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(11) | EP 3 348 661 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | STEEL SHEET AND ENAMELED PRODUCT |
| (57) A steel sheet includes, by mass%: C: 0.0060% or less; Si: 0.0010% to 0.050%; Mn:
0.05% to 0.50%; P: 0.005% to 0.100%; S: 0.0500% or less; Al: 0.0010% to 0.010%; Cu:
0.010% to 0.045%; O: 0.0250% to 0.0700%; N: 0.0010% to 0.0045%; and a remainder: Fe
and impurities, in which a structure of the steel sheet contains a ferrite, an average
grain diameter of the ferrite at a thickness 1/4 position in a through-thickness direction
from a surface is 20.0 µm or less, oxides containing Fe and Mn are contained, among
the oxides, a number density of oxides having a diameter of more than 1.0 µm and 10
µm or less is 1.0×103 grains/mm2 or more and 5.0×104 grains/mm2 or less, and a number density of oxides having a diameter of 0.1 to 1.0 µm is 5.0×103 grains/mm2 or more. |
[Technical Field of the Invention]
[0002] Priority is claimed on Japanese Patent Application No. 2015-179722, filed on September 11, 2015, the content of which is incorporated herein by reference.
[Related Art]
[0003] An enameled product is obtained by firing a glass material on the surface of a steel sheet for vitreous enameling. Enameled products have functions of heat resisting properties, weather resistance, chemical resistance, and water resistance and thus are widely used as materials for kitchen utensils such as pots and sinks, building materials, and the like in the related art. In general, such an enameled product is manufactured by processing a steel sheet into a predetermined shape, then assembling the steel sheet into a product shape by welding or the like, and thereafter performing an enameling treatment (firing treatment) thereon.
[0004] For a steel sheet for vitreous enameling used as the material of the enameled product, firing strain resistance, fishscale resistance, adhesion, bubble and black spot resistance, and the like are required as its properties. In addition, in the manufacturing of the enameled product, press forming is typically performed to obtain a product shape, and thus good formability is required.
[0005] Since corrosion resistance in a severe corrosive environment including sulfuric acid and the like is improved by applying the enameling treatment, the scope of application of enameled products extends to the field of energy such as power generation facilities. In such fields, there is a need for reliability with respect to fatigue and the like in long-term use, and for the purpose of weight reduction of components, a steel sheet to be used requires higher strength. It is known that the reliability with respect to fatigue and the like is influenced by a change in the microstructural morphology of the steel sheet in the manufacturing process including processing of an enameled product and an enameling treatment, that is, a change in strength due to a difference in the microstructural morphology in the steel sheet.
[0006] In the related art, regarding a change in microstructural morphology due to an enameling treatment, a technique for preventing deterioration of fishscale resistance caused by grain diameter coarsening is described, for example, in Patent Document 1. In Patent Document 1, it is described that it is possible to reduce the deterioration of fishscale resistance even in a case where an enameling treatment is repeatedly performed by optimizing the composition, size, shape, proportion, and number of inclusions in known high oxygen steel as the base, by adding a small amount of Ni, Cr, V, and Mo, and further adding Nb, B, and Ti as necessary, and by optimizing manufacturing conditions of a steel sheet.
[0007] In addition, in Patent Document 2, it is described that regarding a problem of deterioration of dimensional accuracy caused by bending during firing due to the decrease in strength with grain growth during enameling treatment for a high oxygen steel, it is effective to decrease the grain diameter distribution by uniformizing the microstructural morphology, that is, ferrite grain diameter, of a steel sheet for vitreous enameling. In Patent Document 2, the addition of Ni and Cr is performed for refinement of the structure of a hot rolled steel sheet in a steel sheet manufacturing process and uniformization of grain growth during annealing.
[0008] However, in both Patent Documents 1 and 2, it is considered that certain properties of the enameled products subjected to the enameling treatment with microstructural change can be secured. However, in both Patent Documents 1 and 2, in order to solve the problem regarding grain growth in the enameling treatment, the addition of Ni is essential. That is, there is a need to add an expensive alloying element to solve the problem. In addition, in Patent Document 2, the uniformity of the ferrite grain diameter is improved, abnormal grain growth is suppressed, and the formation of duplex grains is suppressed by making it difficult to inhibit ferrite grain growth by oxide coarsening through the addition of Cr. However, in this method in which suppression of grain growth by pinning of precipitates or inclusions is not used, a possibility that the grain diameters may become uneven in a case where the temperature in a member is changed during the enameling treatment and thus the required effect may not be obtained is considered. In this case, the strength after the enameling treatment is not stably obtained. Furthermore, in Patent Document 2, an object is to suppress bending of the member after the enameling treatment, and only yield stress before and after the enameling treatment is examined. Therefore, a change in tensile strength affecting fatigue properties is unclear.
[0009] As described above, a high strength steel sheet that sufficiently satisfies the strength properties as an index of fishscale resistance and steel sheet reliability, which are important properties of a steel sheet for vitreous enameling, while considering the manufacturing process, is not provided in a current situation, and objects to further improve the properties still remain.
[Prior Art Document]
[Patent Document]
[0010]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-316760
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2000-063985
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0011] The present invention develops the above-described technique of the steel sheet for vitreous enameling, and an object thereof is to provide a steel sheet capable of obtaining aging resistance, formability, excellent enameling properties (fishscale resistance, adhesion, and external appearance) after an enameling treatment, and strength properties (properties in which the decrease in tensile strength due to an enameling treatment does not occur, or the decrease in tensile strength can be stably suppressed). In addition, another object of the present invention is to provide an enameled product which has excellent enameling properties by including the steel sheet.
[Means for Solving the Problem]
[0012] The present invention has been obtained by performing various examinations in order to overcome the problems of the steel sheet for vitreous enameling in the related art, and is based on findings obtained as a result of examination of the influence of chemical compositions and manufacturing conditions on the fishscale resistance, suppression of the decrease in the strength, and the like of a steel sheet after an enameling treatment.
[0013] That is, the present invention is based on the following findings of 1) to 4).
- 1) Fishscale resistance can be improved by trapping hydrogen in steel, which is a factor of fishscale, by controlling precipitates in the steel through optimization of the steel composition. In particular, it is possible to secure fishscale resistance by allowing oxides of greater than 1.0 µm to 10 µm being present in steel and by optimizing the diameter and number of oxides.
- 2) Nb is a rare metal, and it is environmentally advantageous not to use Nb. However, in a case where Nb is not contained, the decrease in strength after an enameling treatment increases. This is because when Nb is contained, Nb suppresses grain growth during heating and heat retention in the enameling treatment, and in a case where Nb is not contained, this effect cannot be obtained.
- 3) Even if Nb is not contained, by optimizing the steel sheet composition, grain diameter, and the diameter and number of oxides of a steel sheet before the enameling treatment, that is, a raw sheet, it is possible to stably secure the strength after the enameling treatment (that is, the decrease in strength due to the enameling treatment can be suppressed). In particular, in order to suppress grain growth during the enameling treatment, which is a major factor of the decrease in strength due to the enameling treatment, it is effective to optimize the number density of oxides of 0.1 to 1.0 µm.
- 4) By controlling steelmaking conditions, the size of the oxides is controlled, and hot rolling conditions, cold rolling conditions, annealing conditions, and the temper rolling conditions are controlled, whereby it is possible to control the morphology of precipitates in the final product.
[0014] The present invention has been completed on the basis of the above findings, and the gist of the present invention is as follows.
- (1) According to an aspect of the present invention, a steel sheet includes, by mass%: C: 0.0060% or less; Si: 0.0010% to 0.050%; Mn: 0.05% to 0.50%; P: 0.005% to 0.100%; S: 0.0030% to 0.0500%; Al: 0.0010% to 0.010%; Cu: 0.010% to 0.045%; O: 0.0250% to 0.0700%; N: 0.0010% to 0.0045%; and a remainder of Fe and impurities, in which a structure of the steel sheet contains a ferrite, an average grain diameter of the ferrite at a thickness 1/4 position in a through-thickness direction from a surface is 20.0 µm or less, the steel sheet contains oxides containing Fe and Mn, among the oxides, a number density of oxides having a diameter of more than 1.0 µm and 10 µm or less is 1.0×103 grains/mm2 or more and 5.0×104 grains/mm2 or less, and a number density of oxides having a diameter of 0.1 to 1.0 µm is 5.0×103 grains/mm2 or more.
- (2) In the steel sheet according to (1), the impurities may include, by mass%, a total of one or more of B, Cr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg: limited to 0.100%.
