JP2015074807A - Stainless steel excellent in surface quality and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel excellent in surface quality and a method of inexpensively producing the stainless steel by using general purpose equipment.SOLUTION: There is provided the stainless steel containing C:0.1% or less, Si:0.2 to 1%, Mn:0.2 to 2%, S:0.005% or less, Ni:3 to 15%, Cr:13 to 20%, Al:less than 0.005%, Mg:0.0001 to 0.01%, Ca:0.0001 to 0.01%, O:0.0005 to 0.01% and the balance Fe with inevitable impurities, and non-metallic inclusion contained in the stainless steel contains one or more kind of MgO, MgO AlOand CaO-SiO-MgO-AlO-based oxide, the non-metallic inclusion having a length of 5 μm or more is 100 or less per any 1 cmand MgO AlOin the non-metallic inclusion is 50% or less by the number ratio.

Description

The present invention relates to stainless steel having excellent surface quality. Furthermore, regarding the refining method of stainless steel, by controlling the slag basicity and trace components such as Mg, Al, Ca in molten steel, it is possible to generate MgO · Al ₂ O ₃ which is a harmful non-metallic inclusion in molten steel. It suppresses and produces stainless steel excellent in surface quality while preventing adhesion in the nozzle.

  Stainless steel is often used without its coating or coating treatment on the surface due to its excellent corrosion resistance. However, there is a problem that surface defects occur depending on the form of non-metallic inclusions.

There are several disclosures of techniques for detoxifying stainless steel inclusions. For example, in Patent Document 1, MgO · Al ₂ O ₃ that is a harmful non-metallic inclusion is suppressed by using ferrosilicon having a low concentration of Al, Ca, and Mg when refining stainless steel. . In this technique, it is necessary to control the slag basicity to a relatively low value of 1.3 to 2.7 in order to control the inclusion form to the CaO—SiO ₂ —MgO—Al ₂ O ₃ system. Therefore, in some cases, sufficient desulfurization ability may not be obtained, and hot workability may be reduced.

Moreover, in patent document 2, the nonmetallic inclusion in molten steel is controlled by the MgO type inclusion by controlling Al concentration and slag composition in molten steel. Furthermore, in Patent Document 3, nonmetallic inclusions in molten steel are controlled to be MgO inclusions or CaO—Al ₂ O ₃ inclusions by controlling the Al concentration and slag composition in the molten steel.

  Both of the above two techniques require the Al concentration to be adjusted to 0.005% or more. Since the yield of Al is not stable, it is difficult to say that the present technology can be completely implemented. In addition, there is a concern about the quality of the bead part after welding in applications that require welding in order to positively add Al.

In Patent Document 4, the slag composition is controlled, and the non-metallic inclusion composition is changed to MgO · Al ₂ O ₃ , CaO—Al ₂ O ₃ system, MgO, CaO—SiO ₂ —MgO—Al ₂ O ₃ —MnO system oxidation. Techniques for controlling objects are disclosed. According to this, it is shown that stainless steel excellent in corrosion resistance, weldability and surface properties can be obtained. Patent Document 5 discloses a technique for controlling a non-metallic inclusion composition to a CaO—SiO ₂ —MgO—Al ₂ O ₃ —MnO—Cr ₂ O ₃ —FeO-based oxide. According to this, it is shown that stainless steel excellent in corrosion resistance, weldability and surface properties can be obtained.

  The above two techniques have a problem that the control is unstable because the refining method is not clearly shown.

  Patent Document 6 discloses an Fe—Ni—Cr—Mo alloy excellent in impact resistance and surface properties. This technology is Fe-based, and is applicable to alloys containing Ni: 30 to 32%, Cr: more than 26 to 28%, Mo: 6 to 7%, and the basicity C / S of slag is 5 Control is as high as ~ 20.

JP 2001-26811 A Japanese Patent Laid-Open No. 9-256028 Japanese Patent Laid-Open No. 2001-220619 JP 2004-149833 A JP 2007-277727 A JP 2011-97224 A

As described above, in the conventional method, while suppressing the generation of harmful inclusions MgO.Al ₂ O ₃ , Al ₂ O ₃ or CaO, the hot workability is also healthy, It was difficult to ensure quality. An object of the present invention is to provide a stainless steel having excellent surface properties and to propose a method for producing the stainless steel at low cost using general-purpose equipment.

