0.2%. The upper limit is preferably 0.8%, more preferably 0.35%, and still more preferably 0.3%.

P: 0.04% or less
P is an element that promotes grain boundary fracture due to grain boundary segregation, so a lower value is desirable, and the upper limit is made 0.04%. Preferably it is 0.03% or less. More preferably, it is 0.01% or less.

S: 0.01% or less
S is an element that exists as sulfide inclusions such as MnS and reduces ductility, corrosion resistance, and the like. In particular, when the content exceeds 0.01%, those adverse effects are remarkable. Therefore, it is desirable that the S amount be as low as possible. In the present invention, the upper limit of the S amount is 0.01%. More preferably, it is 0.007% or less. More preferably, it is 0.005% or less.

Cr: 15.5-18.0%
Cr is an element having an effect of improving the corrosion resistance by forming a passive film on the steel sheet surface. In order to obtain this effect, the Cr content needs to be 15.5% or more. However, if the Cr content exceeds 18.0%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, the Cr content is in the range of 15.5 to 18.0%. Preferably it is 16.0 to 18.0% of range. Furthermore, it is preferably in the range of 16.0 to 17.0%.

Al: 0.001 to 0.10%
Al, like Si, is an element that acts as a deoxidizer. In order to acquire this effect, 0.001% or more needs to be contained. However, when the Al content exceeds 0.10%, Al-based inclusions such as Al ₂ O ₃ increase, and the surface properties tend to deteriorate. Therefore, the Al content is set to a range of 0.001 to 0.10%. Preferably it is 0.001 to 0.07% of range. More preferably, it is 0.001 to 0.05% of range. Even more preferably, it is in the range of 0.001 to 0.03%.

N: 0.01-0.06%
N, like C and Mn, promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to obtain this effect, the N content needs to be 0.01% or more. However, when the N content exceeds 0.06%, the ductility is remarkably lowered and the corrosion resistance is lowered by promoting the precipitation of Cr nitride. Therefore, the N content is in the range of 0.01 to 0.06%. Preferably it is 0.01 to 0.05% of range. More preferably, it is 0.02 to 0.04% of range.

V: 0.01-0.25%
V is an extremely important element in the present invention. V has a feature that the affinity for C and N is higher than that of Cr. By satisfying V / (Ti + Nb) ≧ 2.0, it is combined with Cr, Ti and Nb during hot rolling (Cr, V, It precipitates as Ti, Nb) (C, N) and suppresses the precipitation of coarse Cr carbonitride. Due to this effect, the generation of austenite phase in which C is excessively concentrated during hot-rolled sheet annealing is suppressed, and a remarkably hard martensite phase is not generated after hot-rolled sheet annealing, resulting in generation of microcracks during cold rolling. Occurrence of the resulting surface line defects is prevented. In order to obtain this effect, the V content needs to be 0.01% or more. However, if the V amount exceeds 0.25%, the workability is lowered and the manufacturing cost is increased. Therefore, the V amount is in the range of 0.01 to 0.25%. Preferably it is 0.03 to 0.20% of range. More preferably, it is 0.05 to 0.15% of range.

Ti: 0.001-0.020%, Nb: 0.001-0.030%, V / (Ti + Nb) ≧ 2.0
Ti and Nb, like V, are elements with higher affinity for C and N than Cr, and when steel contains V, V and Cr and (Cr, V, Ti, Nb) (C, N) Has an effect of suppressing the precipitation of coarse Cr carbonitride during hot rolling. In order to obtain this effect, it is necessary to contain 0.001% or more of Ti and 0.001% or more of Nb and satisfy V / (Ti + Nb) ≧ 2.0. However, if the Ti content exceeds 0.020% or Nb content exceeds 0.030%, it is not (Cr, V, Ti, Nb) (C, N) during hot rolling, but Ti (C, N) and Nb (C, N). ) Precipitates independently, the effect of suppressing coarse Cr carbonitride cannot be obtained, and the predetermined surface properties cannot be obtained. Therefore, the Ti content is in the range of 0.001 to 0.020%, and the Nb content is in the range of 0.001 to 0.030%. The amount of Ti is preferably in the range of 0.001 to 0.015%. More preferably, it is 0.003 to 0.010% of range. The amount of Nb is preferably in the range of 0.001 to 0.025%. More preferably, it is 0.005 to 0.020% of range. When V / (Ti + Nb) is less than 2.0, V required for producing composite carbonitrides is insufficient, and Ti, Nb, and V are independently produced as carbides or nitrides. The formation of Cr carbonitride cannot be sufficiently suppressed. Therefore, V / (Ti + Nb) is set to 2.0 or more. Preferably it is 3.0 or more. More preferably, it is 4.0 or more. On the other hand, when V / (Ti + Nb) exceeds 30.0, even if V, Ti and Nb have a predetermined content, the amount of V existing in a solid solution state without being consumed for formation of composite carbonitride Therefore, the elongation decreases due to the hardening of the steel sheet. Therefore, the upper limit of V / (Ti + Nb) is preferably 30.0.

