JP5636727B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot dip galvanized steel sheet and a method for producing the same. Specifically, the present invention relates to a high-strength hot-dip galvanized steel sheet having excellent bendability and a method for producing the same, and in particular, press forming such as a car body of an automobile. The present invention relates to a high-strength hot-dip galvanized steel sheet suitable for the intended use and a method for producing the same. Here, in the present invention, “hot-dip galvanized steel sheet” includes “alloyed hot-dip galvanized steel sheet”, and “high-strength hot-dip galvanized steel sheet” includes “high-strength galvanized steel sheet”. .

  In recent years, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment, and in order to reduce the weight of the vehicle body and ensure the safety of passengers, high-strength steel sheets with a tensile strength of 590 MPa or more, especially considering rust prevention In materials, there is an increasing need for high-strength hot-dip galvanized steel sheets.

  However, it is not sufficient for a steel sheet to be used for an automobile member to have high strength, and it must satisfy the press formability required at the time of manufacturing parts. In particular, considering the part forming process, bending forming is most frequently used, and thus formed into various shaped parts. Therefore, a high-strength hot-dip galvanized steel sheet having excellent bendability is required. However, the bendability deteriorates as the steel plate strength increases.

  For improving the bendability of a high-strength steel sheet, conventionally, the approach of controlling the steel structure has been taken, and as described in Patent Document 1, the hardness of the low-temperature transformation generation phase is reduced and the hardness with the ferrite phase is reduced. It is considered good to reduce the difference.

  However, when a steel sheet containing a large amount of easily oxidizable elements such as Mn and Si is annealed in a hot dip galvanizing facility for the purpose of increasing the strength, as described in Non-Patent Document 1, Mn is formed on the surface. The wettability of hot dip galvanizing is significantly degraded by the oxide.

  On the other hand, for improving the wettability of high-strength steel sheets, as described in Patent Document 2, Mn and Si are internally oxidized by increasing the dew point of the annealing atmosphere or adding Ni to the steel. Is good. However, as shown in FIG. 1, since the internal oxide in the steel promotes non-uniform deformation, fine cracks are likely to appear on the surface of the processed portion, and the bendability itself is significantly deteriorated. Therefore, even if the techniques disclosed in Patent Document 1 and Patent Document 2 are combined, it is impossible to improve the bendability of the high-strength hot-dip galvanized steel sheet.

  As a high-strength thin steel sheet having excellent bendability, a steel sheet having a ferrite-based structure only in the surface layer portion has been proposed. For example, Patent Document 3 discloses an ultra-high-strength cold-rolled steel sheet that includes 3 to 15% by volume of a soft layer of C: 0.1% by mass or less on one surface in the surface layer portion. This steel sheet has excellent bendability because the surface layer portion is softened by decarburization annealing, thereby allowing close bending. Patent Document 4 discloses an ultra-high-strength cold-rolled steel sheet having a layer mainly composed of ferrite as a surface layer of a steel sheet and a layer mainly composed of martensite and bainite as an inner layer. This steel sheet exhibits excellent ductility and bendability by suppressing local necking due to the fact that the surface layer is mainly composed of ferrite by decarburization annealing. However, as shown in FIG. 2, when the surface of the steel sheet is decarburized, the formation of internal oxides becomes remarkable at the same time, and it becomes easy to obtain a steel sheet that is essentially non-uniformly deformed. A bendable steel sheet cannot be produced. In FIG. 2, the region where crystal grains are growing is ferrite based on decarburization, and the region where the grain boundary on the image is shown with low luminance is an internal oxide layer having an internal oxide. Further, even if close bending is possible by decarburization, a large number of cracks damage the galvanized film, and the corrosion resistance of the bent portion is remarkably deteriorated.

Japanese Patent Laid-Open No. 62-13533 JP-T-2006-517257 JP-A-5-195149 Japanese Patent Laid-Open No. 10-130782

Nisshin Steel Engineering Reports, No. 77 (1998), P1

  An object of the present invention is to provide a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent in bendability and corrosion resistance of a processed part, and a method for producing the same, which have been difficult to manufacture with the conventional technology as described above. And

  Here, “excellent bendability” means that the minimum bend radius of the 180 ° bend test is 2.0 t or less, and no crack appears on the surface after processing at the visual level. Therefore, unless otherwise specified, the bendability in this specification is evaluated by observing such physical properties and actual members. In order to apply the hot-dip galvanized steel sheet according to the present invention to parts having more complicated shapes such as members and pillars, which are typical examples of reinforcing members for automobiles, the minimum bending radius of a 180 ° bending test is used. Is 2.0 t or less, and cracks do not appear on the surface after processing at the visual level, and the (TS × El) value obtained by a tensile test is preferably 16000 MPa ·% or more. On the other hand, in order to further reduce the weight of these parts, the tensile strength is preferably 980 MPa or more.