Inventors repeated earnest research in order to solve the said subject. First, the present inventors studied surface defects generated in an actual machine. That is, the defect was observed by SEM, and the foreign material composition contained therein was specified. As a result, it was found to be either MgO.Al ₂ O ₃ , CaO or Al ₂ O ₃ .

  Furthermore, when investigating the relationship with the operation, these oxides are non-metallic inclusions contained in the molten steel, and are deposited and deposited on the nozzle that supplies the molten steel from the tundish to the mold in the continuous casting machine. It became clear that a large-scale defect was caused by dropping off the part. In order to prevent this, it was found that the basicity of the slag must be controlled and Al in the molten steel must be reduced as much as possible. Therefore, guidance has been obtained that these non-metallic inclusions must be prevented.

At the same time, it was found that when the inclusion composition was MgO or CaO—SiO ₂ —Al ₂ O ₃ —MgO, there was no adhesion to the nozzle and no surface defects. It was also found that MgO.Al ₂ O ₃ does not cause surface defects when the number ratio is 50% or less. Furthermore, when the chemical components were examined in detail, it was found that trace components such as Mg, Ca and O contained in trace amounts had to be controlled.

Therefore, the inventors conducted a laboratory study on the influence of the operating conditions on the trace components and the inclusion composition as follows. First, some alloy components were melted in a vertical resistance furnace using a magnesia crucible in a laboratory. Alloy components are Fe-18% Cr-8% Ni alloy, Fe-18% Cr-12% Ni-2.5Mo alloy, Fe-15% Cr-5% Ni-3% Cu-0.25% Nb alloy , Fe-20% Cr-10% Ni-0.7% Nb alloy was used for the experiment. Si in molten steel, Mn, Al, after the addition to deoxidation any one or more of _Ca, after the addition of _{CaO-SiO 2 -Al 2 O 3} -MgO-F slag The molten steel was collected at a predetermined time to obtain a sample. The chemical composition of this sample and the inclusion composition in the sample were measured, and the influence of the experimental conditions on the trace component and inclusion composition was investigated.

The chemical component in the sample was measured by chemical analysis, and the inclusion composition in the sample was measured by observing the collected sample with SEM / EDS, and arbitrarily selecting 20 inclusions of 5 μm or more. As a result, it was found that it is important to first perform deoxidation with Si and to control Al to less than 0.005%. At the same time, while controlling the Si concentration to 0.2 to 1%, Mg is 0.0001 to 0.01%, Ca is 0.0001 to 0.01%, and O is 0.0005 to 0.01%. By adjusting, a guideline was obtained that can basically control the inclusion composition to MgO or CaO—SiO ₂ —Al ₂ O ₃ —MgO system. Furthermore, it became clear that MgO.Al ₂ O ₃ can be suppressed to 50% or less in terms of the number ratio. The slag composition in that case also gave a guideline that it was necessary to control the slag basicity to less than 2-5.

The present invention has been made based on the above findings, that is, C: 0.1% or less, Si: 0.2-1%, Mn: 0.2-2%, S: 0.005% or less Ni: 3-15%, Cr: 13-20%, Al: less than 0.005%, Mg: 0.0001-0.01%, Ca: 0.0001-0.01%, O: 0.0005 0.01%, nonmetallic inclusions balance contained in the stainless steel in a stainless steel consisting of Fe and unavoidable _{impurities, MgO, MgO · Al 2 O} 3, CaO-SiO 2 -MgO-Al 2 O comprises one or more _3-based oxides, more than a length 5μm of non-metallic inclusions is not more than 100 per ² any 1 cm, MgO · _Al 2 _{O 3} among non-metallic inclusions The stainless steel is characterized in that the number ratio is 50% or less. It is steel.

In the present invention, among the non-metallic inclusions, MgO.Al ₂ O ₃ is preferably 20% or less in number ratio, and more preferably 0% in number ratio.