  The balance is Fe and inevitable impurities.

  Although the effects of the present invention can be obtained by the above component composition, the following elements can be contained for the purpose of further improving manufacturability or material characteristics.

One or more selected from Cu: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-0.5%, Co: 0.01-0.5%
Cu and Ni are both elements that improve the corrosion resistance, and it is effective to contain them particularly when high corrosion resistance is required. Further, Cu and Ni have an effect of promoting the formation of the austenite phase and expanding the two-phase temperature range in which the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. These effects become significant when the content is 0.1% or more. However, if the Cu content exceeds 1.0%, the hot workability is lowered, which is not preferable. Therefore, when it contains Cu, it is 1.0% or less. Preferably it is 0.2 to 0.8% of range. More preferably, it is 0.3 to 0.5% of range. If the Ni content exceeds 1.0%, the workability decreases, which is not preferable. Therefore, when it contains Ni, it is 1.0% or less. Preferably it is 0.1 to 0.6% of range. More preferably, it is 0.1 to 0.3% of range.

  Mo is an element that improves corrosion resistance, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes significant when the content is 0.1% or more. However, if the Mo content exceeds 0.5%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, when it contains Mo, it is 0.1 to 0.5 %% or less. Preferably it is 0.1 to 0.3% of range.

  Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, if the Co content exceeds 0.5%, workability is reduced. Therefore, when it contains Co, it is 0.5% or less. Preferably it is 0.01 to 0.2% of range.

Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050%, REM: 0.01 to 0.10%, Ca: One or more selected from 0.0002 to 0.0020%
Mg: 0.0002-0.0050%
Mg is an element that has the effect of improving hot workability. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Mg content exceeds 0.0050%, the surface quality decreases. Therefore, when it contains Mg, it is set as 0.0002 to 0.0050% of range. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is 0.0005 to 0.0020% of range.

B: 0.0002-0.0050%
B is an effective element for preventing low temperature secondary work embrittlement. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the amount of B exceeds 0.0050%, the hot workability decreases. Therefore, when it contains B, it is set as 0.0002 to 0.0050% of range. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is 0.0005 to 0.0020% of range.

REM: 0.01-0.10%
REM is an element that improves the oxidation resistance, and in particular has the effect of suppressing the formation of an oxide film at the weld and improving the corrosion resistance of the weld. In order to obtain this effect, a content of 0.01% or more is necessary. However, if the content exceeds 0.10%, productivity such as pickling at the time of cold-rolled sheet annealing is lowered. Moreover, since REM is an expensive element, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when it contains REM, it is set as 0.01 to 0.10% of range.

Ca: 0.0002-0.0020%
Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Ca content exceeds 0.0020%, CaS is generated and the corrosion resistance is lowered. Therefore, when it contains Ca, it is set as 0.0002 to 0.0020% of range. Preferably it is 0.0005 to 0.0015% of range. More preferably, it is 0.0005 to 0.0010% of range.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
The ferritic stainless steel of the present invention is a hot-rolled annealed sheet obtained by subjecting a steel slab having the above composition to hot rolling, followed by hot-rolled sheet annealing that is held at a temperature range of 880 to 1000 ° C. for 5 seconds to 15 minutes. Then, after performing cold rolling, it is obtained by performing cold-rolled sheet annealing which is held at a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.

  First, molten steel having the above-described component composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method. This slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled as cast without heating to form a hot-rolled sheet.