  The present invention optimizes hot-dip galvanized steel sheets by reviewing the chemical composition and manufacturing conditions so that bending cracks caused by internal oxides and damage to the plating film can be suppressed, which was difficult with conventional technologies. Thus, as shown in the image shown on the left of FIG. 3, the growth of the decarburized layer can be remarkably accelerated with respect to the growth of the internal oxide layer, and a desired near-surface texture can be obtained. This is based on the knowledge that a hot-dip galvanized steel sheet having excellent bendability and corrosion resistance of the processed part can be obtained.

  Note that the image shown on the left side of FIG. 3 is a temperature range of 700 ° C. or higher and 780 ° C. or lower at 0.5 ° C./second, and the chemical composition is 0.19% C-1.02% Si-1.51% Mn−. 0.012% P-0.002% S-0.029% Al-0.0039% N (remainder Fe and impurities, the same shall apply hereinafter. In this specification, “%” relating to the composition is “mass” unless otherwise specified. It is an image showing a cross-sectional structure of an annealed plate corresponding to an example of the present invention in which a steel plate of) is heated. The steel shown in the left of FIG. 3 has a lower thickness of the internal oxide layer in the ferrite region formed by decarburization than the steel shown in FIG. 2, and decarburization has priority over the growth of internal oxide. It is understood that this occurred.

  3 is a temperature range of 700 ° C. or higher and 780 ° C. or lower at 4 ° C./s and a chemical composition of 0.18% C-0.02% Si-1.49% Mn-0. It is an image which shows the cross-sectional structure | tissue of the annealing board corresponded to the comparative example which heated the steel plate of 011% P-0.002% S-0.032% Al-0.0042% N.

The present invention relates to a hot dip galvanized steel sheet having a hot dip galvanized layer on the surface of the steel sheet and having a tensile strength of 590 MPa or more , and the steel sheet is, by mass, C: 0.03% or more and 0.20% or less, Si: 0. 0.005% or more and 2.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.001% or more and 1.0% or less, N: 0.01% or less, Bi: 0.05% or less, and having a chemical composition containing the balance Fe and impurities, this steel sheet has a plating layer and a mother The maximum depth of the internal oxide layer formed on the steel plate side from the interface with the phase: X (μm) and the average thickness of the region having a steel structure containing 80 area% or more of ferrite formed on the steel plate side from the same interface: Y ( μm) satisfies the following formulas (1) and (2).
Y / X ≧ 4 (1)
Y ≧ 5 (2)

The galvanized steel sheet according to these present invention, the chemical composition, by mass%, Ti: 0.5% or less, Nb: 0.5% or less and V: selected from the group consisting of 0.5% or less It is preferable to further contain one kind or two or more kinds.

  In these hot-dip galvanized steel sheets according to the present invention, the chemical composition was selected from the group consisting of Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less in mass%. It is preferable to further contain one type or two or more types.

  In these hot-dip galvanized steel sheets according to the present invention, the chemical composition is, by mass, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01%. It is preferable to further contain one or more selected from the group consisting of the following.

In these hot-dip galvanized steel sheets according to the present invention, it is preferable that the chemical composition further contains B: 0.01% or less in terms of mass%.
These hot-dip galvanized steel sheets according to the present invention preferably have a steel structure containing residual austenite of 3.0 to 15 area%.

From another point of view, the present invention is a method for manufacturing a hot-dip galvanized steel sheet according to the present invention described above , comprising the following steps.
The average heating rate of the cold-rolled steel sheet, the following temperature range 780 ° C. 700 ° C. or higher: A (° C. / sec) and Si content of the cold-rolled steel: Si (wt%) and the Bi content of cold-rolled steel sheet: Bi ( Mass%) satisfying the following formula (3) and the dew point in the temperature range of 700 ° C. or higher and 780 ° C. or lower is −30 ° C. or higher, followed by recrystallization annealing in a reducing atmosphere. The manufacturing method of the continuous hot-dip galvanized steel sheet which performs hot-dip galvanization after giving.
80 / A × (0.6Si + 2.1 × Bi ^0.25 +1) ≧ 25 (3)

  Furthermore, from another viewpoint, the present invention is characterized by subjecting the hot-dip galvanized steel sheet obtained by the production method according to the present invention to an alloying treatment in a temperature range of 430 ° C. to 600 ° C. It is a manufacturing method of a galvanized steel sheet.