Also, the non-metallic inclusions, MgO · _Al 2 _{O 3} is _{MgO: 10~40%, Al 2 O} 3: a _{60~90%, CaO-SiO 2 -Al} 2 O 3 -MgO based oxide it _{is, CaO: 20~60%, SiO 2} : 10~40%, Al 2 O 3: 30% or less, MgO: more preferably between 5 to 50%.

  In addition to the above components, Mo: 5% or less, Cu: 1-5%, Nb: 0.05-1%, N: 0.01-0.05%, B: 0.01% or less You may include 1 type, or 2 or more types.

Furthermore, the present invention also provides a method for producing the above stainless steel. That is, after melting the raw material, melting a molten stainless steel containing Ni: 3-15% and Cr: 13-20% and then decarburizing in AOD and / or VOD, lime, fluorite, ferrosilicon alloy was poured CaO / SiO ₂ ratio: less than _{2~5, MgO: 3~15%, Al} 2 O 3: CaO-SiO 2 using _{-MgO-Al 2 O} 3 -F-based slag consisting of less than 5%, C: 0.1% or less, Si: 0.2-1%, Mn: 0.2-2%, S: 0.005% or less, Ni: 3-15%, Cr: 13-20%, Al: Stainless steel molten steel comprising less than 0.005%, Mg: 0.0001 to 0.01%, Ca: 0.0001 to 0.01%, O: 0.0005 to 0.01%, the balance being Fe and inevitable impurities For producing stainless steel, characterized by A.

  According to the present invention, by controlling the ratio of alloy components and the absolute number and ratio of inclusions within a specific range, the hot workability is maintained in a healthy state, and the stainless steel has excellent surface properties. Can be provided.

  First, the reasons for limiting the chemical components of the steel used in the present invention will be described. In the following description, “%” means “mass%”.

C: 0.1% or less C is an austenite stabilizing element, but when present in a large amount, it combines with Cr and Mo to form carbides, and reduces the amount of solid solution Cr and Mo contained in the base material, Deteriorates corrosion resistance. Therefore, the C content is set to 0.1% or less. In addition, Preferably it is 0.08% or less, More preferably, it is 0.07%.

Si: 0.2% to 1%
Si is a very important element in the present invention. Si is an element effective for deoxidation, and 0.2% is necessary to control the oxygen concentration to 0.01% or less. Furthermore, there is also a role of reducing CaO and MgO in the CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag and supplying 0.0001% or more of Ca and Mg to the molten steel, respectively. From that point of view, 0.2% is necessary. On the other hand, when it contains exceeding 1%, CaO and MgO in slag will be reduced too much and Ca and Mg will be supplied over 0.01%. As a result, Ca forms inclusions of CaO alone and causes surface defects in the product. Further, Mg has a risk of causing surface defects by forming Mg bubbles in the slab. Therefore, the Si content is defined as 0.2% to 1%. Preferably it is 0.4 to 0.8%.

Mn: 0.2% to 2%
Mn is an element effective for deoxidation. If the Mn content is less than 0.2%, the effect cannot be sufficiently obtained. Conversely, if the Mn content exceeds 2%, the formation of the sigma phase is promoted and embrittlement occurs. Therefore, the Mn content is defined as 0.2% to 2%.

S: 0.005% or less Since S is an element that inhibits hot workability, it should be reduced as much as possible, and the S content is set to 0.005% or less. Preferably it is 0.003% or less. More preferably, it is 0.002% or less. For this purpose, it is necessary to desulfurize using slag at AOD and / or VOD. By setting the basicity C / S of slag to less than 2 to 5 and containing 0.2% to 1% of Si in the molten steel, it is possible to desulfurize and satisfy this range.

Ni: 3% to 15%
Ni has an effect of improving pitting corrosion resistance, crevice corrosion resistance and stress corrosion cracking resistance in a solution environment containing chloride. However, in order to obtain the effect, 3% or more is necessary. However, for the effect, addition of 15% or less is sufficient, and beyond that, the cost increases, which is not preferable. Therefore, the Ni content is defined as 3% to 15%.