- (3) In the steel sheet according to (1) or (2), the impurities include, by mass%, Nb: limited to 0.010% or less.
- (4) In the steel sheet according to any one of (1) to (3), the steel sheet may be a cold rolled steel sheet.
- (5) In the steel sheet according to any one of (1) to (4), the steel sheet may be a steel sheet for vitreous enameling.
- (6) According to another aspect of the present invention, an enameled product includes: the steel sheet according to any one of (1) to (5).
[Effects of the Invention]
[0015] The steel sheet according to the aspect of the present invention is excellent in formability and strength and fishscale resistance after an enameling treatment. In addition, the steel sheet is also excellent in aging resistance, enamel adhesion, and external appearance after the enameling treatment. Therefore, the steel sheet is suitable as a steel sheet for vitreous enameling, which is the substrate of an enameled product applied to kitchen utensils, building materials, and the field of energy.
[0016] Furthermore, the enameled product according to the aspect of the present invention has excellent enameling properties. Therefore, the enameled product is suitable for applications such as kitchen utensils, building materials, and the field of energy.
[Brief Description of the Drawings]
[0017]
FIG. 1 is a photograph showing an example of oxides having a diameter of 0.1 to 1.0 µm.
FIG. 2 is a photograph showing an example of oxides having a diameter of more than 1.0 µm and 10 µm or less.
[Embodiments of the Invention]
[0018] A steel sheet according to an embodiment of the present invention (hereinafter, a steel sheet according to this embodiment) will be described in detail. The steel sheet according to this embodiment is suitably used as the substrate of an enameled product (steel sheet for vitreous enameling).
<Chemical Composition>
[0019] First, the reason for limiting the chemical composition of the steel sheet according to this embodiment will be described. "%" regarding the composition means mass% unless otherwise specified.
<C: 0.0060% or less>
[0020] The lower the C content, the better the ductility. When the C content exceeds 0.0060%, bubble defects are likely to occur. Therefore, the C content is set to 0.0060% or less. In order to improve ductility, the C content is preferably low. However, when the C content is lowered, the steelmaking cost is increased, and thus the C content is preferably 0.0015% or more.
<Si: 0.0010% to 0.050%>
[0021] Si is an element having an effect of controlling the composition of oxides. In order to obtain this effect, the Si content is set to 0.0010% or more. On the other hand, the excess Si content inhibits the enameling properties and simultaneously forms a large amount of Si oxide during hot rolling, and there may be cases where the fishscale resistance decreases. Therefore, the Si content is set to 0.050% or less. From the viewpoint of improving bubble resistance and black spot resistance and obtaining better surface properties after the enameling treatment, it is preferable that the Si content is set to 0.0080% or less.
<Mn: 0.05% to 0.50%>
[0022] Mn relates to the O content and is an important component that affects the composition of oxides having an effect on the fishscale resistance of a steel sheet for vitreous enameling and contributes to the high-strengthening of the steel sheet. Furthermore, Mn is an element that prevents hot embrittlement caused by S during a hot rolling. In order to obtain these effects, the Mn content is set to 0.05% or more. Typically, as the Mn content increases, the enamel adhesion is deteriorated and bubbles and black spots are likely to be generated. However, in a case of the presence of Mn as oxides in steel, the degree of deterioration of these properties is small. However, when the Mn content is excessive, the ductility deteriorates. Therefore, the upper limit of the Mn content is set to 0.50%.
<P: 0.005% to 0.100%>
[0023] P is an element effective in the high-strengthening of the steel sheet. In addition, P also has an effect of suppressing the decrease in strength due to the enameling treatment. In order to obtain these effects, the P content is set to 0.005% or more. In addition, P is an element effective also in suppressing the growth of grains during the enameling treatment by raising the recrystallization temperature. In order to obtain this effect, it is preferable that the P content is set to 0.015% or more. On the other hand, when the P content is excessive, there may be cases where P segregates to the grain boundaries of the steel sheet at a high concentration during the enameling treatment and becomes a factor of bubbles and black spots. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.075% or less.
<S: 0.0500% or less>
[0024] S is an element that forms Mn sulfides. The sulfides may be precipitated with oxides as composite precipitates, and in a case that sulfide is precipitated as composite precipitate, the fishscale resistance can be further improved. In order to obtain this effect, S may be contained. In a case of obtaining the above effect, it is desirable that the S content is set to 0.0030% or more. The S content is more preferably 0.0100% or more, and even more preferably 0.0150% or more. However, when the S content is excessive, there may be case where the effect of Mn necessary for controlling the oxides in steel decreases. Therefore, the upper limit of the S content is set to 0.0500%, and preferably 0.0300% or less.
<Al: 0.0010% to 0.010%>
[0025] Al is a strong deoxidizing element. Therefore, it is necessary to carefully control Al content. When the Al content exceeds 0.010%, it is difficult to contain a necessary amount of O in steel and it is difficult to control the oxides effective in fishscale resistance. Therefore, the Al content is set to 0.010% or less. On the other hand, when the Al content is less than 0.0010%, bubble defects are likely to occur in a slab, and a higher degree of refinement than in the related art is necessary for the slab in the steelmaking stage, and a heavy burden is imposed on the steelmaking process. Therefore, the lower limit of the Al content is set to 0.0010%.
<Cu: 0.010% to 0.045%>
[0026] Cu is an element that improves the enamel adhesion by controlling the reaction between a glass material and steel during the enameling treatment. In order to obtain the effect, the Cu content is set to 0.010% or more. On the other hand, when the Cu content is excessive, not only is the reaction between the glass material and the steel inhibited, but also there may be cases where the ductility is deteriorated. In order to avoid such adverse effects, the Cu content is set to 0.045% or less. The Cu content is preferably 0.029% or less, and more preferably 0.019% or less.
<O: 0.0250% to 0.0700%>
[0027] O is an element that directly affects fishscale resistance and ductility and forms oxides, and affects fishscale resistance in relation to the Mn content. In order to obtain excellent ductility and fishscale resistance, the O content is set to 0.0250% or more. The O content is preferably 0.0400% or more. On the other hand, when the O content is excessively high, the ductility is deteriorated, and the Mn content for forming a necessary amount of oxides is increased, resulting in an increase in the alloy cost. Therefore, the O content is set to 0.0700% or less.
[0028] In this embodiment, the O content is measured by reacting oxygen in about 0.5 g of a steel sample with a graphite crucible in accordance with JIS G 1239, measuring generated CO by an infrared absorption method, and quantifying the concentration.
<N: 0.0010% to 0.0045%>
[0029] N is an interstitial solid solution element, and ductility is deteriorated when a large amount of N is contained. In addition, when the N content is large, the aging resistance is deteriorated. Therefore, the upper limit of the N content is set to 0.0045%. Although there is no need to limit the lower limit, significant costs are incurred by melting to a proportion of less than 0.0010% of N in current techniques, and thus the lower limit of the N content is set to 0.0010%.
[0030] The steel sheet according to this embodiment basically contains the above-described elements, and the remainder of Fe and impurities. The impurities are components incorporated from raw materials such as ore and scrap when steel is industrially manufactured, or by various factors in the manufacturing process and mean components that are allowed in a range in which the steel sheet according to this embodiment is not adversely affected.
[0031] In the steel sheet according to this embodiment, it is preferable to limit the amounts of the elements contained as the impurities to the ranges described later.
Cr, Ni, B, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, Mg: 0.100% or less in Total
[0032] Cr, Ni, B, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg are impurities which are elements that do not need to be positively contained but are unavoidably incorporated. In general, these elements are rarely incorporated independently, and two or more elements are incorporated, for example, like Cr and Ni.
[0033] If these elements are excessively contained, the reaction with the oxide forming elements is not negligible, and it is difficult to perform desired oxide control. Therefore, the total amount of these elements is preferably limited to 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less.
[0034] In addition, in a case where these elements act as deoxidizing elements, the elements affect the value of free oxygen, and there may be cases where it is difficult to adjust the free oxygen. Therefore, it is preferable that the upper limit of each element is set to be in a range in which the value of free oxygen is not affected in a casting stage.
Nb: 0.010% or less
[0035] Nb is a rare metal, and it is environmentally advantageous not to use Nb. Therefore, Nb is not added to the steel sheet according to this embodiment. Nb may be incorporated as an impurity. However, Nb is an element that affects the number of inclusions, and the Nb content is preferably limited to 0.010% or less.
<Oxides>
[0036] Next, the reason for limiting oxides containing Fe and Mn present in the steel sheet according to this embodiment will be described.