  According to the present invention, a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability can be obtained. The hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

It is an image which shows the result of having observed the steel plate surface of a bending part, after carrying out adhesion bending about the hot dip galvanized steel plate equivalent to the invention concerning patent documents 2, and melt | dissolving the hot dip galvanized layer from the steel plate after bending with fuming nitric acid. . It is an image which shows the result of having observed the cross section obtained by grind | polishing the rolling direction cross section of the steel plate corresponded to the invention which concerns on patent document 4 by a well-known method, and etching with a well-known corrosive liquid. On the left is a temperature range of 700 ° C. or more and 780 ° C. or less at 0.5 ° C./second, and the chemical composition is 0.19% C-1.02% Si-1.51% Mn-0.012% P-0.002. It is an image which shows the annealing board cross-sectional structure | tissue corresponding to the example of this invention which heated the steel plate of% S-0.029% Al-0.0039% N, and the right is a temperature range of 700 degreeC or more and 780 degrees C or less at 4 degreeC / s and a steel composition having a chemical composition of 0.18% C-0.02% Si-1.49% Mn-0.011% P-0.002% S-0.032% Al-0.0042% N. It is an image which shows the annealing board cross-sectional structure | tissue equivalent to the heated comparative example.

Hereinafter, modes for carrying out the present invention will be described.
1. Chemical Composition First, the reason why the preferable chemical composition of the hot-dip galvanized steel sheet according to the present invention is specified as described above will be described.

(C: 0.03% to 0.20%)
C is an element contributing to strength improvement, and is contained in an amount of 0.03% or more in order to make the tensile strength of the steel plate 590 MPa or more. However, if the C content exceeds 0.20%, the weldability deteriorates. For this reason, C content shall be 0.03% or more and 0.20% or less. The C content is preferably 0.06% or more. By doing so, it becomes easy to make the tensile strength 980 MPa or more.

(Si: 0.005% to 2.0%)
Si is an element that contributes to improving the strength without deteriorating the ductility so much or improving the ductility. In the present invention, Si is contained in an amount of 0.005% or more. However, if the Si content exceeds 2.0%, the wettability of plating and the adhesion of plating deteriorate. For this reason, Si content shall be 0.005% or more and 2.0% or less. Here, Si is an element that promotes non-uniform deformation. However, as described later, by optimizing the annealing conditions, such adverse effects of Si are alleviated, and deterioration of bendability and corrosion resistance of processed parts is suppressed. Thus, an improvement in strength is achieved. Note that when 0.3% or more of Si is contained, the ductility is further improved by the TRIP effect. For this reason, it is preferable that Si content shall be 0.3% or more.

(Mn: 1.2% to 3.0%)
Mn is an element that contributes to strength improvement, and is contained in an amount of 1.2% or more in order to make the tensile strength of the steel plate 590 MPa or more. However, when Mn is contained exceeding 3.0%, not only the melting and refining of the steel in the converter becomes difficult, but also the weldability deteriorates. For this reason, Mn content shall be 1.2% or more and 3.0% or less. Here, Mn is an element that promotes non-uniform deformation. As will be described later, by optimizing the annealing conditions, such adverse effects of Mn are alleviated, and deterioration of bendability and corrosion resistance of processed parts is suppressed. Thus, an improvement in strength is achieved. In addition, in order to make tensile strength 980 Mpa or more, it is preferable to contain Mn 2.0% or more.

(P: 0.1% or less)
In general, P is an unavoidable impurity, but it is also a solid solution strengthening element and is effective for strengthening the steel sheet. Therefore, P may be actively contained. However, when the P content exceeds 0.1%, the weldability is significantly deteriorated. Therefore, the P content is 0.1% or less. In order to strengthen the steel sheet more reliably, the P content is preferably set to 0.005% or more.

(S: 0.01% or less)
S is an impurity inevitably contained in steel, and is preferably as low as possible from the viewpoints of bendability and weldability. For this reason, S content shall be 0.01% or less. Preferably it is 0.005% or less. More preferably, it is 0.003% or less.

(Sol.Al: 0.001% to 1.0%)
Al is an element added for deoxidizing steel, and is an element that effectively acts to improve the yield of carbonitride-forming elements such as Ti. The Al content is 0.001% or more. However, sol. If the Al content exceeds 1.0%, the weldability deteriorates and the oxide inclusions increase, so the surface properties deteriorate. For this reason, sol. Al content shall be 0.003% or more and 1.0% or less. In addition, Preferably it is 0.02% or more and 0.2% or less.

