Patent Application
20240002969
The present invention relates to a dual phase stainless steel sheet and a dual phase stainless hot-rolled plate, and a method for manufacturing a dual phase stainless steel sheet. Priority is claimed on Japanese Patent Application No. 2020-198585, filed Nov. 30, 2020, the content of which is incorporated herein by reference.
Stainless steel is used as a representative corrosion-resistant material in various applications, and is used not only to simply prevent rusting and hole opening due to corrosion, but in recent years, has also been used in applications that require a favorable appearance after construction.
Therefore, for example, Patent Document 1 discloses a highly corrosion-resistant stainless steel sheet for an exterior building material having excellent ability to prevent the occurrence of a belt-like undulated appearance, which is a bright annealed steel sheet or annealed and pickled steel sheet of a dual phase stainless steel containing in mass %, Cr: 16 to 35%, Ti: 0.05 to 0.5%, Mo: 0 to 6% (including none added), Nb: 0 to 1.0% (including none added), and N: 0.005 to 0.025%, and with a C content that is limited to 0.015% or less, and in which the brightness difference ΔL within the plate width on the surface of the steel sheet in a direction perpendicular to the rolling direction is adjusted to 5 or less.
Japanese Unexamined Patent Application, First Publication No. 2000-129405
Dual phase stainless steel sheets have better weather resistance than austenite-based stainless steel sheets. However, conventional dual phase stainless steel thin sheets have long-term undulation (surface waviness) regarding the surface roughness. In dual phase stainless steel sheets having surface waviness, stripe patterns caused by the surface waviness may be visible. Therefore, when a favorable appearance is required, conventional dual phase stainless steels have room for improvement.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a dual phase stainless steel sheet and dual phase stainless hot-rolled sheet which have a favorable appearance, and a method for manufacturing a dual phase stainless steel sheet.
The inventors found that, when a dual phase stainless steel sheet contains a large number of ferrite crystal grains having a specific crystal orientation, if the dual phase stainless steel sheet is deformed, for example, if mirror polishing is performed, the crystal grains deform differently from crystal grains with other orientations. Accordingly, the inventors found that surface waviness occurs due to different deformation behaviors occurring between different crystal grains. Therefore, the inventors found that the surface waviness is reduced by making the texture of the ferrite random. Specifically, they found that, in the manufacture of a dual phase stainless steel sheet, when a hot-rolled sheet is annealed at a low temperature for a long time, the ferrite is softened, a strain is preferentially introduced into the softened ferrite, and thus the distribution of the ferrite crystal orientation is made uniform. Thus, the inventors conducted extensive studies and completed the present invention.
The gist of the present invention completed based on the above findings is as follows.
[1] A dual phase stainless steel sheet according to one aspect of the present invention is a dual phase stainless steel sheet containing an austenite and a ferrite, in which the dual phase stainless steel sheet includes: in mass %, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50 to 8.00%, Cr: 18.00 to 28.00%, Mo: 5.00% or less, Cu: 0.05 to 1.50%, and N: 0.080 to 0.320%, with the remainder of Fe and impurities, in which, in a center part in a sheet thickness of a cross section in a perpendicular-to-rolling direction which is a direction perpendicular to a rolling direction on a rolled surface and a direction parallel to a sheet thickness direction, an area ratio S<001>/S<111> which is a ratio of an area proportion S<001> of a texture of a ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to an area proportion S<111> of a texture of a ferrite with the <111> direction oriented in the perpendicular-to-rolling direction is 0.90 to 1.10.
[2] The dual phase stainless steel sheet according to [1], in which the surface waviness height in the rolling direction may be 0.3 μm or less.
[3] The dual phase stainless steel sheet according to [1] or [2] may include, in mass %, C: 0.030% or less, Si: 0.75% or less, Mn: 2.00 to 4.00%, P: 0.040% or less, S: 0.0200% or less, Ni: 1.50 to 2.50%, Cr: 18.00 to 21.50%, Mo: 0.60% or less, Cu: 0.50 to 1.50%, and N: 0.150 to 0.200%, with the remainder of Fe and impurities.
[4] The dual phase stainless steel sheet according to any one of [1] to [3] may include, in mass %, in place of some Fe, one or more selected from the group consisting of Al: 0.003 to 0.050%, O: 0.0070% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Co: 0.005 to 0.25%, V: 0.005 to 0.15%, Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%, Ga: 0.001 to 0.050%, Zr: 0.005 to 0.50%, Ta: 0.005 to 0.100%, and B: 0.0002 to 0.0050%.
