stainless steel

Abstract

To provide a stainless steel which is excellent in manufacturability and has a good surface quality.SOLUTION: A stainless steel that is an austenitic stainless steel or a dual-phase stainless steel having prescribed chemical compositions contains inclusions containing at least one or more of S and O and having minor axes of 3 μm or more, wherein number density of the inclusions is 5 pieces / mm2 or less, number density of the inclusion having minor axes of 15 μm or more out of the inclusions is 0.05 pieces / mm2 or less, and when the inclusion satisfying the following [CaO+Al2O3+MgO+ROX+Ta2O5≥90%] and [Al2O3 / MgO≤1.25] out of the inclusions is an MgO-based inclusion, a number ratio of the MgO-based inclusion with respect to the above-firstly-described inclusions is 75% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to stainless steel. [Background technology]

[0002] Stainless steel is prone to the formation of nonmetallic inclusions such as MgO·Al2O3 spinel. Such nonmetallic inclusions are not easily crushed or stretched during processing such as rolling. As a result, they may be exposed on the surface during processing, causing surface defects. Therefore, for example, Patent Document 1 discloses stainless steel in which the composition of the above-mentioned inclusions is controlled to suppress the occurrence of surface defects such as silvering defects.

[0003] Furthermore, Patent Document 2 discloses stainless steel in which MgO·Al2O3 spinel, which degrades surface properties, has been rendered harmless by controlling the slag component to an appropriate range. Similarly, Patent Document 3 discloses stainless steel in which the formation of MgO·Al2O3 spinel has been suppressed by controlling the basicity of the slag and reducing the amount of Al in the steel. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-104950 [Patent Document 2] Japanese Patent Application Publication No. 9-256028 [Patent Document 3] Japanese Patent Publication No. 2015-74807 [Overview of the project] [Problems that the invention aims to solve]

[0005] On the other hand, the stainless steels disclosed in the aforementioned Patent Documents 1 to 3 have room for improvement in the following respects. Specifically, in the stainless steel disclosed in Patent Document 1, it is necessary to control the molten steel composition within a predetermined range in order to control the composition of inclusions, but it is difficult to stably control the molten steel composition.

[0006] In addition, for the stainless steel disclosed in Patent Document 2, in order to detoxify MgO·Al2O3 spinel, it is necessary to reduce SiO2 in the slag. Therefore, there may be restrictions when using Si alloys. Furthermore, for the stainless steel disclosed in Patent Document ③, it is considered that the liquid phase ratio is low during actual production, and the refining efficiency is likely to decrease.

[0007] Thus, the stainless steels disclosed in Patent Documents 1 to 3 above still have room for further improvement from the perspective of manufacturability. Also, there is room for further improvement in terms of surface properties.

[0008] An object of the present invention is to solve the above problems and provide a stainless steel having excellent manufacturability and good surface properties.

Means for Solving the Problems

[0009] [[ID=I7]] The present invention has been made to solve the above problems, and its gist is the following stainless steel and manufacturing method.

[0010] (1) An austenitic stainless steel or duplex stainless steel containing inclusions, The chemical composition is in mass%, C: 0.005 to 0.15%, Si: 0.05 to 4.0%, Mn: 12.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0 to 28.0%, Cr: 14.0 to 28.0%, N: 0.005 to 0.40%, O: 0.0001 to 0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0 to 8.0%, Cu: 0 to 4.0%, REM: 0 to 0.050%, Ta: 0 to 0.10%, B: 0 to 0.004%, W: 0 to 4.0%, V: 0 to 1.0%, Ti: 0 to 0.40%, Nb: 0 to 0.60%, Sn: 0 to 0.1%, Sb: 0 to 0.30%, Co: 0 to 1.0%, Zr: 0 to 0.005%, Ga: 0 to 0.010%, The balance: Fe and impurities, The inclusions contain at least one of S and O, and have a minor diameter of 3 μm or more, The number density of the inclusions is 5 pieces / mm 2 or less, Among the inclusions, the number density of the inclusions with a minor diameter of 15 μm or more is 0.05 pieces / mm 2 or less, Among the inclusions, when the inclusions satisfying the following (i) and (ii) formulas are defined as MgO-based inclusions, A stainless steel in which the number ratio of the MgO-based inclusions to the inclusions is 75% or more. CaO + Al2O3 + MgO + RO X + Ta2O5 ≥ 90% ··· (i) Al2O3 / MgO ≤ 1.25 ··· (ii) However, RO in the above formula (i) x represents REM oxide, and CaO, Al2O3, MgO, RO x in the above formulas (i) and (ii), and Ta2O5 represent the mass% of each oxide in the inclusions, and are set to zero if not contained.

[0011] (2) An austenitic stainless steel, The chemical composition is in mass%, C: 0.005 to 0.15%, Si: 0.2 to 4.0%, Mn: 0.1 to 12.0%, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0~28.0%, Cr: 14.0~28.0%, N: 0.005~0.40%, O: 0.0001~0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0~8.0%, Cu: 0~4.0%, REM: 0~0.050%, Ta: 0~0.10%, B: 0~0.004%, W: 0~2.0%, V: 0~1.0%, Ti: 0~0.40%, Nb: 0~0.60%, Sn: 0~0.1%, Sb: 0~0.30%, Co: 0~1.0%, Zr: 0~0.005%, Ga: 0~0.010%, Stainless steel as described in (1) above, which contains the above.

[0012] (3) Austenitic stainless steel, The aforementioned chemical composition is, in mass%, Mo: 0.01~8.0%, Cu: 0.01~4.0%, REM: 0.0005~0.050%, Ta: 0.001~0.10%, B: 0.0001~0.004%, W: 0.05~2.0%, V: 0.05~1.0%, Ti: 0.001~0.40%, Nb: 0.001~0.60%, Sn: 0.01~0.1%, Sb: 0.005~0.30%, Co: 0.03~1.0%, Zr: 0.0001~0.005%, and Ga: 0.0001~0.010%, The stainless steel described in (2) above, comprising one or more selected from the above.

[0013] (4) Duplex stainless steel, The aforementioned chemical composition is, in mass%, C: 0.005~0.08%, Si: 0.05~1.0%, Mn: 6.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0~14.0%, Cr: 16.0~28.0% N: more than 0.005% and less than 0.35%, O: 0.0001~0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0.05~5.5%, Cu: 0~2.0%, REM: 0~0.050%, Ta: 0~0.10%, B: 0~0.004%, W: 0~4.0%, V: 0~1.0%, Ti: 0~0.40%, Nb: 0~0.10%, Sn: 0~0.1%, Sb: 0~0.30%, Co: 0~1.0%, Zr: 0~0.005%, Ga: 0~0.010%, Stainless steel as described in (1) above, which contains the above.

[0014] (5) Duplex stainless steel, The aforementioned chemical composition is, in mass%, Cu: 0.05~2.0%, REM: 0.0005~0.050%, Ta: 0.001~0.10%, B: 0.0001~0.004%, W: 0.05~4.0%, V: 0.05~1.0%, Ti: 0.001~0.40%, Nb: 0.001~0.10%, Sn: 0.01~0.1%, Sb: 0.005~0.30%, Co: 0.03~1.0%, Zr: 0.0001~0.005%, and Ga: 0.0001~0.010%, The stainless steel according to (4) above, containing one or more selected from

[0015] (6) When inclusions satisfying the following formulas (i) and (iii) among the inclusions are defined as MgO-based inclusions, The stainless steel according to any one of (1) to (5) above, wherein the number ratio of the MgO-based inclusions to the inclusions is 75% or more. CaO + Al2O3 + MgO + RO X + Ta2O5 ≧ 90% ··· (i) Al2O3 / MgO ≦ 0.75 ··· (iii) However, RO in the above formula (i) x represents REM oxide, and CaO, Al2O3, MgO, RO x and Ta2O5 in the above formulas (i) and (iii) represent the mass % of each oxide in the inclusions, and are taken as zero when not contained.