(N: 0.01% or less)
N is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of bendability. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

(Bi: 0.0001% to 0.05%)
Bi is an element that accelerates decarburization and suppresses deterioration of bendability even when Si and Mn are contained in a large amount, and is an optional element that can be contained as necessary. However, when Bi is contained exceeding 0.05%, hot workability deteriorates and hot rolling becomes difficult. For this reason, it is preferable that B content shall be 0.05% or less. In order to acquire the said effect more reliably, it is preferable to contain 0.0001% or more, and it is more preferable to contain 0.0005% or more.

(One or more selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less)
Ti, Nb, and V are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain one or more of Ti, Nb and V. However, if each content exceeds 0.5%, inclusions containing Ti, Nb, and V increase, and the surface properties deteriorate. For this reason, it is preferable that the content of Ti, Nb, and V is 0.5% or less, respectively. In order to obtain the above effect more reliably, it is preferable to contain 0.003% or more of any element of Ti, Nb, and V.

(Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less selected from the group consisting of 1% or less)
Cr, Mo, Cu, and Ni are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain one or more of Cr, Mo, Cu and Ni. However, even if Cr, Mo, Cu, and Ni are contained in excess of 1%, the above effect is saturated and not only economically wasteful, but the hot-rolled steel sheet becomes hard and cold rolling is performed. It becomes difficult to do. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Cr, Mo, Cu, and Ni by said quantity. In order to acquire the said effect more reliably, it is preferable to contain any element 0.01% or more.

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: one or more selected from the group consisting of 0.01% or less)
Ca, Mg, REM, and Zr are all elements that contribute to inclusion control, particularly fine dispersion of inclusions, and further improve bendability, and can be included as necessary. . However, since the surface properties are deteriorated if excessively contained, the content of each element is preferably 0.01% or less. In order to acquire the said effect more reliably, it is preferable to contain any element 0.001% or more.
Here, REM indicates a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(B: 0.01% or less)
B is an element that contributes to strength improvement, and is an optional element that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain B. However, if B exceeds 0.01%, the hot-rolled steel sheet becomes hard and it becomes difficult to perform cold rolling. For this reason, it is preferable that B content shall be 0.01% or less. In order to acquire the said effect more reliably, it is preferable to make it contain 0.0005% or more.

2. Next, the reason for limiting the steel structure of the steel sheet constituting the hot-dip galvanized steel sheet according to the present invention will be described.

The greatest novel feature in the steel structure of the hot dip galvanized steel sheet according to the present invention is that the average depth of the internal oxide layer formed on the steel sheet side from the interface between the hot dip galvanized layer and the steel sheet: X (μm) and the above interface The average thickness of the region having a steel structure containing 80% by area or more of ferrite formed on the steel sheet side: Y (μm) is a point satisfying the following formulas (1) and (2).
Y / X ≧ 4 (1)
Y ≧ 5 (2)

  When Y / X is less than 4 or Y is less than 5, non-uniform deformation is promoted, the tensile strength of the steel sheet is increased to 590 MPa or more, and the bendability and the corrosion resistance of the processed part are improved. It becomes impossible. When Y / X is 4 or more and Y is 5 or more, it becomes possible to achieve a tensile strength of 590 MPa or more while suppressing deterioration of bendability and corrosion resistance of the processed part.

  Furthermore, in order to obtain excellent ductility with a (TS × El) value of 16000 MPa ·% or more, the steel structure of the steel sheet is a fraction evaluated by area ratio, and the ratio of retained austenite is 3.0% or more and 15% or less. It is preferable that Residual austenite contributes to strength improvement without reducing ductility due to the TRIP effect. However, when the area ratio of retained austenite becomes an excessive steel structure, the amount of martensite generated by TRIP increases, and microcracks tend to occur at the structure interface, and the stretch flangeability deteriorates. For this reason, it is preferable that the area ratio of a retained austenite shall be 3.0% or more and 15% or less.

3. Manufacturing method Subsequently, the suitable manufacturing method of the hot dip galvanized steel plate concerning this invention is demonstrated.
It is preferable that the molten steel having the above steel composition is melted by a generally known melting method such as a converter or an electric furnace, and is made into a steel material such as a slab by a continuous casting method. In place of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be employed.

