MANUFACTURING METHOD OF AUSTENITIC STAINLESS STEEL

Abstract

A method of manufacturing an austenitic stainless steel, being dissolved and refined, by providing Si from 0.2 to 2.0% by mass, Mn from 0.3 to 5.0% by mass, S at 0.007% by mass or less, Ni from 7.0 to 15.0% by mass, Cr from 15.0 to 20.0% by mass, Al at 0.005% by mass or less, Ca at 0.002% by mass or less, Mg at 0.001% by mass or less, and 0 from 0.002 to 0.0065% by mass and setting a ratio, during refining, of CaO/SiO.sub.2 in a slag between 1.0 and 2.5. The remainder comprises Fe and unavoidable impurities, and a mass ratio indicated by (Mn+Si)/Al among Mn, Si, and Al is 200 or more. An oxide-based nonmetallic inclusion comprises MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO, where Al.sub.2O.sub.3 is 30% by mass or less, Cr.sub.2O.sub.3 is 5% by mass or less, and MgO is 10% by mass or less, and a sulfide-based nonmetallic inclusion is CaS whose maximum area is 100 μm.sup.2 or less.

Claims

1. A method of manufacturing an austenitic stainless steel, wherein a material is dissolved and refined, comprising the steps of: providing Si from 0.2 to 2.0% by mass, Mn from 0.3 to 5.0% by mass, S at 0.007% by mass or less, Ni from 7.0 to 15.0% by mass, Cr from 15.0 to 20.0% by mass, Al at 0.005% by mass or less, Ca at 0.002% by mass or less, Mg at 0.001% by mass or less, and O from 0.002 to 0.0065% by mass; and setting a ratio, during refining, of CaO/SiO.sub.2 in a slag between 1.0 and 2.5, wherein the remainder comprises Fe and unavoidable impurities, and a mass ratio indicated by (Mn+Si)/Al among Mn, Si, and Al is 200 or more, wherein an oxide-based nonmetallic inclusion comprises MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO, where Al .sub.2O.sub.3 is 30% by mass or less, Cr.sub.2O.sub.3 is 5% by mass or less, and MgO is 10% by mass or less; wherein a sulfide-based nonmetallic inclusion is CaS whose maximum area is 100 μm.sup.2 or less.

Description

DETAILED DESCRIPTION

[0011] An embodiment of the present invention will be explained in detail.

[0012] First, the cause for scratches and stain-like defects was investigated for products obtained by performing bright annealing for stainless steel plates having various plate thicknesses followed by buffing. Upon investigation, it was found that in the stainless steel plates with scratches, the components of the oxide-based nonmetallic inclusion included oxides such as Al.sub.2O.sub.3 at a concentration higher than 30% by mass, Cr.sub.2O.sub.3 at a concentration higher than 5% by mass, and MgO at a concentration higher than 10% by mass. Thus, it was understood that scratches could be prevented from occurring by setting so that the oxide-based nonmetallic inclusion consists mainly of MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO, where Al.sub.2O.sub.3 is 30% by mass or less, Cr.sub.2O.sub.3 is 5% by mass or less, and MgO is 10% by mass or less.

[0013] It was found that the cause for the stain-like defect was CaS, which was a sulfide-based nonmetallic inclusion, and when the area of one CaS was greater than 100 μm.sup.2, it resulted in a stain-like defect that could be observed by visual inspection. Because the cause of occurrence of the stain-like defect is the fact that CaS is a water-soluble sulfide, due to the reaction of CaS with the moisture in the usage environment, CaS is thought to be eluted from the surface of the steel plate. It must be noted that the stain-like defect is determined based on whether or not it can be observed visually, and 100 μm.sup.2 is the reference. A size of 100 μm.sup.2 or less is acceptable in quality as an industrial product. Furthermore, in the usage described in the present invention, the environment was mild from the viewpoint of corrosion resistance, and no deterioration into rusting due to the progress in the stain-like defect was observed.

[0014] Hereinafter, the components included in the stainless steel according to the present invention and their content will be explained.