[0016] (7) When inclusions satisfying the following formulas (i), (ii) and (iv) among the inclusions are defined as MgO-based inclusions, The stainless steel according to any one of (1) to (5) above, wherein the number ratio of the MgO-based inclusions to the inclusions is 90% or more. CaO + Al2O3 + MgO + RO X + Ta2O5 ≧ 90% ··· (i) Al2O3 / MgO ≤ 1.25 ··· (ii) CaO > 20% ··· (iv) However, RO in equation (i) x This represents a REM oxide, and in formulas (i), (ii), and (iv), CaO, Al2O3, MgO, RO x Ta2O5 represents the mass percentage of each oxide in the inclusions, and is set to zero if it is not present. [Effects of the Invention]

[0017] According to the present invention, stainless steel with excellent manufacturability and good surface properties can be obtained. [Modes for carrying out the invention]

[0018] The inventors investigated inclusions that degrade surface properties and obtained the following findings (a) to (c).

[0019] (a) When stainless steel is manufactured, surface defects may occur if MgO·Al2O3 spinel is formed. This is because MgO·Al2O3 spinel is difficult to crush and extend during processes such as rolling, and as the thickness of the steel decreases, it becomes exposed on the surface. As a result, surface defects are formed. Therefore, in order to suppress the occurrence of surface defects and improve the surface properties, it is desirable to suppress the formation of MgO·Al2O3 spinel.

[0020] (b) Therefore, it is desirable to suppress the formation of MgO·Al2O3 spinel while preferentially forming MgO-based inclusions. This is because MgO-based inclusions containing a large amount of MgO are easier to break during processing, and the occurrence of surface defects is suppressed. In order to preferentially form MgO-based inclusions, it is effective to control the activity of MgO and other oxides in the slag to a predetermined range.

[0021] (c) In addition, controlling the size of the inclusions themselves is also effective in suppressing surface defects. Specifically, it is effective to control the number density of inclusions with a short diameter of 3 μm or more, which are likely to cause surface defects. Similarly, it is also effective to control the number density of inclusions with a short diameter of 15 μm or more, which are particularly likely to cause surface defects.

[0022] One embodiment of this invention is based on the above findings. The requirements of this embodiment will be described in detail below.

[0023] 1. Stainless steel type The stainless steel in this embodiment contains inclusions described later. The type of stainless steel is either austenitic stainless steel or duplex stainless steel. Duplex stainless steel refers to stainless steel having a metallic structure in which austenite and ferrite phases are present in equal proportions. Any general duplex stainless steel will suffice.

[0024] 2.Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0025] The chemical composition of the stainless steel in this embodiment is as follows (in mass%): C: 0.005~0.15%, Si: 0.05~4.0%, Mn: 12.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0~28.0%, Cr: 14.0~28.0%, N: 0.005~0.40%, O: 0.0001~0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0. The composition is as follows: 0.10% or less, Mo: 0-8.0%, Cu: 0-4.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0-0.004%, W: 0-4.0%, V: 0-1.0%, Ti: 0-0.40%, Nb: 0-0.60%, Sn: 0-0.1%, Sb: 0-0.30%, Co: 0-1.0%, Zr: 0-0.005%, Ga: 0-0.010%, the remainder being Fe and impurities.

[0026] The chemical composition of austenitic stainless steel is as follows (in mass%): C: 0.005-0.15%, Si: 0.2-4.0%, Mn: 0.1-12.0%, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0-28.0%, Cr: 14.0-28.0%, N: 0.005-0.40%, O: 0.0001-0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.01% Preferably, the composition is 0% or less, Mo: 0-8.0%, Cu: 0-4.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0-0.004%, W: 0-2.0%, V: 0-1.0%, Ti: 0-0.40%, Nb: 0-0.60%, Sn: 0-0.1%, Sb: 0-0.30%, Co: 0-1.0%, Zr: 0-0.005%, Ga: 0-0.010%, with the remainder being Fe and impurities.

[0027] The chemical composition of duplex stainless steel is, in mass %, C: 0.005~0.08%, Si: 0.05~1.0%, Mn: 6.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0~ 14.0%, Cr: 16.0~28.0%, N: over 0.005% and 0.35% or less, O: 0.0001~0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less Preferably, the composition is Mo: 0.05-5.5%, Cu: 0-2.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0-0.004%, W: 0-4.0%, V: 0-1.0%, Ti: 0-0.40%, Nb: 0-0.10%, Sn: 0-0.1%, Sb: 0-0.30%, Co: 0-1.0%, Zr: 0-0.005%, Ga: 0-0.010%, with the remainder being Fe and impurities.

[0028] Below, we will explain the reasons for the limitations on each element, dividing the materials into austenitic stainless steel and duplex stainless steel.

[0029] 2-1. Chemical composition of austenitic stainless steel C: 0.005~0.15% Carbon (C) reduces corrosion resistance by generating chromium carbides. It also reduces processability. For this reason, the carbon content is preferably 0.15% or less. More preferably 0.10% or less, and even more preferably 0.08% or less. On the other hand, excessively reducing the carbon content increases the deoxidation load after decarburization and increases the amount of Al2O3-based inclusions. For this reason, the carbon content is preferably 0.005% or more. More preferably 0.015% or more, and even more preferably 0.035% or more.

[0030] Si: 0.2~4.0% Si is an element that acts as a deoxidizing agent during the melting of stainless steel. For this reason, the Si content is preferably 0.2% or more. A Si content of 0.3% or more is more preferable, and 0.4% or more is even preferable. However, if the Si content is excessive, the workability decreases. For this reason, the Si content is preferably 4.0% or less. A Si content of 2.0% or less is more preferable, and 0.8% or less is even preferable.

[0031] Mn: 0.1~12.0% Mn acts as a deoxidizing agent and also improves hot workability by fixing sulfur as MnS. It is also an element that contributes to the stabilization of austenite. For this reason, the Mn content is preferably 0.1% or more. A more preferable Mn content is 0.5% or more, and even more preferable is 0.7% or more. However, excessive Mn content reduces workability. For this reason, the Mn content is preferably 12.0% or less. A more preferable Mn content is 4.0% or less, and even more preferable is 2.0% or less.

[0032] P:0.04% or less P is an impurity element contained in steel that reduces toughness, hot workability, and corrosion resistance. For this reason, the P content is preferably 0.04% or less. More preferably, the P content is 0.03% or less. It is preferable to reduce the P content as much as possible, but if P is reduced too much, manufacturing costs will increase. For this reason, the P content is preferably 0.01% or more.

[0033] S: 0.0050% or less S is an impurity element contained in steel that reduces toughness, hot workability, and corrosion resistance. For this reason, it is preferable that the S content be 0.0050% or less. It is more preferable that the S content be 0.0030% or less. It is preferable to reduce the S content as much as possible, but if the S content is reduced too much, the manufacturing cost will increase. For this reason, it is preferable that the S content be 0.0001% or more.

