Patent Application
20160194744
This disclosure relates to a method of producing a high-strength hot-dip galvanized steel sheet suitable for automotive body components and has a good surface appearance and good adhesion of the coating and to a method of producing a high-strength galvannealed steel sheet.
In recent years, with the increase in global environmental protection awareness, there has been a strong demand for improved mileage to reduce CO2 emissions from automobiles. To satisfy the demand, a strong movement is under way to strengthen steel sheets used as automotive body component materials to decrease the thickness of automotive body components and thereby decrease the weight of automotive bodies. However, strengthened steel sheets may have low ductility. Thus, it is desirable to develop high-strength high-ductility steel sheets.
Solid-solution strengthening elements such as Si, Mn, and/or Cr have been added to strengthen steel sheets. In particular, Cr in a smaller amount than other elements can strengthen steel sheets. Thus, the addition of Cr is effective in strengthening the material property of steel sheets. However, Cr and such elements are more oxidizable elements than Fe. Thus, there are problems as described below in the production of hot-dip galvanized steel sheets and galvannealed steel sheets based on high-strength steel sheets containing large amounts of such elements.
In general, to produce a hot-dip galvanized steel sheet, a steel sheet is heated and then annealed in a nonoxidizing atmosphere or a reducing atmosphere at a temperature of approximately 600° C. to 900° C. and then subjected to hot-dip galvanizing treatment. Oxidizable elements in steel are selectively oxidized even in a nonoxidizing atmosphere or reducing atmosphere generally employed, are concentrated on the surface, and form oxides on the surface of the steel sheet. Such oxides lower the wettability of molten zinc on the surface of the steel sheet in hot-dip galvanizing treatment and cause an ungalvanized surface. An increase in the concentration of oxidizable elements in steel drastically lowers the wettability and frequently causes ungalvanized surfaces. Even if ungalvanized surfaces are not formed, oxides existing between the steel sheet and the coating reduce adhesion of the coating.
To address this issue, a method of improving the wettability of molten zinc on a surface of a steel sheet is disclosed in Japanese Patent No. 2587724. That method includes heating a steel sheet in an oxidizing atmosphere in advance to rapidly form an iron oxide film on a surface of the steel sheet at an oxidation rate equal to or higher than a predetermined value, thereby preventing oxidation of additive elements on the surface of the steel sheet, and thereafter subjecting the iron oxide film to reduction annealing. However, if a steel sheet is excessively oxidized, iron oxide adheres to a hearth roll and causes a problem of indentation flaws on the steel sheet.
A method disclosed in Japanese Patent No. 3956550 includes pickling a steel sheet after annealing to remove oxides from the surface and thereafter annealing again and hot-dip galvanizing the steel sheet. However, oxides that are insoluble in acids cannot be removed in Japanese Patent No. 3956550. Thus, the appearance of the coating of steel sheets containing Cr that forms an oxide insoluble in acids cannot be improved.
A method disclosed in Japanese Unexamined Patent Application Publication No. 2001-158918 includes immersing a steel sheet in an alkaline molten salt bath after annealing to remove silicon-based oxides, then annealing the steel sheet again, and hot-dip galvanizing the steel sheet. However, electrolytic treatment is not performed in Japanese Unexamined Patent Application Publication No. 2001-158918. Thus, chromium oxide cannot be removed and the appearance of the coating of steel sheets containing Cr cannot be improved.
In view of such situations, it could be helpful to provide a method of producing a high-strength hot-dip galvanized steel sheet having good adhesion of the coating and a good surface appearance and a method of producing a high-strength galvannealed steel sheet.
We thus provide:
We provide a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet each having high strength, a good surface appearance, and good adhesion of the coating. For example, application of a high-strength hot-dip galvanized steel sheet to automobile structural members can improve mileage due to weight reduction of automotive bodies.