[5] A dual phase stainless hot-rolled sheet according to another aspect of the present invention is a dual phase stainless hot-rolled sheet containing an austenite and a ferrite, in which the dual phase stainless hot-rolled sheet includes, in mass %, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50 to 8.00%, Cr: 18.00 to 28.00%, Mo: 5.00% or less, Cu: 0.05 to 1.50%, and N: 0.080 to 0.320% with the remainder of Fe and impurities, in which the difference between the Vickers hardness of the austenite and the Vickers hardness of the ferrite is 50 HV or more.
[6] The dual phase stainless hot-rolled sheet according to [5] may include, in mass %, in place of some Fe, one or more selected from the group consisting of Al: 0.003 to 0.050%, O: 0.0070% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Co: 0.005 to 0.25%, V: 0.005 to 0.15%, Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%, Ga: 0.001 to 0.050%, Zr: 0.005 to 0.50%, Ta: 0.005 to 0.100%, and B: 0.0002 to 0.0050%.
[7] A method for manufacturing a dual phase stainless steel sheet according to still another aspect of the present invention includes a hot rolling process in which a stainless steel containing, in mass %, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50 to 6.80%, Cr: 18.00 to 28.00%, Mo: 5.00% or less, Cu: 0.05 to 1.50% and N: 0.080 to 0.320% with the remainder of Fe and impurities is hot-rolled and coiled at a temperature of 680° C. or higher; a heat treatment process in which the stainless steel after the hot rolling process is heated at a temperature of 500° C. or higher and lower than 600° C. and maintained for 1 hour or longer; and a cold rolling process in which the stainless steel after the heat treatment process is cold-rolled.
[8] In the method for manufacturing a dual phase stainless steel sheet according to [7], the stainless steel may contain, in mass %, in place of some Fe, one or more selected from the group consisting of Al: 0.003 to 0.050%, O: 0.0070% or less, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Co: 0.005 to 0.25%, V: 0.005 to 0.15%, Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%, Ga: 0.001 to 0.050%, Zr: 0.005 to 0.50%, Ta: 0.005 to 0.100%, and B: 0.0002 to 0.0050%.
As described above, according to the present invention, it is possible to provide a dual phase stainless steel sheet and dual phase stainless hot-rolled sheet which have a favorable appearance, and a method for manufacturing a dual phase stainless steel sheet.
A dual phase stainless steel sheet according to the present embodiment is a dual phase stainless steel sheet containing an austenite and a ferrite, including: in mass %, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50 to 8.00%, Cr: 18.00 to 28.00%, Mo: 5.00% or less, Cu: 0.05 to 1.50%, and N: 0.080 to 0.320%, with the remainder of Fe and impurities, in which, in a center part in a sheet thickness of a cross section in a perpendicular-to-rolling direction (TD) which is a direction perpendicular to a rolling direction on a rolled surface and a direction parallel to a sheet thickness direction, an area ratio S<001>/S<111> which is a ratio of the area proportion S<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to the area proportion S<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction is 0.90 to 1.10. Hereinafter, the dual phase stainless steel sheet according to the present embodiment will be described in detail.
First, chemical components of a dual phase stainless steel sheet according to the present embodiment will be described. Here, % indicating a component means mass %.
C: 0.080% or Less
When the C content exceeds 0.080%, corrosion resistance decreases due to Cr carbide precipitation. Therefore, a small C content is desirable, and up to 0.080% or less is acceptable. Therefore, the C content is 0.080% or less. In order to improve corrosion resistance, the C content is preferably 0.030% or less and more preferably 0.025% or less. The lower limit of the C content is not particularly limited, and in consideration of cost, the C content is preferably 0.001% or more and more preferably 0.007% or more.
Si: 1.00% or Less
Si acts as a deoxidizing agent or a desulfurizing agent. When the Si content exceeds 1.00%, since the toughness deteriorates, the Si content is set to 1.00% or less. The Si content is preferably 0.65% or less. In order for Si to sufficiently act as a deoxidizing agent or a desulfurizing agent, the Si content is preferably 0.05% or more. The Si content is more preferably 0.30% or more.
Mn: 4.00% or Less
Although Mn is a relatively inexpensive element, it has an effect of minimizing Cr nitride precipitation by increasing the amount of the austenite in the stainless steel sheet and additionally increasing the solid solubility of nitrogen. On the other hand, an excessive content thereof causes deterioration of corrosion resistance. Therefore, the Mn content is 4.00% or less. The Mn content is preferably 2.50% or less. The Mn content is preferably 0.74% or more, more preferably 0.85% or more, and still more preferably 2.00% or more.
P: 0.040% or Less
P is an element that is inevitably contained in a stainless steel sheet, but since it deteriorates hot workability, the P content is set to 0.040% or less. The P content is preferably 0.035% or less. The lower limit of the P content is not particularly limited, and in consideration of cost, the P content is preferably 0.005% or more.
