JP4824640B2 - Duplex stainless steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a duplex stainless steel and a method for producing the same, and particularly suitable as a material for a steel plate, a steel pipe, a bar wire, a cast product, a forged product, and the like used for a portion that requires high corrosion resistance even in a corrosive environment. The present invention relates to a duplex stainless steel excellent in corrosion resistance and hot workability and its inexpensive manufacturing method.

  Duplex stainless steel is a stainless steel having both the excellent mechanical properties and corrosion resistance of ferritic stainless steel and austenitic stainless steel, which are composed of a two-phase structure of a ferrite phase and an austenitic phase. Since this stainless steel is particularly excellent in corrosion resistance, it is widely used in fields in which highly corrosive gas is generated, such as petrochemical plants and seawater handling equipment, where corrosion of equipment is a problem.

The corrosion resistance of duplex stainless steel is generally expressed by the following formula (1):
Cr + 3.3 (Mo + 0.5W) + 16N (1)
It is known that the higher the value, the better the corrosion resistance. By designing the alloy so as to satisfy the above formula, it is said that a duplex stainless steel having excellent corrosion resistance can be obtained by the component composition of the steel itself. For example, Patent Document 1 discloses a high-strength duplex stainless steel with excellent corrosion resistance, in which the contents of Mo and W are increased so as to satisfy the above formula (1).

  As another technique for improving the corrosion resistance of the duplex stainless steel, for example, Patent Document 2 discloses that the corrosion resistance is improved by controlling the alloy components within an appropriate range and by optimizing the types of non-metallic inclusions. Patent Document 3 discloses a duplex stainless steel that has improved corrosion resistance by forming an austenite-enriched layer that absorbs nitrogen in the steel sheet surface layer in addition to properly designing the alloy. Has been.

  On the other hand, duplex stainless steel is also used as a piping member in chemical plants and the like, and therefore needs to be processed into a thin steel plate or the like. Such a thin steel sheet is generally a steel slab adjusted to a predetermined component composition, heated and then hot-rolled into a hot-rolled sheet as an intermediate product, and then annealed, pickled or cold-rolled as necessary. Manufactured by combining rolling and finish annealing. Here, when the workability of the steel in the above hot rolling is inferior, the plate breaks during the rolling or the surface defect occurs, resulting in an increase in cost due to a decrease in productivity or a decrease in yield, The quality will be reduced. Therefore, the duplex stainless steel needs to be excellent in hot workability.

Regarding the technology for improving the hot workability of duplex stainless steel, for example, in Patent Document 4, solid solution S is reduced, or solid solution S is fixed as sulfide by addition of Ca or REM, and further, an appropriate amount A technique for improving the hot workability by adding Cu is disclosed. Patent Document 5 discloses a technique for improving hot workability by adding Nd and fixing P since P also degrades hot workability as in S.
JP 05-132741 A JP 2004-149833 A JP 2004-353041 A Japanese Patent Laid-Open No. 06-330243 JP 2007-084837 A

  As can be seen from the above equation (1), the corrosion resistance of the duplex stainless steel becomes better as the contents of Cr, Mo, W and N are higher. However, since Cr, Mo, and W are expensive metals, increasing their content causes an increase in manufacturing cost. N is an element that can be obtained at low cost, but there is a limit to the amount of N that can be dissolved when it is dissolved under atmospheric pressure. Furthermore, special equipment such as pressure dissolution is required to dissolve the N concentration exceeding the equilibrium concentration under atmospheric pressure. Moreover, since the strength of the steel increases as the N concentration in the steel increases, it becomes difficult to process.

  Moreover, since the technique which forms the layer excellent in corrosion resistance in the steel plate surface layer needs to perform special heat processing, it raises the manufacturing cost accompanying the process increase, and prolongs the manufacturing days.

  Moreover, in order to improve hot workability, the technique of adding Ca or REM leads to an increase in raw material cost, and since CaO-containing inclusions are generated by the addition of Ca, there is a possibility that the corrosion resistance is lowered. Further, in order to reduce the S concentration, it is necessary to strengthen the deoxidation. However, if a large amount of Al, which is a deoxidizer, is added, the Ca concentration increases. There is.