[0037] The steel structure of the steel sheet according to this embodiment includes oxides containing Fe and Mn as elements of the deoxidation product. Since Nb is not added to the steel sheet according to this embodiment, Nb is not included as an element of the deoxidation product in the oxides. In addition, it is preferable that the oxides do not contain Al, Cr, Si, and the like. This is accomplished by limiting the amounts of the above-mentioned elements or adding the elements so as not to affect the composition of the oxides. However, even when Al, Cr, Si, and the like are not added as deoxidizing agents at the time of adjusting the molten steel components, there may be cases where Al, Cr, Si, and the like are detected from the oxides in a proportion of about 6% or less when the oxides are detected. Since it is considered that Al, Cr, Si, and the like contained as impurity elements are incorporated in the oxides, the components contained in the oxides in a proportion of about 15% or less, and preferably about 6% or less are not counted as elements of the deoxidation product.
[0038] That is, it is preferable that the oxides contained in the steel sheet according to this embodiment are substantially composed only of Fe, Mn, and O (even if Al, Cr, and Si are unavoidably contained, the total amount thereof is 15% or less). However, the oxides may be precipitated as composite oxides with sulfides such as MnS. In a case where the oxides do not contain Nb, Al, Cr, Si, and the like as elements of the deoxidation product, the oxides can be finely dispersed by adjusting free oxygen during casting. On the other hand, in a case where Al, Nb, Cr, Si, and the like are contained as elements of the deoxidation product, it is difficult to control the number and size of the oxides in a desired range by adjusting free oxygen in the casting process of this application.
[0039] In the steel sheet according to this embodiment, it is necessary that among the oxides, the number density of the oxides having a diameter of more than 1.0 µm and 10 µm or less is 1.0×103 grains/mm2 or more and 5.0×104 grains/mm2 or less, and the number density of the oxides having a diameter of 0.1 to 1.0 µm is 5.0×103 grains/mm2 or more.
[0040] The oxides having a diameter of more than 1.0 µm contributes to improve fishscale resistance. In the oxides having smaller diameters than this range, the effect of improving fishscale resistance is reduced. From the viewpoint of the effect of improving fishscale resistance, there is no need to particularly limit the upper limit of the diameter. However, although depending on the amount of the contained oxygen, as the amount of coarse oxides increases, the number density of the oxides decreases, and the effect of inhibiting hydrogen permeation decreases. Furthermore, coarse oxides tend to become crack origin during processing and deteriorate ductility. Therefore, it is preferable that the diameter of the oxides utilized for improving the fishscale resistance is 10 µm or less, and preferably 5 µm or less. That is, in order to improve the fishscale resistance, oxides having a diameter of more than 1.0 to 10 µm are controlled.
[0041] In order to improve the fishscale resistance, it is necessary to include oxides containing Fe and Mn with a diameter of more than 1.0 to 10 µm in a number density of 1.0×103 grains/mm2 or more. In a case of a smaller number density than the above number density, excellent fishscale resistance cannot be secured. On the other hand, when the oxides are present in a number density of more than 5.0×104 grains/mm2, voids are generated at the interface between the oxides and the base steel sheet during processing more than necessary, resulting in the decrease in strength after the enameling treatment. Therefore, the upper limit of the number density is set to 5.0×104 grains/mm2, and preferably 1.0×104 grains/mm2 or less. The oxides having a diameter of more than 1.0 µm have a round shape as shown in FIG. 2 in many cases.
[0042] On the other hand, the oxides having a diameter of 1.0 µm or less have an effect of suppressing grain growth in a heat treatment (enameling treatment) process during the manufacturing of the enameled product. When the diameter thereof is more than 1.0 µm, the effect of suppressing grain growth due to heat input during the heat treatment disappears, and thus the upper limit of the diameter of the oxides utilized for suppressing grain growth is set to 1.0 µm or less. For this effect, it is preferable that the diameter of the oxides in steel is preferably small, preferably 0.8 µm or less, and even more preferably 0.5 µm or less. It is desirable that the diameter of the oxides present in steel is as small as possible. However, when the diameter is too small, it is difficult to analyze the oxides, that is, it is difficult to identify the oxides containing Fe and Mn. Therefore, in the steel sheet according to this embodiment, the lower limit of the diameter of the oxides as an object of controlling the number density is set to 0.1 µm or more. That is, in order to suppress grain growth in the heat treatment process, the oxides having a diameter of 0.1 to 1.0 µm are controlled. The oxides having a diameter of 0.1 to 1.0 µm have an angular shape as shown in FIG. 1 in many cases.
[0043] In the case of suppressing grain growth in the heat treatment process, it is necessary to include oxides containing Fe and Mn with a diameter of 0.1 µm or more and 1.0 µm or less in a number density of 5.0×103 grains/mm2 or more. This is because in a case of a smaller number density than the above number density, the effect of suppressing grain growth in the heat treatment process cannot be sufficiently obtained. From the viewpoint of suppressing grain growth, there is no upper limit of the density of the oxides. However, when the density of the oxides exceeds 1.0×105 grains/mm2, the number density of the oxides having a diameter of more than 1.0 µm consequently decreases, leading to the decrease in fishscale resistance. Therefore, the density of the oxides containing Fe and Mn with a diameter of 0.1 µm to 1.0 µm is set to 1.0×105 grains/mm2 or less.
[0044] In the steel sheet according to this embodiment, by controlling the oxides in the above range, grain growth during the enameling treatment can be suppressed even when Nb is not contained, and the decrease in strength can be suppressed.
[0045] Furthermore, the oxides containing Fe and Mn with a diameter of 0.1 µm to 1.0 µm also have an effect of refining the grain diameter after cold rolling and recrystallization, and thus contribute to bending workability and suppression of breaking and fatigue fracture when a member formed by processing steel is used.
[0046] A method of identifying the above-described oxides is not particularly limited. In this embodiment, since oxides from which Fe, Mn, and O are simultaneously detected are objects, for the identification thereof, for example, field-emission scanning electron microscopy (FE-SEM) and an energy dispersive X-ray dispersive analyzer (EDAX) may be used. The measurement method may be an ordinary method. However, since there is a need to determine the concentration of a particularly fine region, care is needed, for example, the beam diameter of an electron beam is set to be sufficiently small (for example, 0.1 to 0.5 µm).
[0047] The diameter and the number density of the oxides can be measured by the following method. That is, with the FE-SEM, the magnification is set to 5,000-fold or more, the number of visual fields is set to 10 or more, the size and number of the corresponding oxides in the visual fields are measured, and the major axis of the oxide is determined as the diameter of the oxide. The number density is obtained by calculating the number of oxides having a major axis of 0.1 µm to 1.0 µm and the number of oxides having a major axis of more than 1.0 µm to 10.0 µm among the oxides in the visual fields, multiplying the number by a value obtained by dividing the unit area (mm2) by the total area of the visual fields, and thus converting the number into the number per unit area.
[0048] In the steel sheet according to this embodiment, there may be cases where slag and refractory materials incorporated in the manufacturing process are present as inclusions. However, since the composition of the inclusions does not include Mn and Fe and the inclusions do not have an effect of suppressing the decrease in strength, such inclusions are not counted.
[0049] Fe, Mn, and O may be simultaneously detected from the oxides as the measurement objects in this embodiment, and for example, MnS and the like may be precipitated as composite oxides.
<Metallographic Structure>
[0050] Next, the microstructure (metallographic structure) of the steel sheet according to this embodiment will be described.
[0051] The microstructure of the steel sheet according to this embodiment primarily contains ferrite. Therefore, in order to improve the strength, it is effective to reduce the grain diameter.
[0052] In a case where the steel sheet according to this embodiment is processed into an enameled product, the grain diameter changes due to ferrite grain growth in the heat treatment (enameling treatment), and as a result, the strength (tensile strength) decreases. In addition, due to the decrease in strength, the fatigue properties are also deteriorated. Decreasing the grain diameter after the heat treatment is effective for securing the strength of the steel sheet after the heat treatment. In order to decrease the grain diameter after the heat treatment, it is important to decrease the grain diameter before the heat treatment and suppress grain growth due to the heat treatment.
[0053] The average grain diameter of ferrite in the steel sheet microstructure before the heat treatment needs to be 20.0 µm or less at a thickness 1/4 position (1/4t: t is sheet thickness) in a through-thickness direction from the surface of the steel sheet. When the average grain diameter exceeds 20.0 µm, it is difficult to achieve high-strengthening of the steel sheet. In order to achieve the high-strengthening, it is desirable that the average grain diameter is small. However, as the average grain diameter decreases, the ductility is deteriorated. Therefore, it is necessary to determine an optimal grain diameter for a desired product shape. The optimal grain diameter is preferably 15.0 µm or less, more preferably 13.0 µm or less, and even more preferably 11.0 µm or less. The average grain diameter of the ferrite may be measured according to the intercept method described in JIS G 0552 or the like.