S: 0.0300% or Less
Like P, S is an element that is inevitably contained in a stainless steel sheet, but it deteriorates hot workability, toughness, and corrosion resistance. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0200% or less. The lower limit of the S content is not particularly limited, and in consideration of cost, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.
Ni: 1.50 to 8.00%
Ni is an element that improves design properties of a stainless steel sheet in the present invention. When the Ni content is too small, since the solid solution of Ni in the austenite in the hot-rolled sheet is reduced and softened, the difference between the Vickers hardness of the austenite and the Vickers hardness of the ferrite in the hot-rolled sheet to be described below of 50 HV or more is not satisfied. Therefore, when the Ni content is too small, in the present invention, in the steel sheet after cold processing, the following “in a center part in a sheet thickness of a cross section in a perpendicular-to-rolling direction which is a direction perpendicular to a rolling direction on a rolled surface and a direction parallel to a sheet thickness direction, an area ratio S<001>/S<111> which is a ratio of the area proportion S<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to the area proportion S<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction being 0.90 to 1.10” is not satisfied. That is, when the Ni content is too small, since the height of the surface waviness in the rolling direction does not become 0.3 μm or less, an effect of improving design properties by controlling the height of the surface waviness to be small cannot be obtained. Therefore, the Ni content is 1.50% or more. The Ni content is preferably 2.00% or more. On the other hand, when the Ni content is excessive, not only does the cost increase, but the austenite content also becomes excessive, and the hot workability deteriorates. Therefore, the Ni content is 8.00% or less. The Ni content is preferably 6.90% or less, more preferably 6.80% or less, and still more preferably 2.50% or less.
Cr: 18.00 to 28.00%
Cr is an element that improves corrosion resistance of a stainless steel sheet. In consideration of corrosion resistance, the Cr content is 18.00% or more. The Cr content is preferably 20.50% or more and more preferably 21.00% or more. On the other hand, Cr is an element that increases the ferrite content, and when the stainless steel sheet contains an excessive amount of Cr, the amount of ferrite becomes excessive, and the toughness deteriorates. Therefore, the Cr content is 28.00% or less. The Cr content is preferably 24.50% or less and more preferably 21.50% or less.
Mo: 5.00% or Less
Although Mo has a stronger corrosion resistance improving effect than Cr, it is a very expensive element, and when the Mo content is excessive, the manufacturing cost increases. In addition, when the Mo content is excessive, the stainless steel sheet is hardened and the workability deteriorates. Therefore, the Mo content is 5.00% or less. The Mo content is preferably 3.00% or less, more preferably 2.95% or less, and still more preferably 0.60% or less. When the Mo content is less than 0.01%, since the corrosion resistance improving effect of Mo, which is the effect obtained by adding Mo, is poor, the Mo content is thus, for example, 0.01% or more. Preferably, the Mo content is preferably 0.05% or more and more preferably 0.20% or more.
Cu: 0.05 to 1.50%
Like Ni, Cu is an element that minimizes dissolution of a stainless steel sheet in a low pH environment. However, when the stainless steel sheet contains an excessive amount of Cu, since the hot workability is significantly impaired, the Cu content is 1.50% or less. The Cu content is preferably 1.40% or less. On the other hand, the above effect cannot be obtained when the Cu content is less than 0.05%. Therefore, the Cu content is 0.05% or more. The Cu content is preferably 0.60% or more and more preferably 0.70% or more.
N: 0.080 to 0.320%
N is an element that significantly increases corrosion resistance and increases the amount of the austenite. In order to obtain the effect, the N content is 0.080% or more. The N content is preferably 0.150% or more and more preferably 0.155% or more. On the other hand, when the N content exceeds 0.320%, since nitrides are formed in the steel and the corrosion resistance and toughness deteriorates, the N content is 0.320% or less. The N content is preferably 0.200% or less.
In the dual phase stainless steel sheet of the present invention, the remainder other than the above elements is composed of Fe and impurities. However, elements other than the above elements can be contained within a range in which the effects of the present embodiment are not impaired. Here, “impurities” refers to components that are mixed in from raw materials such as ore and scrap and due to various factors in the manufacturing process when the dual phase stainless steel sheet according to the present invention is industrially manufactured, but these are allowable as long as they do not adversely influence the present invention.
The basic components of the dual phase stainless steel sheet according to one embodiment of the present invention have been described above, but in the dual phase stainless steel sheet according to one embodiment of the present invention, other elements to be described below can be appropriately contained in place of some Fe. Here, since the following elements do not have to be contained, the lower limit of the amount of these elements is 0%.
Al: 0.003 to 0.050%
Al is an element having a strong deacidifying action. The Al content is preferably 0.003% or more for the deacidifying action of Al. The Al content is more preferably 0.005% or more. On the other hand, Al easily forms nitrides together with N, and when nitrides are formed, the toughness greatly deteriorates. Therefore, the Al content is preferably 0.050% or less. The Al content is more preferably 0.040% or less.