  Therefore, the present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is more than that of the conventional duplex stainless steel even if the component composition of the conventional duplex stainless steel remains as it is. The object is to provide a duplex stainless steel having both excellent corrosion resistance (particularly pitting corrosion resistance) and excellent hot workability, and to propose an advantageous manufacturing method thereof.

  The inventors pay attention to nonmetallic inclusions (hereinafter sometimes simply referred to as “inclusions”) present in the duplex stainless steel, and the types of inclusions constituting all nonmetallic inclusions. A detailed investigation was conducted on the effect of the composition on the corrosion resistance. As a result, the CaO-containing inclusions contained in all non-metallic inclusions in the duplex stainless steel have a high CaO concentration, the number ratio of CaO-containing inclusions in all non-metallic inclusions is high, and all non-metallic inclusions. When the inclusion has a high CaO concentration, the Ca component is eluted from the CaO-containing inclusion into the aqueous solution to form a gap between the base metal and the inclusion, and pitting corrosion occurs from this gap. In order to improve the corrosion resistance and thus improve the pitting corrosion resistance, the Ca concentration contained in the steel is reduced to 0.0005 mass% or less, and the CaO concentration in individual CaO-containing inclusions is set to 40 mass% or less. In addition, the present inventors have found that the number ratio of CaO-containing inclusions in all nonmetallic inclusions needs to be 40% or less and the CaO concentration in all nonmetallic inclusions needs to be 10 mass% or less.

The inventors further studied in order to achieve improvement in hot workability in parallel with improvement in pitting corrosion resistance. As a result, the slag formed during refining of the duplex stainless steel is CaO—SiO ₂ —MgO— composed of CaO / Al ₂ O ₃ : 3 to 10, CaO / SiO ₂ : 3 to 15, MgO: 3 to 20 mass%. By making the Al ₂ O ₃ -F-based slag and controlling the Al concentration in the steel in the range of 0.001 to 0.05 mass%, the Ca concentration is 0.0005 mass% or less and the S concentration is 0.0015 mass. It has been found that the hot workability can be improved without deteriorating the corrosion resistance by setting the ratio to not more than%, and the present invention has been completed.

That is, the present invention is C: 0.030 mass% or less, Si: 0.01-1 mass%, Mn: 0.01-2 mass%, P: 0.040 mass% or less, S: 0.0015 mass% or less, Ni: 3 to 10 mass%, Cr: 20 to 28 mass%, Mo: 2 to 5 mass%, N: 0.05 to 0.40 mass%, Al: 0.001 to 0.05 mass%, Mg: 0.0001 to 0.0050 mass %, Ca: 0.0005 mass% or less, O: 0.0001 to 0.0050 mass%, the balance is a steel having a component composition consisting of Fe and inevitable impurities, and the nonmetallic inclusions contained in the steel are MgO. _{· Al 2 O 3, Al 2} O 3, MgO, MnO-Al 2 O 3 based oxide, CaO concentration 40 mass% or less of the _CaO-Al 2 _{O 3} based oxide Consists of one or more of the total number ratio of _CaO-Al 2 _{O 3} based oxide to non-metallic inclusions is 40% or less, CaO concentration in the total non-metallic inclusions is not more than 10 mass%, A duplex stainless steel characterized in that the pitting corrosion potential Vc ′ ₁₀ in a 20% NaCl aqueous solution at 60 ° C. is 600 mV (vs SCE) or more.

  In addition to the above component composition, the duplex stainless steel of the present invention may further include W: 0.01 to 1 mass%, Cu: 0.01 to 1 mass%, V: 0.01 to 1 mass%, Co: 0.01 to 1 mass%, Nb: 0.01 to 1 mass%, Ti: 0.01 to 1 mass%, and B: 0.001 to 0.010 mass%, or one or more types selected from .