[0034] Ni: 1.0~28.0% Ni has the effect of enhancing corrosion resistance. It also has the effect of stabilizing the austenite phase. For this reason, the Ni content is preferably 1.0% or more. A Ni content of 4.0% or more is more preferable, and 6.0% or more is even preferable. However, Ni is an expensive element, and excessive inclusion increases manufacturing costs. For this reason, the Ni content is preferably 28.0% or less. A Ni content of 16.0% or less is more preferable, and 12.0% or less is even preferable.

[0035] Cr: 14.0~28.0% Cr has the effect of improving corrosion resistance. For this reason, the Cr content is preferably 14.0% or more. A Cr content of 17.0% or more is more preferable, and 17.5% or more is even preferable. However, if the Cr content is excessive, the processability decreases. For this reason, the Cr content is preferably 28.0% or less. A Cr content of 24.0% or less is more preferable, and 23.0% or less is even preferable.

[0036] N: 0.005~0.40% N has the effect of improving strength and corrosion resistance. For this reason, the N content is preferably 0.005% or more. More preferably 0.010% or more, and even more preferably 0.020% or more. However, if the N content is excessive, intergranular corrosion due to sensitization is likely to occur. For this reason, the N content is preferably 0.40% or less. More preferably 0.25% or less, and even more preferably 0.23% or less.

[0037] O: 0.0001~0.0060% O is an impurity element contained in steel. Furthermore, O constitutes inclusions that cause surface defects. For this reason, the O content is preferably 0.0060% or less. More preferably 0.0030% or less, and even more preferably 0.0015% or less. On the other hand, excessively reducing the O content increases manufacturing costs. For this reason, the O content is preferably 0.0001% or more. More preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0038] Al: 0.20% or less Al is an element that acts as a deoxidizing agent for stainless steel. It also has the effect of improving corrosion resistance. However, if Al is included in excess, the workability decreases. For this reason, it is preferable that the Al content be 0.20% or less. It is more preferable that the Al content be 0.15% or less, and even more preferable that be 0.10% or less. On the other hand, in order to obtain the above effects, it is preferable that the Al content be 0.02% or more.

[0039] Ca:0.0035% or less Ca suppresses the formation of MgO·Al2O3 spinel, which causes surface defects. However, excessive Ca content reduces corrosion resistance. Therefore, it is preferable to have a Ca content of 0.0035% or less. It is more preferable to have a Ca content of 0.0025% or less, and even more preferable to have a Ca content of 0.0020% or less. On the other hand, in order to obtain the above effect, it is preferable to have a Ca content of 0.0003% or more.

[0040] Mg: 0.010% or less Mg is an element that makes up MgO. However, if Mg is included in excess, inclusions tend to form in excess, which contributes to a decrease in properties. For this reason, it is preferable that the Mg content be 0.010% or less. It is more preferable that the Mg content be 0.008% or less, and even more preferable that it be 0.005% or less. On the other hand, in order to obtain the above effects, it is preferable that the Mg content be 0.0005% or more.

[0041] In addition to the elements listed above, one or more elements selected from Mo, Cu, REM, Ta, B, W, V, Ti, Nb, Sn, Sb, Co, Zr, and Ga may be included within the ranges shown below. The reasons for limiting each element are explained below.

[0042] Mo: 0~8.0% Mo has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Mo is included in excess, a sigma phase is formed and the processability decreases. For this reason, the Mo content is preferably 8.0% or less. A Mo content of 6.0% or less is more preferable, and 3.0% or less is even more preferable. On the other hand, in order to obtain the above effect, a Mo content of 0.01% or more is preferable.

[0043] Cu: 0~4.0% Cu has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Cu is included in excess, the hot workability will decrease. For this reason, the Cu content is preferably 4.0% or less. It is more preferably 3.0% or less, and even more preferably 2.0% or less. On the other hand, in order to obtain the above effect, it is preferable that the Cu content be 0.01% or more.

[0044] REM: 0~0.050% Rare-earth metals (REMs) have a high affinity for oxygen (O) and have the effect of suppressing MgO·Al2O3 spinel, which causes surface defects. For this reason, they may be included as needed. However, excessive REM content can cause nozzle clogging during casting, reducing manufacturability. It also makes surface defects more likely to occur. Therefore, it is preferable that the REM content be 0.050% or less. It is more preferable that the REM content be 0.025% or less, and even more preferable that it be 0.010% or less. On the other hand, in order to obtain the above effects, it is preferable that the REM content be 0.0005% or more.

[0045] REM refers to a total of 17 elements including Sc, Y, and lanthanides, and the REM content mentioned above refers to the total content of these elements. Industrially, REM is often added in the form of mischmetal.

[0046] Ta: 0~0.10% Because Ta has a high affinity for O, it suppresses the formation of MgO·Al2O3 spinel, which causes surface defects. For this reason, it may be included as needed. However, excessive Ta content leads to a decrease in room-temperature ductility and toughness. Therefore, it is preferable that the Ta content be 0.10% or less. It is more preferable that the Ta content be 0.05% or less, and even more preferable that be 0.01% or less. On the other hand, in order to obtain the above effect, it is preferable that the Ta content be 0.001% or more.

[0047] B: 0~0.004% B has the effect of increasing the strength of grain boundaries. It also has the effect of improving processability. For this reason, it may be included as needed. However, if B is included in excess, the ductility decreases and the processability actually decreases. For this reason, it is preferable that the B content be 0.004% or less. It is more preferable that the B content be 0.0025% or less, and even more preferable that it be 0.002% or less. On the other hand, in order to obtain the above effects, it is preferable that the B content be 0.0001% or more.

[0048] W: 0~2.0% Water (W) has the effect of enhancing corrosion resistance. For this reason, it may be included as needed. However, water is an expensive element, and excessive amounts of water increase manufacturing costs. Therefore, it is preferable to keep the water content at 2.0% or less. It is more preferable to keep the water content at 1.5% or less, and even more preferable to keep it at 1.0% or less. On the other hand, in order to obtain the above effect, it is preferable to keep the water content at 0.05% or more.

[0049] V: 0~1.0% V has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if V is included in excess, toughness will decrease. For this reason, it is preferable that the V content be 1.0% or less. It is more preferable that the V content be 0.5% or less, and even more preferable that be 0.25% or less. On the other hand, in order to obtain the above effect, it is preferable that the V content be 0.05% or more.

[0050] Ti: 0~0.40% Ti is an element that affects the ferrite phase ratio and may be included as needed. However, if Ti is included in excess, TiN will be generated before casting, which promotes nozzle clogging and reduces manufacturability. It may also degrade the surface properties. For this reason, the Ti content is preferably 0.40% or less. It is more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, in order to obtain the above effects, it is preferable that the Ti content be 0.001% or more.

[0051] Nb: 0~0.60% Nb has the effect of improving moldability and corrosion resistance. For this reason, it may be included as needed. However, if Nb is included in excess, toughness will decrease. For this reason, it is preferable that the Nb content be 0.60% or less. It is more preferable that the Nb content be 0.10% or less, and even more preferable that be 0.08% or less. On the other hand, in order to obtain the above effects, it is preferable that the Nb content be 0.001% or more.

