Patent Application
20160186284
The present invention relates to an Al-coated steel sheet capable of being obtained by modifying a plated layer of a hot-dip Al based alloy plated steel sheet through a heat treatment, and particularly relates to the Al-coated steel sheet that exhibits a high total reflectivity and exhibits good corrosion resistance.
A hot-dip Al based alloy plated steel sheet is being widely used mainly for purposes requiring heat resistance. Many of hot-dip Al based alloy plated steel sheets having been subjected to practical use are manufactured by using an Al based alloy plating bath containing Si. The presence of Si contained may decrease the plating bath temperature, and also may decrease the thickness of the brittle alloy layer (i.e., the initial thickness of the alloy layer) formed between the base steel sheet (i.e., the base sheet for plating) and the Al based alloy plated layer in the process of hot-dip plating.
The alloy layer may grow in some cases in the use of the hot-dip Al based alloy plated steel sheet for purposes requiring heat resistance. In the case where the durability at a high temperature is to be particularly enhanced, such a measure is employed that a heat treatment is performed after the hot-dip plating (i.e., a post heat treatment), so as to form a barrier layer of AlN between the base steel sheet and the alloy layer. In this case, a base steel sheet containing N in such an amount that satisfy the formation of the AlN barrier layer is applied.
A plating bath composition containing approximately from 7 to 12% by mass of Si is effective from the standpoint of the reduction of the bath temperature of the Al based alloy plating bath, and many of the hot-dip Al based alloy plated steel sheets contains 7% by mass or more of Si in the plated layer. However, some of PTLs disclose examples where a post heat treatment is applied with the use of a plating bath having a relatively low Si content of 6% or less (see PTLs 1 to 9).
PTL 1: JP-A-61-124558
PTL 2: JP-A-3-104848
PTL 3: JP-A-6-207262
PTL 4: JP-A-6-330274
PTL 5: Japanese Patent No. 3,383,119
PTL 6: JP-A-8-319549
PTL 7: Japanese Patent No. 3,398,810
PTL 8: Japanese Patent No. 3,485,410
PTL 9: JP-A-2000-290764
A hot-dip Al based alloy plated steel sheet is demanded to have good heat reflection characteristics due to the aforementioned purposes requiring heat resistance. In the application thereof to a building material of a tunnel wall or the like, a reflective plate of a lighting equipment, and the like, it is demanded to have good reflection characteristics with small light absorption. The reflection capabilities of heat and light roughly depend on the total reflectivity. Accordingly, in consideration of the application thereof to purposes requiring heat resistance and purposes utilizing light reflection capability, it is advantageous to have high total reflection characteristics. In the description herein, the fact that an article has high total reflection characteristics is expressed that the article is “having excellent total reflection characteristics”.
Good corrosion resistance is demanded in purposes of a building material and the like. However, a hot-dip Al based alloy plated steel sheet that is manufactured with a plating bath containing Si has a tendency of decreasing the corrosion resistance, as compared to one manufactured with a pure Al based alloy plating bath. Furthermore, it may be assumed that an Al based alloy plated steel sheet is used after subjecting to an anodizing treatment, as similar to an Al alloy material. However, an ordinary Al based alloy plated sheet has a defect that the appearance thereof becomes blackish after an anodizing treatment, and an anodized surface having good design quality is difficult to achieve.
The invention is to provide an Al-coated steel sheet that is excellent in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment, as compared to an ordinary hot-dip Al based alloy plated steel sheet.
The object may be achieved by an Al-coated steel sheet having excellent total reflection characteristics and corrosion resistance, containing a base steel sheet having on a surface thereof an Al-coated layer having an average thickness of 7 μm or more with an Al—Fe—Si based alloy layer intervening therebetween, a surface layer portion of the Al-coated layer extending from the surface thereof to a depth of 3 μm having an average Si concentration of 2.0% by mass or less, and preferably 1.3% by mass or less, and an area ratio of an Al—Fe based intermetallic compound phase occupying the surface of the Al-coated layer being 10% or less. The Al-coated layer may be obtained by modifying a hot-dip Al based alloy plated layer containing Si through a heat treatment. At this time, the Si content of the hot-dip plating bath is preferably 1.5% by mass or more and 6.0% by mass or less, and more effectively 1.5% by mass or more and 3.0% by mass or less, and may be controlled to a range of 1.5% by mass or more and less than 3.0% by mass.