Further, the present invention provides a Cr—Ni—Mo steel containing Cr: 20 to 30 mass%, Ni: 3 to 10 mass%, and Mo: 2 to 5 mass% in an electric furnace in producing the duplex stainless steel. When the steel is refined by any of the methods of AOD treatment, VOD treatment and AOD treatment, after decarburization, lime, fluorite, Al and ferrosilicon are added. _{, CaO / Al 2 O 3:} 3~10, CaO / SiO 2: 3~15, MgO: CaO-SiO 2 is _{3~20mass% -MgO-Al 2 O 3} -F -based slag is formed, Cr reduction A method for producing a duplex stainless steel characterized by deoxidation, desulfurization and component adjustment is proposed.

  The method for producing a duplex stainless steel according to the present invention is characterized in that the refined steel is continuously cast or normally ingoted into a steel slab.

  ADVANTAGE OF THE INVENTION According to this invention, the duplex stainless steel which is excellent in corrosion resistance, especially pitting corrosion resistance, and is excellent also in hot workability can be manufactured at low cost. Therefore, the duplex stainless steel of the present invention can be suitably used as a material such as a thin plate or a steel pipe used in an environment where the occurrence of pitting corrosion is a concern, such as a petrochemical plant or a seawater handling facility.

The component composition that the duplex stainless steel excellent in corrosion resistance and hot workability according to the present invention should have will be described.
C: 0.030 mass% or less Since C is an element that induces sensitization during welding and reduces corrosion resistance, the smaller the amount, the better. However, excessive reduction of C causes an increase in manufacturing cost as well as a decrease in strength. Therefore, in the present invention, the C content is set to 0.030 mass% or less. Preferably, it is 0.025 mass% or less.

Si: 0.01-1 mass%
Si is a component added as a deoxidizer, and it is necessary to add 0.01 mass% or more. However, excessive addition causes saturation of the effect, and leads to an increase in strength and a decrease in ductility, and also promotes the precipitation of intermetallic compounds such as σ phase and χ phase and decreases the corrosion resistance. Therefore, Si is 1 mass. % Or less. A preferable Si content is in the range of 0.01 to 0.5 mass%.

Mn: 0.01-2 mass%
Since Mn is an austenite-forming element, it is an effective component for adjusting the ratio of the austenite phase and the ferrite phase and improving the corrosion resistance and workability of the duplex stainless steel. In the present invention, 0.01 mass% or more Added. However, excessive addition of Mn promotes the precipitation of intermetallic compounds such as σ phase and χ phase and causes a decrease in corrosion resistance. Therefore, the upper limit of the Mn content is set to 2 mass%. A preferable Mn content is in the range of 0.05 to 1.5 mass%.

P: 0.040 mass% or less P is an element that is inevitably mixed in as an impurity, and is easily segregated at the crystal grain boundary and causes a decrease in corrosion resistance and hot workability. However, excessive reduction of the P content causes an increase in manufacturing cost, so the upper limit is made 0.040 mass%. Preferably it is 0.030 mass% or less.

S: 0.0015 mass% or less S, like P, is an element that is inevitably mixed, and is preferably segregated at the crystal grain boundary and decreases corrosion resistance and hot workability. In particular, if the S content exceeds 0.0015 mass%, the harmfulness to hot workability becomes significant, so the upper limit is set to 0.0015 mass%. Preferably it is 0.0010 mass% or less.

Ni: 3 to 10 mass%
Ni is an austenite-forming element, and in the stainless steel of the present invention containing a large amount of ferrite-forming elements such as Cr and Mo, it has a two-phase structure of ferrite phase / austenite phase and imparts high corrosion resistance to 3 mass% or more. Must be added. However, addition exceeding 10 mass% causes an excessive increase in austenite phase and a decrease in ferrite phase, making it difficult to maintain a two-phase structure. Therefore, the Ni content is 3 to 10 mass%. Preferably, it is in the range of 4-8 mass%.

Cr: 20-28 mass%
Since Cr is a ferrite-forming element and an element that improves corrosion resistance, it is positively added. In the present invention, in order to obtain the expected excellent corrosion resistance, it is necessary to contain 20 mass% or more. However, if it exceeds 28 mass%, the formation of intermetallic compounds such as σ phase and χ phase is promoted, and the corrosion resistance is reduced. In addition, the ferrite phase is excessively increased, and it becomes difficult to maintain a two-phase structure. Therefore, in this invention, Cr content shall be 20-28 mass%. Preferably it is the range of 21-27 mass%, More preferably, it is the range of 22-26 mass%.

