JP3872067B2 - Ferritic stainless steel sheet with excellent ridging resistance, formability and secondary work brittleness resistance and method for producing the same - Google Patents

Description

  The present invention relates to a ferritic stainless steel sheet that maintains a beautiful surface without press formability and generation of ridging after processing, and further has excellent secondary work brittleness resistance.

Ferritic stainless steel represented by SUS430 has good workability and corrosion resistance and is relatively inexpensive, and is therefore used in a wide range of fields such as kitchen equipment, electrical products, and automotive materials. However, in recent years, when press-molding ferritic stainless steel sheets, more severe processing is often performed, and a high degree of press formability is required.
In addition, when cold working such as deep drawing and bending is performed on a steel plate produced by rolling a continuous cast piece of ferritic stainless steel, striped undulations called ridging occur along the rolling direction, Product appearance may be significantly impaired. For this reason, it is requested | required that the generation | occurrence | production of a ridging is not conspicuous about the molded article which the beautiful external appearance is requested | required.

Furthermore, molded products formed by subjecting ferritic stainless steel plates to advanced deep drawing and other press processing are subjected to secondary processing such as when a shocking load such as dropping is applied, or further hole expansion and overhanging processing. When trying to apply, it has the problem that it is brittle and easily broken. For this reason, ferritic stainless steel sheets are also required to have excellent secondary work brittleness resistance.
As described above, ferritic stainless steel sheets have excellent press formability capable of advanced processing during press forming, the occurrence of ridging is not noticeable in the processed molded product, and resistance to the molded product after processing. It is anxious to be excellent in secondary processing brittleness.

By the way, ridging that is likely to occur when ferritic stainless steel sheets are press-formed is a plurality of crystal grains with similar crystal orientations because the coarse columnar crystal structure generated during continuous casting is not sufficiently destroyed in the hot rolling process and cold rolling process. It is thought that there is a cause in forming a colony that is a collection of crystal grains.
As a method for suppressing the generation of ridging after the hot rolling step, a method of dividing colonies by repeating cold rolling and annealing a plurality of times to promote recrystallization is effective. However, although the method of fragmenting colonies by repeated cold rolling and annealing is effective in suppressing ridging, it requires multiple cold rolling and annealing processes, increasing the manufacturing cost. I will let you. Therefore, it is not desirable when mass production of inexpensive ferritic steel types is desired. Moreover, it is not easy to completely eliminate the influence of colonies even by repeated cold rolling and annealing.

For this reason, when manufacturing a slab by a continuous casting method, the melting and casting method which increases an equiaxed crystal ratio is developed so that a coarse columnar crystal structure may not develop in a slab. For example, there are a method of casting while maintaining the molten steel temperature at a relatively low temperature, a method of casting the molten steel with electromagnetic stirring, and the like.
However, the low temperature casting method requires casting by lowering the casting temperature to near the solidification temperature of the molten steel, and troubles such as nozzle clogging are likely to occur during operation, and the productivity is remarkably deteriorated. The electromagnetic stirring method is effective for equiaxed crystallization of a solidified structure, but requires expensive equipment, and macrosegregation such as a white band tends to occur depending on the electromagnetic stirring conditions.
Therefore, in order to improve the ridging resistance at a high level, a cold rolling / annealing process having a certain load is inevitably used in combination, resulting in an increase in manufacturing cost.

Recently, a technique has been reported in which Ti is added to ferritic stainless steel and TiN produced in molten steel is used as a nucleation site for ferrite to refine the solidification structure and make it equiaxed.
For example, in Patent Document 1, TiN is contained in a component system adjusted to satisfy Ti × N ≧ 0.0006, Ti ≧ 6 × (C + N) in a range of 0.01 to 0.5% Ti. A method of controlling the number of Mg-containing oxides satisfying the Mg / Ca mass ratio of 0.5 or more has been proposed. Patent Document 2 proposes controlling the number of Mg-based inclusions covered with TiN.
However, even if the components are adjusted according to the techniques of Patent Documents 1 and 2 and the number of Mg-based inclusions is controlled, the effect of increasing the equiaxed crystal ratio is not stable. In addition, TiN used for the nucleation site of equiaxed crystals causes problems such as nozzle clogging at the time of continuous casting, and the origin of surface flaws and work cracks in cold-rolled sheets.

On the other hand, it is necessary to improve the press formability of the ferritic stainless steel sheet.
Many proposals have been made to improve the press formability of ferritic stainless steel sheets. A typical example is a single addition technique or a composite addition technique of Ti and Nb. Ti and Nb added individually or in combination can improve press workability by precipitating C and N dissolved in the matrix as carbonitrides and reducing the C and N concentration of the matrix.