[0054] In order to obtain good ductility, the area ratio of the ferrite is 90% or more, more preferably 95% or more, and even more preferably 99% or more. The remainder is, for example, oxide or iron carbide.
[0055] Typically, breaking or fatigue fracture is likely to occur on the surface of the steel sheet during bending or when a member obtained by processing steel is used. Therefore, in order to improve such properties, it is particularly desirable that the grain diameter of the surface layer of the steel sheet is small. The grain diameter of the steel sheet is greatly influenced by the concentration of elements in steel, particularly P, and as the concentration of P increases, the grain diameter tends to decrease.
[0057] The enameled product according to this embodiment includes the steel sheet according to this embodiment. For example, the enameled product is a product obtained by performing processing, welding, and an enameling treatment on the steel sheet according to this embodiment.
<Manufacturing Method>
[0058] A preferable manufacturing method of the steel sheet according to this embodiment will be described.
[0059] The effect of the steel sheet according to this embodiment is obtained as long as the steel sheet has the above-described configuration, and thus there is no need to limit the manufacturing method. However, as described later, the steel sheet can be stably manufactured according to a manufacturing method including each of steelmaking, casting, hot rolling, cold rolling, continuous annealing, and temper rolling processes, which is preferable.
[0061] The points in the manufacturing are the improvement of fishscale resistance by the oxides containing Fe and Mn and the control of the oxides having the effect of suppressing abnormal grain growth during the enameling treatment. It is preferable that the diameter of the oxides is relatively large in order to improve the fishscale resistance, and it is preferable that the diameter of the oxides is small in order to suppress abnormal grain growth. When the concentration of oxygen in steel is high, oxide having a large diameter is generated. On the other hand, the concentration of oxygen is low, the diameter of oxide is refined.
[0062] Since the oxides of 0.1 to 1.0 µm are angular as shown in FIG. 1, it is considered that the oxides of 0.1 to 1.0 µm are generated by the reaction between free oxygen and the steel components after solidification. Therefore, by stirring the solidification interface through electromagnetic stirring to adjust free oxygen in the steelmaking stage and adjust the concentration of components such as oxygen at the solidification interface, the number of crystallized grains of the oxides of 0.1 to 1.0 µm can be controlled.
[0063] In addition, since inclusions of more than 1.0 to 10 µm have round shapes as shown in FIG. 2, it is considered that the inclusions of more than 1.0 to 10 µm are formed in a liquid state in a molten steel stage in many cases. Therefore, aggregation and floating of the inclusions are controlled by controlling the casting rate, stirring of the molten steel, the degree of overheating of the molten steel, and the like, thereby controlling the number of inclusions of more than 1.0 and 10 µm or less.
<Steelmaking Process and Casting Process>
[0064] It is desirable that ΔT (the degree of overheating of the molten steel) in a mold is set to be in a range of 20°C to 35°C and the casting rate is set to be in a range of 1 to 1.5 m/min. By setting the above-described conditions, inclusions having a large diameter can be aggregated and are allowed to float in the mold, thereby controlling the number of inclusions. In order to promote the aggregation of the inclusions by applying viscous flow in the mold, electromagnetic stirring may be performed in the mold.
[0065] Furthermore, in order to cause fine oxides to be precipitated during solidification or after solidification, it is preferable that free oxygen in the mold is controlled to about 250 to 700 ppm by adding a minute amount of a deoxidizing element to a degree that the deoxidizing element does not affect degassing or the oxide composition during secondary refining and then the resultant is cast by cooling at 1.0 to 5.0 °C/s in a range between 1200°C to 1500°C. By setting the above-described conditions, dissolved oxygen is left at a high temperature and inclusions having a small diameter can be formed at a low temperature.
[0066] That is, by controlling the steelmaking conditions and the casting conditions, it is possible to control the presence states of both the oxides having a large diameter and the oxides having a small diameter.
[0067] The amount of dissolved oxygen (free oxygen) can be measured in a tundish using an oxygen concentration cell. In a case where production during the secondary refining is stable, it is not necessary to measure the amount of dissolved oxygen each time.
<Hot Rolling Process>
[0068] When the slab is heated before hot rolling, the heating temperature is preferably 1150°C to 1250°C. When the heating temperature exceeds 1250°C, the amount of primary scale generated is large, resulting in the decrease in yield. On the other hand, when the heating temperature is lower than 1150°C, due to the decrease in the temperature during rolling, the rolling load increases. In the hot rolling, it is preferable that the rolling reduction ratio is 30% to 90%, and the finishing temperature is Ar3 to 950°C. After the hot rolling, the coiling temperature is preferably 550°C to 750°C. The Ar3 temperature can be obtained by thermal expansion measurement result after applying a thermal history that simulates hot rolling to a small test piece and processing the resultant.
[0069] The oxides containing Fe and Mn produced in the steelmaking process and the casting process are stretched by hot rolling. By setting the hot rolling reduction ratio (cumulative rolling reduction ratio during hot rolling) to 30% or more, it is possible to sufficiently stretch the oxides containing Fe and Mn in steel. When the hot rolling reduction ratio exceeds 90%, there may be cases where the oxides in steel are excessively stretched and good fishscale resistance is not obtained.
[0070] When the finishing temperature in the hot rolling is lower than Ar3, the rolling is performed at a temperature equal to or lower than the transformation point, and mechanical properties such as ductility as a product deteriorate. Simultaneously, the strength of the steel sheet is significantly changed, and thus the rolling tends to be unstable. In addition, in the case where the finishing temperature is lower than Ar3, the microstructure of the hot rolled steel sheet has duplex grains including coarse grains, and there is concern that ridging in a cold-rolling-annealed sheet that uses the hot rolled steel sheet may occur after processing. Therefore, the finishing temperature needs to be set to Ar3 or higher, and is more desirably 900°C or higher. On the other hand, when the finishing temperature exceeds 950°C, the grain diameter becomes coarse, and it is difficult to secure desired strength.
[0071] The coiling temperature after the hot rolling is preferably set to 550°C or higher. When the coiling temperature is lower than 550°C, it is difficult for the microstructural after cold rolling and continuous annealing to secure necessary ductility for processing and r value. In a case where the coiling temperature exceeds 750°C, the grain diameter increases, and it is difficult to secure the desired steel sheet strength.
<Cold Rolling Process>
[0072] After performing pickling on the hot rolled steel sheet as necessary, cold rolling is performed. The cold rolling reduction ratio during the cold rolling is important for determining the properties of the product and is preferably 65% to 85%. The oxides containing Fe and Mn formed in the steelmaking process and the casting process are stretched according to the rolling reduction ratio in the hot rolling process. Thereafter, the oxides are further stretched in the cold rolling process. However, since the cold rolling is a process performed at about 150°C at the maximum and the oxides are hard, the oxides are less likely to be stretched. Therefore, for appropriate stretching, it is preferable that the cold rolling is performed at a cold rolling reduction ratio of 65% or more.
[0073] At this time, voids are generated at both ends in the rolling direction of the oxides. The presence of the voids acts effectively on fishscale resistance but acts adversely on ductility. Therefore, the presence of more voids than necessary causes the decrease in ductility and eventually impairs workability and the strength properties of products after the enameling treatment. Therefore, the upper limit of the cold rolling reduction ratio is set to 85%. In a case of performing cold rolling at a higher cold rolling reduction ratio, it seems that the voids formed at the initial stage of the rolling is crushed and disappeared due to the increase in the cold rolling reduction ratio, in the observed microstructure. However, it is assumed that since the voids are not structurally bonded together, the voids act as the fracture origin due to the introduction of strain during processing and deteriorate the ductility.
<Continuous Annealing Process>
[0074] Continuous annealing is performed on the cold rolled steel sheet. The annealing temperature in the continuous annealing process is preferably set to 700°C to 850°C. For the purpose of imparting features to mechanical properties such as strength, the annealing temperature may be lower than 700°C. On the other hand, when the annealing temperature exceeds 850°C, regarding the mechanical properties, ductility and the like are improved, which is preferable. However, the voids generated in the cold rolling process tend to disappear by diffusion, and thus the fishscale resistance is deteriorated. Therefore, it is preferable that the upper limit of the annealing temperature in the continuous annealing process is set to 850°C.