O: 0.0070% or less
When the amount of O in steel is excessive, oxides are formed and the toughness deteriorates. Therefore, the O content is preferably 0.0070% or less. The O content is more preferably 0.0050% or less. The lower limit of the O content is not particularly limited, and in consideration of cost, the O content is preferably 0.0005% or more. The O content may be 0.001% or more.
Nb: 0.005 to 0.20%
Nb is an element that fixes C and N, prevents a decrease in corrosion resistance due to Cr carbide precipitation and improves corrosion resistance. When the Nb content is 0.005% or more, since its effect is exhibited, the Nb content is preferably 0.005% or more. The Nb content may be 0.01% or more. On the other hand, when the Nb content exceeds 0.20%, since the a phase may harden due to solid solution strengthening and the workability may deteriorate, the Nb content is preferably 0.20% or less. The Nb content may be 0.18% or less.
Ti: 0.005 to 0.20%
Ti is an element that fixes C and N, prevents sensitization due to Cr carbide precipitation, and improves corrosion resistance. When the Ti content is 0.005% or more, since its effect is exhibited, the Ti content is preferably 0.005% or more. The Ti content may be 0.01% or more. On the other hand, when the Ti content exceeds 0.20%, the ferrite is hardened, the toughness is lowered, and additionally, the surface roughness may decrease due to Ti-based precipitates. Therefore, the Ti content is preferably or less. The Ti content may be 0.18% or less.
Co: 0.005 to 0.25%
Co reduces Cr carbide precipitation and reduces a decrease in corrosion resistance. When the Co content is 0.005% or more, since Co exhibits the above effect, the Co content is preferably 0.005% or more. The Co content may be 0.01% or more. On the other hand, since Co is a rare element and expensive, the Co content is preferably or less. The Co content may be 0.20% or less.
V: 0.005 to 0.15%
V is a strong carbide forming element. Therefore, when V, which easily forms carbides in a high-temperature range, is contained, Cr carbide precipitation can be reduced and a decrease in corrosion resistance can be reduced. When the V content is or more, since V exhibits the above effect, the V content is preferably 0.005% or more. The V content may be 0.01% or more. On the other hand, when the V content is large, since hardening is caused, the V content is preferably 0.15% or less. The V content may be 0.12% or less.
Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%
Sn and Sb are elements that improve corrosion resistance, but are also elements that strengthen solid solution of the ferrite. Therefore, each amount of Sn and Sb is preferably 0.20% or less. Each amount of Sn and Sb is more preferably 0.10% or less. When the amount of either Sn or Sb is 0.005% or more, since the effect of improving corrosion resistance is exhibited, each amount of Sn and Sb is preferably 0.005% or more. Each amount of Sn and Sb is more preferably 0.030% or more.
Ga: 0.001 to 0.050%
Ga is an element that contributes to improving corrosion resistance. When the Ga content is 0.001% or more, since a corrosion resistance improving effect is exhibited, the Ga content is preferably 0.001% or more. The Ga content may be 0.005% or more. On the other hand, when the Ga content exceeds 0.050%, a corrosion resistance improving effect reaches saturation and only increases the cost. Therefore, the Ga content is preferably 0.050% or less. The Ga content may be 0.040% or less.
Zr: 0.005 to 0.50%
Zr is an element that contributes to improving corrosion resistance. When the Zr content is 0.005% or more, since a corrosion resistance improving effect is exhibited, the Zr content is preferably 0.005% or more. The Zr content may be 0.01% or more. On the other hand, when the Zr content exceeds 0.50%, the effect reaches saturation. Therefore, the Zr content is preferably 0.50% or less. The Zr content may be 0.40% or less.
Ta: 0.005 to 0.100%
Ta is an element that improves corrosion resistance by modifying inclusions. When the Ta content is 0.005% or more, the above effect is exhibited. Therefore, the Ta content is preferably 0.005% or more. The Ta content may be 0.01% or more. On the other hand, when the Ta content exceeds 0.100%, it may lead to a decrease in the ductility at room temperature and a decrease in toughness. Therefore, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.050% or less.
B: 0.0002 to 0.0050%
B is an element that exhibits an effect of reducing secondary processing embrittlement and deterioration of hot workability. In addition, B is an element that does not influence corrosion resistance. When the B content is 0.0002% or more, since B exhibits the above effect, the B content is preferably 0.0002% or more. The B content may be 0.0005% or more. On the other hand, when the B content exceeds 0.0050% since the hot workability may actually deteriorate, the B content is preferably 0.0050% or less. The B content is more preferably 0.0022% or less, and even more preferably 0.0020% or less.