However, with the advancement of press molding, in recent years, further improvement in press moldability has been demanded. And it is calculated | required to have deep drawability of 2.3 or more by a limit drawing ratio. Furthermore, the technique of simply adding Ti or Nb alone or in combination as described above has not been studied at all for means for avoiding ridging and improving secondary work brittleness resistance.
Regarding the secondary work embrittlement resistance of a low C, N ferritic stainless steel sheet in which press formability is improved by adding Ti or Nb, Patent Document 3 proposes a technique of adding Ti and B.
However, the inclusion of B tends to cause a negative effect that the elongation and the Rankford value r are lowered.

JP 2001-288542 A JP 2000-192199 A Japanese Patent Publication No.2-7391

As described above, when performing advanced press forming on a ferritic stainless steel sheet, three properties of press formability, ridging resistance and secondary work brittleness resistance are required. A ferritic stainless steel sheet that is lightly generated and excellent in secondary work brittleness resistance has not yet been provided.
Therefore, the present invention has been devised to solve such problems, and by controlling the form of TiN and other inclusions / precipitates that are dispersed and precipitated in the steel, the equiaxed crystal ratio is improved. An object of the present invention is to provide a ferritic stainless steel sheet that improves ridging resistance and also improves press formability and secondary work brittleness resistance.

In order to achieve the object, the ferritic stainless steel sheet of the present invention has C: 0.05% by mass or less, Si: 0.01 to 2.0% by mass, Mn: 2.0% by mass or less, S: 0.00%. 0001 to 0.007 mass%, Cr: 9.0 to 40.0 mass%, Nb: 0.05 to 0.8 mass%, B: 0.0001 to 0.03 mass%, Al: 0.05 mass% %, Mg: 0.0001 to 0.001 mass%, Ca: 0.001 mass% or less, and the concentration product of Ti and N (Ti mass% × N mass%) is 0.0007 to 0.004. And the balance is composed of Fe and inevitable impurities, and the following layers (a), (b), and (c) are formed from the inner layers (a), (b), ( c) TiN-based inclusions having a three-layer structure laminated in the order of 10 / mm ² or more in the cross section of the steel sheet It exists in the above frequency.
(A) Layer: inclusion comprising oxide and sulfide satisfying Mg / S ≧ 5 by mass ratio (b) Layer: TiN
(C) Layer: Nb (C + N) and / or TiC and / or TiS

As steel components, Zr: 0.5 mass% or less, V: 0.5 mass% or less, REM: 0.1 mass% or less, Mo: 2.0 mass% or less, Cu: 2.0 mass% or less 1 type or 2 types or more may be included.
In the ferritic stainless steel sheet in which the precipitate having the three-layer structure as described above appears, a refractory having a MgO content of 40% by mass or more is used as a refining vessel in the steelmaking stage of the steel having the above composition. % Is used, and in the cold rolling process, annealing is performed at 880 to 980 ° C. for 20 seconds or more before soaking.

In the present invention, the content of each element such as Al, Si, Mn, Mg, Ti, and N is finely defined, and the cross-sectional structure and dispersion state of the generated TiN inclusions are also finely defined. Ferritic stainless steel sheet with excellent workability, workability and secondary workability, and ridging resistance that does not cause saddle-like undulations even when cold working such as deep drawing and bending is performed An excellent material can be obtained.
For this reason, it is possible to provide a low-cost plate material in a wide range of fields as a material for kitchen equipment, various electric equipments, and automobiles having an excellent appearance.

Based on the knowledge obtained by investigating and researching TiN that is effective as a nucleation site for equiaxed crystals, the inventors have increased the equiaxed crystal ratio by controlling the inclusion composition in a more reliable method, In addition, by controlling the annealing conditions in the cold rolling process, TiN inclusions having a three-layer structure are generated, and by controlling the number of them, the press formability and ridging resistance are excellent, and secondary processing resistance A ferritic stainless steel sheet having excellent brittleness can be obtained.
Details will be described below.

  Generally, it is said that equiaxed crystals increase when TiN inclusions are present when ferritic stainless steel melts. Factors affecting the increase in equiaxed crystallinity by TiN inclusions are: (1) TiN is easily wetted by molten steel, and (2) Ferrite is easily crystallized with TiN as the nucleus, which has a small degree of crystal lattice mismatch. (3) Since TiN is generated near the liquidus temperature of molten steel, it is difficult to float and separate. Therefore, when conditions for TiN generation near the liquidus temperature of molten steel are prepared, it effectively acts as a nucleation site for ferrite before the generated TiN floats and separates as inclusions, thereby increasing the equiaxed crystallinity. Can be said.