The Al-coated layer herein is a layer that has an Al phase as a matrix. The Al-coated layer may contain an Al—Fe based intermetallic compound phase and a Si phase.
The average Si concentration of the surface layer portion of the Al-coated layer extending from the surface thereof to a depth of 3 μm may be obtained by performing EDX analysis (energy dispersive X-ray spectrometry analysis) for the cross sectional surface in parallel to the thickness direction of the Al-coated layer. Specifically, in an SEM observation field of the cross sectional surface with a magnification of 5,000, a rectangular area having a dimension of 3 μm×20 μm with the edge having a length of 3 μm in the thickness direction of the Al-coated layer (i.e., the thickness direction of the steel sheet) is assumed. The rectangular area that entirely overlaps the Al-coated layer (i.e., the rectangular area does not deviate from the Al-coated layer) and has the edge having a length of 20 μm being in contact with at least a part of the outermost surface of the Al-coated layer is set as a measurement area. The measurement area is measured for the average Si concentration (conversion value in terms of percentage by mass) by EDX analysis. The measurement operation above is performed for five or more fields having been randomly selected, and the average value of the average Si concentration values of the measurement areas may be designated as the “average Si concentration of the surface layer portion of the Al-coated layer extending from the surface thereof to a depth of 3 pat”.
The area ratio of an Al—Fe based intermetallic compound phase occupying the surface of the Al-coated layer means the ratio of the area of the portion having the Al—Fe based intermetallic compound phase present therein with respect to the projected area of the observed area obtained by viewing the surface of the Al-coated layer in the thickness direction. The Al—Fe based intermetallic compound phase that is exposed to the surface of the Al-coated layer may be identified as a phase having an Fe content in terms of percentage by mass that is the second highest next to Al.
The invention also provides, as a method for manufacturing the Al-coated steel sheet having excellent total reflection characteristics and corrosion resistance, a manufacturing method containing:
a step of manufacturing a hot-dip Al based alloy plated steel sheet having a plated layer having an average thickness of 7 μm or more, by using a hot-dip Al based alloy plating bath having a Si content of 2.0% by mass or more and 6.0% by mass or less; and
a step of modifying the plated layer to an Al-coated layer having a surface layer portion extending from the surface thereof to a depth of 3 μm that has an average Si concentration of 2.0% by mass or less, by heating and retaining the hot-dip Al based alloy plated steel sheet to a temperature of from 300 to 460° C. to progress diffusion of Si in the plated layer.
As a measure for further improving the total reflection characteristics and the corrosion resistance, the invention also provides a manufacturing method containing:
a step of manufacturing a hot-dip Al based alloy plated steel sheet having a plated layer having an average thickness of 7 μm or more, by using a hot-dip Al based alloy plating bath having a Si content of 1.3% by mass or more and 3.0% by mass or less; and
a step of modifying the plated layer to an Al-coated layer having a surface layer portion extending from the surface thereof to a depth of 3 μm that has an average Si concentration of 1.3% by mass or less, by heating and retaining the hot-dip Al based alloy plated steel sheet to a temperature of from 300 to 460° C. to progress diffusion of Si in the plated layer. In this case, the Si content of the hot-dip Al based alloy plating bath may be controlled to 1.5% by mass or more and less than 3.0% by mass.
According to the invention, an Al-coated steel sheet may be provided that has a high total reflectivity, good corrosion resistance, and an excellent appearance after subjecting to an anodizing treatment, as compared to an ordinary hot-dip Al based alloy plated steel sheet. The Al-coated steel sheet is particularly excellent in heat reflection characteristics and light reflection characteristics due to the high total reflectivity thereof, and thus is considerably useful for purposes requiring heat resistance and purposes utilizing light reflection capability. The Al-coated steel sheet may be obtained by subjecting a hot-dip Al based alloy plated steel sheet capable of being manufactured with an ordinary hot-dip plating line, as a base material, to a post heat treatment. Accordingly, the invention contributes to the enhancement of the purposes of a hot-dip Al based alloy plated steel sheet.