The dual phase stainless steel sheet according to the present embodiment has the above chemical components, but preferably contains C: 0.030% or less, Si: 0.75% or less, Mn: 2.00 to 4.00%, P: 0.040% or less, S: 0.0200% or less, Ni: 1.50 to 2.50%, Cr: 20.50 to 21.50%, Mo: 0.60% or less, Cu: 0.50 to 1.50%, and N: 0.150 to 0.200%. When the dual phase stainless steel sheet has the chemical components, it has better corrosion resistance.
In the dual phase stainless steel sheet according to the present embodiment, in a center part in the sheet thickness of the cross section in the perpendicular-to-rolling direction, an area ratio S<001>/S<111> which is a ratio of the area proportion S<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to the area proportion S<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction is 0.90 to 1.10.
Here, with reference to
In the center part in the sheet thickness of the cross section in the perpendicular-to-rolling direction obtained by cutting at the position of the center of the sheet width, 3 or more fields of view of SEM images are acquired at an observation magnification of 1,000. Here, the center part in the sheet thickness refers to a range of 2t/5 to 3t/5 in the sheet thickness direction from the surface of the steel sheet when the sheet thickness of the steel sheet is t. For each SEM image, the crystal orientation of the measurement point is analyzed with a measurement interval of 1 μm. A crystal orientation measurement target is a crystal grain having a CI value (Confidence Index) of 0.1 or more, which is an index indicating the likelihood of the calculated crystal orientation. Therefore, a texture in which the orientation difference in the <111> direction with respect to the perpendicular-to-rolling direction is within 15° is defined as a texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction. In addition, a texture in which the orientation difference in the <001> direction with respect to the perpendicular-to-rolling direction is within 15° is defined as a texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction. Similarly, the texture of the ferrite with the <101> direction oriented in the perpendicular-to-rolling direction and the texture of the ferrite with the <411> direction oriented in the perpendicular-to-rolling direction are defined. Therefore, for example, an inverse pole figure crystal orientation map (Inverse Pole Figure (IPF) map) shown in
From the obtained IPF map, the area of each texture is calculated from the equivalent circle diameter of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction and the equivalent circle diameter of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction. The area of each texture is subjected to image analysis. From the area of each texture, the total area of the texture of the ferrite with the oriented <111> direction and the total area of the texture of the ferrite with the oriented <001> direction are calculated. Using the calculated total area of the crystal grains in respective directions, the area proportion S′<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction and the area proportion S′<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction with respect to the area of one entire field of view are calculated, and the area ratio S′<001>/S′<111> is calculated. The area ratio S′<001>/S′<111> is calculated for each field of view of the acquired SEM image, and the average value thereof is defined as the area ratio S<001>/S<111>.
Generally, the grain size of the austenite is about several μm, and the grain size of the ferrite is about 10 μm. Therefore, in the SEM image acquired at an observation magnification of 1,000, it can be said that sufficient crystal grains are projected so that the area ratio S<001>/S<111> can be used as an index of the degree of randomization of the texture.
When the area ratio S<001>/S<111> is less than 0.90, the texture of the ferrite indicates that the <111> direction is strongly oriented in the perpendicular-to-rolling direction. In addition, when the area ratio S<001>/S<111> exceeds 1.10, the texture of the ferrite indicates that the <001> direction is strongly oriented in the perpendicular-to-rolling direction. When a steel sheet having a texture of a ferrite with the <111> direction oriented in TD (perpendicular-to-rolling direction) or a texture of a ferrite with the <001> direction oriented in TD (perpendicular-to-rolling direction) is deformed, these crystal grains deform with different behaviors from other textures. Thereby, waviness occurs on the surface of the steel sheet. As a result, stripe patterns occur on the surface of the steel sheet. Therefore, the area ratio S<001>/S<111> is 0.90 to 1.10. The area ratio S<001>/S<111> may be 0.92 or more or 1.00 or more. In addition, the area ratio S<001>/S<111> may be 1.00 or less or 1.08 or less.
The dual phase stainless steel sheet according to the present embodiment preferably has a surface waviness height R of 0.3 μm or less in the rolling direction. Here, with reference to
When the surface waviness height R in the rolling direction is 0.3 μm or less, the appearance of the stainless steel sheet has less visible stripe patterns, and the appearance becomes more favorable. Therefore, the surface waviness height R is preferably 0.3 μm or less. More preferably, the surface waviness height R in the rolling direction of the stainless steel sheet to which a tensile stress is applied and a strain of 16% is applied is preferably 1.8 μm or less. When the surface waviness height R in the rolling direction of the stainless steel sheet to which a tensile stress is applied and a strain of 16% is applied is 1.8 lam or less, even if a stainless steel sheet is processed for actual use, stripe patterns become even less visible.