By the way, TiN-based inclusions are roughly classified into a single type (a in FIG. 1) generated by TiN alone and a composite type (b in FIG. 1) generated using inclusions such as oxides and sulfides as nuclei. . According to “Iron and Steel”, 87 (2001), 707, the effect on the formation of equiaxed crystals varies depending on the type of oxide that forms the core of TiN, and the nuclei of MgO · Al ₂ O ₃ are highly equiaxed crystals. It is said that it is preferable for efficiency.
However, in industrial-based production, the influence of refractories, slag, deoxidation products derived from deoxidizers, etc. is large, and it is difficult to produce pure MgO · Al ₂ O ₃ as inclusions in molten steel. .

Therefore, as a result of investigating and examining the inclusions that become the core of TiN in detail, when the molten steel components satisfy specific conditions, inclusions that are likely to become the core of TiN are generated even in industrial-based production. It has been found that a higher equiaxed crystal ratio can be achieved.
In this component system, oxides and sulfides having a mass ratio of Mg / S: 5 or more, which are the cores of TiN, are generated by quantitatively regulating Al, Mg, Ca, and S. As shown in FIG. 2, (b) TiN layer 2 and (c) Nb around (a) inclusion 1 made of oxide and sulfide satisfying Mg / S ≧ 5 in mass ratio as a nucleus. A layer 3 of (C + N) and / or TiC and / or TiS is sequentially deposited. Of these, the TiN of the (b) layer produced with the layer (a) as a nucleus is produced in the solidification stage, effectively acting as a nucleation site for equiaxed crystals, and increasing the equiaxed crystal ratio.

(A) In order to produce oxides and sulfide inclusions satisfying Mg / S ≧ 5 in the mass ratio of layers, Mg: 0.0001 to 0.001 mass%, Al: 0.05 mass% Hereinafter, regulation of Ca: 0.001 mass% or less and S: 0.0001 to 0.007 mass% is necessary. When Mg is insufficient and Al, Ca, and S are excessive, the generated inclusions are unlikely to become TiN nuclei in layer (b) and cannot effectively act as equiaxed crystal production sites. The rate is lowered.
Note that TiN up to (b) layer with oxide / sulfide inclusions as the core in (a) Mg / S mass ratio of less than 5 may also be observed, but this is It is thought that it was not formed but formed after solidification, and is not considered to function effectively as a nucleation site.

Also, even when TiN is produced up to the layer (b) having oxide / sulfide inclusions as the core with a mass ratio of Mg / S of 5 or more, if there is no more than a predetermined number, it is effectively higher Axial crystallisation cannot be achieved. When the conditions necessary for increasing the equiaxed crystal ratio were examined, as a component condition of Ti and N, when the concentration product of Ti and N (Ti mass% × N mass%) was 0.0007 or more, (a) (B) TiN of the layer having a Mg / S mass ratio of 5 or more and sulfide inclusions as the core is dispersed at a rate of 10 pieces / mm ² or more in the steel cross section, and equiaxed crystals are formed. Increased.

Furthermore, in addition to the above components, Nb: 0.8% by mass or less, B: 0.0001 to 0.03% by mass, solidified around TiN up to (b) layer generated in the solidification stage In the subsequent cooling, hot rolling step, and annealing step, the Nb (C + N) and / or TiC and / or TiS layer of the (c) layer is deposited so as to surround it.
That is, in order from the inner layer (a) inclusions composed of oxides and sulfides satisfying Mg / S ≧ 5 by mass ratio, (b) TiN, (c) Nb (C + N) and / or TiC and / or TiS It has been found that TiN inclusions having a three-layer structure have an effect more than the improvement of ridging resistance due to the improvement of equiaxed crystallinity by the action of conventional single type TiN or composite type TiN.

  Furthermore, the present inventors have found that this three-layered TiN-based inclusion has a remarkable improvement effect in press formability mainly composed of deep drawability and secondary work brittleness resistance, which has not been achieved in the past. However, in order to effectively improve the press formability mainly composed of deep drawability and the resistance to secondary work brittleness, uniformly (c) around TiN up to the (b) layer formed in the solidification stage. It is necessary to deposit the Nb (C + N) and / or TiC and / or TiS layer of the layer. In the hot-rolled sheet after the hot-rolling step, TiN in a form in which only a part of the Nb (C + N) and / or TiC and / or TiS layer is deposited is also observed. Even if such a hot-rolled sheet is produced by a normal production method, ridging resistance can be improved, but the effect of improving the press formability and the secondary work brittleness resistance mainly with deep drawability is small.