[0015] [Si: 0.2 to 2.0% By Mass]

[0016] Si is a component used for deoxidation of molten steel and constitutes the oxide-based nonmetallic inclusion as SiO.sub.2. When the content of Si is less than 0.2% by mass, insufficient deoxidation occurs, the content of O in the stainless steel exceeds 0.01% by mass, and the Cr.sub.2O.sub.3 concentration in the oxide-based nonmetallic inclusion becomes more than 5% by mass, of which oxide, the cause for scratches, is generated. In addition, if the content of O in the steel exceeds 0.01% by mass, the S concentration in the steel increases in most cases, and a coarse sulfide-based nonmetallic inclusion is generated that causes stain-like defects. On the other hand, if the content of Si exceeds 2.0% by mass, the steel plate becomes hard, a large number of passes are required for rolling the steel plate to a predetermined thickness at the time of manufacturing a thin plate by cold working, and also an annealing step may be needed for certain plate thicknesses, thereby leading to a decline in productivity and increase in production costs. Therefore, the content of Si was set to 0.2% by mass or more and 2.0% by mass or less.

[0017] [Mn: 0.3 to 5.0% By Mass]

[0018] The same as Si, Mn is a component used for deoxidation of molten steel and constitutes the oxide-based nonmetallic inclusion as MnO. If the content of Mn is less than 0.3% by mass, it becomes difficult to generate MnO, which is an oxide component for preventing scratches. On the other hand, if the content of Mn exceeds 5.0% by mass, coarse MnS-based sulfides are generated easily when the S concentration is high. In such a case, cracks in bending may occur easily. Therefore, the content of Mn was set to 0.3% by mass or more and 5.0% by mass or less.

[0019] [S: 0.007% by Mass or Less]

[0020] S reacts with Ca to form a sulfide-based nonmetallic inclusion. If the content of S exceeds 0.007% by mass, a large sulfide having an area of 100 μm.sup.2 or more per sulfide is generated, which may cause stain-like defects to occur. Therefore, the content of S was set to 0.007% by mass or less.

[0021] [Ni: 7.0 to 15.0% By Mass]

[0022] Ni is the main component of austenitic stainless steel and must be present in an amount of 7.0% by mass or more in order to secure corrosion resistance and processability. However, since Ni is a relatively expensive element, in view of the production cost, the content of Ni was set to 7.0% by mass or more and 15.0% by mass or less.

[0023] [Cr: 15.0 to 20.0% By Mass]

[0024] Cr is the main component of stainless steel and must be present in an amount of 15.0% by mass or more in order to secure corrosion resistance. On the other hand, if the content of Cr exceeds 20.0% by mass, it may cause the material to harden and the processability to deteriorate. Therefore, the content of Cr was set to 15.0% by mass or more and 20.0% by mass or less.

[0025] [Al: 0.005% by Mass or Less]

[0026] Al has a stronger oxygen affinity than Si and Mn, and if the content exceeds 0.005% by mass, an oxide-based nonmetallic inclusion containing Al.sub.2O.sub.3 at more than 30% by mass, which becomes the source of occurrence of scratches, is generated. Therefore, the content of Al was set to 0.005% by mass or less.

[0027] [Ca: 0.002% by Mass or Less]

[0028] Ca is an element that greatly affects the composition of the oxide-base nonmetallic inclusion and the sulfide-based nonmetallic inclusion. Furthermore, Ca forms CaS, which is a cause of occurrence of stain-like defects. If the content of Ca exceeds 0.002% by mass, a large sulfide having a size of 100 μm.sup.2 or more is generated, which may cause stain-like defects to occur. Therefore, the content of Ca was set to 0.002% by mass or less.

[0029] [Mg: 0.001% by Mass or Less]

[0030] Mg has an even stronger oxygen affinity than Al, and if the content exceeds 0.001% by mass, the MgO present in the oxide-based nonmetallic inclusion becomes more than 10% by mass, resulting in scratches. Therefore, the content of Mg was set to 0.001% by mass or less.

[0031] [0: 0.002 to 0.01% By Mass]

[0032] O is a constituent element of the oxide-based nonmetallic inclusion, however, if the content of O is less than 0.002% by mass, MgO, which is the primary oxide of a lining refractory used during ladle and continuous casting, is likely to be reduced, and the content of Mg may exceed the upper limit of 0.001% by mass. On the other hand, if the content of O exceeds 0.01% by mass, a Cr.sub.2O.sub.3-based inclusion, which is the cause for scratches, is generated. Therefore, the content of O was set to 0.002% by mass or more and 0.01% by mass or less.