Annealing a steel sheet forms chromium oxide on the surface of the steel sheet and forms a low Cr concentration region in a surface layer of the steel sheet. If chromium oxide on the surface can be removed, because of the low concentration of Cr in the surface layer of the steel sheet, another annealing forms less chromium oxide, that is, a suppressed amount of chromium oxides on the surface of the steel sheet. However, chromium oxides are insoluble in acids, as described above. Thus, to solve problems caused by chromium oxide by a method of removing chromium oxide from the surface, chromium oxide on the surface must be removed by a method other than acid dissolution. The term a “surface layer” of steel sheets, as used herein, refers to a layer having a thickness of 10 μm or less directly under the surface of the steel sheets.
In an electric potential-pH diagram of Cr, chromic acid is stable in a wide acidic to alkaline region on a high electric potential side. That is, chromium oxide can be converted into chromic acid in an aqueous solution by application of a high electric potential to be dissolved in the aqueous solution. Chromic acid is formed at a lower electric potential in an alkaline region than in an acidic region. Thus, we extensively studied the assumption that the appearance of the coating of steel sheets containing Cr can be improved by removing chromium oxide by converting it into chromic acid dissolved in an alkaline aqueous solution. We found that chromium oxide can be removed by electrolytic treatment using a steel sheet as an anode in an alkaline aqueous solution. Chromium oxide on a surface of a steel sheet can be removed even in a short electrolytic treatment time. Further, chromium oxide at grain boundaries in steel sheets can also be removed in an electrolytic treatment time of 2 seconds or more. This particularly improves the adhesion of the coating of hot-dip galvanized steel sheets.
Our methods and steel sheets will be described below. This disclosure, however, is not limited to the examples. The symbol “%” that represents the amount of component refers to “% by mass”.
First, a steel sheet that is used as a raw material in a production method according to a first example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 5.00% or less, P: 0.100% or less,
S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. In addition to these components, the steel sheet may contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. In addition to these components, the steel sheet may contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. Each of the components listed above will be described below.
C is an austenite formation element, forms a composite microstructure in an annealed sheet, and is effective in improving strength and ductility. The C content is 0.040% or more to improve strength and ductility. However, a C content of more than 0.500% results in marked hardening of a welded portion and a heat-affected zone, degradation of the mechanical characteristics of the welded portion, poor spot weldability, and poor arc weldability. Thus, the C content is 0.500% or less.
Si: 1.00% or less
Si is a ferrite formation element and is effective in improving the solid-solution strengthening and work hardenability of ferrite in annealed sheets. A Si content of more than 1.00% results in formation of silicon oxide on a surface of a steel sheet during annealing and consequently poor coatability. Thus, the Si content is 1.00% or less. The Si content may be 0%.
Cr is an austenite formation element and effective in securing the strength of annealed sheets. The Cr content is 0.10% or more to secure the strength. However, a Cr content of more than 2.00% results in insufficient removal of chromium oxide from a surface layer of a steel sheet resulting in a poor appearance of the coating. Thus, the Cr content is 2.00% or less.
In the production method according to the first example, the remainder may be Fe and incidental impurities. As described above, the steel sheet used as a raw material in the production method according to the first example may contain the following components in addition to the components described above.
Mn: 5.00% or less
Mn is an austenite formation element and effective in securing the strength of annealed sheets. The Mn content is preferably 0.80% or more to secure the strength. However, a Mn content of more than 5.00% may result in a poor appearance of the coating due to a large amount of oxide formed in a surface layer of a steel sheet during annealing. Thus, the Mn content is preferably 5.00% or less.
P: 0.100% or less
P is an element effective in strengthening steel. However, a P content of more than 0.100% may result in embrittlement due to intergranular segregation and low impact resistance. Thus, the P content is preferably 0.100% or less.
S: 0.0100% or less
S forms an inclusion such as MnS, and thereby lowers impact resistance or causes a crack along a metal flow in a welded portion. Thus, the S content is preferably minimized. The S content is preferably 0.0100% or less.