  Therefore, in the cold rolling annealing step, by annealing at 880 to 980 ° C. for 20 seconds or more before finish rolling, uniformly around the TiN up to the layer (b) generated in the solidification stage (c) It has been found that when Nb (C + N) and / or TiC and / or TiS layers of the layer are deposited, an improvement effect of press formability mainly composed of ridging resistance and deep drawability and secondary work brittleness resistance can be obtained. .

  TiN-based inclusions having a three-layer structure are considered to act more effectively on the degree of strain accumulation in the cold rolling process than single-type or composite TiN-based inclusions. And it is thought that a colony is destroyed by accumulation of this distortion. Moreover, it is thought that the TiN-based inclusion having a three-layer structure develops a {111} texture that affects the cold-rolled texture and is effective for deep drawability. Furthermore, the Nb (C + N) and / or TiC and / or TiS layer of the (c) layer, which is the outermost layer of the TiN-based inclusion having a three-layer structure, relieves stress concentration during secondary processing, and is resistant to secondary It is thought to improve work brittleness.

In the stainless steel defined in the present invention, for example, a deoxidizer is added in a vacuum atmosphere or an inert atmosphere, and a slag mainly composed of a CaO—Al ₂ O ₃ system is brought into contact with the molten steel while a slag / metal is added. A refining method with stirring is adopted. The slag may contain a faux material such as CaF ₂ . As the deoxidizer, Al, Si, Mn, Ti, REM, or the like, or an alloy containing one or more of them is used.
As a refining vessel, refining is performed using a refractory having an MgO content of 40% by mass or more for 50% or more of the molten steel contact surface. Since a smelting vessel lined with a refractory containing MgO is used, MgO in the refractory is reduced by a deoxidizing agent and once dissolved in molten steel as metallic Mg. Then, Mg: 0.0001 to 0.001 mass%, and the metal Mg in the molten steel becomes a suitable inclusion formation source as a core of TiN.

The molten stainless steel is sufficiently stirred by gas stirring for 3 minutes or more in a refining vessel, Al: 0.05 mass% or less, Mg: 0.0001 to 0.001 mass%, Ca: 0.001 mass% or less, S : Including 0.0001 to 0.007 mass%, and the components are adjusted so that the concentration product of Ti and N (Ti mass% × N mass%) is in the range of 0.0007 to 0.004.
Although the component-adjusted stainless molten steel is continuously cast through a tundish, nozzle clogging is prevented by maintaining the superheat degree of the molten steel in the tundish at 30 to 70 ° C.
The continuously cast slab becomes a hot-rolled steel strip through a hot rolling process, but the hot-rolling process may be performed under conditions according to a conventional method. A cold-rolled annealed steel strip is manufactured by combining the hot-rolled steel strip with annealing and cold rolling. In the present invention, it is essential to perform annealing before finish rolling at 880-980 ° C. for 20 seconds or longer. And annealing and cold rolling other than this annealing are in accordance with conventional methods.

Next, the alloy components, contents, and the like of the ferritic stainless steel targeted by the present invention will be described in detail.
C: 0.05% by mass or less There is an effect of increasing the strength of the ferritic stainless steel. However, since it will degrade corrosion resistance and manufacturability if it is contained excessively, the C content is specified to be 0.05% by mass or less.

Si: 0.01-2.0 mass%
Although it is an alloy component that improves corrosion resistance and strength, if it is contained excessively, it causes deterioration of manufacturability, so the Si content is specified in the range of 0.01 to 2.0 mass%.
Mn: 2.0 mass% or less Although it is an alloy element that improves the corrosion resistance and strength in the same manner as Si, its effect deteriorates if it is contained in excess, so the upper limit of Mn content is specified to 2.0 mass% did.

S: 0.0001 to 0.007 mass%
This is an important component for producing TiN inclusions having a three-layer structure. (C) In order to generate TiS of the layer, 0.0001% by mass or more of S is required. However, when more than 0.007 mass% S is contained, Mg / S in the inclusions in the layer (a) becomes smaller than 5 and is in an effective form as an equiaxed nucleation site ( b) The required amount of TiN up to the layer cannot be generated. Therefore, the content of S is specified in the range of 0.0001 to 0.007% by mass.