[0075] After the annealing, temper rolling may be performed mainly for the purpose of shape control. In the temper rolling, the amount of strain introduced into the steel sheet varies depending on the temper rolling reduction ratio as well as the shape control. At this time, when the temper rolling reduction ratio increases, that is, when the amount of strain introduced into the steel sheet increases, abnormal grain growth during the enameling treatment is promoted. Therefore, the upper limit of the temper rolling reduction ratio is set to a rolling reduction ratio at which the shape control is possible, and it is not desirable that more strain than necessary is imparted. From the viewpoint of shape control, the temper rolling reduction ratio is preferably 1.5% or less.
[0076] Accordingly, a steel sheet having desired properties, specifically, a steel sheet for vitreous enameling can be obtained.
[0077] The enameled product according to this embodiment is obtained by processing the steel sheet according to this embodiment into a predetermined shape, then assembling the steel sheet into a product shape by welding or the like, and thereafter performing an enameling treatment thereon. The enameling treatment may be performed under known conditions, and for example, a steel sheet coated with a glaze is heated to, for example, 800°C to 850°C and is held for 1 to 10 minutes to adhere the glass material of the glaze and the steel sheet to each other.
[Examples]
[0078] Steels having the compositions shown in Tables 1 and 2 were melted in a converter and continuously cast into slabs. During the casting, ΔT in a mold and the casting rate were set as shown in Tables 3 and 4, the cooling rate in a range of 1200°C to 1500°C and the amount of dissolved oxygen were controlled in the ranges of Tables 3 and 4 using electromagnetic stirring, thereby controlling the number and density of oxides and the amount of oxygen. The amount of dissolved oxygen (free oxygen) was checked by the method described above. These slabs were heated in a heating furnace at a temperature of 1150°C to 1250°C, were subjected to hot rolling at a finishing temperature of 900°C or higher, and were coiled at 700°C to 750°C into hot rolled steel sheets. In addition, after pickling, the rolling reduction ratio of cold rolling was changed in the ranges in Tables 3 and 4 to produce cold rolled steel sheets, and the cold rolled steel sheets were further subjected to continuous annealing at 780°C and thereafter subjected to temper rolling, thereby producing steel sheets having a sheet thickness of 0.8 mm. In order to control the sheet thickness after the temper rolling to be constant, the sheet thickness of the hot rolled steel sheets were changed with respect to the rolling reduction ratio for the cold rolling.
[Table 1]
| No. | Composition (mass %) * the remainder includes Fe and impurities | ||||||||||
| C | Si | Mn | P | S | Al | Cu | O | N | Nb | Other components | |
| A1 | 0.0030 | 0.002 | 0.23 | 0.045 | 0.0203 | 0.002 | 0.035 | 0.0589 | 0.0032 | 0.001 | - |
| A2 | 0.0025 | 0.012 | 0.46 | 0.075 | 0.0243 | 0.003 | 0.025 | 0.0483 | 0.0028 | 0.002 | - |
| A3 | 0.0048 | 0.005 | 0.26 | 0.075 | 0.0350 | 0.003 | 0.029 | 0.0533 | 0.0032 | 0.001 | - |
| A4 | 0.0038 | 0.006 | 0.48 | 0.042 | 0.0273 | 0.002 | 0.026 | 0.0630 | 0.0028 | 0.003 | - |
| A5 | 0.0043 | 0.004 | 0.21 | 0.083 | 0.0432 | 0.004 | 0.030 | 0.0463 | 0.0027 | 0.001 | - |
| A6 | 0.0038 | 0.012 | 0.35 | 0.065 | 0.0325 | 0.003 | 0.031 | 0.0445 | 0.0032 | 0.003 | - |
| A7 | 0.0023 | 0.006 | 0.43 | 0.053 | 0.0125 | 0.005 | 0.038 | 0.0536 | 0.0024 | 0.002 | - |
| A8 | 0.0053 | 0.003 | 0.42 | 0.038 | 0.0283 | 0.003 | 0.025 | 0.0486 | 0.0025 | 0.003 | - |
| A9 | 0.0036 | 0.006 | 0.13 | 0.068 | 0.0267 | 0.003 | 0.032 | 0.0368 | 0.0033 | 0.001 | - |
| A10 | 0.0045 | 0.004 | 0.32 | 0.048 | 0.0243 | 0.002 | 0.028 | 0.0430 | 0.0021 | 0.002 | - |
| A11 | 0.0038 | 0.007 | 0.43 | 0.068 | 0.0256 | 0.003 | 0.036 | 0.0342 | 0.0019 | 0.003 | - |
| A12 | 0.0036 | 0.004 | 0.26 | 0.053 | 0.0276 | 0.004 | 0.028 | 0.0356 | 0.0028 | 0.006 | - |
| A13 | 0.0012 | 0.005 | 0.28 | 0.041 | 0.0243 | 0.002 | 0.032 | 0.0533 | 0.0041 | 0.002 | - |
| A14 | 0.0017 | 0.003 | 0.21 | 0.021 | 0.0182 | 0.003 | 0.023 | 0.0452 | 0.0018 | 0.001 | - |
| A15 | 0.0020 | 0.003 | 0.22 | 0.023 | 0.0090 | 0.006 | 0.023 | 0.0432 | 0.0023 | 0.003 | - |
| A16 | 0.0021 | 0.003 | 0.23 | 0.024 | 0.0093 | 0.005 | 0.024 | 0.0442 | 0.0021 | 0.001 | - |
| A17 | 0.0029 | 0.045 | 0.25 | 0.033 | 0.0099 | 0.002 | 0.031 | 0.0476 | 0.0019 | 0.002 | - |
| A18 | 0.0020 | 0.006 | 0.09 | 0.060 | 0.0212 | 0.005 | 0.019 | 0.0463 | 0.0020 | 0.004 | - |
| A19 | 0.0023 | 0.005 | 0.12 | 0.019 | 0.0193 | 0.005 | 0.033 | 0.0327 | 0.0026 | 0.005 | - |
| A20 | 0.0021 | 0.004 | 0.22 | 0.011 | 0.0234 | 0.002 | 0.031 | 0.0562 | 0.0023 | 0.009 | - |
| A21 | 0.0023 | 0.003 | 0.25 | 0.006 | 0.0181 | 0.003 | 0.033 | 0.0502 | 0.0026 | 0.002 | - |
| A22 | 0.0027 | 0.005 | 0.21 | 0.068 | 0.0050 | 0.005 | 0.030 | 0.0377 | 0.0028 | 0.001 | - |
| A23 | 0.0020 | 0.005 | 0.25 | 0.073 | 0.0129 | 0.009 | 0.035 | 0.0440 | 0.0031 | 0.003 | - |
| A24 | 0.0021 | 0.006 | 0.17 | 0.024 | 0.0147 | 0.001 | 0.015 | 0.0288 | 0.0022 | 0.001 | - |
| A25 | 0.0022 | 0.004 | 0.26 | 0.065 | 0.0091 | 0.004 | 0.027 | 0.028 | 0.0020 | 0.002 | - |
| A26 | 0.0017 | 0.006 | 0.39 | 0.076 | 0.0119 | 0.005 | 0.020 | 0.0557 | 0.0013 | 0.001 | - |
[Table 2]
| No. | Composition (mass%) * the remainder includes Fe and impurities | ||||||||||
| C | Si | Mn | P | S | Al | Cu | O | N | Nb | Other components | |
| A27 | 0.0017 | 0.008 | 0.32 | 0.034 | 0.0181 | 0.005 | 0.040 | 0.0456 | 0.0041 | 0.004 | - |