[0052] Sn: 0~0.1% Sn has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Sn is included in excess, the processability will decrease. For this reason, it is preferable that the Sn content be 0.1% or less. It is more preferable that the Sn content be 0.05% or less, and even more preferable that be 0.02% or less. On the other hand, in order to obtain the above effect, it is preferable that the Sn content be 0.01% or more.

[0053] Sb: 0~0.30% Sb has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Sb is included in excess, it may promote TiN formation and cause surface defects, so it is preferable that the Sb content be 0.30% or less. It is more preferable that the Sb content be 0.10% or less, and even more preferable that it be 0.05% or less. On the other hand, in order to obtain the above effect, it is preferable that the Sb content be 0.005% or more.

[0054] Co: 0~1.0% Co has the effect of enhancing corrosion resistance. For this reason, it may be included as needed. However, since Co is a very expensive element, excessive Co content increases manufacturing costs. For this reason, it is preferable to keep the Co content at 1.0% or less. It is more preferable to keep the Co content at 0.7% or less, and even more preferable to keep it at 0.4% or less. On the other hand, in order to obtain the above effect, it is preferable to keep the Co content at 0.03% or more.

[0055] Zr: 0~0.005% Zr has the effect of improving corrosion resistance by fixing sulfur. For this reason, it may be included as needed. However, if Zr is included in excess, coarse sulfides will form in the molten steel, which will actually decrease corrosion resistance. For this reason, the Zr content is preferably 0.005% or less. It is more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, in order to obtain the above effect, it is preferable that the Zr content be 0.0001% or more.

[0056] Ga: 0~0.010% Ga has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Ga is included in excess, it is preferable that the Ga content be 0.010% or less. It is more preferable that the Ga content be 0.005% or less, and even more preferable that it be 0.003% or less. On the other hand, in order to obtain the above effect, it is preferable that the Ga content be 0.0001% or more.

[0057] In the chemical composition of the austenitic stainless steel of this embodiment, the remainder consists of Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of stainless steel due to various factors in the raw materials such as ore and scrap, and in the manufacturing process, and which are acceptable as long as they do not adversely affect the steel of this embodiment.

[0058] 2-2. Chemical composition of duplex stainless steel C: 0.005~0.08% Carbon (C) reduces corrosion resistance by generating chromium carbides. It also reduces processability. For this reason, the C content is preferably 0.08% or less. More preferably 0.065% or less, and even more preferably 0.025% or less. On the other hand, if the C content is reduced excessively, the deoxidation load after decarburization increases, and the amount of Al2O3-based inclusions increases. For this reason, the C content is preferably 0.005% or more. More preferably 0.010% or more, and even more preferably 0.015% or more.

[0059] Si: 0.05~1.0% Si is an element that acts as a deoxidizing agent during the melting of stainless steel. For this reason, the Si content is preferably 0.05% or more. A Si content of 0.2% or more is more preferable, and 0.5% or more is even preferable. However, if the Si content is excessive, the workability decreases. For this reason, the Si content is preferably 1.0% or less. A Si content of 0.9% or less is more preferable, and 0.8% or less is even preferable.

[0060] Mn: 6.0% or less Mn acts as a deoxidizing agent and also improves hot workability by fixing sulfur as MnS. It is also an element that contributes to the stabilization of austenite. However, excessive Mn content reduces workability. For this reason, the Mn content is preferably 6.0% or less. More preferably 5.5% or less, and even more preferably 3.5% or less. On the other hand, to obtain the above effects, the Mn content is preferably 0.5% or more, and more preferably 1.0% or more.

[0061] P:0.04% or less P is an impurity element contained in steel that reduces toughness, hot workability, and corrosion resistance. For this reason, the P content is preferably 0.04% or less. More preferably, the P content is 0.03% or less. It is preferable to reduce the P content as much as possible, but if P is reduced too much, manufacturing costs will increase. For this reason, the P content is preferably 0.01% or more.

[0062] S: 0.0050% or less S is an impurity element contained in steel that reduces toughness, hot workability, and corrosion resistance. For this reason, it is preferable that the S content be 0.0050% or less. It is more preferable that the S content be 0.0030% or less. It is preferable to reduce the S content as much as possible, but if the S content is reduced too much, the manufacturing cost will increase. For this reason, it is preferable that the S content be 0.0001% or more.

[0063] Ni: 1.0~14.0% Ni has the effect of improving corrosion resistance. It also has the effect of stabilizing the austenite phase. For this reason, the Ni content is preferably 1.0% or more. A Ni content of 2.0% or more is more preferable, and 4.0% or more is even preferable. However, Ni is an expensive element, and excessive inclusion increases manufacturing costs. For this reason, the Ni content is preferably 14.0% or less. A Ni content of 10.0% or less is more preferable, and 7.0% or less is even preferable.

[0064] Cr: 16.0~28.0% Cr has the effect of improving corrosion resistance. For this reason, the Cr content is preferably 16.0% or more. A Cr content of 20.0% or more is more preferable, and 22.0% or more is even preferable. However, if the Cr content is excessive, the processability decreases. For this reason, the Cr content is preferably 28.0% or less. A Cr content of 26.0% or less is more preferable, and 25.5% or less is even preferable.

[0065] N: More than 0.005% and less than 0.35% N has the effect of improving strength and corrosion resistance. For this reason, the N content is preferably greater than 0.005%. The N content is more preferably 0.010% or more, and even more preferably 0.10% or more. However, if the N content is excessive, intergranular corrosion due to sensitization is likely to occur. For this reason, the N content is preferably 0.35% or less. The N content is more preferably 0.25% or less, and even more preferably 0.18% or less.

[0066] O: 0.0001~0.0060% O is an impurity element contained in steel. Furthermore, O constitutes inclusions that cause surface defects. For this reason, the O content is preferably 0.0060% or less. More preferably 0.0030% or less, and even more preferably 0.0015% or less. On the other hand, excessively reducing the O content increases manufacturing costs. For this reason, the O content is preferably 0.0001% or more. More preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0067] Al: 0.20% or less Al is an element that acts as a deoxidizing agent for stainless steel. It also has the effect of improving corrosion resistance. However, if Al is included in excess, the workability decreases. For this reason, it is preferable that the Al content be 0.20% or less. It is more preferable that the Al content be 0.10% or less, and even more preferable that be 0.08% or less. On the other hand, in order to obtain the above effects, it is preferable that the Al content be 0.01% or more.

[0068] Ca:0.0035% or less Ca suppresses the formation of MgO·Al2O3 spinel, which causes surface defects. However, excessive Ca content reduces corrosion resistance. Therefore, it is preferable to have a Ca content of 0.0035% or less. It is more preferable to have a Ca content of 0.0025% or less, and even more preferable to have a Ca content of 0.0020% or less. On the other hand, in order to obtain the above effect, it is preferable to have a Ca content of 0.0003% or more.

[0069] Mg: 0.010% or less Mg is an element that makes up MgO. However, if Mg is included in excess, inclusions tend to form in excess, which contributes to a decrease in properties. For this reason, it is preferable that the Mg content be 0.010% or less. It is more preferable that the Mg content be 0.008% or less, and even more preferable that it be 0.005% or less. On the other hand, in order to obtain the above effect, it is preferable that the Mg content be 0.001% or more.

