STEEL SHEET AND ENAMELED PRODUCT

Abstract

This steel sheet has a predetermined chemical composition, and, on a surface parallel to a surface at a ¼ position of a sheet thickness in a sheet thickness direction from the surface, with respect to a total area of three types of oxides of MnO, Cr.sub.2O.sub.3 and Al.sub.2O.sub.3 having a major axis of more than 1.0 μm, a total area ratio of the MnO and the Cr.sub.2O.sub.3 is 98.0% or more, and an area ratio of the Al.sub.2O.sub.3 is 2.0% or less.

Claims

1. A steel sheet comprising, as a chemical composition, by mass %: C: 0.0050% or less; Si: 0.050% or less; Mn: 0.007% to 1.00%; P: 0.003% to 0.050%; S: 0.005% to 0.050%; Al: 0.010% or less; O: 0.0300% to 0.1000%; Cu: 0.010% to 0.060%; N: 0.0050% or less; Cr: 0.01% to 1.00%; and a remainder of Fe and an impurity, wherein, on a plane parallel to a surface at a ¼ position of a sheet thickness in a sheet thickness direction from the surface, with respect to a total area of three types of oxides of MnO, Cr.sub.2O.sub.3 and Al.sub.2O.sub.3 having a major axis of more than 1.0 μm, a total area ratio of the MnO and the Cr.sub.2O.sub.3 is 98.0% or more, and an area ratio of the Al.sub.2O.sub.3 is 2.0% or less.

2. The steel sheet according to claim 1, wherein a total number density of the MnO and the Cr.sub.2O.sub.3 having a major axis of more than 1.0 μm and 10 μm or less is 5.0×10.sup.2/mm.sup.2 or more and 5.0×10.sup.4/mm.sup.2 or less.

3. The steel sheet according to claim 1, wherein a number density of MnO having a major axis of 0.1 to 1.0 μm is 1.0×10/mm.sup.2 or more and 5.0×10.sup.2/mm.sup.2 or less.

4. The steel sheet according to claim 1, further comprising, as the chemical composition, by mass %: a total of 0.100% or less of one or more selected from the group of B, Ni, Nb, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg.

5. The steel sheet according to claim 1, wherein, when, by mass %, a Cu content is indicated by [Cu], a P content is indicated by [P], and a S content is indicated by [S], [Cu]/[P] is 1.0 to 4.0, and [P]/[S] is 0.2 to 2.0.

6. The steel sheet according to claim 1, wherein the steel sheet is a cold-rolled steel sheet.

7. The steel sheet according to claim 1, wherein the steel sheet is a steel sheet for vitreous enameling.

8. An enameled product comprising: the steel sheet according to claim 1.

9. The steel sheet according to claim 2, wherein a number density of MnO having a major axis of 0.1 to 1.0 μm is 1.0×10/mm.sup.2 or more and 5.0×10.sup.2/mm.sup.2 or less.

10. The steel sheet according to claim 2, wherein, when, by mass %, a Cu content is indicated by [Cu], a P content is indicated by [P], and a S content is indicated by [S], [Cu]/[P] is 1.0 to 4.0, and [P]/[S] is 0.2 to 2.0.

Description

EXAMPLES

Example 1

[0103] Steels having a chemical composition shown in Table 1 (the remainder was Fe and an impurity) were melted in a converter, in secondary refining, Al, Cr, and Mn were added in the order of B1 or B2 in Table 2, and then continuous casting was carried out to produce slabs.

[0104] These slabs were heated at 1150° C. to 1250° C., then, hot-rolled at a finish temperature of 900° C. or higher, and coiled at 600° C. to 700° C. to produce hot-rolled steel sheets. In addition, the hot-rolled steel sheets were pickled and then cold-rolled at a rolling reduction of 70% to 85% to produce cold-rolled steel sheets. The cold-rolled steel sheets were continuously annealed at 650° C. to 750° C. and then temper-rolled to produce steel sheets having a sheet thickness of 0.7 mm (cold-rolled steel sheets).