Cr: 9-40 mass%
It is an important alloying component for maintaining the corrosion resistance of ferritic stainless steel, and it is required to contain 9% by mass or more. However, if a large amount of Cr exceeding 40% by mass is contained, productivity is deteriorated. For this reason, Cr content was prescribed | regulated in the range of 9-40 mass%.
Nb: 0.05 to 0.8 mass%
It is an element necessary for generating Nb (C + N) in the (c) layer of the TiN-based inclusion having a three-layer structure, and it is necessary to contain 0.05% by mass or more. However, an excessive content exceeding 0.8% by mass decreases the manufacturability and workability, so the Nb content is regulated to 0.8% by mass or less.

B: 0.0001 to 0.03 mass%
It is an element that is indirectly necessary for generating the TiS layer of the (c) layer of TiN-based inclusions having a three-layer structure, and inclusion of 0.0001% by mass or more suppresses grain boundary segregation of S ( c) TiS of layer is generated. Moreover, although it has the effect | action which improves the secondary workability of a product, excessive inclusion exceeding 0.03 mass% will cause a hot workability and a secondary workability to deteriorate conversely. For this reason, B content shall be the range of 0.0001-0.03 mass%.

Al: 0.05% by mass or less Al is a component that affects the form of the layer (a) serving as the nucleus of a TiN-based inclusion having a three-layer structure. In order to produce TiN up to the (b) layer in a form effective as a nucleation site for equiaxed crystals, the Al content needs to be regulated to 0.05% by mass or less. When the Al content exceeds 0.05% by mass, the mass ratio of Mg / S in the inclusion is less than 5, and it becomes difficult to generate TiN up to the required amount of the (b) layer.

Mg: 0.0001 to 0.001 mass%
It is a component that affects the form of the layer (a) which is the nucleus of the TiN-based inclusion having a three-layer structure. In order to produce TiN up to layer (b) in an effective form as an equiaxed nucleation site, 0.0001% by mass or more of Mg is required. When the Mg content exceeds 0.001% by mass, MgS is generated, and the mass ratio of Mg / S is less than 5, making it difficult to generate TiN up to the required amount of the (b) layer. Further, TiS cannot be generated in the layer (c). Therefore, the upper limit of the Mg content is regulated to 0.001% by mass.
By using a refractory with a MgO content of 40% by mass or more as 50% or more of the molten steel contact surface as a refining vessel, the dissolution of Mg from the refractory shall be adjusted to 0.001% by mass or less. Can do.

Ca: 0.001% by mass or less Ca is a component that affects the form of the layer (a) serving as the nucleus of the TiN-based inclusion having a three-layer structure. In order to generate TiN up to the layer (b) in an effective form as an equiaxed nucleation site, 0.001% by mass or more of Ca is necessary. If the Ca content exceeds 0.001% by mass, the Mg / S mass ratio is less than 5, making it difficult to produce TiN up to the required amount of the (b) layer.

Concentration product of Ti and N (Ti mass% × N mass%): 0.0007 to 0.004
Ti is an alloy component necessary for producing TiN up to the (b) layer produced during solidification in molten steel. In this component system, oxides and sulfide inclusions having a Mg / S mass ratio of 5 or less in the layer (a), which is the core of TiN due to restrictions on the content of Al, Mg, Ca, and S, are dispersed in the molten steel. Therefore, TiN up to the layer (b) is generated using this inclusion as a nucleus. In order to generate TiN up to the (b) layer that is effective as a nucleation site for equiaxed crystals, the concentration product of Ti and N (Ti mass% × N mass%) is adjusted to 0.0007 or more. There is a need. However, if excessive Ti and N are contained, TiN becomes a cluster and causes nozzle clogging and surface flaws during casting, so the upper limit of the concentration product of Ti and N is set to 0.004. .

Zr: 0.5 mass% or less An alloy element added as necessary. Fixes C and N in steel to form precipitates, and has the effect of improving corrosion resistance and workability. However, since excessive addition exceeding 0.5 mass% will reduce manufacturability and workability, also when adding, it shall be 0.5 mass% or less.
V: 0.5 mass% or less An alloy element added as necessary. Fixes C and N in steel to form precipitates, and has the effect of improving corrosion resistance and workability. However, since excessive addition exceeding 0.5 mass% will reduce manufacturability and workability, also when adding, it shall be 0.5 mass% or less.
REM (rare earth element): 0.1% by mass or less An alloy element added as necessary. Has the effect of improving hot workability. However, excessive addition exceeding 0.1 mass% conversely reduces hot workability, so even when REM (rare earth element) is added, the content is made 0.1 mass% or less.