[0070] Mo: 0.05~5.5% Mo has the effect of improving corrosion resistance. For this reason, the Mo content is preferably 0.05% or more. A more preferable Mo content is 0.3% or more, and even more preferable is 1.5% or more. However, if Mo is included in excess, a sigma phase is formed, and the processability decreases. For this reason, the Mo content is preferably 5.5% or less. A more preferable Mo content is 4.0% or less, and even more preferable is 3.0% or less.

[0071] In addition to the elements listed above, one or more elements selected from Cu, REM, Ta, B, W, V, Ti, Nb, Sn, Sb, Co, Zr, and Ga may be included within the ranges shown below. The reasons for limiting each element are explained below.

[0072] Cu: 0~2.0% Cu has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Cu is included in excess, the hot workability will decrease. For this reason, the Cu content is preferably 2.0% or less. It is more preferably 1.0% or less, and even more preferably 0.8% or less. On the other hand, in order to obtain the above effect, it is preferable that the Cu content be 0.05% or more.

[0073] REM: 0~0.050% Rare-earth metals (REMs) have a high affinity for oxygen (O) and have the effect of suppressing the formation of MgO·Al2O3 spinel, which causes surface defects. For this reason, they may be included as needed. However, excessive REM content can cause nozzle clogging during casting, reducing manufacturability. Therefore, it is preferable to have a REM content of 0.050% or less. It is more preferable to have a REM content of 0.025% or less, and even more preferable to have a REM content of 0.010% or less. On the other hand, in order to obtain the above effect, it is preferable to have a REM content of 0.0005% or more.

[0074] Ta: 0~0.10% Because Ta has a high affinity for O, it suppresses the formation of MgO·Al2O3 spinel, which causes surface defects. For this reason, it may be included as needed. However, excessive Ta content leads to a decrease in room-temperature ductility and toughness. For this reason, the Ta content is preferably 0.10% or less. A Ta content of 0.05% or less is more preferable, and 0.01% or less is even more preferable. On the other hand, in order to obtain the above effects, a Ta content of 0.001% or more is preferable.

[0075] B: 0~0.004% B has the effect of increasing the strength of grain boundaries. It also has the effect of improving processability. For this reason, it may be included as needed. However, if B is included in excess, the ductility decreases and the processability actually decreases. For this reason, it is preferable that the B content be 0.004% or less. It is preferable that the B content be 0.0025% or less, and more preferably 0.002% or less. On the other hand, in order to obtain the above effects, it is preferable that the B content be 0.0001% or more.

[0076] W: 0~4.0% Water (W) has the effect of enhancing corrosion resistance. For this reason, it may be included as needed. However, water is an expensive element, and excessive amounts of water increase manufacturing costs. Therefore, it is preferable to keep the water content at 4.0% or less. It is more preferable to keep the water content at 2.0% or less, and even more preferable to keep it at 1.0% or less. On the other hand, in order to obtain the above effect, it is preferable to keep the water content at 0.05% or more.

[0077] V: 0~1.0% V has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if V is included in excess, toughness will decrease. For this reason, it is preferable that the V content be 1.0% or less. It is more preferable that the V content be 0.5% or less, and even more preferable that be 0.25% or less. On the other hand, in order to obtain the above effect, it is preferable that the V content be 0.05% or more.

[0078] Ti: 0~0.40% Ti is an element that affects the ferrite phase ratio and may be included as needed. However, if Ti is included in excess, TiN will be generated before casting, which promotes nozzle clogging and reduces manufacturability. It may also degrade the surface properties. For this reason, the Ti content is preferably 0.40% or less. It is more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, in order to obtain the above effects, it is preferable that the Ti content be 0.001% or more.

[0079] Nb: 0~0.10% Nb has the effect of improving moldability and corrosion resistance. For this reason, it may be included as needed. However, if Nb is included in excess, toughness will decrease. For this reason, it is preferable that the Nb content be 0.10% or less. It is more preferable that the Nb content be 0.05% or less, and even more preferable that be 0.04% or less. On the other hand, in order to obtain the above effects, it is preferable that the Nb content be 0.001% or more.

[0080] Sn: 0~0.1% Sn has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Sn is included in excess, the processability will decrease. For this reason, it is preferable that the Sn content be 0.1% or less. It is more preferable that the Sn content be 0.05% or less, and even more preferable that be 0.02% or less. On the other hand, in order to obtain the above effect, it is preferable that the Sn content be 0.01% or more.

[0081] Sb: 0~0.30% Sb has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Sb is included in excess, it may promote TiN formation and cause surface defects, so it is preferable that the Sb content be 0.30% or less. It is more preferable that the Sb content be 0.10% or less, and even more preferable that it be 0.05% or less. On the other hand, in order to obtain the above effect, it is preferable that the Sb content be 0.005% or more.

[0082] Co: 0~1.0% Co has the effect of enhancing corrosion resistance. For this reason, it may be included as needed. However, since Co is a very expensive element, excessive Co content increases manufacturing costs. For this reason, it is preferable to keep the Co content at 1.0% or less. It is more preferable to keep the Co content at 0.7% or less, and even more preferable to keep it at 0.4% or less. On the other hand, in order to obtain the above effect, it is preferable to keep the Co content at 0.03% or more.

[0083] Zr: 0~0.005% Zr has the effect of improving corrosion resistance by fixing sulfur. For this reason, it may be included as needed. However, if Zr is included in excess, coarse sulfides will form in the molten steel, which will actually decrease corrosion resistance. For this reason, the Zr content is preferably 0.005% or less. It is more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, in order to obtain the above effect, it is preferable that the Zr content be 0.0001% or more.

[0084] Ga: 0~0.010% Ga has the effect of improving corrosion resistance. For this reason, it may be included as needed. However, if Ga is included in excess, it is preferable that the Ga content be 0.010% or less. It is more preferable that the Ga content be 0.005% or less, and even more preferable that it be 0.003% or less. On the other hand, in order to obtain the above effect, it is preferable that the Ga content be 0.0001% or more.

[0085] In the chemical composition of the duplex stainless steel of this embodiment, the remainder consists of Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of stainless steel due to various factors in the raw materials such as ore and scrap, and in the manufacturing process, and which are acceptable as long as they do not adversely affect this embodiment.

[0086] 3.Inclusions In this embodiment, the stainless steel is controlled in terms of inclusions that affect the surface properties as follows. While there are various classifications of inclusions in general, in this embodiment, the inclusions are compounds that contain at least one of S and O and have a short diameter of 3 μm or more.

[0087] 3-1. Number density of inclusions When many large inclusions are formed, surface defects are more likely to occur. Therefore, the number density of the aforementioned inclusions is 5 inclusions / mm². 2The following conditions apply: The number density of inclusions is 5 inclusions / mm³. 2 If the number density is too high, surface defects are more likely to occur, and the surface quality deteriorates. Therefore, the number density of inclusions should be 5 pieces / mm 2 The following applies: 3 pieces / mm 2 The following is preferable. While there is no particular lower limit to the number density of inclusions, it is typically 0.8 inclusions / mm³. 2 In most cases, the result will be as described above.

[0088] Furthermore, an increase in the number of inclusions with a short diameter of 15 μm or more increases the likelihood of surface defects. Therefore, the number density of inclusions with a short diameter of 15 μm or more is 0.05 pieces / mm². 2 The following applies: The number density of inclusions with a short diameter of 15 μm or more is 0.03 particles / mm³. 2 The following is preferable. While there is no particular lower limit to the number density of inclusions with a short diameter of 15 μm or more, it is typically 0.005 inclusions / mm³. 2 In most cases, this is the case.