| A28 | 0.0045 | 0.004 | 0.38 | 0.056 | 0.0245 | 0.004 | 0.032 | 0.0389 | 0.0022 | 0.003 | Cr: 0.012, Ni: 0.023 |
| A29 | 0.0029 | 0.002 | 0.24 | 0.046 | 0.0211 | 0.002 | 0.036 | 0.0546 | 0.0034 | 0.002 | Sn: 0.007, Ca: 0.005, Sb: 0.003 |
| A30 | 0.0038 | 0.006 | 0.42 | 0.053 | 0.0239 | 0.003 | 0.021 | 0.0466 | 0.0034 | 0.003 | La: 0.052, Ce: 0.019 |
| A31 | 0.0034 | 0.007 | 0.36 | 0.068 | 0.0245 | 0.003 | 0.024 | 0.0422 | 0.0024 | 0.001 | Mo: 0.025, W: 0.007, Ta: 0.005 |
| A32 | 0.0018 | 0.003 | 0.09 | 0.018 | 0.0123 | 0.004 | 0.018 | 0.0352 | 0.0019 | 0.002 | |
| A33 | 0.0025 | 0.006 | 0.25 | 0.031 | 0.0012 | 0.003 | 0.017 | 0.0591 | 0.0021 | 0.005 | - |
| B1 | 0.0100 | 0.007 | 0.34 | 0.053 | 0.0265 | 0.003 | 0.031 | 0.0322 | 0.0032 | 0.003 | - |
| B2 | 0.0022 | 0.0007 | 0.12 | 0.041 | 0.0240 | 0.005 | 0.027 | 0.0489 | 0.0023 | 0.004 | - |
| B3 | 0.0016 | 0.064 | 0.19 | 0.036 | 0.0095 | 0.002 | 0.021 | 0.0626 | 0.0030 | 0.003 | - |
| B4 | 0.0025 | 0.005 | 0.62 | 0.073 | 0.0228 | 0.002 | 0.034 | 0.0557 | 0.0029 | 0.001 | - |
| B5 | 0.0035 | 0.003 | 0.33 | 0.048 | 0.0253 | 0.004 | 0.026 | 0.0754 | 0.0038 | 0.003 | - |
| B6 | 0.0031 | 0.003 | 0.29 | 0.062 | 0.0345 | 0.003 | 0.028 | 0.0215 | 0.0026 | 0.004 | - |
| B7 | 0.0025 | 0.004 | 0.23 | 0.004 | 0.0193 | 0.003 | 0.023 | 0.0592 | 0.0026 | 0.002 | - |
| B8 | 0.0017 | 0.010 | 0.24 | 0.132 | 0.0092 | 0.004 | 0.043 | 0.0495 | 0.0028 | 0.003 | - |
| B9 | 0.0016 | 0.004 | 0.30 | 0.080 | 0.0632 | 0.003 | 0.033 | 0.0657 | 0.0026 | 0.002 | - |
| B10 | 0.0016 | 0.010 | 0.13 | 0.064 | 0.0139 | 0.0008 | 0.020 | 0.0677 | 0.0027 | 0.006 | - |
| B11 | 0.0021 | 0.005 | 0.31 | 0.043 | 0.0098 | 0.0122 | 0.034 | 0.0178 | 0.0020 | 0.001 | - |
| B12 | 0.0022 | 0.005 | 0.13 | 0.019 | 0.0247 | 0.003 | 0.008 | 0.0580 | 0.0029 | 0.002 | - |
| B13 | 0.0025 | 0.007 | 0.21 | 0.028 | 0.0116 | 0.001 | 0.051 | 0.0430 | 0.0018 | 0.003 | - |
| B14 | 0.0025 | 0.008 | 0.18 | 0.045 | 0.0173 | 0.005 | 0.021 | 0.0311 | 0.0052 | 0.002 | - |
| B15 | 0.0035 | 0.003 | 0.25 | 0.045 | 0.0236 | 0.004 | 0.024 | 0.0152 | 0.0023 | 0.004 | La: 0.068, Ce: 0.058 |
| B16 | 0.0023 | 0.002 | 0.23 | 0.024 | 0.0093 | 0.004 | 0.028 | 0.0455 | 0.0024 | 0.002 | - |
| B17 | 0.0024 | 0.003 | 0.21 | 0.025 | 0.0132 | 0.003 | 0.031 | 0.0439 | 0.0022 | 0.001 | - |
| B18 | 0.0038 | 0.006 | 0.48 | 0.042 | 0.0273 | 0.002 | 0.026 | 0.0630 | 0.0028 | 0.003 | - |
| B19 | 0.0038 | 0.006 | 0.48 | 0.042 | 0.0273 | 0.002 | 0.026 | 0.0630 | 0.0028 | 0.001 | - |
| B20 | 0.0024 | 0.004 | 0.15 | 0.069 | 0.0251 | 0.004 | 0.028 | 0.0381 | 0.0021 | 0.002 | - |
| B21 | 0.0028 | 0.003 | 0.13 | 0.071 | 0.0178 | 0.002 | 0.031 | 0.0377 | 0.0023 | 0.001 | - |
[Table 3]
| No. | Steelmaking and casting processes | Cold rolling reduction ratio (%) | Temper rolling reduction ratio (%) | |||
| ΔT (°C) | Casting rate (m/min) | Cooling rate between 1200°C to 1500°C (°C/s) | Amount of dissolved oxygen (free oxygen) (ppm) | |||
| A1 | 25 | 1.5 | 1.4 | 576 | 78 | 0.8 |
| A2 | 30 | 1.1 | 3.2 | 465 | 78 | 0.7 |
| A3 | 31 | 1.3 | 3.4 | 515 | 81 | 1.2 |
| A4 | 30 | 1.3 | 4.1 | 616 | 81 | 1.4 |
| A5 | 27 | 1.5 | 3.3 | 436 | 78 | 1.2 |
| A6 | 28 | 1.3 | 1.2 | 430 | 77 | 1.0 |
| A7 | 30 | 1.4 | 1.7 | 509 | 78 | 0.8 |
| A8 | 31 | 1.4 | 3.4 | 464 | 78 | 0.8 |
| A9 | 28 | 1.5 | 2.2 | 343 | 78 | 0.7 |
| A10 | 22 | 1.4 | 3.9 | 409 | 78 | 0.8 |
| A11 | 24 | 1.1 | 1.3 | 323 | 68 | 1.0 |
| A12 | 25 | 1.5 | 2.8 | 337 | 83 | 1.1 |
| A13 | 29 | 1.4 | 2.1 | 518 | 78 | 1.2 |
| A14 | 29 | 1.5 | 3.3 | 438 | 75 | 0.6 |
| A15 | 31 | 1.0 | 3.4 | 420 | 79 | 0.7 |
| A16 | 31 | 1.0 | 3.4 | 432 | 58 | 0.7 |
| A17 | 29 | 1.5 | 2.6 | 459 | 72 | 0.9 |
| A18 | 25 | 1.3 | 2.3 | 436 | 68 | 1.3 |
| A19 | 31 | 1.3 | 2.9 | 305 | 82 | 1.4 |
| A20 | 25 | 1.5 | 3.6 | 539 | 78 | 1.0 |
| A21 | 24 | 1.3 | 3.6 | 539 | 78 | 1.0 |
| A22 | 33 | 1.3 | 4.1 | 356 | 77 | 0.9 |
| A23 | 32 | 1.5 | 3.9 | 419 | 76 | 1.2 |
| A24 | 31 | 1.4 | 3.4 | 267 | 74 | 0.7 |
| A25 | 28 | 1.2 | 1.3 | 255 | 85 | 1.3 |
| A26 | 24 | 1.0 | 1.9 | 543 | 79 | 0.7 |
[Table 4]
| No. | Steelmaking and casting processes | Cold rolling reduction ratio (%) | Temper rolling reduction ratio (%) | |||
| ΔT (°C) | Casting rate (m/min) | Cooling rate between 1200°C to 1500°C (°C/s) | Amount of dissolved oxygen (free oxygen) (ppm) | |||
| A27 | 22 | 1.4 | 2.6 | 427 | 74 | 0.8 |
| A28 | 32 | 1.5 | 2.9 | 375 | 78 | 1.2 |
| A29 | 33 | 1.5 | 3.0 | 524 | 81 | 1.3 |
| A30 | 32 | 1.3 | 2.6 | 449 | 78 | 1.0 |
| A31 | 21 | 1.1 | 2.6 | 395 | 68 | 0.8 |
| A32 | 21 | 1.4 | 3.4 | 342 | 72 | 0.8 |
| A33 | 24 | 1.3 | 4.4 | 566 | 79 | 1.4 |
| B1 | 24 | 1.2 | 3.2 | 293 | 78 | 0.8 |
| B2 | 25 | 1.4 | 3.6 | 471 | 72 | 0.7 |
| B3 | 31 | 1.2 | 2.7 | 615 | 69 | 1.5 |
| B4 | 32 | 1.3 | 1.2 | 530 | 81 | 1.4 |
| B5 | 28 | 1.4 | 3.5 | 743 | 78 | 1.4 |
| B6 | 27 | 1.1 | 2.4 | 192 | 81 | 1.2 |
| B7 | 25 | 1.3 | 3.6 | 539 | 78 | 1.0 |
| B8 | 29 | 1.3 | 1.7 | 473 | 70 | 1.1 |
| B9 | 25 | 1.2 | 1.3 | 644 | 82 | 0.7 |
| B10 | 33 | 1.4 | 1.3 | 649 | 82 | 1.2 |
| B11 | 31 | 1.1 | 1.7 | 165 | 67 | 0.8 |
| B12 | 32 | 1.4 | 3.6 | 552 | 78 | 1.4 |
| B13 | 32 | 1.4 | 2.9 | 408 | 73 | 1.4 |
| B14 | 25 | 1.3 | 3.0 | 283 | 75 | 1.4 |
| B15 | 28 | 1.2 | 4.1 | 140 | 78 | 0.8 |
| B16 | 41 | 1.0 | 3.4 | 431 | 66 | 1.3 |
| B17 | 13 | 1.2 | 3.4 | 414 | 68 | 1.1 |
| B18 | 32 | 1.7 | 4.1 | 610 | 80 | 0.9 |
| B19 | 29 | 0.8 | 4.1 | 620 | 66 | 1.0 |
| B20 | 27 | 1.4 | 0.7 | 352 | 78 | 0.7 |
| B21 | 29 | 1.5 | 6.2 | 363 | 78 | 0.7 |
<Mechanical Properties>
[0080] Regarding the mechanical properties, tensile strength (TS) and fracture elongation (EL) were measured by a tensile test using JIS No. 5 test pieces according to JIS Z 2241. The test piece having a fracture elongation of 30% or more was evaluated as having excellent formability.