Mo: 2.0% by mass or less Mo is an alloy element added as necessary. Has the effect of improving corrosion resistance and strength. However, excessive addition exceeding 2.0% by mass decreases productivity and workability, so even when added, the content is made 2.0% by mass or less.
Cu: 2.0% by mass or less An alloy element added as necessary. Has the effect of improving corrosion resistance and strength. However, excessive addition exceeding 2.0% by mass decreases productivity and workability, so even when added, the content is made 2.0% by mass or less.

In addition to other alloy components, Y: 0.05% by mass or less, W: 1.0% by mass or less, Ag: 0.5% by mass or less, Sn: 0.5% by mass or less, Co: 1 You may add 1 type (s) or 2 or more types, such as 0.0 mass% or less and Zn: 0.5 mass% or less. As long as P contained as an impurity is regulated to 0.05% by mass or less, it does not affect the characteristics.

(A) layer of TiN-based inclusions having a three-layer structure: When dispersing TiN up to (b) layer effective as an nucleation site of an equiaxed crystal with a mass ratio of Mg / S of 5 or more , (A) It is weight to generate oxides and sulfide inclusions having a Mg / S mass ratio ≧ 5 as the core of the layer. If the Mg / S mass ratio of the inclusions is less than 5, high equiaxed crystallinity cannot be expected.
The mass ratio of Mg / S is determined by degassing with a deoxidizing agent, then stirring the gas for 3 minutes or more, adjusting the components after desulfurizing S, and setting Al, Mg, Ca, and S as specified components. This can be achieved by making it within the range.
Inclusions with Mg / S mass ratio ≧ 5 are oxides and sulfides containing Al, Ca, O, Si, Mn, Cr, etc. in addition to Mg and S, and the mass ratio of Mg / S. Other than the above, there are no restrictions on content. The composition of inclusions can be measured by quantitative analysis using, for example, an X-ray microanalyzer.

Number of TiN-based inclusions having a three-layer structure: 10 pieces / mm ² or more The influence of TiN-based inclusions having a three-layer structure on increasing the equiaxed crystal ratio is caused by the formation of TiN in the solidification stage (b) It depends on the number of inclusions formed. In addition, after solidification cooling, hot rolling, and cold rolling, (c) layer Nb (C + N) and / or TiC and / or TiS is formed, and finally becomes a TiN-based inclusion having a three-layer structure. The layer (c) is also effective in improving ridging, press formability, and secondary work brittleness. The improvement effect is, for example, that the number of TiN-based inclusions having a three-layer structure satisfying the conditions of the layers (a), (b) and (c) measured by quantitative analysis using an X-ray microanalyzer is 10 / mm ^2. It appears remarkably when it is finally dispersed in the steel cross section of the molded product at the above ratio.
In addition, even if the TiN of the layer (b) contains a small amount of C, O, S, etc., it has a three-layer structure TiN which affects ridging resistance, press formability mainly composed of deep drawing, and secondary work brittleness resistance. The effect of system inclusions remains the same. The layer (c) is mainly composed of Nb (C + N) and / or TiC and / or TiS, but may contain other carbonitride-forming elements.

The dispersion ratio of the TiN inclusion having a three-layer structure is such that after deoxidation in a vacuum atmosphere or an inert atmosphere, gas stirring is performed for 3 minutes or more at a flow rate of 100 to 500 NL / min, and the titanium or ferrometal It can be controlled by adding a Ti-containing alloy such as titanium. Increasing the concentration product of Ti and N, that is, increasing the addition amount of the Ti-containing alloy, or using an N ₂ -containing gas as the stirring gas increases the number of dispersed TiN-based inclusions.

Annealing before finish rolling in the cold rolling step: (880 to 980 ° C.) × soaking for 20 seconds or more The present invention is characterized by the formation of TiN-based inclusions having a three-layer structure. In order to effectively utilize TiN-based inclusions having a three-layer structure for improving ridging resistance, deep drawing mainly for press forming and secondary work brittleness resistance, (b) TiN up to layer (c) It is necessary to deposit the Nb (C + N) and / or TiC and / or TiS layers of the layer uniformly. For this purpose, it is necessary to perform annealing at 880 to 980 ° C. for 20 seconds or more in the cold rolling process. If the soaking temperature is less than 880 ° C., the (c) layer does not deposit uniformly on TiN up to the (b) layer, and if it exceeds 980 ° C., the (c) layer partially dissolves again. If the soaking time does not reach 20 seconds, the (c) layer is not uniformly deposited on the TiN up to the (b) layer.
This heat treatment for layer deposition (c) is performed at the time of annealing before finish rolling in order to improve the press formability mainly composed of deep drawability. In finish annealing in which a conventional continuous annealing pickling line is passed, the Nb (C + N) and / or TiC and / or TiS layer of the (c) layer is partially re-dissolved, but the TiN up to the (b) layer It does not dissolve completely from the surroundings.