[0089] Here, the number density of the inclusions mentioned above is measured by the following procedure: When observing the rolled surface (a cross-section parallel to the rolling direction and perpendicular to the thickness direction of the steel sheet), any inclusions containing at least one of S and O and larger than 3 μm are identified as inclusions. From the identified inclusions, 100 or more are randomly selected and the number density is calculated from the total field of view within the observation area (mm²). 2 Identify the inclusions. Next, calculate the number density from the total number of extracted inclusions and the observation area. Similarly, for inclusions with a minor axis of 15 μm or more, further extract inclusions with a minor axis of 15 μm or more from the observation area and calculate the number density. The minor axis of an inclusion is the shortest length in the direction perpendicular to the major axis of the inclusion. The major axis of an inclusion is the length of the longest straight line when connecting two points on the outer circumference of the inclusion.

[0090] 3-2.MgO inclusions In the stainless steel of this embodiment, the formation of MgO·Al2O3 spinel, which causes surface defects, is suppressed, and MgO-based inclusions are preferentially formed. By preferentially forming these MgO-based inclusions, surface defects can be suppressed and the surface properties can be improved. Here, MgO-based inclusions are inclusions among the above-mentioned inclusions that satisfy the following formulas (i) and (ii), or inclusions that satisfy the following formulas (i) and (iii), or inclusions that satisfy the following formulas (i), (ii), and (iv).

[0091] CaO + Al2O3 + MgO + RO X +Ta2O5≧90% ··· (i) Al2O3 / MgO ≤ 1.25 ··· (ii) (i) RO in formula x This represents a REM oxide, and in formulas (i) and (ii), CaO, Al2O3, MgO, RO X Ta2O5 represents the mass percentage of each oxide in the inclusions, and is set to zero if it is not present.

[0092] MgO-based inclusions satisfy equation (i), i.e., CaO, Al2O3, MgO, RO X The total amount of , and Ta2O5 is 90% or more by mass. Note that if equation (i) is not satisfied, it indicates the presence of many lower oxides such as SiO2 and MnO, which increases the likelihood of oxides that lead to the formation of surface defects and internal defects during processing.

[0093] MgO-based inclusions are usually composed of oxides of Ca, Al, and Mg, but when REM, Ta, etc. are included, REM oxide (RO) is present. x ), and / or oxides of Ta may be included, because oxides of REM and Ta are generally unlikely to contribute to the occurrence of the defects described above.

[0094] Furthermore, MgO-based inclusions satisfy condition (ii), that is, the ratio of Al2O3 to MgO is 1.25 or less. This is because, compared to MgO·Al2O3 spinel, MgO is more likely to form as a crystallized phase during continuous casting if the ratio of Al2O3 to MgO is 1.25 or less.

[0095] The number proportion of MgO-based inclusions satisfying equations (i) and (ii) above should be 75% or more of the total number of inclusions. If the number proportion of MgO-based inclusions satisfying equations (i) and (ii) is less than 75%, a crystalline phase mainly composed of Al2O3MgO spinel is formed instead of a crystalline phase mainly composed of MgO, making it prone to surface defects. It is preferable that the number proportion of MgO-based inclusions be 85% or more, and more preferably 90% or more.

[0096] In addition to equations (i) and (ii) above, it is preferable that the MgO-based inclusion satisfies equation (iii) below, and that the number ratio of this MgO-based inclusion to the aforementioned inclusions is 75% or more. Here, since an MgO-based inclusion that satisfies equation (iii) inevitably satisfies equation (ii), this MgO-based inclusion satisfies equations (i) and (iii). Al2O3 / MgO ≤ 0.75 ··· (iii) However, in formula (iii) above, Al2O3 and MgO represent the mass percentages of Al2O3 and MgO, respectively, in the inclusions.

[0097] This is because the formation of MgO·Al2O3 spinel is further suppressed when the number ratio of MgO-based inclusions satisfying equations (i) and (iii) to the total number of inclusions becomes 75% or more.

[0098] Furthermore, in addition to equations (i) and (ii), it is preferable that the MgO-based inclusions satisfy equation (iv) below, and that the number ratio of these MgO-based inclusions to the above-mentioned inclusions is 90% or more. CaO > 20% ··· (iv) However, in formula (iv) above, CaO represents the mass percentage of CaO in the inclusions, and if it is not present, it is treated as zero.

[0099] This is because the formation of MgO·Al2O3 spinels is further suppressed by ensuring that the number ratio of MgO-based inclusions satisfying equations (i), (ii), and (iv) is 90% or more of the total inclusions.

[0100] The method for calculating the number proportion of each MgO-based inclusion is as follows: Observation is performed under the same conditions as when calculating the number density of the inclusions described above, and 100 or more inclusions are randomly selected. This is used as the population, and the elements contained in the compound of the population are analyzed by SEM-EDS. At this time, MgO-based inclusions are identified among the inclusions. In the determination, those that satisfy equations (i) and (ii), those that satisfy equations (i) and (iii), and those that satisfy equations (i), (ii), and (iv) are considered MgO-based inclusions.

[0101] Furthermore, the content of each element in the inclusions was identified, and using the identified elemental analysis values, Ca, Al, Mg, REM, and Ta were all CaO, Al2O3, MgO, and RO. X Assuming it is TaO2, the oxides CaO, Al2O3, MgO, RO X The TaO2 content is expressed as (mass %). X As mentioned above, this refers to REM oxides, and for example, if the REM is Ce, it indicates the content of the oxide of the contained REM element, such as Ce2O3. Furthermore, for each MgO-based inclusion, the proportion of each MgO-based inclusion relative to 100 or more inclusions in the population is calculated as the number percentage (%) of each MgO-based inclusion.

[0102] 4. Manufacturing method A preferred manufacturing method for the stainless steel according to this embodiment will be described. The stainless steel according to this embodiment can be stably manufactured by, for example, the following manufacturing method.

[0103] First, it is preferable to melt the main raw material, such as iron ore, and perform primary refining. The conditions for melting and primary refining are not particularly limited; they can be carried out according to conventional methods. Subsequently, it is preferable to perform secondary refining. In the stainless steel of this embodiment, it is preferable to perform preliminary deoxidation before normal deoxidation. For preliminary deoxidation, it is preferable to use one or more selected from Si, Mn, and Al as a deoxidizing agent. By performing this preliminary deoxidation, it is preferable to reduce the O concentration in the molten steel to 0.006% or less in the initial stage. As a result, the number density of inclusions and the number density of inclusions with a short diameter of 15 μm or more can be controlled within the range described above.

[0104] Next, towards the end of the secondary refining process, an alloy of Al, Si, Ti, or REM is added to deoxidize the mixture. While it is possible to add multiple alloys of the above elements, it is common to add an alloy of only one of these elements. At this stage, it is preferable to control the activity of MgO in the slag and the activity of the oxides of the elements (Al, Si, Ti, REM) used as deoxidizers towards the end of the secondary refining process. Additionally, Ca may be added at this stage for the usual purpose of creating a multi-component system of inclusions.