[0033] [Mass Ratio of Mn, Si, and Al in the Stainless Steel]

[0034] The composition of the oxide-based nonmetallic inclusion that causes scratches can be controlled by restricting the relationship of the mass ratio of Mn, Si, and Al in the stainless steel. That is, as regards the mass ratio of Mn, Si, and Al, if the value of (Mn+Si)/Al is less than 200, the concentration of Al.sub.2O.sub.3 in the oxide increases, and an oxide-based nonmetallic inclusion, which is the cause for scratches, is generated. Therefore, as regards the relationship of the mass ratio of Mn, Si, and Al in the stainless steel, the value of (Mn+Si)/Al was set to 200 or more.

[0035] [Components of Oxide-Based Nonmetallic Inclusion in Stainless Steel]

[0036] Scratches can be prevented if the oxide-based nonmetallic inclusion in the stainless steel consists mainly of MnO—SiO.sub.2—Al.sub.20.sub.3—CaO, and in the oxide-based nonmetallic inclusion, Al.sub.2O.sub.3 is at 30% by mass or less, Cr.sub.2O.sub.3 is at 5% by mass or less, and MgO at 10% by mass or less. If the Al.sub.2O.sub.3 concentration, the Cr.sub.2O.sub.3 concentration, and the MgO concentration in the oxide exceed the upper limit described above, a rigid inclusion is formed, and scratches occur at the time of buffing because the inclusion is harder than the material. If the Al.sub.2O.sub.3 concentration, the Cr.sub.2O.sub.3 concentration, and the MgO concentration in the oxide are confined within the upper limit described above, the melting point of the oxide-based nonmetallic inclusion lowers, of which at the time of hot working of the ingot, a viscous deformation of the inclusion occurs at the temperature of hot working, and the inclusion is dispersed into extremely minute inclusions at the time of cold working, and as a result, scratches are no longer observed during buffing.

[0037] [Size of One Sulfide in Sulfide-Based Nonmetallic Inclusion in Stainless Steel]

[0038] The sulfide-based nonmetallic inclusion observed in the above austenitic stainless steel is CaS. As described above, the sulfides are water soluble of which if a sulfide is large with an area of more than 100 μm.sup.2, stain-like defects, which can easily be determined visually, occur. Thus, if the area per one sulfide-based nonmetallic inclusion is controlled below 100 μm.sup.2, stain-like defects cannot be determined visually, which enables practical use as a product. In order to thus control the size of the sulfide-based nonmetallic inclusion, the composition of the stainless steel must be such that S≦0.007% by mass, Ca≦0.002% by mass, and O≦0.01% by mass.

EXAMPLES

[0039] 80 tons of austenitic stainless steel having each of the compositions of the 14 charges shown in Table 1 were melted to form a slab through each of the processes of the electric furnace, converter reactor, vacuum oxygen decarburization (VOD) refining, and CC process, in that order. It must be noted that during reducing refining in VOD, while the basicity of the CaO/SiO.sub.2 of the slag used in accordance with the charge was varied up to 1.0 to 2.5, the concentration of Mn, Si, and Al used as the desalting agents was also varied.

TABLE-US-00001 TABLE 1 Components of stainless steel of each charge that is melted in example (% by mass) Charge NO. Si Mn Ni Cr Al S O Ca Mg A001 0.2 0.5 8.0 18.1 0.001 0.0012 0.0025 0.0011 0.0010 A002 0.6 0.3 8.2 18.8 0.001 0.0038 0.0054 0.0020 0.0007 A003 2.0 0.3 9.1 17.5 0.005 0.0014 0.0048 0.0018 0.0004 A004 1.0 2.5 7.0 17.9 0.002 0.0044 0.0065 0.0014 0.0003 A005 0.5 5.0 8.4 18.3 0.001 0.0068 0.0098 0.0007 0.0001 A006 0.6 0.8 8.3 18.2 0.004 0.0024 0.0080 0.0016 0.0009 A007 0.4 0.7 15.0 15.1 0.002 0.0009 0.0031 0.0015 0.0005 B001 0.1 3.0 8.6 18.5 0.002 0.0075 0.0107 0.0018 0.0001 B002 0.4 0.6 8.3 18.2 0.008 0.0008 0.0020 0.0025 0.0015 B003 0.2 0.9 9.7 17.1 0.006 0.0012 0.0035 0.0014 0.0009 B004 0.5 0.6 14.8 15.9 0.002 0.0024 0.0010 0.0025 0.0012 B005 0.3 0.2 8.7 18.2 0.003 0.0017 0.0023 0.0012 0.0006 B006 0.8 5.1 8.5 18.6 0.001 0.0092 0.0087 0.0006 0.0001 B007 0.3 0.6 8.6 18.1 0.001 0.0068 0.0112 0.002 0.0008