This steel material is hot-rolled to obtain a hot-rolled steel sheet, and this hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
In hot rolling, cast steel material is not cooled to room temperature but directly fed into a heating furnace while being heated and heated and then rolled, or directly after a little heat retention Rolling may be performed, or the steel material may be once cooled and then reheated for rolling. At this time, it is preferable to equalize the entire length of the rough bar after the rough rolling and before the finish rolling by induction heating or the like because the characteristic variation can be suppressed.

  Preferably, the slab obtained by the above continuous casting process is subjected to a homogenization treatment for 20 minutes or more in a temperature range of 1200 ° C. to 1350 ° C., and then a finishing temperature: 800 ° C. to 950 ° C., a coiling temperature : Hot rolling at 400 ° C. or higher and 750 ° C. or lower to obtain a hot rolled steel sheet, and cold rolling to the hot rolled steel sheet to obtain a cold rolled steel sheet.

(Homogenization treatment temperature: 1200 ° C to 1350 ° C, homogenization treatment time: 20 minutes to 3 hours)
The slab to be subjected to hot rolling is preferably subjected to a homogenization treatment in which the slab is kept in a temperature range of 1200 ° C. or higher and 1350 ° C. or lower for 20 minutes or longer. By holding the slab used for hot rolling in a temperature range of 1200 ° C. or higher for 20 minutes or more, the non-uniform structure due to segregation of Mn is further eliminated, and the bendability can be further improved. The homogenization temperature is preferably 1350 ° C. or lower from the viewpoints of suppressing scale loss, preventing heating furnace damage, and improving productivity. Moreover, it is also preferable that the homogenization treatment time is 3 hours or less from the viewpoint of suppressing scale loss and improving productivity.

(Finish temperature: 800 ° C or higher and 950 ° C or lower)
The finishing temperature is preferably 800 ° C. or higher and 950 ° C. or lower. By setting the finishing temperature to 800 ° C. or higher, deformation resistance during hot rolling is reduced, and operation can be performed more easily. In addition, by setting the finishing temperature to 950 ° C. or less, wrinkles due to scale can be more reliably suppressed, and good surface properties can be ensured.

(Winding temperature: 400 ° C or higher and 750 ° C or lower)
The winding temperature is preferably 400 ° C. or higher and 750 ° C. or lower. By setting the coiling temperature to 400 ° C. or higher, generation of hard bainite and martensite is suppressed, and subsequent cold rolling becomes easy. Moreover, by setting the coiling temperature to 750 ° C. or lower, oxidation of the steel sheet surface is suppressed, and good surface properties can be ensured.

In the hot rolling process, it is preferable to equalize the temperature of the entire length of the rough bar by induction heating or the like with respect to the rough bar after the rough rolling and before the finish rolling, because the characteristic variation can be suppressed.
In order to make the structure of the steel sheet after continuous annealing uniform, it is preferable that the rolling reduction of cold rolling is 30% or more. Moreover, it is preferable from the viewpoint of ensuring flatness to correct the shape by performing mild rolling with a rolling reduction of 5% or less before or after pickling. In addition, when such mild rolling is performed before pickling, pickling performance is improved, removal of surface concentrating elements is promoted, and surface properties can be improved.

  In the cold rolled steel sheet obtained by the hot rolling process and the cold rolling process, the average heating rate in the temperature range of 700 ° C. or higher and 780 ° C. or lower: A (° C./second), Si content: Si (mass%) and Bi content: Bi is heated under conditions where Bi (% by mass) satisfies the following formula (3) and the dew point in the temperature range of 700 ° C. or higher and 780 ° C. or lower is −30 ° C. or higher. Is subjected to recrystallization annealing, followed by hot dip galvanizing. When the alloying treatment is performed, it is performed in a temperature range of 430 ° C. or more and 600 ° C. or less.

(Relationship between the average heating rate in the temperature range of 700 ° C. or higher and 780 ° C. or lower, the Si content and the Bi content: within the range of the formula (3) and the dew point in the temperature range of 700 ° C. or higher and 780 ° C. or lower: −30 ° C. or higher )
Average heating rate in the temperature range of 700 ° C. or more and 780 ° C. or less: A (° C./second), Si content: Si (mass%) and Bi content: Bi (mass%) satisfy the following formula (3), And it is preferable that the dew point of the temperature range of 700 degreeC or more and 780 degrees C or less shall be -30 degreeC or more.
80 / A × (0.6Si + 2.1 × Bi ^0.25 +1) ≧ 25 (3)

  When A, Si, and Bi satisfy the range of the following formula (3), the growth of the decarburized layer can be remarkably promoted than the growth of the internal oxide layer, and the bendability can be improved. Furthermore, the wettability of plating can be improved by setting the dew point in the temperature range of 700 ° C. or higher and 780 ° C. or lower to −30 ° C. or higher. In order to give a good appearance to the hot-dip galvanized steel sheet, the hydrogen concentration during annealing is preferably set to 2% by volume or more and 15% by volume or less.