[0105] Specifically, it is preferable that the activity of MgO in the slag be 0.9 or higher, based on pure solid MgO. This is because if the activity of MgO in the slag is less than 0.9, based on pure solid MgO, the number of MgO-based inclusions decreases, resulting in poor surface properties. Furthermore, the activities of the oxides of the elements used in the above deoxidation should be 0.005 or less for Al2O3, 0.0005 or less for SiO2, 0.0005 or less for Ti2O3, and RO X It is preferable that the activity of 0.5 or less. X In terms of activity, for example, when multiple REMs are added, it is sufficient to control the activity of a representative element. This is because controlling the activity in this way allows the proportion of MgO-based inclusions to be controlled within the range described above.

[0106] In this case, the activity of MgO and other oxides in the slag can be calculated by measuring the composition of the slag and using thermodynamic data sets and commercial thermodynamic calculation software. If the MgO and other oxides in the slag do not satisfy the above range, the activity can be adjusted by the following methods. To increase the activity of MgO, it is preferable to add CaO and / or MgO to the molten steel, for example. Also, to decrease the activity of the above oxides other than MgO, it is preferable to add CaO to the molten steel, for example.

[0107] While methods such as adding Mg alloys or Ti can be considered to increase the Mg concentration in the molten steel, it should be noted that this may make it difficult to adjust the chemical composition within the range of this embodiment. Furthermore, the conditions for secondary refining are not particularly limited beyond the conditions described above. Conventional methods can be followed, and the conditions should be adjusted to satisfy the range of the steel's chemical composition according to this embodiment.

[0108] During secondary refining, it is preferable to stir the molten steel. Furthermore, the time from the addition of the deoxidizer at the end of secondary refining to the completion of casting is preferably in the range of 20 to 240 minutes. In addition, in order to ensure that the number percentage of MgO-based inclusions satisfying equations (i) and (iii) is 90% or more, it is preferable that this time be 150 minutes or less.

[0109] In the stainless steel of this embodiment, it is preferable to use aluminum (Al) as the deoxidizing agent. That is, it is desirable that Al contributes to deoxidation throughout the entire process, through preliminary deoxidation or deoxidation at the end of secondary refining. However, as mentioned above, this does not preclude the use of deoxidizing agents other than Al.

[0110] Subsequently, cast slabs or slabs may be produced as appropriate by continuous casting or other methods. The obtained cast slabs or slabs may then be subjected to necessary processing, heat treatment, cooling, pickling, etc., to obtain the desired microstructure, specifically austenitic stainless steel or duplex stainless steel. The shape of the steel is not particularly limited, but possible shapes include, for example, steel wire rods, wire rods, steel bars, steel plates, and other structural steel. Processing, heat treatment, cooling, pickling, etc., may be performed according to the desired shape.

[0111] The stainless steel according to this embodiment will be described in more detail below with reference to examples, but this embodiment is not limited to these examples. [Examples]

[0112] Molten steel having the chemical composition shown in Table 1 was poured into a tundish and cast using a continuous casting machine to obtain a cast slab. During the production of the molten steel, deoxidation was performed as follows: Preliminary deoxidation was carried out in the secondary refining using one or more of Al, Si, and Mn to reduce the O concentration in the molten steel to 0.006% or less. Next, the slag composition was controlled to adjust the activity of the components in the slag as shown in Table 2, and then deoxidation was performed using one of Al, Si, Ti, or REM. In this process, the slag components used to control the activity were, as shown in Table 2, essentially MgO, with the others being oxides of components corresponding to the deoxidizing agent added last (at the end of the secondary refining).

[0113] In other words, the activity of the oxide of the REM used was controlled: SiO2 for Si, Al2O3 for Al, Ti2O3 for Ti, and REM for REM. When multiple REMs were added simultaneously, the activity of a representative component was adjusted. The activity of the slag in each example is shown in Table 2 below, and the activity in the table was calculated using thermodynamic equilibrium calculation software based on a pure solid standard. Other conditions were adjusted as appropriate to produce stainless steel. Note that the chemical composition shown in Table 1 is the chemical composition after the adjustment of the component range of each element has been completely completed, and for each example, γ is written for austenitic stainless steel and DP is written for duplex stainless steel.

[0114] [Table 1]

[0115] The obtained cast slabs were heated to 1200-900°C, hot-rolled, and then hot-rolled sheet annealed at 980-1100°C for 0-60 seconds, followed by pickling. The obtained hot-rolled sheets were cold-rolled to a rolling ratio of 80%, cold-rolled sheet annealed at 1000-1100°C for 0-60 seconds, and then pickled to obtain 1.0 mm thick steel sheets. The obtained steel sheets were subjected to inclusion measurement and surface defect evaluation according to the following procedure.

[0116] (Evaluation of inclusions) The rolled surface was used as the observation surface, and compounds containing at least one of S and O, and larger than 3 μm, were identified as inclusions. From the identified inclusions, more than 100 were randomly selected, and the observation area (mm) was determined from the total number of fields of view. 2 The inclusions were identified, and the number density of inclusions of a predetermined size was calculated. The minor axis was observed and measured. SEM-EDS was used for observation and analysis. The magnification during analysis was approximately 1000x.

[0117] While measuring the number density described above, the composition of the inclusions was measured using the attached EDS. Using the randomly selected population of 100 or more inclusions as described above, the type of MgO-based inclusion was determined, and the percentage of MgO-based inclusions was calculated. The percentage was defined as the proportion of each MgO-based inclusion to the population of 100 or more inclusions. Other measurement conditions were as described above.

[0118] (Evaluation of surface defects) Surface defects were evaluated by visually inspecting the entire length of the obtained steel plate, divided into 1m intervals. Areas with surface defects of 10mm or more in length were rated as follows: ◎ if 1.0% or less of the total area, ○ if between 1.0% and 2.0%, △ if between 2.0% and 5.0%, and × if over 5.0%. Surfaces rated ○ and ◎ were evaluated as having good surface quality.

[0119] [Table 2]

[0120] Samples B1 to B21, which satisfied the requirements of this embodiment, had good surface properties. On the other hand, samples No. b1 to b5, which did not satisfy the requirements of this embodiment, had poor surface properties. Because b1 had an MgO activity of less than 0.9, the proportion of MgO-based inclusions was small, resulting in surface defects.

[0121] b2 had a higher SiO2 activity than 0.0005, resulting in a lower proportion of MgO-based inclusions and surface defects. b3 had a high O concentration, a high number density of inclusions with a short diameter of 3 μm or more and 15 μm or more, and surface defects. b4 had a high O concentration and a high number density of inclusions 15 μm or more. In addition, its SiO2 activity was higher than 0.0005, resulting in a lower proportion of MgO-based inclusions and surface defects. b5 had a higher Al2O3 activity than 0.005, resulting in a lower proportion of MgO-based inclusions and surface defects. [Examples]

[0122] Similar to Example 1, secondary refining was performed to obtain a cast slab with a predetermined chemical composition. In secondary refining, the time from the timing of the final blow-blowing in which the deoxidizing agent was added to the completion of continuous casting was adjusted to a range of 90 to 240 minutes, as shown in Table 4. Other manufacturing conditions were as shown in Table 4, and similar to Example 1, the activity of the components in the slag was adjusted in secondary refining. The obtained cast slab was subjected to hot rolling, hot-rolled sheet annealing, and pickling, followed by cold rolling, cold-rolled sheet annealing, and pickling to obtain steel sheets. The obtained steel sheets were evaluated for inclusions and surface defects according to the same criteria as in Example 1. The chemical composition of each steel sheet is shown in Table 3. Table 4 shows the evaluation results for slag activity, inclusions, and surface defects.