Although the method for manufacturing a dual phase stainless steel sheet according to the present invention will be described below in detail, the dual phase stainless steel sheet according to the present invention is manufactured by performing cold rolling on the stainless hot-rolled sheet manufactured according to a predetermined treatment. The difference ΔHV (=HVγ−HVα) between the Vickers hardness HVγ of the austenite and the Vickers hardness HVα of the ferrite of the stainless hot-rolled sheet manufactured during the manufacturing of the dual phase stainless steel sheet according to the present invention is 50 HV or more, preferably 60 HV or more, and more preferably 65 HV or more, or 70 HV or more. The upper limit of ΔHV is not particularly limited. ΔHV may be, for example, 65 HV or less, 70 HV or less, or 75 HV or less. The Vickers hardness HVγ of the austenite and the Vickers hardness HVα of the ferrite are measured under a load of 0.01 kgf according to JIS Z 2244: 2009. The austenite and the ferrite are each measured at five points at the center part in the sheet thickness, and an average value thereof is used as a representative value.
When ΔHV is 50 HV or more, a strain is preferentially introduced into the ferrite in subsequent processes, for example, a cold rolling process, a temper rolling process or a processing process for actual use. As a result, the area ratio S<001>/S<111> of the stainless steel sheet becomes 0.90 to 1.10. As a result, waviness is reduced on the surface of the steel sheet, and the occurrence of stripe patterns on the surface of the steel sheet is reduced. When ΔHV is 65 or more, since the hardness difference between the austenite and the ferrite of the hot-rolled sheet is large, the grain size of the soft phase is subdivided during cold rolling. Thereby, the difference in deformability of the crystal orientation is less likely to occur, and even if a stainless steel sheet is processed for actual use, stripe patterns become even less visible.
The sheet thickness of the dual phase stainless steel sheet according to the present embodiment is, for example, 0.30 mm or more and 2.00 mm or less. The sheet thickness of the dual phase stainless steel sheet may be 0.50 mm or more or 0.80 mm or more. In addition, the sheet thickness of the dual phase stainless steel sheet may be 1.80 mm or less or 1.50 mm or less. Within such a range, a more remarkable effect of reducing stripe patterns can be obtained, and a dual phase stainless steel sheet having a favorable appearance can be obtained.
Next, an example of a method for manufacturing a dual phase stainless steel sheet according to the present embodiment will be described.
The method for manufacturing a dual phase stainless steel sheet according to the present embodiment includes a hot rolling process in which a stainless steel having the above chemical components is subjected to hot rolling and coiled at a temperature of 680° C. or higher, a heat treatment process in which the stainless steel after the hot rolling process is heated at a temperature of 500° C. or higher and lower than 600° C. and maintained for 1 hour or longer, and a cold rolling process after the heat treatment process. The dual phase stainless steel sheet according to the present embodiment is manufactured by performing, for example, a steelmaking process, the hot rolling process, the heat treatment process, a hot-rolled sheet pickling process, the cold rolling process, a heat treatment process after cold rolling, and a cold-rolled sheet pickling process, in this order. For processes other than the hot rolling process and the heat treatment process, manufacturing conditions are not particularly limited, and known methods can be applied. Hereinafter, the hot rolling process, the heat treatment process and the cold rolling process will be described.
In this process, a stainless steel having the above chemical components is subjected to hot rolling and coiled at a temperature of 680° C. or higher. As the stainless steel to be hot-rolled, for example, a stainless steel piece obtained by continuous casting may be used.
It is preferable to heat the stainless steel to 1,150 to 1,250° C. before hot rolling. When the heating temperature is lower than 1,150° C., ear cracking may occur during hot rolling. On the other hand, when the heating temperature exceeds 1,250° C., a steel piece may be deformed in a heating furnace or scratches may easily occur during hot rolling.
After the heating, the stainless steel is hot-rolled. The reduction rate is preferably 50% or less. When the reduction rate is more than 50%, the difference in the structure shape for each rolling direction is promoted, and regardless of the rolling direction, uniform fracture surface properties may not be obtained.
Hot rolling may be performed over a plurality of passes, and when a plurality of passes are performed, the reduction rate per pass is 50% or less.
The coiling temperature of the stainless steel after rolling is 680° C. or higher. Between the ferrite and the austenite, recovery and recrystallization of the ferrite occur first. When the coiling temperature of the stainless steel increases, recovery of the ferrite during coiling occurs, and recrystallization occurs in a part of the ferrite. When the coiling temperature is lower than 680° C., the ferrite is not sufficiently recovered during coiling. Therefore, the coiling temperature is 680° C. or higher. The coiling temperature is preferably 700° C. or higher. On the other hand, the coiling temperature is preferably 750° C. or lower. When the coiling temperature is 750° C. or lower, it is possible to further reduce recovery and recrystallization of the austenite.