(Recrystallization annealing temperature: 800 ° C or higher and 950 ° C or lower)
The annealing temperature is preferably 800 ° C. or higher and 950 ° C. or lower. By setting the annealing temperature to 800 ° C. or higher, the remaining of unrecrystallized is suppressed, a uniform structure can be obtained reliably, and the bendability can be further improved. Moreover, by setting the annealing temperature to 950 ° C. or lower, it is possible to suppress damage to the annealing furnace and improve productivity.

  In order to completely remove unrecrystallized crystals and to ensure good bendability stably, the annealing time is preferably set to 10 seconds or more. Further, from the viewpoint of productivity, it is preferable that the annealing time is within 300 seconds.

  After recrystallization annealing, cooling is performed in the process of immersing in a galvanizing bath. In this case, the average cooling rate is from 1 ° C./s to 50 ° C./s from the highest temperature to 700 ° C. Subsequently, 700 The temperature from 0 ° C. to the cooling stop temperature is preferably 3 ° C./s or more and 50 ° C./s or less. By cooling to 700 ° C. at 1 ° C./s or more and 50 ° C./s or less, ferrite is generated, and a desired austenite fraction can be easily obtained. On the other hand, the strength reduction can be suppressed by cooling from 700 ° C. to the cooling stop temperature at 3 ° C./s or more. Further, in order to cool the cooling stop temperature to more than 50 ° C./s, it is necessary to remodel the continuous hot dip galvanizing equipment, and the manufacturing cost is increased.

  The cooling stop temperature is preferably in a temperature range of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less. When the cooling stop temperature is less than [zinc plating bath temperature −20 ° C.], heat removal at the time of entering the plating bath is large, and operation becomes difficult. On the other hand, if the cooling stop temperature is higher than [zinc plating bath temperature + 100 ° C.], the operation becomes difficult as the temperature of the plating bath increases. Hot dip galvanization is performed by immersing the annealed steel sheet in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower according to a conventional method.

(Alloying temperature: 430 ° C or higher and 600 ° C or lower)
When the alloying treatment is performed, it is performed in a temperature range of 430 ° C. or more and 600 ° C. or less after immersion in the plating bath. When the alloying treatment temperature is less than 430 ° C., unevenness of the unalloyed part occurs, and the surface properties of the steel sheet deteriorate. On the other hand, when the alloying treatment temperature exceeds 600 ° C., most of the hard second phase is tempered and a desired tensile strength cannot be obtained. When the alloying temperature exceeds 580 ° C., the decomposition of austenite is promoted, and a desired austenite fraction cannot be obtained. Furthermore, when the alloying treatment temperature is 500 ° C. or more and 530 ° C. or less and the alloying treatment time is 10 seconds or more and 60 seconds or less, the degree of alloying (Fe content of the plating layer) is 8 mass% or more and 13 mass%. Since it becomes easy to improve the adhesiveness of plating as below, it is preferable.

  After the continuous hot dip galvanizing treatment, it is preferable to perform temper rolling with an elongation of 0.05% or more and 1% or less. Temper rolling can suppress the occurrence of yield point elongation and can prevent seizure and galling during pressing.

In the hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet by the above-described manufacturing method, the average depth of the internal oxide layer formed on the steel sheet side from the interface between the hot-dip galvanized layer and the steel sheet: X (μm) And the average thickness of the region having a steel structure containing 80% by area or more of ferrite formed on the steel sheet side from the interface: Y (μm) satisfies the following formulas (1) and (2): A hot-dip galvanized steel sheet can be manufactured easily.
Y / X ≧ 4 (1)
Y ≧ 5 (2)

  Thus, the present invention provides a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability, which has been difficult to manufacture by conventional techniques, and a method for manufacturing the same.

Furthermore, the present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted in a converter to produce a slab having a thickness of 245 mm. Furthermore, hot rolling was performed under the conditions shown in Table 2, and then pickling was performed, and further cold rolling was performed under the conditions shown in Table 2 to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm.