<Observation of Microstructure and Precipitates>
[0081] Regarding precipitates in steel, cross-sections parallel to the cold rolling direction were observed by SEM, and the diameter and number density of oxides were measured by the above-described method. The average grain diameter of ferrite was measured using a cutting method described in JIS G 0552.
<Strength Properties after Enameling Treatment>
[0082] In addition, in order to evaluate the decrease in strength due to grain growth after the enameling treatment, the steel sheet was subjected to a heat treatment that simulates enameling at a furnace temperature of 830°C for five minutes, the tensile strength was obtained by a tensile test in the above-described manner, and the ratio of the strength after the heat treatment to the strength before the heat treatment was obtained.
[0083] In addition, in consideration of the stability of the strength after the heat treatment, the Vickers hardness of the steel was measured before and after the heat treatment, and the ratio before and after the heat treatment was also obtained for the minimum value of the measurement result.
[0084] Specifically, in each of the steels before and after the heat treatment, the Vickers hardnesses of five points at a thickness 1/4 position under a load of 0.98 N were measured, and the average value thereof was taken as the hardness at the measurement position. Furthermore, the above measurement was performed at 10 or more positions with intervals of 20 mm or more therebetween, and the minimum value of the measurement result (hardness) was obtained before and after the heat treatment. The ratio between the minimum values of the measurement results before and after the heat treatment was obtained.
[0085] In a case where the tensile strength after the enameling treatment was equal to or more than 0.85 (85%) of the tensile strength before the enameling treatment and the minimum value of hardness after the enameling treatment was equal to or more than 0.85 of the minimum value of the hardness before the enameling treatment, it was determined that the decrease in strength due to the enameling treatment can be stably suppressed.
<Aging Resistance>
[0086] The aging resistance was evaluated by the aging index. The aging index is the difference in yield stress between before and after aging at 100°C for 60 minutes by applying 10% prestrain by tension using a JIS No. 5 tensile test piece. In a case where the difference in yield stress was 30 MPa or less, it was determined that the aging resistance was excellent (OK).
<Fishscale Resistance>
[0088] The fishscale resistance for a steel sheet which was coated with a glaze to 100 µm by a dry powder electrostatic coating method and fired at a furnace temperature of 830°C for five minutes in the air was evaluated. The steel sheet after the enameling treatment was subjected to a fishscale acceleration test in which the steel sheet was put in a thermostat at 160°C for 10 hours, and the fishscale generation state was visually evaluated as four stages, A: excellent, B: slightly better, C: normal, and D: problematic. The case of D was rejected.
<Enamel Adhesion>
[0089] The enamel adhesion was evaluated by dropping a 2-kg weight with a spherical head from a height of 1 m on the steel sheet subjected to the enameling treatment as described above, measuring the enamel peeled state of the deformed portion with 169 contact probes, and obtaining the area ratio of the non-peeled portion. When the area ratio of the non-peeled portion was 40% or more, there was no problem, and when the area ratio thereof is less than 40%, the adhesion was evaluated as poor.
<External Appearance>
[0090] The steel sheet subjected to the enameling treatment as described above was visually observed, the condition of bubbles and black spots was observed, and the outer appearance after the enameling treatment was evaluated as five stages, "very good", "excellent", "normal", "slightly inferior", and "significantly inferior". It was determined that in the stages of "very good", "excellent", "normal", and "slightly inferior", there was no problem, and in the case of "significantly inferior", bubbles and black spots were generated.
[0091] The evaluation results are shown in Tables 5 and 6. In the examples of the present invention, no precipitates having a diameter of more than 10 µm were observed in the oxides containing Fe and Mn in the steel. In addition, it was confirmed that one in which the number of oxides having a diameter of more than 1.0 µm and 10 µm or less among the oxides containing Fe and Mn per unit area is within the range of the present invention satisfied fishscale resistance. Furthermore, it was confirmed that one in which the number of oxides having a diameter of 1.0 µm or less among the oxides containing Fe and Mn per unit area is within the range of the present invention cases less decrease in strength due to grain growth after the enameling treatment. In the description regarding the oxide densities in Tables 5 and 6, E represents an index, and for example, 1.0E+03 represents 1.0×103.
[0093] From the results of Tables 5 and 6, it was confirmed that, in the ranges of the present invention, it was possible to provide a steel sheet for vitreous enameling capable of having excellent fishscale resistance and stably suppressing the decrease in tensile strength due to the enameling treatment without impairing aging resistance, enamel adhesion, and external appearance compared to the steel sheet for vitreous enameling in the related art.
[Table 5]
| No. | Mechanical | Aging resistance | Average grain diameter (µm) | Density of oxides of more than 1.0 to 10 µm or less (grains/mm2) | Density of oxides of 0.1 to 1.0 µm or less (grains/mm2) | Strength properties | Enameling properties | Others | ||||
| TS (MPa) | EL (%) | After enameling treatment / before enameling treatment TS | After enameling treatment / before enameling treatment HV | Fishscale resistance | External appearance and adhesion | |||||||
| A1 | 370 | 38 | OK | 13.3 | 4.3E+04 | 6.8E+04 | 0.92 | 0.92 | A | No problem | ||
| A2 | 412 | 33 | OK | 12.9 | 7.8E+03 | 5.2E+04 | 0.91 | 0.90 | B | No problem | ||
| A3 | 399 | 35 | OK | 12.2 | 3.8E+04 | 6.3E+04 | 0.88 | 0.87 | A | No problem | ||
| A4 | 384 | 36 | OK | 13.1 | 4.9E+04 | 7.1E+04 | 0.91 | 0.91 | A | No problem | ||
| A5 | 403 | 34 | OK | 12.2 | 8.1E+03 | 5.3E+04 | 0.95 | 0.93 | B | No problem | ||
| A6 | 397 | 35 | OK | 12.3 | 7.3E+03 | 4.8E+04 | 0.92 | 0.94 | B | No problem | ||
| A7 | 390 | 36 | OK | 13.3 | 4.7E+04 | 6.5E+04 | 0.93 | 0.94 | A | No problem | ||
| A8 | 377 | 37 | OK | 12.8 | 8.2E+03 | 5.3E+04 | 0.91 | 0.91 | B | No problem | ||
| A9 | 384 | 36 | OK | 13.4 | 1.4E+03 | 2.2E+04 | 0.94 | 0.95 | C | No problem | ||