Mo: 2-5 mass%
Mo is a useful element that improves the corrosion resistance. In order to acquire the effect, it is necessary to add 2 mass% or more. However, if added in excess of 5 mass%, precipitation of intermetallic compounds is promoted and, on the contrary, corrosion resistance and hot workability are reduced, so the upper limit is made 5 mass%. The Mo content is preferably 2.5 to 4.0 mass%.

N: 0.05-0.40 mass%
N is a powerful austenite-forming element and is a useful element that has an effect of improving corrosion resistance, so it is positively added in the present invention. If the N content is less than 0.05 mass%, the steel to which a large amount of Cr or Mo, which is a ferrite-forming element, is added is not sufficient to generate an austenite phase, and the effect of improving corrosion resistance cannot be expected. On the other hand, if the N content exceeds 0.40 mass%, the austenite phase increases too much, and in addition to deviating from the range of the duplex stainless steel, refining because N approaches the melting limit of molten steel. Since the time is remarkably long, the cost increases. Therefore, in the present invention, the upper limit of the N content is set to 0.40 mass%. The preferred N content is in the range of 0.10 to 0.30 mass%.

Al: 0.001 to 0.05 mass%
Al is an effective deoxidizing element and is a component necessary for controlling non-metallic inclusions generated in the steel to Al ₂ O ₃ or MgO · Al ₂ O ₃ . In addition, desulfurization is promoted by deoxidation, S is reduced, and hot workability is improved. In order to obtain these effects, it is necessary to add 0.001 mass% or more. However, if the Al addition amount exceeds 0.05 mass%, not only the deoxidation effect is saturated, but also precipitation of intermetallic compounds is promoted, or CaO in the slag is reduced to reduce the Ca concentration in the molten steel. It causes an increase, promotes the formation of CaO-containing inclusions, and causes a decrease in corrosion resistance. Therefore, in the present invention, the upper limit of Al is set to 0.05 mass%. A preferable Al content is in the range of 0.005 to 0.03 mass%.

Mg: 0.0001 to 0.0050 mass%
Mg is an element effective for improving hot workability, and is an element necessary for making the composition of inclusions in steel MgO.Al ₂ O ₃ and MgO. In order to obtain these effects, it is necessary to add 0.0001 mass% or more. However, if Mg is added excessively, the hot workability is rather deteriorated, so the upper limit of the Mg content is set to 0.0050 mass%.

Ca: 0.0005 mass% or less Ca is an element that generates CaO-containing inclusions, and in the present invention whose main purpose is to improve corrosion resistance, the smaller the Ca, the more preferable. However, in the refining process, Ca is mixed even when CaO in slag and refractory is reduced, and at present, adopting a manufacturing process using a refractory that does not contain CaO is substantially effective. Since it is difficult, mixing of Ca is inevitable. Therefore, in the present invention, the upper limit is 0.0005 mass%, which is an allowable mixing limit for obtaining a desired pitting corrosion potential.

O: 0.0001 to 0.0050 mass%
O reacts with Al, Mg, Ca, Mn and the like to generate oxide inclusions (non-metallic inclusions). In particular, when the O concentration exceeds 0.0050 mass%, a large number of oxide-based inclusions are generated in the steel, and this inclusion increases, resulting in deterioration of the corrosion resistance and hot workability of the steel. Thus, a desired pitting corrosion potential and a healthy hot-rolled plate cannot be obtained. On the other hand, when the O concentration is less than 0.0001 mass%, CaO inclusions are generated, leading to deterioration of corrosion resistance. Therefore, in the present invention, the lower limit of the O concentration is set to 0.0001 mass%.

In addition to the essential components described above, the following components can be added to the duplex stainless steel of the present invention as necessary.
W: 0.01 to 1 mass%, Cu: 0.01 to 1 mass%, V: 0.01 to 1 mass%, Co: 0.01 to 1 mass%, Nb: 0.01 to 1 mass%, and Ti: 0.01 One element or two or more elements selected from ˜1 mass% These elements are effective elements for improving the corrosion resistance, and in order to obtain the effect, each element is preferably added in an amount of 0.01 mass% or more. However, if added in excess of 1 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted, and in addition to deterioration of corrosion resistance, hot workability is deteriorated. Therefore, the upper limit of each is preferably set to 1 mass%. More preferably, it is the range of 0.05-0.5 mass%, respectively.