The Al-coated steel sheet of the invention can be achieved by modifying a plated layer of a hot-dip Al based alloy plated steel sheet manufactured by using an Al based alloy plating bath containing Si, through a post heat treatment. It is important herein that the post heat treatment largely enhances the diffusion of Si in the plated layer, as compared to a post heat treatment having been ordinarily performed, and thereby the Si concentration of the surface layer portion of the plated layer is reduced. Furthermore, for reducing the Si concentration of the surface layer portion of the plated layer, it is considerably effective to use a hot-dip Al based alloy plating bath having a relatively low Si content.
Even in the case where a hot-dip Al based alloy plated steel sheet manufactured by using an ordinary hot-dip Al based alloy plating bath containing Si in an amount, for example, of 7% by mass or more is subjected to a post heat treatment for a certainly long period of time, the Si phase 6 remains in a large amount in the Al-coated layer 30, as shown in
The hot-dip Al based alloy plated steel sheet obtained by using a hot-dip Al based alloy plating bath having a relatively small Si content is different in structure state of the plated layer 3 from the ordinary hot-dip Al based alloy plated steel sheet (
According to the detailed investigations by the inventors, it has been found that in the case where the plated steel sheet manufactured by using a hot-dip Al based alloy plating bath having a small Si content as in
In the Al-coated layer 30 that has no Si phase observed therein or has an extremely small content of the remaining Si phase, as shown in
It is considered that the total reflection characteristics are improved by reducing the Si concentration of the surface layer portion since the Al purity of the surface layer portion of the plated layer is increased, and the reflection characteristics that are closer to pure Al may be imparted thereto.
On the surface 10 of the Al-coated layer 30, a portion where the Al—Fe based intermetallic compound phase 5 is exposed is formed. It has been found that the Al—Fe based intermetallic compound phase 5 present on the surface may deteriorate the appearance after subjecting to an anodizing treatment. The exposed Al—Fe based intermetallic compound phase 5 may also be a factor of deteriorating the total reflection characteristics and the corrosion resistance. However, in the case where a hot-dip Al based alloy plating bath containing Si in an amount of 1.5% by mass or more is used, the Al—Fe based intermetallic compound phase 5 is liable to be formed on the side close to the alloy layer 2, and the amount of the Al—Fe based intermetallic compound phase 5 formed on the side close to the surface 10 is small. As a result of various investigations, in the case where the area ratio of the Al—Fe based intermetallic compound phase occupying the surface 10 of the Al-coated layer 30 is suppressed to 10% or less, coupled with the reduction of the Si concentration in the surface layer portion described above, the appearance after subjecting to an anodizing treatment may be considerably improved. The heat reflection characteristics and the corrosion resistance may also be improved. The condition that the area ratio of the Al—Fe based intermetallic compound phase is suppressed to 10% or less may be controlled by using a hot-dip Al based alloy plating bath having a Si content of 1.5% by mass or more.
In the hot-dip Al based alloy plated steel sheet having the cross sectional structure shown in
Both the upper layer and the lower layer have a Si concentration of less than 3.0% by mass intact after plating, and the major intermetallic compound constituting the phases is estimated as an Al—Fe based intermetallic compound, as shown in Table 1.
It is understood that the Si content of the upper layer is greatly increased through the post heat treatment. Such a phenomenon may be observed that Si present in the plated layer is incorporated in the upper layer, and the upper layer is changed to the structure containing mainly an Al—Fe—Si based intermetallic compound.
The base steel sheet as the base sheet for plating applied to the invention may be various steel species that have been applied to hot-dip Al based alloy plated steel sheets. For purposes requiring heat resistance, a steel having a N content of from 0.04 to 0.015% by mass is preferably used for preventing the alloy layer from growing. Specific examples of the contents of the steel components comprise from 0.001 to 0.06% of C, 0.5% or less of Si, 1.0% or less of Mn, 0.016% or less of P, 0.007% or less of S, 0.012% or less of Al, 0.015% or less of N, and from 0 to 0.03% of Ti (all in terms percentage by mass), with the balance of Fe and unavoidable impurities.