  Next, hot rolling is performed. In winding, the winding temperature is preferably 500 ° C. or higher and 850 ° C. or lower. If it is less than 500 ° C., recrystallization after winding is insufficient, and ductility after cold-rolled sheet annealing may be lowered, which is not preferable. When it winds up above 850 degreeC, a particle size will become large and rough skin may generate | occur | produce at the time of press work. Accordingly, the winding temperature is preferably in the range of 500 to 850 ° C.

  Then, hot-rolled sheet annealing is performed for 5 seconds to 15 minutes at a temperature of 880 to 1000 ° C. that is a two-phase region temperature of a ferrite phase and an austenite phase.

  Hot-rolled sheet annealing is an important process for the present invention to obtain predetermined surface properties and formability. If the hot-rolled sheet annealing temperature is less than 880 ° C., sufficient recrystallization does not occur and the ferrite single-phase region is formed, so that the effects of the present invention that are manifested by two-phase region annealing may not be obtained. However, if the annealing temperature exceeds 1000 ° C, solid solution of the carbide is promoted, so C concentration in the austenite phase is promoted, and an extremely hard martensite phase is generated after hot-rolled sheet annealing. The property cannot be obtained.

  Moreover, when the hot-rolled sheet annealing temperature exceeds 1000 ° C., the amount of austenite phase produced decreases. Therefore, the amount of martensite phase formed after hot-rolled sheet annealing is reduced, and the metal due to the concentration of rolling strain on the ferrite phase in the vicinity of the martensite phase by cold rolling the metal structure containing the ferrite phase and martensite phase. The effect of relaxing the anisotropic structure cannot be sufficiently obtained, and the predetermined | Δr | cannot be obtained.

  When the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, generation of austenite phase and recrystallization of the ferrite phase do not occur sufficiently, so that desired formability cannot be obtained. On the other hand, when the annealing time exceeds 15 minutes, a part of (Cr, V, Ti, Nb) (C, N) is dissolved and C concentration in the austenite phase is promoted. A predetermined surface texture cannot be obtained.

  Further, if the annealing time exceeds 15 minutes, excessive C concentration occurs in the martensite phase generated by transformation of the austenite phase after hot-rolled sheet annealing by the above mechanism. This martensite phase decomposes into carbide and ferrite phases during cold-rolled sheet annealing, but if the C concentration is excessively large, the martensite phase changes to a ferrite phase containing a large amount of carbides. This results in a mixed grain structure of ferrite grains with few carbides in the grains and on the grain boundaries after cold-rolled sheet annealing and ferrite grains with too much carbides in the grains and on the grain boundaries. In such a metal structure, a difference in hardness occurs between grains with few carbides and grains with many carbides, so deformation strain concentrates at the interface between both grains, and large voids are likely to occur starting from carbides on the grain boundaries. Thus, ductility is reduced.

  Therefore, hot-rolled sheet annealing is held at a temperature of 880 to 1000 ° C. for 5 seconds to 15 minutes. Preferably, it is held at a temperature of 900 to 1000 ° C. for 15 seconds to 15 minutes. More preferably, it is held at a temperature of 900 to 1000 ° C. for 15 seconds to 3 minutes.

  Next, cold rolling and cold rolled sheet annealing are performed. Pickle the product as necessary to make a product.

  Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoints of formability and shape correction. In the present invention, cold rolling and annealing may be repeated twice or more, and a stainless foil having a thickness of 200 μm or less may be formed by cold rolling.

  Cold-rolled sheet annealing is performed at a temperature of 800 to 950 ° C. for 5 seconds to 5 minutes in order to obtain good formability.

Cold-rolled sheet annealing is an important process for making a two-phase structure of a ferrite phase and a martensite phase formed by hot-rolled sheet annealing into a ferrite single-phase structure. If the cold-rolled sheet annealing temperature is less than 800 ° C., sufficient recrystallization does not occur and the predetermined ductility and average r value cannot be obtained. On the other hand, when the cold-rolled sheet annealing temperature exceeds 950 ° C, the steel component becomes hard because the martensite phase is formed after the cold-rolled sheet annealing in the steel component in which the temperature is a two-phase temperature range of the ferrite phase and the austenite phase. A predetermined ductility cannot be obtained. Moreover, even if the temperature is a steel component in the ferrite single-phase temperature range, the glossiness of the steel sheet is lowered due to marked coarsening of crystal grains, which is not preferable from the viewpoint of surface quality. When the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, the ferrite phase is not sufficiently recrystallized, so that the predetermined ductility and average r value cannot be obtained. If the annealing time exceeds 5 minutes, the crystal grains become extremely coarse and the glossiness of the steel sheet is lowered, which is not preferable from the viewpoint of surface quality. Therefore, cold-rolled sheet annealing is held in the range of 800 to 950 ° C. for 5 seconds to 5 minutes. Preferably, it is held at 850 ° C. to 900 ° C. for 15 seconds to 3 minutes. In order to obtain more gloss, BA annealing (bright annealing) may be performed.
In order to further improve the surface properties, grinding or polishing may be performed.