Al: 0.100% or less
Excessive addition of Al results in low surface quality or poor formability due to an increased oxide inclusion and is responsible for increased costs. Thus, the Al content is preferably 0.100% or less, more preferably 0.050% or less.
Mo is an austenite formation element and effective in securing the strength of annealed sheets. The Mo content is preferably 0.01% or more to secure the strength. Owing to high alloy costs of Mo, a high Mo content may be responsible for increased costs. Thus, the Mo content is preferably 0.50% or less.
Nb is an element that contributes to improved strength due to solid-solution strengthening or precipitation strengthening. The Nb content is preferably 0.010% or more to produce this effect. However, a Nb content of more than 0.100% may result in low ductility and poor workability of steel sheets. Thus, the Nb content is preferably 0.100% or less.
B improves hardenability and contributes to improved strength of steel sheets. The B content is preferably 0.0001% or more to produce this effect. However, an excessively high B content may result in low ductility and poor workability. An excessively high B content is also responsible for increased costs. Thus, the B content is preferably 0.0050% or less.
Ti, together with C or N, forms fine carbide or fine nitride in steel sheets and contributes to improved strength of the steel sheets. The Ti content is preferably 0.010% or more to produce this effect. This effect levels off at a Ti content of more than 0.100%. Thus, the Ti content is preferably 0.100% or less.
Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less
Cu, V, and Ni are effective in strengthening steel and may be used to strengthen steel within the ranges specified herein. To strengthen steel, the Cu content is preferably 0.05% or more, the V content is preferably 0.005% or more, and the Ni content is preferably 0.05% or more. However, an excessively high Cu content of more than 1.00%, an excessively high V content of more than 0.500%, or an excessively high Ni content of more than 0.50% may result in low ductility due to markedly increased strength. An excessively high content with respect to these elements may be responsible for increased costs. Thus, when these elements are added, the Cu content is preferably 1.00% or less, the V content is preferably 0.500% or less, and the Ni content is preferably 0.50% or less.
N: 0.0100% or less
N reduces the anti-aging effects of steel and is preferably minimized. AN content of more than 0.0100% may result in significantly reduced anti-aging effects. Thus, the N content is preferably 0.0100% or less.
Sb: 0.10% or less, Sn: 0.10% or less
Sb and Sn can suppress nitriding in the vicinity of a surface of a steel sheet. To suppress nitriding, the Sb content is preferably 0.005% or more, and the Sn content is preferably 0.005% or more. This effect levels off at a Sb content or a Sn content of more than 0.10%. Thus, the Sb content is preferably 0.10% or less, and the Sn content is preferably 0.10% or less.
Ca: 0.0100% or less
Ca is effective in improving ductility due to the shape control of a sulfide such as MnS. The Ca content is preferably 0.001% or more to produce this effect. This effect levels off at a Ca content of more than 0.0100%. Thus, the Ca content is preferably 0.0100% or less.
REM: 0.010% or less
REM controls the shape of sulfide-base inclusions and contributes to improved workability. The REM content is preferably 0.001% or more to produce the effect of improving workability. A REM content of more than 0.010% may result in an increased amount of inclusions and poor workability. Thus, the REM content is preferably 0.010% or less.
The remainder is Fe and incidental impurities.
The method of producing the steel sheet used as a raw material in the production method according to the first example is not particularly limited. For example, a steel slab having the composition as described above is heated and then subjected to rough rolling and finish rolling in a hot-rolling step. Then, scales are removed from a surface layer of the hot-rolled steel sheet in a pickling step, and the hot-rolled steel sheet is subjected to cold rolling. The conditions for the hot-rolling step and the conditions for the cold-rolling step are not particularly limited and may be appropriately determined.
The steel sheet used as a raw material is typically produced through such common steps of steel making, casting, and hot rolling as described above. However, for example, part or all of the hot-rolling step may be omitted by using strip casting or the like.