B: 0.001 to 0.010 mass%
B is an extremely effective element for improving hot workability. However, the effect is small if it is less than 0.001 mass%. On the other hand, if added excessively, hot workability is adversely affected. Therefore, in this invention, when adding B, it is preferable to make 0.010 mass% which is a range which does not inhibit hot workability into an upper limit. More preferably, it is the range of 0.001-0.005 mass%.

Non-metallic inclusions in steel The duplex stainless steel of the present invention controls the non-metallic inclusions present in the steel within an appropriate range in order to impart excellent corrosion resistance and hot workability. the non-metallic inclusions in _steel, of _{MgO · Al 2 O 3, Al} 2 O 3, MgO, MnO-Al 2 O 3 based oxide and CaO concentration 40 mass% or less of _CaO-Al 2 _{O 3} based oxide The number ratio of CaO—Al ₂ O ₃ -based oxides composed of any one or two or more of them is 40% or less with respect to all nonmetallic inclusions, and the average CaO of all nonmetallic inclusions It is necessary to control the concentration to 10 mass% or less. The reason will be described below.

Al ₂ O ₃ , MgO, MgO.Al ₂ O ₃ , MnO—Al ₂ O ₃ -based oxides These oxide-based inclusions (non-metallic inclusions) are generated when Al deoxidation is performed. Deoxidation product. In the duplex stainless steel of the present invention, Mg is added in the range of 0.0001 to 0.0050 mass% in order to improve hot workability. Therefore, when the content of Al and Mg is large, Oxide inclusions such as Al ₂ O ₃ and MgO, MgO.Al ₂ O ₃ are generated. Conversely, when both the contents of Al and Mg are low, MnO—Al ₂ O ₃ inclusions Generated. Since these oxide inclusions do not contain CaO, the corrosion resistance is not deteriorated. Therefore, in order to obtain the pitting potential (pitting corrosion resistance) as intended in the present invention, it is preferable that the inclusions contained in the steel basically consist of these inclusions.

CaO-Al ₂ O ₃ type oxide with CaO concentration of 40 mass% or less The oxide inclusions containing CaO are basically dissolved in water because the Ca component dissolves in water and reduces the pitting corrosion resistance of steel. The existence itself is not preferable. In the present invention, as described above, the Ca content is set to 0.0005 mass% or less. However, even if the Ca content is reduced to 0.0005 mass% or less, a small number of CaO—Al ₂ O ₃ -based oxides containing CaO are produced. Therefore, the inventors investigated the influence of the CaO-containing inclusions on pitting corrosion resistance. As a result, if the CaO concentration contained in the CaO-Al ₂ O ₃ -based inclusions is 40 mass% or less, It was found that there was little elution and the adverse effect on pitting corrosion resistance was small. Therefore, in the present invention, even when a CaO—Al ₂ O ₃ -based inclusion is present in the nonmetallic inclusion, the inclusion needs to have a CaO concentration of 40 mass% or less.

CaO—Al ₂ O ₃ Oxide Number Ratio of 40% or Less As described above, since a slight amount of Ca is unavoidably mixed in steel, CaO—Al ₂ O ₃ oxide is slightly generated. The adverse effect of this CaO—Al ₂ O ₃ inclusion can be made harmless to some extent by reducing the CaO concentration in the inclusion. However, the inventors have found that in order to further improve the pitting corrosion resistance, in addition to reducing the CaO content, it is important to reduce the number of the existence. Therefore, in the present invention, the number ratio of CaO—Al ₂ O ₃ inclusions is limited to 40% or less with respect to the total number of nonmetallic inclusions.