The thickness of the base sheet for plating may be in a range of from 0.1 to 3.5 mm, and may be controlled to a range of from 0.2 to 1.6 mm.
The hot-dip Al based alloy plated steel sheet that is applied to the invention may be manufactured with an ordinary hot-dip plating line. The plating bath composition is preferably an Al based alloy plating bath having a Si content of 1.5% by mass or more and 6.0% by mass or less. When the Si content of the bath is too large, it may be difficult to reduce sufficiently the Si concentration of the surface layer portion through the post heat treatment as the subsequent process step. When the Si content is too small, the structure of the plated layer becomes close to pure Al plating to enhance the tendency that the Al—Fe based intermetallic compound phase 5 is formed on the side close to the surface 10 as shown in
Fe is generally incorporated in the bath. The Fe content is preferably controlled 3.0% by mass or less, and more preferably 2.5% by mass or less. As other elements in the bath, the bath may contain at least one of 1.0% by mass or less of Ti, 1.0% by mass or less of B, 1.0% by mass or less of Zr, 1.0% by mass or less of Sr, and 5.0% by mass or less of Mg. Ti, B and Zr are effective for enhancing the surface appearance through miniaturization of the spangle size, Sr is effective for miniaturizing the Si phase, and Mg is effective for enhancing the corrosion resistance. The balance of the aforementioned elements may be Al and unavoidable impurities.
The plating deposition amount is preferably 7 μm or more, and more preferably 20 μm or more, in terms of the thickness of the plated layer per one surface (exclusive of the alloy layer). The upper limit thereof is not particularly determined, and in general, the average thickness may be in a range of 50 μm or less, and may be controlled to 40 μm or less.
The hot-dip Al based alloy plated steel sheet is subjected to a heat treatment for providing the Al-coated layer having a low Si concentration through modification of the Al based alloy plated layer. The heat treatment is performed after the hot-dip plating, and thus is referred to as a “post heat treatment” in the description herein. For improving the total reflection characteristics and the like, the Al based alloy plated layer is preferably modified to the Al-coated layer having an average Si concentration of the surface layer portion extending from the surface thereof to a depth of 3 μm of 2.0% by mass or less, and more preferably 1.3% by mass or less.
As a result of various investigations, in the case where an Al-coated layer having an average Si concentration of the surface layer portion extending from the surface thereof to a depth of 3 μm of 2.0% by mass or less is to be obtained through the modification, a hot-dip Al based alloy plated steel sheet manufactured by using a hot-dip Al based alloy plating bath having a Si content of 2.0% by mass or more and 6.0% by mass or less may be used. It may be controlled to use a hot-dip Al based alloy plated steel sheet manufactured by using a hot-dip Al based alloy plating bath having a Si content of more than 2.0% by mass and 6.0% by mass or less.
In the case where an Al-coated layer having an average Si concentration of the surface layer portion extending from the surface thereof to a depth of 3 μm of 1.3% by mass or less as a preferred embodiment is to be obtained through the modification, it is effective to use a hot-dip Al based alloy plated steel sheet manufactured by using a hot-dip Al based alloy plating bath having a Si content of 1.5% by mass or more and 3.0% by mass or less. It may be controlled to use a hot-dip Al based alloy plated steel sheet manufactured by using a hot-dip Al based alloy plating bath having a Si content of 1.5% by mass or more and less than 3.0% by mass.
The heating temperature of the post heat treatment may be set in a range of from 300 to 460° C., and more effectively in a range of from 380 to 460° C. When the heating temperature is too low, the Si concentration of the surface layer portion of the plated layer may be difficult to reduce. When the heating temperature is too high, the alloy layer may excessively grow. The atmosphere of the post heat treatment may be in the air.