TABLE-US-00001 TABLE 1 Steel Chemical component (mass %), remainder: Fe and impurity No. C Si Mn P S Al O Cu Cr N Others [Cu]/[P] [P]/[S] A1 0.0050 0.008 0.23 0.025 0.034 0.002 0.0712 0.035 0.22 0.0028 1.4 0.7 A2 0.0028 0.050 0.28 0.013 0.022 0.003 0.0588 0.028 0.61 0.0025 Nb: 0.03, Ti: 0.03 2.2 0.6 A3 0.0012 0.003  0.007 0.005 0.008 0.004 0.0655 0.020 0.06 0.0013 Mg: 0.005, As: 0.002 4.0 0.6 A4 0.0011 0.015 1.00 0.019 0.013 0.003 0.0801 0.028 0.12 0.0018 B: 0.005, Mg: 0.01 1.5 1.5 A5 0.0019 0.006 0.42 0.003 0.007 0.006 0.0376 0.013 0.27 0.0035 4.3 0.4 A6 0.0032 0.007 0.22 0.050 0.026 0.002 0.0524 0.058 0.08 0.0022 1.2 1.9 A7 0.0023 0.004 0.29 0.007 0.005 0.003 0.0621 0.027 0.04 0.0028 3.9 1.4 A8 0.0018 0.024 0.26 0.028 0.050 0.005 0.0456 0.029 0.19 0.0033 Sn: 0.03, Sb: 0.003 1.0 0.6 A9 0.0028 0.009 0.34 0.004 0.038 0.010 0.0473 0.015 0.77 0.0019 Ca: 0.004, Se: 0.002 3.8 0.1 A10 0.0014 0.013 0.65 0.028 0.048 0.006 0.0300 0.034 0.35 0.0017 1.2 0.6 A11 0.0027 0.007 0.18 0.045 0.036 0.003 0.1000 0.035 0.04 0.0022 Mo: 0.008, W: 0.003, 0.8 1.3 Ta: 0.002 A12 0.0019 0.004 0.41 0.004 0.013 0.004 0.0489 0.010 0.43 0.0036 2.5 0.3 A13 0.0013 0.003 0.32 0.022 0.019 0.002 0.0577 0.060 0.18 0.0019 Ni: 0.05 2.7 1.2 A14 0.0028 0.036 0.76 0.011 0.012 0.007 0.0545 0.028 0.01 0.0028 2.5 0.9 A15 0.0013 0.005 0.21 0.027 0.020 0.005 0.0421 0.043 0.50 0.0016 1.6 1.4 A16 0.0027 0.008 0.55 0.041 0.019 0.003 0.0746 0.052 0.26 0.0050 La: 0.001, Ce: 0.07 1.3 2.2 A17 0.0018 0.002 0.28 0.012 0.018 0.005 0.0547 0.028 0.06 0.0024 2.3 0.7 A18 0.0015 0.003 0.37 0.018 0.012 0.004 0.0636 0.033 0.05 0.0022 1.8 1.5 a1 0.0100 0.004 0.12 0.015 0.016 0.004 0.0435 0.031 0.31 0.0021 2.1 0.9 a2 0.0014 0.062 0.42 0.012 0.019 0.002 0.0548 0.034 0.17 0.0011 2.8 0.6 a3 0.0029 0.003  0.004 0.011 0.008 0.003 0.0787 0.022 0.21 0.0042 2.0 1.4 a4 0.0019 0.036 1.28 0.036 0.019 0.002 0.0473 0.052 0.05 0.0038 1.4 1.9 a5 0.0021 0.004 0.65 0.002 0.022 0.003 0.0543 0.016 0.09 0.0016 8.0 0.09 a6 0.0018 0.017 0.29 0.065 0.018 0.005 0.0564 0.028 0.47 0.0021 0.4 3.6 a7 0.0012 0.002 0.35 0.022 0.004 0.004 0.0489 0.047 0.69 0.0046 2.1 5.5 a8 0.0017 0.024 0.47 0.006 0.067 0.002 0.0531 0.018 0.13 0.0019 3.0 0.09 a9 0.0014 0.005 0.31 0.014 0.250 0.021 0.0639 0.022 0.04 0.0025 1.6 0.06 a10 0.0032 0.003 0.29 0.029 0.032 0.003 0.0233 0.013 0.26 0.0034 0.4 0.9 a11 0.0013 0.008 0.53 0.034 0.019 0.005 0.1276 0.055 0.81 0.0016 1.6 1.8 a12 0.0027 0.006 0.33 0.016 0.013 0.007 0.0623 0.004 0.06 0.0011 0.3 1.2 a13 0.0018 0.017 0.37 0.011 0.016 0.004 0.0545 0.102 0.19 0.0038 9.3 0.7 a14 0.0019 0.003 0.66 0.021 0.022 0.002 0.0497 0.035  0.003 0.0023 1.7 1.0 a15 0.0015 0.009 0.39 0.019 0.019 0.006 0.0413 0.023 1.34 0.0026 1.2 1.0 a16 0.0027 0.011 0.22 0.004 0.009 0.003 0.0577 0.014 0.35 0.0061 Sn: 0.04, Sb: 0.03, 3.5 0.4 Mo: 0.035

TABLE-US-00002 TABLE 2 Order of elements added to molten Refining steel after decarburization No. 1 2 B1 Cr, Mn Al B2 Al Cr, Mn

[0105] In the obtained steel sheets, oxides that were present in a 10 mm×10 mm range on a plane parallel to the surface of the steel sheet at a position of t/4 (t: sheet thickness) in the sheet thickness direction from the surface were analyzed using Metals Quality Analyzer (MQA: registered trademark), and the proportions of the oxides were measured.