[0040] Thereafter, each slab was subjected to hot rolling.fwdarw.cold rolling.fwdarw.acid pickling.fwdarw.cold rolling.fwdarw.bright annealing to form a cold-rolled coil of 1.0 mm thickness, and a steel plate was extracted from the coil. After performing buffing and mirror finishing for each steel plate obtained in this manner, the scratch occurrence status was investigated. To investigate the stain-like defects occurrence status, the CASS test was performed for a specimen. In the CASS test, 5% NaCl was adjusted with acetic acid to a pH value of 3, the liquid temperature was set to 50° C., and then sprayed on the specimen for 16 hours. After the completion of spraying, the occurrence status of stain-like defects was investigated.

[0041] Table 2 shows the components of each specimen, the composition of the oxide-based nonmetallic inclusion, the type and area of the sulfide-based nonmetallic inclusion, and the occurrence status of scratches and stain-like defects. The area of the sulfide-based nonmetallic inclusion was determined by photographing the sulfide with an electron microscope, and then tracing the outer periphery of the sulfide with a planimeter.

TABLE-US-00002 TABLE 2 Components of each specimen, composition of the oxide-based nonmetallic inclusion, type and area of the sulfide-based nonmetallic inclusion, and occurrence status of scratch and stain-like defect. Ratio of Sample Si Mn Al (Mn + Si)/ S O Ca Mg Classification No. (%) (%) (%) Al (%) (%) (%) (%) Present A001 0.2 0.5 0.001  700 0.0012 0.0025 0.0011 0.0010 Example A002 0.6 0.3 0.001  900 0.0038 0.0054 0.0020 0.0007 A003 2.0 0.3 0.005  200 0.0014 0.0048 0.0018 0.0004 A004 1.0 2.5 0.002 1750 0.0044 0.0065 0.0014 0.0003 A005 0.5 5.0 0.001 1750 0.0068 0.0098 0.0007 0.0001 A006 0.6 0.8 0.004  433 0.0024 0.0080 0.0016 0.0009 A007 0.4 0.7 0.002  550 0.0009 0.0031 0.0015 0.0005 Comparative B001 0.1 3.0 0.002 1550 0.0075 0.0107 0.0018 0.0001 example B002 0.4 0.6 0.008  125 0.0008 0.0020 0.0025 0.0015 B003 0.2 0.9 0.006  183 0.0012 0.0035 0.0014 0.0009 B004 0.5 0.6 0.002  550 0.0024 0.0010 0.0025 0.0012 B005 0.3 0.2 0.003  167 0.0017 0.0023 0.0012 0.0006 B006 0.8 5.1 0.001 4300 0.0092 0.0087 0.0006 0.0001 B007 0.3 0.6 0.001  900 0.0068 0.0112 0.0020 0.0008 Concentration of each Type of Presence oxide in oxide-based sulfide-based Max. area Presence of Components Of Oxide-based nonmetallic inclusion nonmetallic of CaS of stain-like Classification nonmetallic inclusion Al.sub.2O.sub.3 Cr.sub.2O.sub.3 MgO inclusion (μm.sup.2/piece) scratch defect Present MnO—SiO.sub.2—Al.sub.2O.sub.3—MgO 20.5 <0.1 9.4 CaS  28 No No Example MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO—MgO 15.6 <0.1 3.8  53 No No MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO—MgO 29.1 0.2 0.8  23 No No MnO—SiO.sub.2—Al.sub.2O.sub.3—Cr.sub.2O.sub.3 22.8 2.5 0.1  79 No No MnO—SiO.sub.2—Al.sub.2O.sub.3—Cr.sub.2O.sub.3 5.4 4.7 <0.1  90 No No MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO—Cr.sub.2O.sub.3— 18.7 3.9 5.9  85 No No MgO MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO—MgO— 9.4 0.3 1.3  35 No No Cr.sub.2O.sub.3 Comparative MnO—Cr.sub.2O.sub.3 <0.1 5.5 <0.1 MnS Not Yes *2 example observed Al.sub.2O.sub.3—MgO—CaO 68.6 <0.1 24.7 CaS 252 Yes Yes Al.sub.2O.sub.3—MgO 69.7 <0.1 30.2 CaS  32 Yes No Al.sub.2O.sub.3—MgO 67.2 <0.1 31.8 CaS 125 Yes Yes Al.sub.2O.sub.3—MgO 68.8 <0.1 30.9 CaS  38 Yes No MnO—SiO.sub.2—Al.sub.2O.sub.3—Cr.sub.2O.sub.3 10.4 4.1 <0.1 MnS Not No *2 MnO—SiO.sub.2—Cr.sub.2O.sub.3 <0.1 8.4 <0.1 MnS observed Yes *2 *1 The underlined components in the comparative examples indicate concentrations out of the conditions of the present invention *2 As regards comparative examples B001, B006, and B007, at the time of bending, long linear MnS-based sulfides existed in the bent ridgeline, and therefore, a flex crack occurred.