In this process, the stainless steel after the hot rolling process is heated at a temperature of 500° C. or higher and lower than 600° C. and maintained for 1 hour or longer.
The heat treatment temperature is 500° C. or higher and lower than 600° C. When the heat treatment temperature is lower than 500° C., recovery and recrystallization of the ferrite are insufficient, and the ferrite is not softened. When the ferrite is not softened, strain is not preferentially introduced into the ferrite in the subsequent cold rolling process, and the ferrite has an oriented structure in which crystal orientation is not randomized. Therefore, the heat treatment temperature is 500° C. or higher. The heat treatment temperature is preferably 550° C. or higher. On the other hand, when the heat treatment temperature is 600° C. or higher, the austenite is also softened, and the ferrite has an oriented structure in which crystal orientation is not randomized. Therefore, the heat treatment temperature is lower than 600° C., and preferably 585° C. or lower.
The heat treatment time is 1 hour or longer. When the heat treatment time is shorter than 1 hour, recovery and recrystallization of the ferrite are insufficient, and the ferrite is not softened. When the ferrite is not softened, strain is not preferentially introduced into the ferrite in the subsequent cold rolling process, and the ferrite has an oriented structure in which crystal orientation is not randomized. Therefore, the heat treatment time is 1 hour or longer. On the other hand, the upper limit of the heat treatment time is not particularly limited. However, in consideration of crystal grain coarsening, the heat treatment time is preferably 2 hours or shorter.
According to the manufacturing process up to the heat treatment process, a hot-rolled sheet in which the difference ΔHV (=HVγ−HVα) between the Vickers hardness HVγ of the austenite and the Vickers hardness HVα of the ferrite is 50 HV or more is manufactured.
In this process, the stainless steel after the heat treatment process (hot-rolled sheet according to the present embodiment) is subjected to cold rolling. Cold rolling conditions are not particularly limited and known conditions may be used. For example, cold rolling may be performed in one pass or a plurality of passes. The cumulative cold rolling reduction rate may be 30 to 80%, and the cold rolling temperature may be, for example, room temperature or higher and 200° C. or lower.
The stainless steel to be cold-rolled may be a stainless steel that has been pickled after the heat treatment process.
According to the cold rolling process, a large amount of rolling strain is introduced into the ferrite softened in the heat treatment process. Thereby, crystal grains having various crystal orientations of the ferrite are generated, and the texture of the ferrite is randomized. As a result, waviness is reduced on the surface of the steel sheet, and the occurrence of stripe patterns on the surface of the steel sheet is reduced.
The method for manufacturing a dual phase stainless steel sheet according to the present embodiment has been described above. As described above, a dual phase stainless steel sheet is manufactured in which, in a center part in the sheet thickness of the cross section in the perpendicular-to-rolling direction, an area ratio S<001>/S<111> which is a ratio of the area proportion S<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to the area proportion S<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction is 0.90 to 1.10. Since the dual phase stainless steel sheet according to the present embodiment has a random texture of the ferrite, the surface waviness height in the rolling direction of the dual phase stainless steel sheet is reduced. As a result, it is possible to reduce the occurrence of visible stripe patterns.
Hereinafter, embodiments of the present invention will be described in detail with reference to examples. Here, the examples shown below are only examples of the present invention, and the present invention is not limited to the following examples.
Stainless steels having the chemical components shown in Table 1 were hot-rolled at a reduction rate of 70% and the rolled stainless steels were coiled at a coiling temperature shown in Table 2. Next, the heat treatment process was performed at a heat treatment temperature and for a heat treatment time shown in Table 2 to manufacture hot-rolled sheets. Then, each of the manufactured hot-rolled plates was cold-rolled at a reduction rate of 80% at room temperature to produce stainless steel sheets. Here, in Table 1, “-” indicates that it was not added intentionally.
The Vickers hardness HVγ of the austenite and the Vickers hardness HVα of the ferrite of the manufactured hot-rolled sheet were measured according to JIS Z 2244: 2009 under a load of 0.01 kgf. The austenite and the ferrite were each measured at five points at the center part in the sheet thickness, and an average value thereof was used as a representative value.