Cr: 13% to 20%
Cr is an element that forms a passive film, which is indispensable for ensuring corrosion resistance, on the surface of the steel sheet, and is a matrix for improving acid resistance, pitting corrosion resistance, crevice corrosion resistance and stress corrosion cracking resistance. As a constituent component of the material, it is the heaviest element. However, if the Cr content is less than 13%, sufficient corrosion resistance cannot be obtained. On the other hand, if the content exceeds 20%, a sigma phase is generated and embrittlement occurs. For the above reason, the Cr content is defined as 13% to 20%.

Al: Less than 0.005% Al is an element that forms 50% or more of MgO.Al ₂ O ₃ that causes surface defects due to clusters and also forms alumina inclusions. Therefore, Al is an element that must be reduced as much as possible. is there. Furthermore, it is an element that deteriorates the quality of the weld bead portion. Therefore, the Al content is specified to be less than 0.005%. Preferably it is 0.004% or less. Of course, in order to control within this range, it is of utmost importance not to use Al as a deoxidizer.

Mg: 0.0001% to 0.01%
Mg is the composition of nonmetallic inclusions in the steel, without the formation of clusters, effective to control the oxide MgO or _{CaO-SiO 2 -Al 2 O 3} -MgO based oxide no adverse effect on surface quality Element. The effect cannot be obtained when the content is less than 0.0001%. Conversely, when the content exceeds 0.01%, Mg bubbles are formed in the slab, resulting in surface defects in the final product. Therefore, the Mg content is defined as 0.0001% to 0.01%. Preferably, it is 0.0002 to 0.005%. More preferably, it is 0.0003 to 0.003%.

In order to effectively add Mg into the molten steel, it is preferable to use the following reaction.
2 (MgO) + Si = (SiO ₂ ) +2 Mg (1)
The parentheses indicate the components in the slag, and the underline indicates the components in the molten steel.
In order to control Mg within the above range, the slag basicity may be controlled to be less than 2 to 5 and the MgO concentration in the slag may be adjusted to 3 to 15%.

Ca: 0.0001% to 0.01%
Ca is an effective element for controlling the composition of non-metallic inclusions in steel to a CaO—SiO ₂ —Al ₂ O ₃ —MgO-based oxide that does not form clusters and does not adversely affect the surface quality. The effect cannot be obtained when the content is less than 0.0001%. Conversely, when the content exceeds 0.01%, inclusions of simple CaO are formed, resulting in surface defects in the final product. Therefore, the Ca content is defined as 0.0001% to 0.01%. Preferably, it is 0.0002 to 0.005%. More preferably, it is 0.0003 to 0.003%.
In order to effectively add Ca to the molten steel, it is preferable to use the following reaction.
2 (CaO) + Si = (SiO ₂ ) +2 Ca (2)
What is necessary is just to control slag basicity to less than 2-5 in order to control Ca to said range.

O: 0.0005% to 0.01%
If O is present in the steel in an amount exceeding 0.01%, desulfurization is inhibited, and the S concentration in the molten steel exceeds 0.005%. On the other hand, if the content is less than 0.0005%, Si excessively enhances the ability to reduce MgO and CaO in the slag. That is, when the reactions of the above formulas (1) and (2) proceed too much, Mg and Ca in the molten steel become higher than 0.01%, respectively. Therefore, the O content is defined as 0.0005% to 0.01%. In order to control within this range, it is necessary to adjust the Si concentration to 0.2% to 1% and to adjust the basicity of the slag to less than 2 to 5. Preferably, it is 0.0006 to less than 0.005%, More preferably, it is 0.001 to 0.004%.

Further, the steel of the present invention may contain one or more of the following elements.
Cu: 1 to 5%
Cu is a useful element because it is less prone to processing effects and improves formability. It is also an element that improves antibacterial properties and corrosion resistance to sulfuric acid. However, when added in a large amount, hot workability is lowered and toughness is also lowered. Therefore, 1 to 5% is desirable. More desirably, it is 2 to 4%. As an effect on refining, Cu has an effect of increasing the solubility of Mg in molten steel and facilitating the formation of MgO inclusions. At the same time, on the opposite side, since Cu strengthens the action of Al in the molten steel, it also has an action of reacting with Mg to easily form MgO.Al ₂ O ₃ spinel inclusions. Therefore, the application of the present invention is extremely effective for Cu-containing steel.