Hereinafter, the present invention will be described in detail with reference to examples.
Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 h and then hot rolled to form 3.5 mm thick hot rolled sheets. Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing under the conditions shown in Table 2, and then the surfaces were descaled by shot blasting and pickling. The pickling was performed by immersing in a solution of 20 mass% sulfuric acid at a temperature of 80 ° C for 120 seconds, and then immersed in a mixed acid solution consisting of 15 mass% nitric acid and 3 mass% hydrofluoric acid at a temperature of 55 ° C for 60 seconds. Furthermore, after cold-rolled sheet annealing was performed under the conditions shown in Table 2 to a thickness of 0.7 mm by cold rolling, electrolysis under conditions of 25 C / dm ^{2 in} a water temperature of 80 ° C. and an 18 mass% Na ₂ SO ₄ aqueous solution. Pickling and descaling treatment by electrolytic pickling under conditions of 30 C / dm ² in a 10 mass% HNO ₃ aqueous solution at a water temperature of 50 ° C. were performed to obtain a cold rolled pickling annealed plate.

  The following evaluation was performed about the cold-rolled pickling annealing board obtained in this way.

(1) Evaluation of surface properties After cold-rolled sheet annealing, the number of linear wrinkles having a length of 5 mm or more present per 1 m ^{2 of} steel sheet was measured. The case where the number of linear wrinkles recognized on the surface of the cold-rolled annealed plate was 5 or less per 1 m ^{2 of the} steel plate was accepted, and the case where it was more than 5 was rejected.

(2) Evaluation of ductility From a cold-rolled pickled and annealed sheet, a JIS 13B tensile test piece was taken at right angles to the rolling direction, the tensile test was performed in accordance with JIS Z2241, the elongation at break was measured, and the elongation at break was 25 The case of% or more was regarded as pass (○), and the case of less than 25% was rejected (×).

(3) Evaluation of average r value and | Δr | From the cold-rolled pickled and annealed sheet, JIS 13B in a direction parallel to the rolling direction (L direction), 45 ° (D direction) and perpendicular to C direction (C direction) Tensile test specimens were collected, the tensile test according to JIS Z2241 was interrupted to a strain of 15%, the r value in each direction was measured, and the average r value (= (r _L + 2r _D + r _C ) / 4) and The absolute value (| Δr |) of the in-plane anisotropy (Δr = (r _L −2r _D + r _C ) / 2) of the r value was calculated. Here, r _L , r _D , and r _C are r values in the L direction, the D direction, and the C direction, respectively. As for the average r value, 0.65 or more was regarded as acceptable (◯), and less than 0.65 was regarded as unacceptable (x). For | Δr |, 0.30 or less was accepted (◯), and more than 0.30 was rejected (x).

(4) Evaluation of corrosion resistance A 60mm x 100mm test piece was collected from the cold rolled pickled and annealed plate, and the test piece was prepared by polishing the surface with # 600 emery paper and sealing the end face. Subjected to the prescribed salt spray cycle test. The salt spray cycle test consists of salt spray (35 ℃, 5% NaCl, spray 2h) → drying (60 ℃, relative humidity 40%, 4h) → wet (50 ℃, relative humidity ≧ 95%, 2h) as one cycle. 3 cycles were performed.
Photograph the surface of the test piece after 3 cycles of salt spray cycle test, measure the rusting area on the surface of the test piece by image analysis, and calculate the rusting area ratio ((in the test piece Rust area / total area of test piece) × 100 [%]) was calculated. Rust area ratio of 10% or less passed with excellent corrosion resistance (◎), more than 10% passed 25% or less (○), more than 25% rejected (×) The results are shown in Table 2 together with the cold rolling annealing conditions.