CaO concentration of all non-metallic inclusions is 10 mass% or less As described above, the presence of inclusions containing CaO is not preferable from the viewpoint of ensuring corrosion resistance. According to the inventors' investigation, the inclusions contained in the duplex stainless steel are composed of various types of inclusions, but the average CaO concentration in all non-metallic inclusions contained in the steel is averaged. When the value exceeds 10 mass%, it has been found that the pitting potential is lowered due to the dissolution of the Ca component into water. Therefore, in the present invention, the CaO concentration in all non-metallic inclusions is defined as 10 mass% or less.

Pitting corrosion potential: 600 mV or more The duplex stainless steel according to the present invention realizes excellent pitting corrosion resistance by controlling the nonmetallic inclusions in the steel to have the above-mentioned suitable composition. As a result, duplex stainless steel of the present invention, a defined pitting potential measurement test in JIS G0577, as high as 60 ° C., the pitting potential Vc when measured in an aqueous solution of 20% NaCl _'10 is more than 600mV Shows pitting corrosion resistance. Furthermore, by reducing the average CaO concentration of nonmetallic inclusions to 9 mass% or less, the pitting potential Vc ′ ₁₀ can be set to an extremely high pitting potential of 900 mV or more.

  Conventionally, as a material of a member used in a corrosive environment that cannot be endured by ordinary duplex stainless steel, for example, seawater environment or chemical plant, super stainless steel with increased Ni or Cr concentration has been used. . However, since both Ni and Cr are expensive, an increase in content causes a significant increase in cost. In this regard, the duplex stainless steel of the present invention can improve corrosion resistance, particularly pitting corrosion resistance, by appropriately controlling the inclusions in the steel without excessively adding expensive metals, There is no increase in manufacturing cost. Therefore, the duplex stainless steel of the present invention can be suitably used as an alternative to conventional super stainless steel.

Next, a method for producing a duplex stainless steel according to the present invention will be described.
The method for producing the duplex stainless steel of the present invention is to charge a raw material of stainless steel into an electric furnace or the like and melt it to obtain Cr: 20 to 30 mass%, Ni: 3 to 10 mass%, Mo: 2 to 5 mass%. Decomposition, lime, fluorite, Al, and ferrosilicon using any one of AOD treatment, VOD treatment, and AOD treatment followed by VOD treatment by melting Cr-Ni-Mo steel containing Etc. are added to form CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag, Cr reduction, deoxidation and desulfurization, and the steel component composition is finally adjusted to the predetermined range described above. It is the manufacture characterized by this. Thereafter, the steel is cast by a continuous casting method, or is made into an ingot by a normal ingot casting method and then forged or split rolled to form a steel slab. Here, the CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag is composed of CaO / Al ₂ O ₃ : 3 to 10, CaO / SiO ₂ : 3 to 15, MgO: It is necessary to satisfy the condition of 3 to 20 mass%.

CaO / Al ₂ O ₃ : 3 to 10
In order to efficiently deoxidize and desulfurize molten steel, it is necessary to control the CaO / Al ₂ O ₃ ratio of the CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag to a range of 3 to 10. If this ratio is less than ₃ , the activity of Al ₂ O ₃ is high, and sufficient deoxidation and desulfurization cannot be performed. Therefore, it is difficult to control the S concentration and O concentration in the steel within the range of the present invention. Become. On the other hand, when the CaO concentration in the slag increases and the CaO / Al ₂ O ₃ ratio exceeds 10, CaO is reduced, and the Ca concentration in the molten steel increases, exceeding the range of the present invention. Therefore, in the present invention, to control the CaO / _Al 2 _{O 3} ratio of the slag in the range of 3-10. Preferably, it is the range of 4-8.
In order to control the CaO / Al ₂ O ₃ ratio, it is preferable to use lime or fluorite as the CaO component and alumina produced by oxidation of Al as a deoxidizer as the Al ₂ O ₃ component. . Further, the Al ₂ O ₃ component may be used alumina or lime aluminate material.