[0106] The results are shown in Table 3.

[0107] In addition, for the obtained steel sheets, the enamel characteristics (fish scaling resistance, enamel adhesion, and external appearance after an enameling treatment) were evaluated as described below. The results are shown in Table 3.

[0108] [Fish Scaling Resistance]

[0109] As a pretreatment, a sample having sizes of 150 mm×100 mm was collected from the steel sheet, and the sample was alkali-degreased, then, immersed in 15 g/l of a nickel sulfate solution at 70° C. for 7 minutes, and then neutralized. After that, a 102 # glaze manufactured by Ferro Enamels (Japan) Limited was glazed 100 μm on both surfaces and baked at 860° C. for 5 minutes in an atmosphere having a dew point of 35° C.

[0110] The baked sample was heated by being held at 150° C. for 20 hours, and the status of the occurrence of fish scaling was visually observed and evaluated. The occurrence status was evaluated by the average of four samples. The evaluation criteria were as described below, “A” indicates “excellent”, “B” indicates “normal”, C indicates “problematic”, and C was regarded as fail.

[0111] A: The number of fish scales generated per surface is 10 or less.

[0112] B: The number of fish scales generated per surface is 11 to 20.

[0113] C: The number of fish scales generated per surface is 21 or more.

[0114] [Enamel Adhesion]

[0115] As a pretreatment, four samples having sizes of 150 mm×100 mm were collected from the steel sheet, and the samples were alkali-degreased, then, immersed in a 10% sulfuric acid solution at 70° C. for 10 minutes, then, immersed in 15 g/l of a nickel sulfate solution at 70° C. for 7 minutes, and then neutralized. Furthermore, a 102 # glaze manufactured by Ferro Enamels (Japan) Limited was glazed 100 μm on both surfaces and baked at 860° C. for 5 minutes in an atmosphere having a dew point of 35° C.

[0116] A weight with a 2 kg sphere head was dropped from a height of 1 m onto the baked sample, and the enamel peeling status of the distorted portion was measured with 169 palpation needles and evaluated with the area ratio of the unpeeled portion. The area ratio was evaluated by the average of four samples.

[0117] The evaluation criteria were as described below, “A” indicates “excellent”, “B” indicates “normal”, C indicates “problematic”, and C was regarded as fail.

[0118] A: The area ratio of the unpeeled portion is 90% or more.

[0119] B: The area ratio of the unpeeled portion is 40% or more and less than 90%.

[0120] C: The area ratio of the unpeeled portion is less than 40%.

[0121] [External Appearance]

[0122] As a pretreatment, 10 samples having sizes of 150 mm×100 mm were collected from the steel sheet, and the samples were alkali-degreased, then, immersed in 15 g/l of a nickel sulfate solution at 70° C. for 7 minutes, and then neutralized. Furthermore, a 102 # glaze manufactured by Ferro Enamels (Japan) Limited was glazed 100 μm on both surfaces and baked at 860° C. for 5 minutes in an atmosphere having a dew point of 35° C.

[0123] The external appearance of the baked sample was visually observed, and the bubble and black point status was evaluated. In a case where a bubble and a black point were generated in 3 or more of 10 sheets, a bubble and a black point were regarded as being generated, and, in a case where a bubble and a black point were generated in 2 or less of 10 sheets, the external appearance was regarded as having no problems.