Ferritic stainless steel (80 tons / charge) having the composition shown in Table 1 was melted through an electric furnace, converter, and VOD process, and continuously cast into a slab. In the vacuum refining of the VOD process, Ar was blown into the molten steel at a flow rate of 100 to 500 NL / min through a porous plug in a vacuum furnace maintained at a vacuum degree of 50 to 200 Pa. As shown in Table 2, the refining vessel used was a Magdro-type refractory containing 40% or more of MgO and Magcro-type refractory except for Steel No. 8, which was used for 50% or more of the entire molten steel contact surface. Steel No. 8 used was a magdro and magcro refractory containing 30% MgO used for 40% of the entire molten steel contact surface.
Several ratios of the thickness of the equiaxed crystal zone to the slab thickness of the test piece cut out from the obtained slab were measured, and the measured values were averaged to calculate the equiaxed crystal ratio. The average value of the calculation results is shown in Table 3 as the equiaxed crystal ratio.

Next, the slab was hot-rolled according to a conventional method to produce a hot-rolled sheet. Cold rolling and annealing were combined to produce a cold rolled annealed plate with a thickness of 0.8 mm.
Each cold-rolled annealed plate was evaluated for the dispersion ratio, ridging resistance, and secondary workability of the three-layered TiN inclusions in the following tests.

The shape of the TiN inclusions having a three-layer structure and a dispersed state of the specimens cut from the cold-rolled annealed plate are polished to a cross section parallel to the rolling direction and the plate thickness direction, and using an X-ray microanalyzer (a) The inclusions serving as the core of the layer and (c) layer were quantitatively analyzed. From the analysis results, (a) the layer is an inclusion having an Mg / S mass ratio of 5 or more, and (b) TiN of the layer and (c) uniform Nb (C + N) and / or TiC and / or Alternatively, the inclusion in which the TiS layer is generated is a TiN inclusion having a three-layer structure.
The analysis was performed on at least 10 total TiNs, and the ratio of TiN inclusions having a three-layer structure to the total TiNs was determined. Further, the number of dispersed inclusions was counted with an optical microscope, and the number of dispersed TiN per unit area was calculated. Then, the dispersion ratio of the TiN-based inclusions having the three-layer structure was calculated as the product of the ratio of the TiN-based inclusions having the three-layer structure and the number of dispersed TiNs per unit area.

After imparting 20% tensile deformation to a JIS No. 5 tensile test piece cut out from a ridging-resistant cold-rolled annealed plate, the ridging property was determined by visual observation of the surface of the test piece.
A case in which ridging did not occur or was very slight was determined as a decision 1, and was evaluated in 5 stages along with deterioration of ridging. A ridging level of 2 or less can be said to be a ridging level having no practical problem.

Limit drawing ratio In order to evaluate the substantial press formability, a cup forming test was conducted by changing the blank diameter in a forming tester, and the drawing ratio (LDR) of each cold-rolled annealed plate was obtained. The punch diameter was 40 mm, the punch R was 3 mm, the die diameter was 42 mm, and the die R was 5 mm. A stainless steel press oil was applied to the back surface of the cold-rolled annealed plate and tested.
Under these conditions, the limit drawing ratio of the conventional steel sheet was about 2.1, so that the limit drawing ratio of 2.3 or more was judged to be excellent press formability.

Secondary work brittleness resistance In the secondary work brittleness resistance test, a cup having a drawing ratio of 4 and a diameter of 15 mm was prepared by three-stage drawing. The prepared cup is cut at the ear and kept at -10 ° C, and then the cup is placed on a conical punch having a vertex of 5 degrees, and 1 kg of heavyweight is dropped from the height of 20 cm to expand the tube. A shocking strain was applied in the direction, and the presence or absence of brittle cracks appearing on the side wall of the cup was examined.
Impulsive strain is applied to 5 cups. If there are no cracks in all, ○, occurrence of minor cracks of 2 mm or less is △, and cracks of 2 mm or more are 1 When it occurred, it was judged as x. ○ was judged as a practically acceptable level.