The Si phase formed in the plated layer tends to be distributed on the side close to the surface. In the post heat treatment, the Si phase is consumed for the reaction of increasing the Si content of the alloy layer, and thereby the Si concentration of the surface layer portion of the plated layer is reduced. It is important to determine the heating time for sufficiently progressing the diffusion of Si in the plated layer and the reaction of increasing the Si content of the alloy layer. The suitable heating time may be determined by comprehending in advance the relationship between the heating temperature and the heating time that are sufficient for reducing the Si concentration of the surface layer portion of the plated layer (see
As a base steel sheet, a cold-rolled annealed steel sheet having the following chemical composition having a thickness of 0.8 mm was prepared.
0.0033% of C, less than 0.01% of Si, 0.23% of Mn, less than 0.01% of P, 0.013% of S, 0.01% of Al, 0.0027% of 0, and 0.0025% of N, with the balance of Fe and unavoidable impurities
By using the base steel sheet was used as a base sheet for plating, hot-dip Al based alloy plated steel sheets having an average thickness of the plated layer (exclusive of the alloy layer) in a range approximately from 30 to 50 μm were manufactured.
Si content in Al bath: shown in Tables 3 and 4
Fe content in Al bath: approximately 2% by mass
Contents of additional elements other than Si and Fe in Al bath: shown in Tables 3 and 4
Components in bath other than above: Al and unavoidable impurities
Plating bath temperature: 660° C.
Plating bath dipping time: 2 seconds
Average cooling rate until complete solidification of plated layer: 13° C. per second
The resulting hot-dip Al based alloy plated steel sheet was subjected to a post heat treatment at the heating temperature for the heating time shown in Tables 3 and 4 to prepare a specimen, which was then investigated as follows. The atmosphere of the post heat treatment was in the air. For comparison, a specimen that was not subjected to a post heat treatment was prepared.
EDX analysis was performed for the cross sectional surface in parallel to the thickness direction of the specimen. In an SEM observation field with a magnification of 5,000, a rectangular area having a dimension of 3 μm×20 μm with the edge having a length of 3 μm in the thickness direction of the Al-coated layer was assumed. The rectangular area that entirely overlapped the Al-coated layer and had the edge having a length of 20 μm being in contact with at least a part of the outermost surface of the Al-coated layer was set as a measurement area. The measurement area was measured for the average Si concentration (conversion value in terms of percentage by mass) by EDX analysis. The measurement operation above was performed for five fields having been randomly selected, and the average value of the average Si concentration values of the measurement areas was designated as the average Si concentration of the surface layer portion of the Al-coated layer extending from the surface thereof to a depth of 3 μm.
The surface of the Al-coated layer of the specimen was observed with SEM in the thickness direction thereof, and the area ratio of the Al—Fe based intermetallic compound phase occupying the projected area of the observed area obtained by viewing the surface of the Al-coated layer in the thickness direction was obtained. The Al—Fe based intermetallic compound phase exposed to the surface may be identified by EDX analysis. The area ratio was measured for five fields having been randomly selected, and the average value thereof was designated as the area ratio (%) of the Al—Fe based intermetallic compound phase occupying the surface.
The average thickness of the alloy layer was measured by observing the cross section of the specimen in parallel to the thickness direction with SEM. The alloy layer exhibited a multi-phase structure including an upper layer and a lower layer, except for some instances using a plating bath having a high Si content.
On the cross sectional surface in parallel to the thickness direction of the specimen, ten measurement points having been randomly selected in the vicinity of the center in the thickness direction of the upper layer, as similar to the measurement points e and f shown in
The surface of the Al-coated layer of the specimen was measured for total reflectivity. The measurement was performed with MCP 3100, produced by Shimadzu Corporation, under conditions of a reflection angle of 8° and a measurement wavelength of 550 nm, and the total reflection characteristics were evaluated by the following standard. The evaluation of
◯ or better was judged as passed.
⊙: total reflectivity of 75% or more
◯: total reflectivity of 70% or more and less than 75%
Δ: total reflectivity of 65% or more and less than 70%
X: total reflectivity of less than 65%
The specimen was subjected to a humidity test of retaining in an environment of a temperature of 90° C. and a relative humidity of 95% for 500 hours, and the rust formation ratio was measured from the area where rust was formed on the surface, and evaluated for the corrosion resistance by the following standard. The evaluation of ◯ or better was judged as passed.