Mo: 5% or less Mo is an element that improves corrosion resistance. If it exceeds 5%, the tendency to form a σ phase becomes strong and tends to become brittle. Therefore, it is desirable to keep it at 5% or less. Preferably it is 3% or less.

Nb: 0.05 to 1%
Nb is an element necessary for precipitation hardening stainless steel, contributes to hardening, and fixes C to enhance corrosion resistance. In order to obtain such an effect, 0.05% or more is necessary. On the other hand, if the content of these elements is excessive, a large amount of ferrite is formed at the solution heat treatment temperature, and the hardness after age hardening decreases. Therefore, the total content of these elements should be 1% or less. Therefore, 0.05 to 1% is a desirable range. Preferably it is 0.1 to 0.7%.

N: 0.01% to 0.05%
N is an interstitial element and improves the hardness and corrosion resistance of steel, so addition of 0.01% or more is preferable. However, when the N content is excessive, a nitride is formed together with Nb and Cr, which adversely affects workability. Therefore, the N content needs to be 0.05% or less. Therefore, 0.01% to 0.05% is desirable. Preferably, it is 0.015 to 0.04%.

B: 0.01% or less B is an element that improves hot workability on the relatively low temperature side of about 900 ° C. However, addition exceeding 0.01% inhibits hot workability on the relatively high temperature side of about 1200 ° C. For this reason, the addition should be limited to 0.01% or less. Preferably it is 0.005% or less.

Non-metallic inclusions In the present invention, the non-metallic inclusion composition includes one or more of MgO, MgO.Al ₂ O ₃ , CaO—SiO ₂ —MgO—Al ₂ O ₃ -based oxide, A preferred embodiment is that Al ₂ O ₃ is 50% or less in terms of the number ratio. Hereinafter, the grounds for limiting the number ratio of non-metallic inclusions will be shown.

The non-metallic inclusion composition includes one or more of MgO, MgO.Al ₂ O ₃ , CaO—SiO ₂ —MgO—Al ₂ O ₃ based oxide, and MgO · Al ₂ O ₃ in a number ratio. 50% or less The stainless steel according to the present invention is made of MgO, MgO.Al ₂ O ₃ , CaO—SiO ₂ —MgO—Al ₂ O ₃ -based oxides according to the contents of Si, Al, Mg and Ca in the steel. Contains one or more. The reason for including these inclusions is that MgO has a high melting point of 2800 ° C., and therefore does not sinter in the immersion nozzle of the continuous casting machine, and therefore does not adhere and deposit. Therefore, no surface defects are caused. Since the CaO—SiO ₂ —MgO—Al ₂ O ₃ -based oxide has a low melting point of about 1300 ° C., it is not sintered either. Therefore, no surface defects are caused.
Since MgO.Al ₂ O ₃ is an inclusion that causes surface defects, it is preferable that it be as small as possible. However, if the content is 50% or less in terms of the number ratio, MgO.Al ₂ O ₃ does not adhere in the nozzle, so the number ratio was determined to be 50% or less.

The reason why the constituent components of MgO.Al ₂ O ₃ are specified will be described.
_{MgO: 10~40%, Al 2 O} 3: 60~90%
MgO.Al ₂ O ₃ is a compound having a relatively wide solid solution. Since it became a solid solution in the above range, it was determined in this way.

The reason for defining each component of the CaO—SiO ₂ —Al ₂ O ₃ —MgO-based oxide will be described.
_{CaO: 20~60%, SiO 2:} 10~40%, Al 2 O 3: 30% or less, MgO: 5 to 50%
Basically, in order to keep the melting point of the CaO—SiO ₂ —Al ₂ O ₃ —MgO-based oxide at about 1300 ° C. or less, the above range was set. In addition, when CaO is less than 20%, the melting point becomes high, and when CaO exceeds 60%, CaO inclusions coexist. If SiO ₂ is less than 10% or more than 40%, the melting point becomes high. If Al ₂ O ₃ exceeds 30%, pure Al ₂ O ₃ inclusions coexist. If MgO is less than 5% or more than 40%, the melting point becomes high. From the above, CaO: 20 to 60%, SiO ₂ : 10 to 40%, Al ₂ O ₃ : 30% or less, and MgO: 5 to 50%.