  A test material for plating was collected from the obtained cold-rolled steel sheet, annealed and cooled under the conditions shown in Table 3, dipped in a molten zinc bath, and plated. Some samples were subsequently alloyed at the temperature in Table 3 and held at this temperature for 10 seconds. The appearance of the plated steel sheet thus obtained was visually observed, and the plating property was evaluated from the viewpoint of whether or not there is a portion where plating is repelled and unplated. As the evaluation criteria, those with non-plated parts were judged as defective and those without were judged good.

A test steel sheet for evaluation other than the evaluation of plating property was obtained by the following method.
A test material for heat treatment was sampled from the cold-rolled steel sheet obtained by the above cold rolling, and annealed corresponding to the heat pattern in the continuous hot dip galvanizing equipment shown in Table 3. In the heat treatment after holding at the cooling stop temperature, the assumed plating bath temperature is set to 460 ° C., cooling to this temperature over 5 seconds, holding at this temperature for 10 seconds, and then cooling to room temperature at a cooling rate of 10 ° C./s. did. In order to simulate the alloying treatment, after maintaining the above-mentioned assumed plating bath temperature, the temperature is further raised to the alloying treatment temperature over 5 seconds, kept at this temperature for 10 seconds, and then a cooling rate of 10 ° C./s. At room temperature.

  For the annealed cold-rolled steel sheets obtained under various production conditions, the maximum depth of the internal oxide layer and the average thickness of the region having a steel structure containing 80% by area or more of ferrite were evaluated by structure observation. Since the steel structure is affected by the steel composition and the heat treatment up to the plating immersion, the same steel structure as when the plating process is performed can be obtained without performing the plating process. Therefore, the maximum depth of the internal oxide layer formed on the steel plate side from the interface between the plating layer and the parent phase is 80% by area or more of the maximum depth of the internal oxide layer from the steel plate surface and ferrite formed on the steel plate side from the same interface. The average thickness of the region having the steel structure to be contained is the average thickness of the region having the steel structure containing 80% by area or more of ferrite from the steel sheet surface.

The obtained annealed cold-rolled steel sheet (hereinafter abbreviated as “annealed sheet”) was subjected to a tensile test and a bending test in which the bending ridge line is in the rolling direction to evaluate mechanical properties.
(Maximum depth of internal oxide layer)
Test pieces were taken from the rolling direction of various annealed plates, and the internal oxides were confirmed by EPMA. Specifically, an O (oxygen) mapping image was measured for one region of the cross section with EPMA, and the same region in the cross section was observed with an optical microscope or an electron microscope to obtain an image. By comparing these images, the structure in which the oxide was formed at the grain boundary was specified, and the depth of the internal oxide layer, which is a region by the specified structure, was measured by an image using an optical microscope or an electron microscope. This operation was performed in the range of the visual field length of 200 μm, and the maximum depth of the internal oxide layer in the range was measured.

(Average thickness of a region having a steel structure containing 80% by area or more of ferrite)
Test pieces were taken from the rolling direction of various annealed plates, and the structure of the cross section in the rolling direction was photographed with an optical microscope or an electron microscope. The area ratio of ferrite was measured by image analysis, and the average thickness of a region having a steel structure containing 80 area% or more of the area ratio was measured.

(Measurement of area ratio of retained austenite)
Various annealed plates are subjected to chemical polishing to reduce the thickness by (1/4), X-ray diffraction is applied to the surface after chemical polishing, the profile obtained is analyzed, and the area ratio of residual austenite is determined. It was measured.

(Tensile test)
From various annealed plates, JIS No. 5 tensile test specimens were collected from the direction perpendicular to the rolling direction, and the tensile strength (TS) and total elongation (El) were measured.

(Bending test)
Bending test pieces (width 40 mm × length 100 mm × sheet thickness 1.2 mm) having a longitudinal direction in the direction perpendicular to the rolling direction were collected from various annealed plates so that the bending ridge line was in the rolling direction. A 180 ° bending test with a 4.8 mm steel plate sandwiched was performed, and the presence or absence of cracks was confirmed visually. A 180 ° bending test was performed on a test piece having no cracks by sandwiching a steel plate having a thickness of 3.6 mm thinner than the previous one by 1.2 mm, and the presence or absence of cracks was similarly confirmed. Furthermore, a 180 ° bending test was performed on the test piece without cracks by sandwiching a 2.4 mm steel plate thinner by 1.2 mm than the previous time, and the presence or absence of cracks was similarly confirmed. Furthermore, a 180 ° bending test was performed on the test piece without cracks by sandwiching a 1.2 mm steel plate thinner by 1.2 mm than the previous time, and the presence or absence of cracks was similarly confirmed. Furthermore, when there was no crack, the adhesive bending which does not pinch | interpose a steel plate was performed, and the presence or absence of the crack was confirmed similarly.