⊙: rust formation ratio of less than 10%
◯: rust formation ratio of 10% or more and less than 20%
Δ: rust formation ratio of 20% or more and less than 50%
X: rust formation ratio of 50% or more
Evaluation of Appearance after Subjecting to Anodizing Treatment
The specimen was subjected to an anodizing treatment, and the resulting anodized surface was measured for L value (luminosity). The anodizing treatment conditions were a treatment solution containing 150 g/L of sulfuric acid and 5 g/L of aluminum sulfate, a treatment temperature of 25° C., an electric current density of 5 A/dm2, and a treating time of 10 minutes. The appearance after subjecting to the anodizing treatment was evaluated, and the evaluation of 0 or better was judged as passed.
⊙: L value of 90 or more
◯: L value of 88 or more and less than 90
Δ: L value of 85 or more and less than 88
X: L value of less than 85
The specimen was subjected to a cylinder drawing process, and the vertical wall portion of the processed article was evaluated for peeled off state of the Al-coated layer. The cylinder drawing conditions were a drawing ratio of 2.0, a blank diameter of 80 mm, a die diameter of 42 mm, a die corner radius of 5 mm, a punch diameter of 40 mm, and a punch corner radius of 5 mm. The workability was evaluated by the following standard, and the evaluation of ◯ or better was judged as passed.
◯: no peel-off of Al-coated layer found
X: peel-off of Al-coated layer found
The surface of the Al-coated layer of the specimen was evaluated for refinement of the spangle in terms of the spangle density. The evaluation of ◯ or better was judged as passed.
⊙: spangle density of 200 per cm2 or more
◯: spangle density of 50 per cm2 or more and less than 200 per cm2
X: spangle density of less than 50 per cm2
The result is shown in
The specimens according to the invention were improved in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment, and also were good in workability and surface appearance. In particular, the specimens having an average Si concentration of the surface layer extending to a depth of 3 μm of the Al-coated layer of 1.3% by mass or less were considerably excellent in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment.
On the other hand, Nos. 31 and 32 as comparative examples were manufactured by using a pure Al plating bath, in which a large amount of an Al—Fe based intermetallic compound phase was formed in the vicinity of the surface of the plated layer. The Al—Fe based intermetallic compound phase remained substantially unchanged after subjecting to the post heat treatment (No. 32). These specimens had no Si present in the surface layer portion of the Al-coated layer, but were not improved in total reflection characteristics and corrosion resistance and were inferior in appearance after subjecting to an anodizing treatment, due to the large area ratio of the Al—Fe based intermetallic compound phase occupying the surface. The specimens were also inferior in workability due to the alloy layer formed to have a large thickness.
No. 33 maintained the tendency that a large amount of an Al—Fe based intermetallic compound phase was formed in the vicinity of the surface of the plated layer, due to the too small Si content in the plating bath. Accordingly, the specimen was inferior in various characteristics as similar to the specimens using a pure Al bath. This specimen was not subjected to a post heat treatment. However, it is difficult to reduce the area ratio of the Al—Fe based intermetallic compound phase occupying the surface through a post heat treatment.
Nos. 34, 37, 38 and 41 had a high average Si concentration of the surface layer portion of the Al-coated layer since a post heat treatment was not performed, and the heating conditions were not proper, although the plating bath had the appropriate Si content. As a result, the specimens were inferior in total reflection characteristics and appearance after subjecting to an anodizing treatment and were insufficient in improvement of corrosion resistance.
Nos. 35 and 39 were hot-dip Al based alloy plated steel sheets manufactured by using a plating bath having a high Si content, and were inferior in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment due to the large average Si concentration in the surface layer portion of the Al-coated layer (the Al-coated layers in these specimens were the Al based alloy plated layers intact after plating).
Nos. 36 and 40 were obtained by subjecting a hot-dip Al based alloy plated steel sheet manufactured by using a plating bath having a high Si content to a post heat treatment, but the average Si concentration of the surface layer portion of the Al-coated layer did not sufficiently reduced, and thus the specimens were not improved in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-168478 | Aug 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/068035 | 7/7/2014 | WO | 00 |