Manufacturing Method The present invention also proposes a method for manufacturing stainless steel. First, raw materials are melted, molten stainless steel containing Ni: 3 to 15% and Cr: 13 to 20%, and then decarburized in AOD and / or VOD, and then lime, fluorite, ferrosilicon alloy was poured CaO / SiO ₂ ratio: less than _{2~5, MgO: 3~15%, Al} 2 O 3: using a _{CaO-SiO 2 -MgO-Al 2} O 3 -F -based slag consisting of less than 5% This is a method for refining molten steel. According to this, it is possible to effectively reduce the S concentration in the molten stainless steel of the present invention to 0.005% or less. Further, the non-metallic inclusions also include one or more of MgO, MgO.Al ₂ O ₃ , CaO—SiO ₂ —MgO—Al ₂ O ₃ based oxide, and MgO · Al ₂ O ₃ in a number ratio. By controlling to 50% or less, it becomes possible to prevent surface defects in the final product and ensure good surface properties.

The method for producing a stainless alloy according to the present invention is characterized by the composition of the slag as described above. Hereinafter, the basis of the slag composition defined in the present invention will be described.
CaO / SiO ₂ ratio: It is necessary to control the CaO / SiO ₂ ratio of slag in order to efficiently deoxidize and desulfurize molten alloy of less than 2 to 5 and to control the composition of non-metallic inclusions within the scope of the present invention. There is. When the value of this ratio exceeds 5, the activity of CaO in the slag becomes high, and the reaction of the formula (2) proceeds too much. Therefore, the Ca concentration reduced in molten steel exceeds 0.01%, non-metallic inclusions of CaO alone are generated, adhere to the nozzle, and cause surface defects in the final product. Therefore, the upper limit was set to 5 (less than). On the other hand, when the CaO / SiO ₂ ratio is less than 2, deoxidation and desulfurization do not proceed, and the S concentration and O concentration ranges in the present invention cannot be controlled. Therefore, the lower limit is set to 2. In order to control to such a CaO / SiO2 ratio, it can be adjusted by adding lime or fluorite as a CaO component. On the other hand, the SiO2 component can be obtained by oxidation of Si as a deoxidizer. That is, when an FeSi alloy is introduced during the Cr reduction period and the Cr oxide is reduced, SiO ₂ silica is formed in the slag. Although there is no limitation, if there is a shortage, silica sand may be appropriately added as the SiO ₂ component. Therefore, the basicity was determined to be less than 2-5. Preferably, it is more than 2.7 to 4.9.

MgO: 3-15%
MgO in the slag is an important element in order to control the Mg concentration contained in the molten steel to the concentration range described in the claims, and to control the nonmetallic inclusions to a composition preferable for the present invention. Is also an important element. Therefore, the lower limit was made 3%. On the other hand, if the MgO concentration exceeds 15%, the reaction of the formula (2) proceeds too much, the Mg concentration in the molten steel becomes high, and Mg bubbles are formed in the slab, resulting in surface defects in the final product. . Therefore, the upper limit of the MgO concentration is set to 15%. MgO in the slag is in a predetermined range by dissolving dolomite bricks or magcro bricks used in AOD refining or VOD refining into the slag. Or in order to control to a predetermined range, you may add the waste brick of a dolomite brick or a magchrom brick.

_{Al 2 O 3: Al 2 O} 3 of less than 5% in the slag is higher the Al concentration in the molten steel is also increased to 0.005% or more, MgO · _Al 2 _{O 3} is to produce more than 50% by number. Moreover, since alumina inclusions are also formed, it is necessary to reduce the Al ₂ O ₃ concentration in the slag as much as possible. Therefore, the upper limit was made 5% (less than). In order to satisfy the upper limit, it is important not to use Al as a deoxidizer.