The production method according to the first example will be described below. The production method includes a first heating step, a cooling step, an electrolytic treatment step, a second heating step, and a coating treatment step.
The first heating step includes holding the steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H2 concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C. In the first heating step, while Fe is not oxidized, Cr is oxidized on the surface of the steel sheet. Consequently, a surface layer containing chromium oxide is formed.
The H2 concentration should be high enough to suppress the oxidation of Fe and is 0.05% by volume or more. A H2 concentration of more than 25.0% by volume results in increased costs. Thus, the H2 concentration is 25.0% by volume or less.
A dew point of less than −45° C. results in suppressed oxidation of Cr. A dew point of more than 0° C. results in oxidation of Fe. Thus, the dew point ranges from −45° C. to 0° C.
A steel sheet temperature of less than 700° C. results in no oxidation of Cr. A steel sheet temperature of more than 900° C. results in high heating costs. Thus, the heating temperature of the steel sheet (steel sheet temperature) is a temperature of 700° C. to 900° C. Holding in the first heating step may be holding of the steel sheet at a constant temperature or may be holding of the steel sheet at varying temperatures.
A holding time of less than 20 seconds results in insufficient formation of chromium oxide on the surface. A holding time of more than 600 seconds causes low electrolytic treatment efficiency due to excessive formation of chromium oxide and results in low production efficiency. Thus, the holding time ranges from 20 to 600 seconds.
The electrolytic treatment step includes subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a Coulomb density (charge density) of 1.0 to 400 C/dm2, where the steel sheet acts as an anode.
The electrolytic treatment step is performed to remove chromium oxide formed in the first heating step from the surface layer. Therefore, the steel sheet acts as an anode for electrolytic treatment. A Coulomb density of less than 1.0 C/dm2 results in insufficient removal of chromium oxide. A Coulomb density of more than 400.0 A/dm2 results in greatly increased costs. Thus, the Coulomb density is 1.0 to 400.0 C/dm2. A short electrolytic treatment time may result in insufficient removal of chromium oxide formed at grain boundaries in the steel sheet and poor adhesion of the coating. The electrolytic treatment time is preferably 2 seconds or more, more preferably 5 seconds or more to further improve the adhesion of the coating. Although the electrolytic treatment time has no particular upper limit, a long treating time results in high costs, and therefore the electrolytic treatment time is preferably 60 seconds or less.
Examples of the alkaline aqueous solution used in the electrolytic treatment step include aqueous solutions containing NaOH, Ca(OH)2, or KOH.
The second heating step includes holding the resultant steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H2 concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less. The second heating step is performed to facilitate coating (particularly hot-dip coating) of the steel sheet.
The H2 concentration should be high enough to suppress oxidation of Fe and is 0.05% by volume or more. A H2 concentration of more than 25.0% by volume results in increased costs. Thus, the H2 concentration is 25.0% by volume or less.
A dew point of more than 0° C. results in oxidation of Fe. Thus, the dew point is 0° C. or less. The dew point has no particularly lower limit. The dew point is preferably −60° C. or more in terms of industrial practice.
A steel sheet temperature of less than 650° C. results in no activation of the surface of the steel sheet and poor molten zinc wettability. A steel sheet temperature of more than 900° C. results in formation of an oxide of Cr on the surface during annealing, formation of a surface layer containing chromium oxide, and poor wettability of the steel sheet with molten zinc. Thus, the heating temperature of the steel sheet (steel sheet temperature) is a temperature of 650° C. to 900° C. In the second heating step, the steel sheet may be held at a constant temperature or at varying temperatures.
A holding time of less than 15 seconds results in insufficient activation of the surface of the steel sheet. A holding time of 300 seconds or more results in formation of an oxide of Cr on the surface of the steel sheet again, formation of a surface layer containing chromium oxide, and poor molten zinc wettability. Thus, the holding time is 15 to 300 seconds. Coating Treatment Step
For example, the coating treatment step includes cooling the steel sheet after the treatment described above and immersing the steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing.