In Invention Examples Nos. 1 to 23, 33 to 46, and 52 to 63 satisfying the scope of the present invention, the number of linear wrinkles recognized after cold-rolled sheet annealing is 5 or less per 1 m ² and has good surface properties. Obtained. Further, good moldability was obtained with a breaking elongation of 25% or more, an average r value of 0.65 or more, and | Δr | of 0.30 or less. Furthermore, regarding corrosion resistance, the rusting area ratio on the surface of the test piece after performing the salt spray cycle test for 3 cycles was 25% or less, and good characteristics were obtained.

  In particular, in steels L, M, N and BM (No. 17, 18, 19, 52, 61) containing Cu, Ni and Mo, the rusting area ratio after the salt spray cycle test is 10% or less. Corrosion resistance has been further improved.

  On the other hand, in Comparative Example No. 24 in which the V content is below the range of the present invention and does not satisfy V / (Ti + Nb) ≧ 2.0, and Comparative Example No. 26 in which Ti and Nb exceed the range of the present invention, Due to insufficient precipitation of (Cr, V, Ti, Nb) (C, N), solid solution C and N were not sufficiently fixed during hot-rolled sheet annealing. Martensite phase was generated, and a large amount of linear flaws were generated after cold-rolled sheet annealing.

  In Comparative Example No. 25 in which the V content exceeds the range of the present invention, the predetermined average r value and | Δr | were obtained, but the predetermined ductility was obtained because the steel sheet was hardened by excessive V content. I could not.

  In Comparative Example No. 27 where the Cr content is below the range of the present invention, the predetermined surface properties and ductility, the average r value and | Δr | were obtained, but the predetermined corrosion resistance was obtained because the Cr content was insufficient. There wasn't.

  In Comparative Example No. 28 in which the Cr content exceeds the range of the present invention, sufficient corrosion resistance was obtained, but since an excessive amount of Cr was contained, an austenite phase was not generated during hot-rolled sheet annealing, a predetermined ductility, average The r value and | Δr | could not be obtained.

  In Comparative Example No. 29, in which the amount of C exceeds the range of the present invention, the amounts of V, Ti and Nb were within the range of the present invention, but C in the steel was changed to (Cr, V, Ti, Nb) (C, Since N was not sufficiently fixed as N) and solid solution C remained, an extremely hard martensite phase was formed after hot-rolled sheet annealing, and the predetermined surface properties were not obtained. Moreover, since the amount of solute C increased, the strength of the steel sheet increased remarkably, and the predetermined ductility was not obtained.

  On the other hand, in Comparative Example No. 30 where the amount of C is below the range of the present invention, since the austenite phase was not sufficiently stabilized by C, a sufficient amount of austenite phase during hot-rolled sheet annealing in the two-phase region. Was not generated, and the predetermined average r value and | Δr | were not obtained.

  In Comparative Examples No. 31 and No. 32 where V / (Ti + Nb) is below the range of the present invention, precipitation of (Cr, V, Ti, Nb) (C, N) during hot rolling is not sufficient. As a result, a large amount of coarse Cr carbonitride precipitates, and a remarkably hard martensite phase is generated after hot-rolled sheet annealing. There wasn't.

  No. 47 and No. 64 are comparative examples in which V / (Ti + Nb) is below the range of the present invention and the hot-rolled sheet annealing temperature is higher than the range of the present invention. Since V / (Ti + Nb) is below the range of the present invention, C concentration in the austenite phase accompanying the solid solution of coarse carbides precipitated during hot rolling is promoted, and extremely hard after hot-rolled sheet annealing. Since the martensite phase was generated, a large amount of linear wrinkles was generated, and the predetermined surface properties could not be obtained. Furthermore, since the hot-rolled sheet annealing temperature was higher than the range of the present invention, the amount of austenite phase generated during annealing decreased, and the amount of martensite phase generated after hot-rolled sheet annealing decreased. The anisotropic relaxation effect of the metal structure could not be obtained, and the predetermined | Δr | was not obtained.