CaO _/ SiO 2: 3~15
In order to deoxidize and desulfurize molten steel, it is necessary to control the CaO / SiO ₂ ratio of the CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag to a range of 3 to 15. If the ratio is less than 3, the SiO ₂ activity is relatively high, so that sufficient deoxidation and desulfurization cannot be performed, and the S concentration and O concentration in the steel are controlled within the range of the present invention. Becomes difficult. On the other hand, when the CaO concentration in the slag increases and the CaO / SiO ₂ ratio exceeds 15, in addition to the reduction of CaO and the increase in the Ca concentration in the molten steel, the fluidity of the slag deteriorates and the slag is removed. Since acid and desulfurization do not proceed efficiently, it becomes difficult to control the Ca concentration, S concentration and O concentration in the steel within the range of the present invention. Therefore, in the present invention, to control the CaO / SiO ₂ ratio of the slag in the range of 3-15. Preferably, it is the range of 4-12.
In order to control the CaO / SiO ₂ ratio, it is preferable to use lime or fluorite as the CaO component and silica produced by oxidation of Si as a deoxidizer as the SiO ₂ component. Silica sand may be used as the SiO ₂ component.

MgO: 3 to 20 mass%
MgO in the slag is a component necessary for controlling the Mg concentration contained in the molten steel within an appropriate range. Moreover, MgO also has the function which lowers melting | fusing point of slag and promotes hatching by containing in slag. In order to acquire such an effect, MgO in slag needs to be 3 mass% or more. On the other hand, if the MgO concentration exceeds 20 mass%, on the contrary, the melting point of the slag becomes high, the fluidity deteriorates, and deoxidation and desulfurization do not proceed efficiently. It becomes difficult to realize. Therefore, in this invention, MgO density | concentration in slag is made into the range of 3-20 mass%. Preferably, it is in the range of 4 to 15 mass%.
In addition, MgO in the slag is almost within the predetermined range as dolomite bricks and magcro bricks used in the AOD treatment and / or VOD treatment are eluted into the slag, but are controlled to a more appropriate range. For this reason, waste bricks such as dolomite bricks and magcro bricks may be added.

  Stainless steel raw materials such as stainless steel scrap, iron scrap, ferronickel, ferrochrome, pure Ni, Fe-Ni alloy scrap and molybdenum are charged into an electric furnace with a capacity of 60 tons and melted, Cr: 20-30 mass%, Ni: 3 10% by mass, Mo: 2-5% by mass Cr-Ni-Mo steel is melted, and then, as shown in Table 1, AOD treatment, VOD treatment, and any one of the methods of VOD treatment after AOD treatment After decarburization by blowing with Ar gas or N gas and oxygen gas using the method, lime, fluorite, Al and ferrosilicon are added to form slag having the composition shown in Table 1, Cr Reduction, deoxidation and desulfurization were carried out, and finally the components of the steel were adjusted. A molten steel having a component composition of 1 to 14 was obtained, and then the molten steel was made into an ingot by the ordinary ingot casting method and then forged or split rolled, or cast by a continuous casting method to obtain a steel slab. Here, no. Nos. 1 to 9 are invention examples, and 10-14 show the comparative examples.

  Next, the above-mentioned various steel slabs were heated and soaked at a temperature of 1100 to 1200 ° C. in a heating furnace, and then hot-rolled to form a hot-rolled sheet having a thickness of 5.5 mm. Pickling, cold rolling, and finish annealing were performed to obtain a cold rolled sheet (product) having a sheet thickness of 1 to 3 mm.

The following investigation was conducted for each of the hot-rolled sheet and the cold-rolled sheet of the invention example and the comparative example obtained as described above.
<Component analysis of steel>
About the cold-rolled sheet, the component composition of steel was quantitatively analyzed by fluorescent X-ray analysis.
<Composition of non-metallic inclusions>
Using an EDS (energy dispersive analyzer), 10 non-metallic inclusions present in each hot-rolled sheet were arbitrarily selected and quantitatively analyzed, and component analysis and classification of the non-metallic inclusions were performed.
<Measurement of pitting potential>
After wet-polishing the surface of the test piece (plate thickness × 25 mm × 20 mm) collected from the cold-rolled plate with SiC # 800 abrasive paper, the entire surface is covered with a silicone sealant leaving a measurement surface of about 1 cm ² , Immediately before measuring the pitting potential, the surface was wet-polished with SiC # 600 abrasive paper. Next, the test piece was immersed in a 20% NaCl aqueous solution maintained at a temperature of 60 ° C. for 10 minutes, and then polarized from the natural potential toward the anode at a sweep rate of 20 mV / min, and the current density was 10 μA / cm ² . the highest potential in excess was determined as the pitting potential Vc _'10. In this pitting corrosion potential measurement, a saturated calomel electrode (SCE) was used as a reference electrode, and other conditions were in accordance with JIS G0577 (2005).
<Heat-rolled plate surface defects>
The surface of the hot-rolled sheet was visually observed to investigate the presence of surface defects.