For production of hot-dip galvanized steel sheets, the bath temperature preferably is 440° C. to 550° C., and the concentration of Al in the galvanizing bath preferably is 0.13% to 0.24%. The symbol “%” with respect to the Al concentration refers to “% by mass”.
A bath temperature of less than 440° C. may be inappropriate because temperature variations in the bath may cause solidification of Zn in a low-temperature portion. A bath temperature of more than 550° C. may result in rapid evaporation from the bath and deposition of vaporized Zn on the inner side of the furnace, thereby causing operational problems. This also tends to result in over-alloying because alloying proceeds during coating.
In the production of a hot-dip galvanized steel sheet, when the concentration of Al in the bath is less than 0.14%, this may result in poor adhesion of the coating due to Fe-Zn alloying.
When the concentration of Al in the bath is more than 0.24%, aluminum oxide may cause a defect.
A method of producing a high-strength hot-dip galvanized steel sheet may include other steps, provided that these steps do not have a negative impact. For example, another step may be performed between the steps, before the first heating step, or after the coating treatment step. A specific example that includes another step will be described below in each of the second example and thereafter. Another step is not limited to the step described in each of the second example and examples thereafter.
First, a steel sheet that is used as a raw material in a production method according to a second example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 8.00%, P: 0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. The steel sheet may further contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. The steel sheet may further contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
The components other than Mn are the same as in the first example and will not be described. The method of producing the steel sheet used as a raw material is also the same as in the first example and will not be described. Mn: 8.00% or less
Like Cr, Mn is oxidized on the surface of the steel sheet in the annealing step and forms a surface layer containing manganese oxide. Because use of Cr increases production costs, Mn, which is similar to Cr in the effects on the material quality, is often added together with Cr. Manganese oxide cannot be removed by alkaline electrolytic treatment, which can remove chromium oxide. However, oxides containing Mn are soluble in acids and can therefore be removed by pickling the surface of the steel sheet after the electrolytic treatment.
Mn is an austenite formation element and effective in securing the strength of annealed sheets. The Mn content is preferably 0.80% or more to secure the strength. However, a Mn content of more than 8.00% may result in insufficient removal of manganese oxide from the surface by pickling. A Mn content of more than 8.00% results in oxidation of a large amount of Mn on the surface of the steel sheet during reannealing, formation of a surface layer containing a large amount of oxide, and a poor appearance of the coating. Thus, the Mn content is 8.00% or less.
A production method according to the second example will be described below. The production method according to the second example includes a first heating step, a cooling step, an electrolytic treatment step, a pickling step after the electrolytic treatment, a second heating step, and a coating treatment step.
Unlike the production method in the first example, the production method in the second example includes the pickling step after the electrolytic treatment. The first heating step, cooling step, electrolytic treatment step, second heating step, and coating treatment step are the same as in the first example and will not be described.
The pickling step after the electrolytic treatment includes, before the second heating step, pickling the surface of the steel sheet after the electrolytic treatment step such that the pickling weight loss is 0.05 to 5 g/m2 on an Fe basis. This step is performed to clean the surface of the steel sheet. This step is also performed to remove oxides formed on the surface of the steel sheet in the first heating step and are soluble in acids.
A pickling weight loss of less than 0.05 g/m2 on an Fe basis may result in insufficient removal of oxides. A pickling weight loss of more than 5 g/m2 may result in dissolution of not only oxides on the surface layer of the steel sheet but also an inner portion of the steel sheet where Cr concentration has been decreased, and thus may fail to suppress formation of chromium oxide in the second heating step. Thus, the pickling weight loss is 0.05 to 5 g/m2 on an Fe basis.