  The minimum bend radius standardized by the plate thickness (t) by dividing the plate thickness of the steel plate that is not cracked after the test by twice the plate thickness of the bent specimen (2.4 mm) (indicated as Rmin in Table 4) Was calculated.

The results of the above evaluation are shown in Table 4.
In addition, the numerical value underlined in Tables 1-4 has shown that content, conditions, or a mechanical characteristic shown by the numerical value is outside the range of this invention.

Sample No. in Table 4 3 to 4 , 9 , 15, 18 to 20, 22, 25 , 27 to 30 , 33, and 35 to 37 all satisfy the above formulas (1) and (2), have a tensile strength of 590 MPa or more, and are bent. It was excellent in nature.

On the other hand, specimen No. No. 1 had poor plating properties because the Si content exceeded the upper limit.
Specimen No. 5, 8, 11 and 26 were out of the range of the above formula (3) and did not satisfy the above formula (1) or (2), so the bendability was poor.

  Specimen No. No. 12 is lower than the lower limit of the range specified by the present invention for the C content. No. 34 had a Mn content below the lower limit of the range defined in the present invention, so the desired tensile strength could not be obtained. The test material No. No. 12 had an area ratio of ferrite of 80% or more over the entire observation region.

Specimen No. No. 16 was unable to obtain the desired tensile strength because the alloying temperature exceeded the upper limit of the range defined in the present invention.
Specimen No. No. 32 had poor plating properties because the dew point exceeded the upper limit of the range defined in the present invention.

Among the steel sheets of the present invention examples, the specimen No. 1 in which the Si content is in the above-described preferable range and the area ratio of retained austenite is 3% or more . 9 , 18 to 20, 22, 25 , 28 to 30 , 33, and 35 to 37 have a tensile strength of 590 MPa or more, a TS × El value of 16000 MPa ·% or more, excellent ductility, and excellent bendability. The preferred steel plate.

Furthermore, test pieces 2 5, 27 and 29-30 of the C content and the Mn amount is in the preferred range, the preferred steel sheet tensile strength is very good over the bendability 980 MPa.

Claims (8)

In a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet and having a tensile strength of 590 MPa or more , the steel sheet is, by mass, C: 0.03% to 0.20%, Si: 0.005% or more 2.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.001% or more and 1.0% or less, N: 0.01% or less, Bi: 0.05% or less, having a chemical composition containing the remaining Fe and impurities, the hot-dip galvanized layer and the steel plate The maximum thickness of the internal oxide layer formed on the steel plate side from the interface with X: μm (μm) and the average thickness of the region having a steel structure containing 80 area% or more of ferrite formed on the steel plate side from the interface : Y (μm) satisfies the following formulas (1) and (2).
Y / X ≧ 4 (1)
Y ≧ 5 (2) The chemical composition further contains, in mass%, one or more selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less. The hot dip galvanized steel sheet according to claim 1 . The chemical composition further contains, in mass%, one or more selected from the group consisting of Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less, The hot-dip galvanized steel sheet according to claim 1 or 2 . The chemical composition is, in mass%, one selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less The hot-dip galvanized steel sheet according to any one of claims 1 to 3 , further comprising two or more kinds. The hot-dip galvanized steel sheet according to any one of claims 2 to 4 , wherein the chemical composition further contains B: 0.01% or less in terms of mass%. The hot-dip galvanized steel sheet according to any one of claims 1 to 5 , wherein the steel sheet has a steel structure containing 3.0 to 15 area% residual austenite. The average heating rate of the cold-rolled steel sheet, the following temperature range 780 ° C. 700 ° C. or higher: A (° C. / sec) and Si content of the cold-rolled steel sheet: Si (wt%) and the Bi content of the cold-rolled steel sheet: Heat is applied under the condition that Bi (mass%) satisfies the following formula (3) and the dew point in the temperature range of 700 ° C. or higher and 780 ° C. or lower is −30 ° C. or higher, and then recrystallization annealing in a reducing atmosphere The method for producing a hot dip galvanized steel sheet according to any one of claims 1 to 6 , wherein hot dip galvanizing is performed thereafter.
80 / A × (0.6Si + 2.1 × Bi ^0.25 +1) ≧ 25 (3) A method for producing a hot-dip galvanized steel sheet, comprising subjecting the hot-dip galvanized steel sheet obtained by the production method according to claim 7 to an alloying treatment in a temperature range of 430 ° C to 600 ° C.