In addition, in the center part in the sheet thickness of the cross section in the perpendicular-to-rolling direction obtained by cutting at the position of the center of the sheet width, an area ratio S<001>/S<111> which is a ratio of the area proportion S<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction to the area proportion S<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction was calculated by the following method. In the center part in the sheet thickness of the cross section in the perpendicular-to-rolling direction, 3 fields of view of SEM images were acquired at an observation magnification of 1,000, and the crystal orientation of the measurement point was analyzed at a measurement interval of 1 μm for each SEM image. When the orientation difference in crystal orientation between adjacent measurement points was within 15°, it was assumed that the texture was the same. The area of each texture was calculated from the equivalent circle diameter of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction and the equivalent circle diameter of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction. For each texture, the area was calculated from the calculated equivalent circle diameter, the total area of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction and the total area of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction were calculated. Using the calculated total area of each texture, the area proportion S′<111> of the texture of the ferrite with the <111> direction oriented in the perpendicular-to-rolling direction and the area proportion S′<001> of the texture of the ferrite with the <001> direction oriented in the perpendicular-to-rolling direction with respect to the area of the entire field of view were calculated, and the area ratio S′<001>/S′<111> was calculated. The area ratio S′<001>/S′<111> was calculated for each field of view of the acquired SEM image, and the average value thereof was defined as the area ratio S<001>/S<111>.
The surface waviness height R in the rolling direction of the manufactured dual phase stainless steel was measured by the following method. According to JIS B 0601: 2013, using a surface roughness measuring machine (SV-3000CNC, commercially available from Mitutoyo Corporation), a surface roughness curve of the stainless steel sheet was obtained at a measurement interval of 0.02 mm for a length of 10 mm in the perpendicular-to-rolling direction at the center position of the dual phase stainless steel sheet, and the surface waviness height R in the rolling direction was measured.
In addition, for the stainless steel sheet after applying a tensile stress to the manufactured stainless steel sheet in the rolling direction, and applying a strain of 16%, at the center position of the steel sheet, the surface waviness height R in the rolling direction was measured.
The appearance was evaluated by visual observation. Specifically, simulating an example of the produced stainless steel sheet and processing for actual use, the surface of the stainless steel sheet that had been subjected to overhang processing with a cylindrical punch was minor-polished, the minor-polished surface was observed in various directions, and the presence of stripe patterns was checked. Here, when no stripe patterns were observed on the stainless steel sheet after the above processing was performed, the appearance was evaluated as very good (A), when no stripe patterns were observed on the stainless steel sheet before a strain of 16% was applied, the appearance was evaluated as good (B), and when stripe patterns were observed on the stainless steel sheet before a strain of 16% was applied, the appearance was evaluated as poor (C).
Table 2 shows the evaluation results. Here, in Table 2, Rε=0 indicates the surface waviness height in the rolling direction of the stainless steel sheet to which no strain was applied, and Rε=16 indicates the surface waviness height in the rolling direction of the stainless steel sheet after a tensile stress was applied in the rolling direction and a strain of 16% was applied. In addition, underlined values in Table 2 are outside the scope of the present invention.
650
1.23
600
1.41
550
1.37
480
1.39
700
0.86
Steel type
0.87
16
650
0.87
600
0.89
Steel type
1.38
17
The chemical composition of each of the obtained steel sheet was substantially the same as the chemical composition of each stainless steel. In addition, when the obtained hot-rolled sheets and stainless steel sheets were observed under an SEM, all the steel sheets were dual phase stainless steel sheets.
As shown in Table 2, in the hot-rolled sheet manufactured under conditions of a coiling temperature in the hot rolling process of 680° C. or higher, a heat treatment temperature in the heat treatment process of 500° C. or higher and lower than 600° C., and a heat treatment time of 1 hour or longer, ΔHV was 50 or more. In addition, in the stainless steel sheet obtained by cold rolling the hot-rolled sheet with ΔHV of 50 or more (in Table 2, referred to as “cold-rolled sheet”), the area ratio S<001>/S<111> was 0.90 to 1.10. In the dual phase stainless steel sheet with an area ratio S<001>/S<111> of 0.90 to 1.10, the surface waviness height Rε=0 in the rolling direction was 0.3 μm or less, and the surface waviness height Rε=16 in the rolling direction was also a small value. Here, the dual phase stainless steel sheets with an area ratio S<001>/S<111> of 0.90 to 1.10 had good appearance evaluation results.
In addition, in the example No. 1 and the example No. 2, ΔHV of the hot-rolled sheet was 65 or more, and the appearance evaluation result was very good (A). This was thought to be caused by the fact that, since ΔHV of the hot-rolled sheet was large, the grain size of the soft phase during cold rolling was subdivided, and as a result, the difference in deformability of the crystal orientation was less likely to occur.
While preferable embodiments of the present invention have been described above in detail, the present invention is not limited to these examples. It can be clearly understood by any person with ordinary knowledge in the field of technology to which the present invention belongs that various alternations or modifications can be made within the scope of the technical idea described in the scope of the claims, and these also naturally belong to the technical scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-198585 | Nov 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/043464 | 11/26/2021 | WO |