Next, although an Example is shown and the structure and effect of this invention are clarified more, this invention is not limited only to a following example.
Using an electric furnace with a capacity of 60 tons, ferronickel, pure nickel, ferrochrome, iron scrap, stainless steel scrap, Fe-Ni alloy scrap, and the like were melted as raw materials. In some steel types, FeMo, FeNb or Cu was also added as a raw material. Thereafter, oxygen blowing (oxidative refining) for removing C in AOD or VOD is performed, limestone and fluorite are added, and CaO—SiO ₂ —Al ₂ O ₃ —MgO—F-based slag is generated. The FeSi alloy was added, Cr reduction was performed, and then deoxidation was performed. Thereafter, desulfurization was further carried out by stirring with Ar. In AOD and VOD, magcro bricks were lined. Thereafter, the steel was taken out in a ladle, temperature adjustment and component adjustment were performed, and a slab was produced by a continuous casting machine.

  The manufactured slab was ground and heated at 1200 ° C. to perform hot rolling, thereby manufacturing a tropics having a thickness of 6 mm. Thereafter, annealing and pickling were performed to remove the scale on the surface. Finally, cold rolling was performed to manufacture a thin coil having a thickness of 1 mm, a width of 1 m, and a length of 1000 m.

  Tables 1 and 2 show the chemical composition of the obtained stainless steel, the slag composition at the end of AOD or VOD refining, the nonmetallic inclusion composition, and the form and quality evaluation of the inclusion. In Table 1, “-” indicates that it was below the analysis limit because it was not added.

In addition, various items described in Tables 1 and 2 were obtained as follows.
1) Chemical composition and slag composition of alloy: Quantitative analysis was performed using a fluorescent X-ray analyzer, and oxygen concentration of the alloy was quantitatively analyzed by an inert gas impulse melting infrared absorption method.
2) Nonmetallic inclusion composition: Immediately after the start of casting, a sample collected by tundish was mirror-polished, and inclusions having a size of 5 μm or more were randomly measured using SEM-EDS at 20 points.
3) Number ratio of spinel inclusions: The number ratio was evaluated from the measurement result of 2) above.
4) Quality evaluation: The surface of the thin plate produced by rolling is visually observed, and surface defects caused by non-metallic inclusions (linear flaws are generated near the center of the plate width, linear defects) and hot workability are reduced. The presence or absence of surface defects (turned wrinkles on the edge of the plate, ear cracks) was determined. The total length of the coil was observed to indicate the number of defects.

  Since Examples 1 to 9 of the invention satisfied the scope of the present invention, the surface of the final product had no or very few defects due to inclusions (11 or less), and good quality could be obtained. .

  On the other hand, since the comparative example deviated from the scope of the present invention, surface defects occurred. Each example will be described below. In Comparative Example 10, the basicity was higher than 5.45 and 5, so the oxygen concentration was too low, and the Ca concentration was 0.0117% and higher than 0.01%. Further, the MgO concentration in the slag was 1.7%, which was lower than 3.0%, and the Mg concentration was below the analysis limit. As a result, non-metallic inclusions of simple CaO were generated, and defects due to the inclusions occurred in the final product.

Since Comparative Example 11 had a basicity of 1.73 and less than 2, deoxidation and desulfurization did not proceed. Therefore, inclusions number has become much greater than the 156 / cm ² and 100 / cm ^2. Furthermore, Ca was low below the analysis limit, MnO in the inclusions was as high as 47.5%, and at the same time, the S concentration was 0.0058%, which was higher than 0.005%. As a result, hot workability decreased and surface defects occurred.

In Comparative Example 12, since Si was higher than 1.23% and 1.0%, and Al concentration was higher than 0.006% and 0.005%, MgO. A lot of Al ₂ O ₃ inclusions were formed. As a result, the spinel ratio exceeded 95% and 50%, and surface defects occurred.