Equiaxial crystal ratio, annealing temperature and soaking time of annealing before finish rolling, (a) per unit area of TiN-based inclusion having a three-layer structure in which the layer is an inclusion with a mass ratio of Mg / S of 5 or more Table 3 shows the evaluation results of the number and material characteristics.
Sample numbers 1A, 2A, and 2B where the composition is steel numbers 1 to 5 (examples of the present invention) within the scope of the present invention and the annealing conditions before finish cold rolling satisfy the conditions of a temperature of 880 to 980 ° C and a soaking rate of 20 seconds or more. , 3A, 4A, 5A, and 5B, the number of TiN-based inclusions having a three-layer structure in which (a) layer is an inclusion having a mass ratio of Mg / S of 5 or more is 10 pieces / mm ² or more, and cold rolling All evaluations of ridging judgment, limit drawing ratio and secondary work brittleness resistance of the annealed sheet were good.

On the other hand, even if the component composition is steel numbers 1 to 5 (invention examples) within the scope of the present invention, the annealing conditions before finish cold rolling were out of the specified range. Sample numbers 1B, 1C, 2C, In 2D, 2E, 3B, 3C, and 4B, the number of TiN inclusions having a three-layer structure as defined in the claims is insufficient, and a desired limit drawing ratio or the like cannot be obtained.
Steel No. 6 has a low B content and a high Ca content. Steel No. 7 has a high Al content and a low concentration product of Ti and N (Ti mass% × N mass%). Steel No. 8 had a low MgO content in the refractory used in the smelting vessel and a low MgO refractory usage rate on the molten steel contact surface, so the Mg content in the steel was low. Furthermore, Steel No. 9 had a low B content and a low concentration product of Ti and N (Ti mass% × N mass%). As described above, in steel that does not satisfy the component composition range specified in the present invention, even if annealing is performed before finish rolling under the specified conditions, (a) the layer has a mass ratio of Mg / S of 5 or more. The number of TiN inclusions having a three-layer structure, which is an object, has decreased. For this reason, the limit drawing ratio is small, and the evaluation of secondary work brittleness resistance is also poor. Moreover, since the equiaxed crystal ratio is low, the ridging judgment is also poor.

The figure explaining the section of single type TiN and the TiN inclusion of the two-layer structure The figure explaining the section of the TiN inclusion of the three-layer structure

Explanation of symbols

1: Oxide / sulfide inclusions 2: TiN 3: Nb (C + N), TiC, TiS

Claims (3)

C: 0.05 mass% or less, Si: 0.01-2.0 mass%, Mn: 2.0 mass% or less, S: 0.0001-0.007 mass%, Cr: 9.0-40. 0 mass%, Nb: 0.05 to 0.8 mass%, B: 0.0001 to 0.03 mass%, Al: 0.05 mass% or less, Mg: 0.0001 to 0.001 mass%, Ca : 0.001% by mass or less, and the concentration product of Ti and N (Ti mass% × N mass%) is adjusted to satisfy the relationship of 0.0007 to 0.004, with the balance being Fe and inevitable impurities And a TiN-based inclusion having a three-layer structure in which the following layers (a), (b), and (c) are laminated in the order of (a), (b), and (c): Is present at a frequency of not less than 10 pieces / mm ² in the cross section of the steel sheet, and ridging resistance, formability and Ferritic stainless steel plate with excellent secondary processing brittleness.
(A) Layer: inclusion comprising oxide and sulfide satisfying Mg / S ≧ 5 by mass ratio (b) Layer: TiN
(C) Layer: Nb (C + N) and / or TiC and / or TiS   As steel components, Zr: 0.5 mass% or less, V: 0.5 mass% or less, REM: 0.1 mass% or less, Mo: 2.0 mass% or less, Cu: 2.0 mass% or less The ferritic stainless steel sheet excellent in ridging resistance, formability, and secondary work brittleness resistance according to claim 1, wherein the ferritic stainless steel sheet is one or more kinds.   In the steelmaking stage of the steel having the composition according to claim 1 or 2, using a refractory having an MgO content of 40% by mass or more as a refining vessel for 50% or more of the molten steel contact surface, cold rolling The ferrite excellent in ridging resistance, formability and secondary work brittleness resistance according to claim 1 or 2, wherein annealing is performed at 880 to 980 ° C for 20 seconds or more before soaking in the process. Of manufacturing stainless steel sheet.

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