| A10 | 379 | 37 | OK | 12.6 | 7.0E+03 | 5.1E+04 | 0.92 | 0.93 | B | No problem | ||
| A11 | 404 | 34 | OK | 13.7 | 3.3E+03 | 8.5E+03 | 0.93 | 0.91 | C | No problem | ||
| Example of the | A12 | 379 | 37 | OK | 12.9 | 1.8E+03 | 1.2E+04 | 0.91 | 0.91 | C | No problem | |
| Invention | A13 | 369 | 38 | OK | 16.8 | 4.1E+04 | 6.1E+04 | 0.92 | 0.92 | A | No problem | |
| A14 | 312 | 46 | OK | 18.3 | 1.2E+04 | 3.0E+04 | 0.88 | 0.87 | A | No problem | ||
| A15 | 332 | 43 | OK | 13.9 | 1.2E+04 | 3.0E+04 | 0.88 | 0.87 | A | No problem | ||
| A16 | 338 | 42 | OK | 12.7 | 1.3E+04 | 3.5E+04 | 0.89 | 0.90 | C | No problem | ||
| A17 | 346 | 41 | OK | 13.3 | 1.4E+04 | 6.1E+04 | 0.89 | 0.87 | A | No problem | ||
| A18 | 355 | 40 | OK | 14.0 | 2.2E+04 | 4.0E+04 | 0.93 | 0.91 | A | No problem | ||
| A19 | 323 | 44 | OK | 14.0 | 6.7E+03 | 1.2E+04 | 0.88 | 0.88 | B | No problem | ||
| A20 | 304 | 47 | OK | 14.3 | 4.3E+04 | 6.4.E+04 | 0.87 | 0.88 | A | No problem | ||
| A21 | 286 | 49 | OK | 17.3 | 3.8E+04 | 5.6.E+04 | 0.86 | 0.87 | A | No problem | ||
| A22 | 370 | 38 | OK | 13.4 | 7.7E+03 | 2.1E+04 | 0.94 | 0.93 | B | No problem | ||
| A23 | 377 | 37 | OK | 12.9 | 5.0E+04 | 5.2E+04 | 0.95 | 0.94 | A | No problem | ||
| A24 | 330 | 43 | OK | 12.7 | 8.5E+03 | 5.0E+03 | 0.87 | 0.88 | B | No problem | ||
| A25 | 370 | 38 | OK | 13.5 | 3.4E+03 | 5.0E+03 | 0.91 | 0.92 | C | No problem | ||
| A26 | 432 | 31 | OK | 10.3 | 3.2E+04 | 5.7E+04 | 0.94 | 0.95 | A | No problem | ||
[Table 6]
| No. | Mechanical properties | Aging resistance | Average grain diameter (µm) | Density of oxides of more than 1.0 to 10 µm or less (grains/mm2) | Density of oxides of 0.1 to 1.0 µm or less (grains/mm2) | Strength properties | Enameling properties | Others | ||||
| TS (MPa) | EL (%) | After enameling treatment / before enameling treatment TS | After enameling treatment / before enameling treatment HV | Fishscale resistance | External appearance and adhesion | |||||||
| Example of the Invention | A27 | 348 | 41 | OK | 13.9 | 1.4E+04 | 4.5E+04 | 0.91 | 0.93 | A | No problem | |
| A28 | 390 | 36 | OK | 12.3 | 3.6E+03 | 2.3E+04 | 0.93 | 0.92 | C | No problem | ||
| A29 | 372 | 38 | OK | 12.7 | 4.2E+04 | 6.6E+04 | 0.94 | 0.93 | A | No problem | ||
| A30 | 390 | 36 | OK | 13.0 | 7.8E+03 | 4.6E+04 | 0.92 | 0.89 | B | No problem | ||
| A31 | 400 | 35 | OK | 12.7 | 7.2E+03 | 3.8E+04 | 0.93 | 0.91 | B | No problem | ||
| A32 | 280 | 49 | OK | 20.0 | 3.1E+03 | 6.1E+03 | 0.88 | 0.89 | A | No problem | ||
| A33 | 341 | 42 | OK | 14.6 | 3.6E+04 | 9.1E+04 | 0.88 | 0.89 | C | No problem | ||
| Comparative Example | B1 | 421 | 28 | OK | 11.1 | 2.8E+03 | 6.2.E+03 | 0.94 | 0.95 | C | Bubbles occurred | |
| B2 | 341 | 42 | OK | 14.0 | 2.1E+04 | 4.9E+04 | 0.93 | 0.91 | D | No problem | ||
| B3 | 347 | 41 | OK | 13.2 | 4.3E+04 | 8.5E+04 | 0.91 | 0.91 | D | No problem | ||
| B4 | 488 | 26 | OK | 12.8 | 3.4E+04 | 6.0E+04 | 0.94 | 0.93 | A | No problem | ||
| B5 | 380 | 23 | OK | 12.1 | 6.2E+04 | 8.5.E+03 | 0.93 | 0.94 | A | No problem | ||
| B6 | 389 | 36 | OK | 13.8 | 3.4E+02 | 6.2E+02 | 0.82 | 0.83 | D | No problem | ||
| B7 | 271 | 52 | OK | 18.2 | 4.8E+04 | 5.9E+04 | 0.81 | 0.80 | A | No problem | ||
| B8 | 426 | 32 | OK | 12.1 | 2.1E+04 | 5.4E+04 | 0.94 | 0.92 | A | Bubbles and black sports occurred | ||
| B9 | 385 | 28 | OK | 12.6 | 6.4E+04 | 6.5E+04 | 0.95 | 0.94 | A | No problem | ||
| BIO | 361 | 27 | OK | 13.4 | 6.3E+04 | 6.5E+04 | 0.94 | 0.92 | A | No problem | High steelmaking load | |
| B11 | 355 | 40 | OK | 13.2 | 3.2E+02 | 4.8E+02 | 0.81 | 0.82 | D | No problem | ||
| B12 | 323 | 44 | OK | 13.4 | 4.3E+04 | 6.8E+04 | 0.88 | 0.89 | A | Inferior adhesion | ||
| B13 | 478 | 27 | OK | 11.5 | 1.1E+04 | 3.5E+04 | 0.91 | 0.92 | A | No problem | ||
| B14 | 349 | 41 | NG | 13.3 | 3.6E+03 | 5.8E+03 | 0.91 | 0.91 | C | No problem | ||
| B15 | 372 | 38 | OK | 13.3 | 8.7E+02 | 3.5E+03 | 0.83 | 0.83 | D | No problem | ||
| B16 | 334 | 43 | OK | 13.4 | 6.3E+02 | 3.0E+04 | 0.91 | 0.92 | D | No problem | ||
| B17 | 372 | 28 | OK | 13.0 | 5.9E+04 | 4.0E+04 | 0.89 | 0.89 | A | No problem | ||
| B18 | 422 | 27 | OK | 13.1 | 6.1E+04 | 7.2E+04 | 0.91 | 0.91 | A | No problem | ||
| B19 | 384 | 36 | OK | 13.1 | 8.1E+02 | 6.8E+04 | 0.94 | 0.93 | D | No problem | ||
| B20 | 386 | 36 | OK | 13.4 | 1.4E+03 | 3.8E+03 | 0.81 | 0.82 | C | No problem | ||
| B21 | 386 | 36 | OK | 13.4 | 8.0E+02 | 1.8E+05 | 0.98 | 0.97 | D | No problem | ||
[Industrial Applicability]
[0094] In a case where the steel sheet according to the aspect of the present invention is applied to kitchen utensils, building materials, the field of energy, and the like after being subjected to an enameling treatment, the steel sheet is excellent in formability, fishscale resistance after the enameling treatment, and strength properties. Therefore, the steel sheet is suitable as a steel sheet for vitreous enameling and has high industrial applicability.
1. A steel sheet comprising, by mass%:
C: 0.0060% or less;
Si: 0.0010% to 0.050%;
Mn: 0.05% to 0.50%;
P: 0.005% to 0.100%;
S: 0.0500% or less;
Al: 0.0010% to 0.010%;
Cu: 0.010% to 0.045%;
O: 0.0250% to 0.0700%;
N: 0.0010% to 0.0045%; and
a remainder of Fe and impurities,
wherein a structure of the steel sheet contains a ferrite, and an average grain diameter of the ferrite at a thickness 1/4 position in a through-thickness direction from a surface is 20.0 µm or less, and
the steel sheet contains oxides containing Fe and Mn, among the oxides, a number density of oxides having a diameter of more than 1.0 µm and 10 µm or less is 1.0×103 grains/mm2 or more and 5.0×104 grains/mm2 or less, and a number density of oxides having a diameter of 0.1 to 1.0 µm is 5.0×103 grains/mm2 or more.
2. The steel sheet according to claim 1,
wherein the impurities include, by mass%,
a total of one or more of B, Cr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg: limited to 0.100%.
wherein the impurities include, by mass%,
a total of one or more of B, Cr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg: limited to 0.100%.
3. The steel sheet according to claim 1,
wherein the impurities include, by mass%,
Nb: limited to 0.010% or less.
wherein the impurities include, by mass%,
Nb: limited to 0.010% or less.
4. The steel sheet according to claim 1 or 2,
wherein the steel sheet is a cold rolled steel sheet.
wherein the steel sheet is a cold rolled steel sheet.
5. The steel sheet according to claim 1 or 2,
wherein the steel sheet is a steel sheet for vitreous enameling.
wherein the steel sheet is a steel sheet for vitreous enameling.
6. An enameled product comprising:
the steel sheet according to any one of claims 1 to 3.
the steel sheet according to any one of claims 1 to 3.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description