TABLE-US-00003 TABLE 3 Area ratio of oxides having major axis of more than 1.0 μm Enamel characteristics Steel Refining MnO + Cr.sub.2O.sub.3 Al.sub.2O.sub.3 Fish scaling External Sign No. No. (%) (%) resistance Adhesion appearance Invention C1 A1 B1 99.5 0.5 A A No problem Example C2 A2 B1 99.3 0.7 A A No problem C3 A3 B1 99.1 0.9 A A No problem C4 A4 B1 99.3 0.7 A A No problem C5 A5 B1 98.8 1.2 B B No problem C6 A6 B1 99.5 0.5 A A No problem C7 A7 B1 99.3 0.7 A A No problem C8 A8 B1 99.0 1.0 A A No problem C9 A9 B1 98.1 1.9 B B No problem C10 A10 B1 98.8 1.2 A A No problem C11 A11 B1 99.3 0.7 A B No problem C12 A12 B1 99.1 0.9 A A No problem C13 A13 B1 99.5 0.5 A A No problem C14 A14 B1 98.6 1.4 A A No problem C15 A15 B1 99.0 1.0 A A No problem C16 A16 B1 99.3 0.7 A B No problem C17 A17 B1 99.0 1.0 A A No problem C18 A18 B1 99.1 0.9 A A No problem Comparative c1 a1 B2 96.9 3.1 C A Bubble generated Example c2 a2 B2 97.1 2.9 C A Bubble and black point generated c3 a3 B2 97.2 2.8 C A No problem c4 a4 B2 97.4 2.6 C A No problem c5 a5 B2 97.1 2.9 C C No problem c6 a6 B2 96.7 3.3 C C Bubble and black point generated c7 a7 B2 96.8 3.2 C B No problem c8 a8 B2 97.3 2.7 C B No problem c9 a9 B2 95.6 4.4 C B No problem c10 a10 B2 97.2 2.8 C B No problem c11 a11 B2 96.5 3.5 C A No problem c12 a12 B2 96.4 3.6 C B No problem c13 a13 B2 96.8 3.2 C B No problem c14 a14 B2 97.3 2.7 C A No problem c15 a15 B2 96.4 3.6 C A Black point generated c16 a16 B2 97.1 2.9 C C No problem c17 A17 B2 96.8 3.2 C A No problem c18 A18 B2 97.3 2.7 C A No problem c19 a9 B1 96.2 3.8 C B No problem c20 a12 B1 97.3 2.7 C B No problem

[0124] As is clear from Table 1 to Table 3, in C1 to C18, which were invention examples, since the chemical composition and the oxides were within the scope of the present invention, the enamel characteristics were excellent.

[0125] On the other hand, in c1 to c20, which were comparative examples, the enamel characteristics were poor.

[0126] When C4 in which A4 containing 0.12% of Cr was used, C6 in which A6 containing 0.08% of Cr was used, C17 in which A17 containing 0.06% of Cr was used, and c17 were compared, in the sample C4, a black point was generated in 2 of 10 sheets, and, in all of the samples C6, C17, and c17, no black points were observed.

Example 2

[0127] Cr, Mn, and Al were added in the addition order shown in the refining No. B1 in Table 2, and slabs having a chemical composition of Steel No. A5, A9, All, or A17 in Table 1 were obtained.

[0128] From these slabs, steel sheets were manufactured under the same conditions as in Example 1.

[0129] In the obtained steel sheets, oxides that were present in a 10 mm×10 mm range on a plane parallel to the surface of the steel sheet at a position of t/4 in the sheet thickness direction from the surface were analyzed using Metals Quality Analyzer (MQA: registered trademark), and the proportion of oxides having a major axis of more than 1.0 μm, the number density of MnO and Cr.sub.2O.sub.3 having a major axis of more than 1.0 μm and 10 μm or less, and the number density of MnO having a major axis of 0.1 to 1.0 μm were measured.

[0130] The results are shown in Table 4.

[0131] In addition, for the obtained steel sheets, the enamel characteristics (fish scaling resistance, enamel adhesion, and external appearance after an enameling treatment) were evaluated in the same manner as in Example 1. In addition, decreases in the tensile strengths due to enameling treatments were measured as described below. The results are shown in Table 4.

[0132] [Tensile Strength Before and After Enameling Treatment]

[0133] The tensile strengths of the obtained steel sheets were measured. The tensile strengths (TS) were measured by carrying out a tensile test according to JIS Z 2241: 2011 using a JIS No. 5 test piece.

[0134] In addition, a heat treatment simulating an enameling treatment at a furnace temperature of 830° C. for 5 minutes was carried out on the obtained steel sheets, and a tensile test was carried out in the same manner as described above to determine the tensile strengths.

[0135] From the results, the proportions of the strength after the heat treatment in the strength before the heat treatment was calculated.

[0136] In a case where the tensile strength after the heat treatment was 0.85 (85%) or more of the tensile strength before the heat treatment, it is determined that a decrease in strength due to the enameling treatment could be stably suppressed.