First, a steel sheet that is used as a raw material in a production method according to a third example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 3.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. The steel sheet may further contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. The steel sheet may further contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
The above described steel sheet used in the third example is substantially the same as the steel sheet used as a raw material in the production method in the second example. Thus, the components other than Cr and the production method of the steel sheet will not be described. Cr: 0.10% to 3.00%
Cr is an austenite formation element and effective in securing the strength of annealed sheets. The Cr content is 0.10% or more to secure the strength. A Cr content of more than 3.00% results in insufficient removal of chromium oxide from the surface of the steel sheet even utilizing the third example, resulting in a poor appearance of the coating. Thus, the Cr content is 3.00% or less.
A production method according to the third example will be described below. The production method according to the third example includes a first heating step, a cooling step, a pickling step before electrolytic treatment, an electrolytic treatment step, a second heating step, and a coating treatment step. The first heating step, cooling step, electrolytic treatment step, second heating step, and coating treatment step are the same as in the first example and will not be described.
The pickling step before electrolytic treatment includes, before the electrolytic treatment step, pickling a surface of the steel sheet after the cooling step such that the pickling weight loss is 0.05 to 5 g/m2 on an Fe basis. This step is performed to clean the surface of the steel sheet. This step is also performed to remove oxides that are formed on the surface of the steel sheet in the first heating step and are soluble in acids.
A surface layer containing manganese oxide and chromium oxide is formed on the surface of the steel sheet after annealing. Manganese oxide is formed closer to the outermost side, and chromium oxide is formed closer to the steel sheet. Thus, chromium oxide can be more effectively removed in the electrolytic treatment step by pickling the surface of the steel sheet before the electrolytic treatment step to remove manganese oxide.
A pickling weight loss of less than 0.05 g/m2 on an Fe basis may result in insufficient removal of oxides. A pickling weight loss of more than 5 g/m2 may result in dissolution of not only oxides on the surface layer of the steel sheet but also an inner portion of the steel sheet that has a low Cr concentration, thus failing to suppress formation of chromium oxide in the second heating step. Thus, the pickling weight loss is 0.05 to 5 g/m2 on an Fe basis.
The production method may include both the pickling step after the electrolytic treatment described in the production method according to the second example and the pickling step before electrolytic treatment.
A production method according to a fourth example further includes an alloying treatment step after the coating treatment step of the first, second, or third example.
In the production method in the fourth example, the alloying treatment step is preferably performed after the following coating treatment step.
For production of high-strength galvannealed steel sheets, the bath temperature preferably is 440° C. to 550° C., and the concentration of Al in the galvanizing bath preferably is 0.10% to 0.20%.
A bath temperature of less than 440° C. may be inappropriate because variations in the bath temperature may cause solidification of Zn in a low-temperature portion. A bath temperature of more than 550° C. may result in rapid evaporation from the bath and deposition of vaporized Zn on the furnace, thereby causing operational problems. A bath temperature of more than 550° C. also tends to result in over-alloying because alloying proceeds during coating.
When the concentration of Al in the bath is less than 0.10%, this may result in formation of a large amount of phase and a poor powdering property. When the concentration of Al in the bath is more than 0.20%, Fe-Zn alloying may not proceed.
Although the conditions for the alloying treatment are not particularly limited, the alloying treatment temperature is most preferably more than 460° C. and less than 580° C. An alloying treatment temperature of 460° C. or less results in slow alloying. An alloying treatment temperature of 580° C. or more causes excessive formation of a hard and brittle Zn-Fe alloy layer at an interface with the base metal due to over-alloying and results in poor adhesion of the coating.