[0042] The oxide-based nonmetallic inclusion observed in sample No. A001 through A007, which constitutes the present example according to the present invention, was a MnO—SiO.sub.2—Al.sub.2O.sub.3—CaO—MgO system, with the Al.sub.2O.sub.3 concentration in the oxide at 30% by mass or less, the Cr.sub.2O.sub.3 concentration at 5% by mass or less, and the MgO concentration at 10% by mass or less, and no scratches were observed on the surface of the steel plate. Furthermore, the maximum size of CaS observed as the sulfide-based nonmetallic inclusion was 100 μm.sup.2 or less, and a stain-like defect was not observed.

[0043] On the other hand, in sample No. B001, which constitutes the comparative example, the Si concentration was low which led to insufficient deoxidation, the O concentration in the steel was 0.0107% by mass, which is outside the specified range, and in addition, desulfurization was insufficient and the S concentration was 0.0075% by mass. As a result, a rigid MnO—Cr.sub.2O.sub.3 was generated as the oxide-based nonmetallic inclusion, and the inclusion was the cause for scratches.

[0044] In sample No. B002, the Al concentration was high at 0.008% by mass, the value of (Mn+Si)/Al was less than 200, and the Mg concentration was high at 0.0015% by mass. As for the oxide-based nonmetallic inclusion, a rigid Al .sub.2O.sub.3—MgO based inclusion was observed, and scratches were seen on the surface of the steel plate. In addition, the Ca concentration was 0. 0025% by mass, which exceeded the specified range, and therefore, CaS having an area in excess of 100 μm.sup.2 was generated, and a stain-like defect was also observed.

[0045] In sample No. B003, the Al concentration was 0.006% by mass, and therefore, an Al.sub.2O.sub.3—MgO based inclusion was observed. A scratch was observed on the surface of the steel plate.

[0046] In sample No. B004, although the Al concentration in the steel satisfied the specified conditions, the O concentration in the steel was low and the oxygen potential in the molten steel was also low, of which a reduction reaction (MgO=Mg +O) of the MgO included at approximately 40% by mass in the refractory was promoted, the Mg picked up in the molten steel and Mg concentration exceeded the specified range to become 0.0012% by mass. Therefore, an Al .sub.2O.sub.3—MgO based inclusion was generated, and a scratch and a stain-like defect were seen.

[0047] In sample No. B005, Mn was low at 0.2% by mass, the value of (Mn+Si)/Al was less than 200, an Al.sub.2O.sub.3—MgO based inclusion was generated, and a scratch was observed.

[0048] In sample No. B006, the Mn concentration was 5.1% by mass and exceeded the specified range, and also, the S concentration was high at 0.0092% by mass.

[0049] Therefore, a MnS-based sulfide was generated, and a crack in bending occurred.

[0050] In sample No. B007, the O concentration was 0.0112% by mass which led to insufficient deoxidation, and as a result, the Cr.sub.2O.sub.3 concentration in the oxide-based nonmetallic inclusion exceeded 5.0% by mass, and a rigid inclusion was generated. A scratch occurred as a result.

[0051] The present invention, for example, is employed as austenitic stainless steel for use in home equipment and electrical and electronic device components where surface glossiness is considered important.