In Comparative Example 13, since Si was 0.11% and less than 0.2%, deoxidation did not proceed, and the number of inclusions increased to 127 / cm ² and exceeded 100 / cm ² . The Mg and Ca concentrations were below the analytical limit, and the MnO concentration in the inclusions was as high as 36.3%. Furthermore, deoxidation and desulfurization did not proceed, and the S concentration was 0.0068%, which was higher than 0.005%. As a result, hot workability decreased and surface defects occurred.

  In Comparative Example 14, the alumina concentration in the slag was as high as 8.5%, exceeding 5%. Therefore, the Al concentration in the molten steel was increased beyond 0.025% and 0.005%, and the non-metallic inclusions of the alumina alone were Produced surface defects.

In Comparative Example 15, the alumina concentration in the slag is higher than 7.5% and 5%, the Al concentration in the molten steel is higher than 0.007% and 0.005%, and the MgO · Al ₂ O ₃ ratio is 60 % And over 50%, and surface defects occurred.

  In Comparative Example 16, the MgO concentration in the slag is higher than 17.5% and 15%, the Mg concentration in the molten steel is higher than 0.0112% and 0.01%, and Mg bubbles are formed in the slab. It resulted in surface defects in the final product.

In Comparative Example 17, Si was higher than 1.02% and 1.0%. Furthermore, since the alumina concentration in the slag was as high as 5.5%, exceeding 5%, the Al concentration in the molten steel was higher than 0.008% and 0.005%. As a result, only MgO.Al ₂ O ₃ inclusions were formed and surface defects were generated.

Claims (6)

C: 0.1% or less, Si: 0.2-1%, Mn: 0.2-2%, S: 0.005% or less, Ni: 3-15%, Cr: 13-20%, Al: Stainless steel with less than 0.005%, Mg: 0.0001 to 0.01%, Ca: 0.0001 to 0.01%, O: 0.0005 to 0.01%, the balance being Fe and inevitable impurities In
Nonmetallic inclusions contained in the stainless steel include one or more of MgO, MgO.Al ₂ O ₃ , CaO—SiO ₂ —MgO—Al ₂ O ₃ oxide,
Among the non-metallic inclusions, those having a length of 5 μm or more are 100 or less per 1 cm ² ,
Stainless steel characterized in that MgO.Al ₂ O ₃ is 50% or less in number ratio among the non-metallic inclusions. _2. The stainless steel according to claim 1, wherein MgO · Al ₂ O ₃ is 20% or less in number ratio among the non-metallic inclusions. _2. The stainless steel according to claim 1, wherein among the non-metallic inclusions, MgO · Al ₂ O ₃ is 0% in number ratio. Among the non-metallic inclusions, MgO · Al ₂ O ₃ is MgO: 10 to 40%, Al ₂ O ₃ : 60 to 90%, and CaO—SiO ₂ —Al ₂ O ₃ —MgO-based oxide is _{CaO: 20~60%, SiO 2:} 10~40%, Al 2 O 3: 30% or less, MgO: stainless steel according to claim 1 or 2, characterized in that 5 to 50%.   Furthermore, Mo: 5% or less, Cu: 1-5%, Nb: 0.05-1%, N: 0.01-0.05%, B: 0.01% or less The stainless steel according to any one of claims 1 to 4, wherein the stainless steel is included. It is a manufacturing method of stainless steel given in any 1 paragraph of Claims 1-5, Comprising: A raw material is melted, Melting stainless steel containing Ni: 3-15% and Cr: 13-20%, Next, after decarburizing in AOD and / or VOD, lime, fluorite, and ferrosilicon alloy are added and CaO / SiO ₂ ratio: less than 2-5, MgO: 3-15%, Al ₂ O ₃ : less than 5% A CaO—SiO ₂ —MgO—Al ₂ O ₃ —F-based slag composed of C: 0.1% or less, Si: 0.2 to 1%, Mn: 0.2 to 2%, S: 0.00. 005% or less, Ni: 3-15%, Cr: 13-20%, Al: less than 0.005%, Mg: 0.0001-0.01%, Ca: 0.0001-0.01%, O: 0.0005 to 0.01%, the balance being Fe and inevitable impurities Method for producing stainless steel and adjusting the less molten steel.