Steel having the composition listed in Table 1, the remainder being Fe and incidental impurities, was produced in a converter and formed into a slab in a continuous casting process. The slab was heated to 1200° C., was then hot-rolled to a thickness of 2.3 to 4.5 mm, and coiled. The hot-rolled steel sheet was then pickled and, if necessary, cold-rolled. The first heating step, electrolytic treatment step, and second heating step were then performed under the heat-treatment conditions listed in Table 2 (Tables 2-1 and 2-2 are collectively referred to as Table 2) or Table 3 (Tables 3-1 and 3-2 are collectively referred to as Table 3) in a furnace in which the atmosphere could be adjusted (the temperature in the first heating step and the temperature in the second heating step were in the temperature ranges around the temperatures listed in the tables (the temperatures listed in the tables ±20° C.), respectively). Hot-dip galvanizing treatment was subsequently performed in a Zn bath containing 0.13% to 0.24% Al to produce a hot-dip galvanized steel sheet. After hot-dip galvanizing treatment was performed in a Zn bath containing 0.10% to 0.20% Al, alloying treatment was performed under the conditions listed in Table 2 to produce a galvannealed steel sheet.
The surface appearance and adhesion of the coating of the hot-dip galvanized steel sheet and galvannealed steel sheet thus produced were examined by the following methods. Surface Appearance
The steel sheets were visually inspected for appearance deficiencies such as ungalvanized surfaces and pinholes. Steel sheets having no appearance deficiencies were rated as good (circle). Steel sheets having generally good appearances with a few appearance deficiencies were rated as fair (triangle). Steel sheets having appearance deficiencies were rated as poor (cross).
Adhesion of the coating of galvannealed steel sheets was evaluated in terms of powdering resistance. More specifically, a cellophane adhesive tape was applied to a galvannealed steel sheet. The surface to which the tape was applied was bent at an angle of 90 degrees and was bent back. A cellophane adhesive tape having a width of 24 mm was applied to and pressed against the inside of a processed portion (compressed side) parallel to the bent-processed portion and was then peeled off. The amount of zinc adhered to a portion of the cellophane adhesive tape having a length of 40 mm was measured as a Zn count using fluorescent X-rays. The Zn count per unit length (1 m) was rated according to the following criteria. The rank 1 is good (circle), the rank 2 is fair (triangle), and the rank 3 or more are poor (cross).
Fluorescent X-rays count rank
Unalloyed hot-dip galvanized steel sheets were subjected to a ball impact test. Adhesion of the coating was evaluated by peeling off a processed portion with a cellophane adhesive tape and by visually inspecting the processed portion for peeling of the coated layer. In the ball impact test, the mass of the ball was 1.8 kg, and the drop height was 100 cm.
Table 2 shows the evaluation results.
S
10.80
T
3.45
U
2.84
S
T
U
650
800
950
550
850
−50
680
S
T
U
The high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.
Hot-dip galvanized steel sheets and galvannealed steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Table 3 were employed. Evaluation was performed in the same way as in Example 1. Table 3 shows the evaluation results.
650
800
950
550
850
−50
680
S
T
U
The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.
Hot-dip galvanized steel sheets and galvannealed steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Table 4 (Tables 4-1 and 4-2 are collectively referred to as Table 4) were employed. Evaluation was performed in the same way as in Example 1. Table 4 shows the evaluation results.
650
800
950
550
850
−50
680
S
T
U
The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.
Hot-dip galvanized steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Tables 5 to 7 were employed. Evaluation was performed in the same way as in Example 1 except for adhesion of the coating of unalloyed hot-dip galvanized steel sheets.
Unalloyed hot-dip galvanized steel sheets were subjected to a ball impact test. Adhesion of the coating was evaluated by peeling off a processed portion with a cellophane adhesive tape and by visually inspecting the processed portion for peeling of the coated layer. In the ball impact test, the mass of the ball was 1.8 kg, and the drop height was 100 cm. The diameters of the impact portions were ¾ and ⅜ inches.
Tables 5 to 7 show the evaluation results.
The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.
We provide a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet each having high strength, a good surface appearance, and good adhesion of the coating. For example, application of a high-strength hot-dip galvanized steel sheet or high-strength galvannealed steel sheet to automobile structural members can improve mileage due to weight reduction of automotive bodies.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-167385 | Aug 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/004176 | 8/12/2014 | WO | 00 |