The results are shown in Tables 3 and 4. From these tables, the slag composition at the time of steel refining, the chemical composition (component composition) of steel, and the composition of nonmetallic inclusions are all within the scope of the present invention. 1-9 of duplex stainless steel (Inventive Example 1-9) immediately, are excellent in hot workability, the pitting potential, Vc _'10 has also exceeded 600 mV, has excellent pitting resistance I understand that. On the other hand, no. In the duplex stainless steel plates of Comparative Examples 10 to 14 (Comparative Examples 1 to 5), any one or more items of the chemical composition of the steel, the slag composition, and the composition of the nonmetallic inclusions depart from the scope of the present invention. As a result, a surface defect is generated in the hot-rolled sheet, so that a material that cannot proceed to the next process (Comparative Example 4) is obtained, or the pitting corrosion potential Vc ′ ₁₀ is low. It can be seen that is not obtained.

  Since the duplex stainless steel of the present invention is excellent in pitting corrosion resistance and hot workability, in addition to petrochemical plants and seawater handling equipment, sodium hypochlorite storage tanks and pulp bleaching equipment for tap water sterilization Such a hypochlorite aqueous solution can be suitably used for containers and devices for storing and using.

Claims (4)

C: 0.030 mass% or less,
Si: 0.01-1 mass%,
Mn: 0.01-2 mass%,
P: 0.040 mass% or less,
S: 0.0015 mass% or less,
Ni: 3 to 10 mass%,
Cr: 20 to 28 mass%,
Mo: 2 to 5 mass%,
N: 0.05-0.40 mass%,
Al: 0.001 to 0.05 mass%,
Mg: 0.0001 to 0.0050 mass%,
Ca: 0.0005 mass% or less,
O: 0.0001 to 0.0050 mass%,
The balance is steel with a component composition consisting of Fe and inevitable impurities,
Non-metallic inclusions contained in the _{steel, MgO · Al 2 O 3,} Al 2 O 3, MgO, MnO-Al 2 O 3 based oxide, CaO concentration 40 mass% or less of the _CaO-Al 2 _{O 3} based oxide The number ratio of CaO—Al ₂ O ₃ -based oxide to all nonmetallic inclusions is 40% or less, and the CaO concentration in all nonmetallic inclusions is 10 mass% or less. A duplex stainless steel having a pitting corrosion potential Vc ′ ₁₀ in a 20% NaCl aqueous solution at 60 ° C. of 600 mV (vs SCE) or more. In addition to the above component composition, W: 0.01 to 1 mass%, Cu: 0.01 to 1 mass%, V: 0.01 to 1 mass%, Co: 0.01 to 1 mass%, Nb: 0.01 to The duplex stainless steel according to claim 1, comprising one or more selected from 1 mass%, Ti: 0.01 to 1 mass%, and B: 0.001 to 0.010 mass%. steel. In producing the duplex stainless steel according to claim 1 or 2, a Cr-Ni-Mo system containing Cr: 20-30 mass%, Ni: 3-10 mass%, and Mo: 2-5 mass% in an electric furnace. When steel is melted and refined by any of the methods of AOD treatment, VOD treatment, and AOD treatment followed by VOD treatment, lime, fluorite, Al, and ferrosilicon are added after decarburization treatment. CaO / Al ₂ O ₃ : 3 to 10; CaO / SiO ₂ : 3 to 15; MgO: 3 to 20 mass% CaO—SiO ₂ —MgO—Al ₂ O ₃ —F based slag is formed, Cr A method for producing a duplex stainless steel, characterized by reduction, deoxidation, desulfurization, and component adjustment. The method for producing a duplex stainless steel according to claim 3, wherein the refined steel is continuously cast or normally ingoted to form a steel slab.