  No. 48 and No. 65 are comparative examples in which V / (Ti + Nb) is below the range of the present invention and the hot-rolled sheet annealing temperature is lower than the range of the present invention. V / (Ti + Nb) is below the range of the present invention, but the hot-rolled sheet annealing temperature was in the ferrite single-phase temperature range and the austenite phase was not generated, resulting in the formation of a significantly hard martensite phase. There was almost no occurrence of linear wrinkles, and good surface properties were obtained. However, since the hot-rolled sheet annealing temperature was lower than the range of the present invention, sufficient recrystallization did not occur, and no martensite phase was formed after the hot-rolled sheet annealing, so that the predetermined ductility, average r value and | Δr | was not obtained.

  No. 66 is a comparative example in which V / (Ti + Nb) is below the range of the present invention and the hot-rolled sheet annealing time is longer than the range of the present invention. Therefore, as a result of excessive C concentration in the austenite phase accompanying the solid solution of coarse carbides precipitated during hot rolling, a remarkably hard martensite phase was generated after hot-rolled sheet annealing, resulting in linear flaws. It was generated in a large amount and a predetermined surface property could not be obtained. Furthermore, because the metal structure after cold-rolled sheet annealing was a mixed grain structure consisting of ferrite crystal grains with excessive carbide in grains and on grain boundaries, and ferrite grains with few carbides on grain boundaries and grain boundaries. During tensile deformation, local strain concentration occurred at the interface between the two crystal grains, and the predetermined ductility was not obtained.

    No. 67 is a comparative example in which V / (Ti + Nb) is below the range of the present invention and the cold-rolled sheet annealing temperature is lower than the range of the present invention. Since V / (Ti + Nb) was below the range of the present invention, a large amount of linear wrinkles occurred, and the predetermined surface properties could not be obtained. Furthermore, because the cold-rolled sheet annealing temperature was lower than the range of the present invention, the recrystallization in the cold-rolled sheet annealing was insufficient and the work structure at the time of cold rolling remained, so the predetermined ductility and average r value could not be obtained. .

  No. 68 is a comparative example in which V / (Ti + Nb) is below the range of the present invention and the cold-rolled sheet annealing temperature is higher than the range of the present invention. Since V / (Ti + Nb) was below the range of the present invention, a large amount of linear wrinkles occurred and the predetermined surface properties could not be obtained. Furthermore, because the cold-rolled sheet annealing temperature was higher than the range of the present invention, it became the annealing in the two-phase temperature range of the ferrite phase and the austenite phase, so the austenite phase was generated again, and after the cold-rolled sheet annealing, the martensite phase Due to the transformation, the steel sheet was remarkably hardened and the predetermined ductility was not obtained.

The ferritic stainless steel obtained by the present invention is particularly suitable for press-molded products mainly composed of a drawing and applications requiring high surface beauty, such as kitchen utensils and tableware.

Claims (5)

  In mass%, C: 0.005-0.05%, Si: 0.02-0.50%, Mn: 0.05-1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5-18.0%, Al: 0.001-0.10% , N: 0.01 to 0.06%, V: 0.01 to 0.25%, Ti: 0.001 to 0.020%, Nb: 0.001 to 0.030%, the balance is Fe and inevitable impurities, and V / (Ti + Nb) ≧ 2.0 Satisfies ferritic stainless steel.   In mass%, C: 0.01-0.05%, Si: 0.02-0.50%, Mn: 0.2-1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 16.0-18.0%, Al: 0.001-0.10% , N: 0.01 to 0.06%, V: 0.01 to 0.25%, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.025%, the balance is Fe and inevitable impurities, and V / (Ti + Nb) ≧ 2.0 Satisfies ferritic stainless steel.   2. The composition according to claim 1, further comprising one or more selected from Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 0.5%, and Co: 0.01 to 0.5%. 2. Ferritic stainless steel according to 2.   The composition further comprises one or more selected from the group consisting of Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050%, REM: 0.01 to 0.10%, Ca: 0.0002 to 0.0020%. The ferritic stainless steel as described in any one of -3.   The steel slab having the component composition according to any one of claims 1 to 4 is hot-rolled and then annealed in a temperature range of 880 to 1000 ° C for 5 seconds to 15 minutes to perform hot rolling. A method for producing a ferritic stainless steel, which is annealed and then cold-rolled and then cold-rolled sheet annealed at 800 to 950 ° C. for 5 seconds to 5 minutes.