TABLE-US-00004 TABLE 4 Number density Decrease in Area ratio of oxides of oxides having Number density tensile strength having major axis major axis of of oxides having due to enamel- of more than 1.0 μm more than 1.0 μm major axis of Enamel characteristics ing treatment Re- MnO + and 10 μm or less 0.1 to 1.0 μm Fish (TS after enamel Steel fining Cr.sub.2O.sub.3 Al.sub.2O.sub.3 MnO + Cr.sub.2O.sub.3 MnO scaling External baking/TS before Sign No. No. (%) (%) (molecules/mm.sup.2) (molecules/mm.sup.2) resistance Adhesion appearance enameling) Invention D1  A5 B1 98.8 1.2 8.7 × 10.sup.2 1.8 × 10.sup.2 B B No problem 0.91 Example D2 A11 B1 98.8 1.2 4.5 × 10.sup.4 2.3 × 10.sup.  A B No problem 0.87 D3 A17 B1 99.0 1.0 3.8 × 10.sup.3 8.9 × 10.sup.  A A No problem 0.89 D4  A9 B1 98.1 1.9 6.2 × 10.sup.2 3.6 × 10.sup.2 B B No problem 0.95

[0137] As is clear from Table 1, Table 2, and Table 4, in a case where the total number density of MnO and Cr.sub.2O.sub.3 having a major axis of more than 1.0 μm and 10 μm or less was 5.0×10.sup.2/mm.sup.2 or more and 5.0×10.sup.4/mm.sup.2 or less, the fish scaling resistance was also evaluated as B or higher, and, in a case where the total number density of MnO and Cr.sub.2O.sub.3 having a major axis of more than 1.0 μm and 10 μm or less was 1.0×10.sup.3/mm.sup.2 or more, the fish scaling resistance was evaluated as A.

[0138] In addition, in a case where the number density of the MnO oxide having a major axis of 0.1 to 1.0 μm was within an appropriate range, a decrease in tensile strength due to the enameling treatment was further suppressed. In particular, as the number density became larger, the decrease in tensile strength became smaller. In addition, the MnO oxide having a major axis of 0.1 to 1.0 μm was substantially spherical.

Example 3

[0139] Cr, Mn, and Al were added in the addition order shown in the refining No. B1 in Table 2, and slabs having a chemical composition of Steel No. A3, A8, A9, All, A17, or A18 in Table 1 were obtained.

[0140] From these slabs, steel sheets were manufactured under the same conditions as in Example 1.

[0141] For the obtained steel sheets, the proportions of oxides having a major axis of more than 1.0 μm and the number densities of MnO and Cr.sub.2O.sub.3 having a major axis of more than 1.0 μm and 10 μm or less were measured in the same manner as in Example 2.

[0142] The results are shown in Table 5.

[0143] In addition, for the obtained steel sheets, the enamel characteristics (fish scaling resistance, enamel adhesion, and external appearance after an enameling treatment) were evaluated in the same manner as in Example 1. The results are shown in Table 5.

TABLE-US-00005 TABLE 5 Number density of Area ratio of oxides oxides having major having major axis of axis of more than 1.0 μm Enamel characteristics more than 1.0 μm and 10 μm or less Fish Steel Refining MnO + Cr.sub.2O.sub.3 Al.sub.2O.sub.3 MnO + Cr.sub.2O.sub.3 Component ratio scaling External Sign No. No. (%) (%) (molecules/mm.sup.2) [Cu]/[P] [P]/[S] resistance Adhesion appearance Invention E1  A8 B1 99.0 1.0 1.1 × 10.sup.3 1.0 0.6 A A No problem Example E2  A3 B1 99.1 0.9 7.9 × 10.sup.3 4.0 0.6 A A No problem E3  A9 B1 98.1 1.9 6.2 × 10.sup.2 3.8 0.1 B B No problem E4 A11 B1 99.3 0.7 3.3 × 10.sup.3 0.8 1.3 A B No problem E5 A17 B1 99.0 1.0 3.8 × 10.sup.4 2.3 0.7 A A No problem E6 A18 B1 99.1 0.9 7.1 × 10.sup.3 1.8 1.5 A A No problem

[0144] As is clear from Table 1, Table 2, and Table 5, in a case where [Cu]/[P] was 1.0 to 4.0 and [P]/[S] was 0.2 to 2.0, the enamel adhesion was more superior.

INDUSTRIAL APPLICABILITY

[0145] According to the present invention, it is possible to provide a steel sheet being excellent in terms of fish scaling resistance after an enameling treatment, enamel adhesion, and external appearance after an enameling treatment. This steel sheet is preferable as a steel sheet for vitreous enameling that is a substrate for enameled products that are applied to kitchen appliances, building materials, energy fields, and the like. Therefore, the present invention is highly industrially applicable.