[0123] [Table 3]

[0124] [Table 4]

[0125] F1 to F5 had good surface properties, satisfying the requirements of this embodiment. In addition, F1 to F3, which satisfy the preferred range of this embodiment, had extremely good surface properties compared to other examples. [Examples]

[0126] Similar to Example 1, secondary refining was performed to obtain a cast slab with the specified chemical composition. When producing the molten steel, a Ca alloy was added to increase the CaO content in the inclusions. Other manufacturing conditions are as described in Table 6. Similar to Example 1, the activity of the components in the slag was adjusted during secondary refining, followed by stirring and casting. The obtained cast slab was subjected to hot rolling, hot-rolled annealing, and pickling under the same conditions as in Example 1, followed by cold rolling, cold-rolled annealing, and pickling to obtain steel sheets. The obtained steel sheets were evaluated for inclusions and surface defects according to the same criteria as in Example 1. Table 5 shows the chemical composition of each steel sheet. Table 6 shows the evaluation results for slag activity, inclusions, and surface defects.

[0127] [Table 5]

[0128] [Table 6]

[0129] D1 to D4 had good surface properties, satisfying the requirements of this embodiment. In addition, D3, which satisfies the preferred range of this embodiment, had extremely good surface properties compared to the other examples.

Claims

1. Austenitic stainless steel or duplex stainless steel containing inclusions, The chemical composition is expressed in mass percent. C: 0.005-0.15%, Si: 0.05-4.0%, Mn: 12.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0 to 28.0%, Cr: 14.0-28.0%, N: 0.005-0.40%, O: 0.0001 to 0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0-8.0%, Cu: 0-4.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0 to 0.004%, W: 0-4.0%, V: 0-1.0%, Ti: 0 to 0.40%, Nb: 0 to 0.60%, Sn: 0 to 0.1%, Sb: 0 to 0.30%, Co: 0 to 1.0%, Zr: 0 to 0.005%, Ga: 0 to 0.010%, The remainder consists of Fe and impurities. The aforementioned inclusions contain at least one of S and O, and have a short diameter of 3 μm or more. The number density of the aforementioned inclusions is 5 inclusions / mm³. ２ The following: Of the aforementioned inclusions, the number density of inclusions with a short diameter of 15 μm or more is 0.05 pieces / mm ２ The following: Of the aforementioned inclusions, those satisfying the following equations (i) and (ii) are defined as MgO-based inclusions, Stainless steel in which the number ratio of MgO-based inclusions to the total number of inclusions is 75% or more. CaO+Al ２ Oh ３ +MgO+RO Ｘ +That ２ Oh ５ ≧90% ・・・ (i) Al ２ O ３ / MOO≦1.25 ・・・ (ii) However, RO in equation (i) ｘ This represents a REM oxide, and in formulas (i) and (ii), CaO and Al ２ O ３ MgO, RO ｘ Ta ２ O ５ This represents the mass percentage of each oxide in the inclusion, and is set to zero if it is not present.

2. Austenitic stainless steel, The aforementioned chemical composition, in mass%, C: 0.005-0.15%, Si: 0.2-4.0%, Mn: 0.1 to 12.0%, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0 to 28.0%, Cr: 14.0-28.0%, N: 0.005-0.40%, O: 0.0001 to 0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0-8.0%, Cu: 0-4.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0 to 0.004%, W: 0-2.0%, V: 0-1.0%, Ti: 0 to 0.40%, Nb: 0 to 0.60%, Sn: 0 to 0.1%, Sb: 0 to 0.30%, Co: 0 to 1.0%, Zr: 0 to 0.005%, Ga: 0 to 0.010%, The stainless steel according to claim 1, comprising the same material.

3. Austenitic stainless steel, The aforementioned chemical composition, in mass%, Mo: 0.01-8.0%, Cu: 0.01 to 4.0%, REM: 0.0005-0.050%, Ta: 0.001 to 0.10%, B: 0.0001 to 0.004%, W: 0.05-2.0%, V: 0.05-1.0%, Ti: 0.001 to 0.40%, Nb: 0.001 to 0.60%, Sn: 0.01 to 0.1%, Sb: 0.005 to 0.30%, Co: 0.03 to 1.0%, Zr: 0.0001–0.005%, and Ga: 0.0001 to 0.010%, The stainless steel according to claim 2, comprising one or more selected from the following.

4. It is a duplex stainless steel, The aforementioned chemical composition, in mass%, C: 0.005-0.08%, Si: 0.05-1.0%, Mn: 6.0% or less, P: 0.04% or less, S: 0.0050% or less, Ni: 1.0 to 14.0%, Cr: 16.0-28.0%, N: more than 0.005% and less than 0.35%, O: 0.0001 to 0.0060%, Al: 0.20% or less, Ca: 0.0035% or less, Mg: 0.010% or less, Mo: 0.05-5.5%, Cu: 0-2.0%, REM: 0-0.050%, Ta: 0-0.10%, B: 0 to 0.004%, W: 0-4.0%, V: 0-1.0%, Ti: 0 to 0.40%, Nb: 0 to 0.10%, Sn: 0 to 0.1%, Sb: 0 to 0.30%, Co: 0 to 1.0%, Zr: 0 to 0.005%, Ga: 0 to 0.010%, The stainless steel according to claim 1, comprising the same material.

5. It is a duplex stainless steel, The aforementioned chemical composition, in mass%, Cu: 0.05-2.0%, REM: 0.0005-0.050%, Ta: 0.001 to 0.10%, B: 0.0001 to 0.004%, W: 0.05-4.0%, V: 0.05-1.0%, Ti: 0.001 to 0.40%, Nb: 0.001 to 0.10%, Sn: 0.01-0.1%, Sb: 0.005 to 0.30%, Co: 0.03-1.0%, Zr: 0.0001–0.005%, and Ga: 0.0001 to 0.010%, The stainless steel according to claim 4, comprising one or more selected from the following.

6. Of the aforementioned inclusions, those satisfying equations (i) and (iii) below are defined as MgO-based inclusions. The stainless steel according to any one of claims 1 to 5, wherein the number ratio of the MgO-based inclusions to the total number of inclusions is 75% or more. CaO+Al ２ Oh ３ +MgO+RO Ｘ +That ２ Oh ５ ≧90% ・・・ (i) Al ２ O ３ / M2O≦0.75 ・・・ (iii) However, RO in equation (i) above ｘ This represents a REM oxide, and in formulas (i) and (iii) above, CaO and Al ２ O ３ MgO, RO ｘ Ta ２ O ５ This represents the mass percentage of each oxide in the inclusion, and is set to zero if it is not present.

7. Of the aforementioned inclusions, those satisfying the following equations (i), (ii), and (iv) are defined as MgO-based inclusions. The stainless steel according to any one of claims 1 to 5, wherein the number ratio of the MgO-based inclusions to the total number of inclusions is 90% or more. CaO+Al ２ Oh ３ +MgO+RO Ｘ +That ２ Oh ５ ≧90% ・・・ (i) Al ２ O ３ / MOO≦1.25 ・・・ (ii) CaO>20%... (iv) However, RO in equation (i) ｘ This represents a REM oxide, and in formulas (i), (ii), and (iv), CaO and Al ２ O ３ MgO, RO ｘ Ta ２ O ５ This represents the mass percentage of each oxide in the inclusion, and is set to zero if it is not present.

Images