JP5228722B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet and a method for producing the same, and more specifically, to a high-strength alloyed hot-dip galvanized steel sheet having excellent plating adhesion and a method for producing the same. The present invention relates to a high-strength galvannealed steel sheet excellent in plating adhesiveness suitable for press forming, of which, in particular, bending forming, which has been difficult in the past, is indispensable, and a method for producing them.

  In recent years, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment. In order to reduce the weight of the vehicle body and ensure the safety of passengers, high-strength steel sheets having a tensile strength of 540 MPa or more, particularly considering rust prevention In materials, there is an increasing need for high-strength galvannealed 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 various performances required at the time of component molding, such as press formability and weldability. In particular, considering the part forming process, bending molding is used most frequently, and since it is formed into parts of various shapes depending on the combination thereof, a high-strength galvannealed steel sheet having excellent bendability is required.

  In order to improve the bendability of a high-strength steel sheet, conventionally, an approach of controlling the steel structure has been adopted. For example, in Patent Document 1, the hardness of the low-temperature transformation generation phase is reduced to reduce the hardness difference from the ferrite phase. Is disclosed. On the other hand, Patent Document 2 and Patent Document 3 disclose that by making the ferrite crystal grains ultrafine, it is possible to achieve both stretch flangeability and high strength that require local deformability as well as bendability. .

  However, in a high-strength steel sheet containing a large amount of Mn for increasing the strength, as shown in FIG. 1, the fluctuation of Mn concentration strongly affects the local chemical composition, particularly the formation of low-temperature transformation phase, due to solidification segregation. And a heterogeneous structure corresponding to the fluctuation is formed. For this reason, with the technique disclosed in Patent Document 1, it is extremely difficult to precisely control the hardness of the ferrite phase and the low-temperature transformation phase in the entire steel sheet, as well as dealing with local chemical composition fluctuations. As shown in FIG. 2, due to the uneven structure, remarkable unevenness that can be visually observed appears on the surface of the processed part. This unevenness further promotes the uneven deformation and induces cracking, and the bendability itself is improved. Deteriorate. Moreover, even if it does not lead to a crack, the unevenness present in the processed part not only causes a poor appearance but also deteriorates the collision performance as a part.

  In addition, the transformation phenomenon locally changes due to solidification segregation and the crystal grain size becomes non-uniform, so the techniques disclosed in Patent Document 2 and Patent Document 3 cannot improve the bendability. In particular, the techniques described in these documents contain a large amount of Mn and Ni that are prone to solidification and segregation in the steel, so that not only the bendability at the time of molding as described above, but also the impact performance as a part deteriorates. There is concern.

  Moreover, in order to improve the bendability of a high-strength steel sheet, there is an ultimate approach of a single-phase structure from the viewpoint of homogenizing the structure. Patent Document 4 discloses a martensite single-phase structure that is the ultimate uniform structure. It is disclosed that bendability is also improved. However, if the steel structure is a martensite single-phase structure as in the technique disclosed in Patent Document 4, the flatness of the steel sheet is impaired, so that it cannot be applied as an automobile part that also requires dimensional accuracy.

  Therefore, in order to achieve both excellent bendability and high strength, it is necessary to satisfy both seeming contradictions that a uniform structure can be obtained even if a large amount of Mn is contained for high strength.

  There is also an approach to eliminate solidification segregation itself, which is the origin of a heterogeneous structure, by diffusion. Patent Document 5 discloses that a steel material is homogenized by reducing solidification segregation by performing a solute treatment for holding the steel material at a high temperature of 1250 ° C. or higher for a long time of 10 hours or longer. However, the process of holding at a high temperature for a long time brings about a significant increase in manufacturing cost and a decrease in productivity, and is not a realistic process.

  On the other hand, the alloyed hot-dip galvanized steel sheet itself has the following problems. In particular, alloyed hot-dip galvanized steel sheets that use high-strength steel sheets as the base material rapidly increase the surface pressure applied to the coating layer of the coating during press forming. The press factor has a greater effect on the film peeling behavior than the galvanized steel sheet. Therefore, an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet as a base material requires measures against coating damage during press forming, that is, improvement in plating adhesion.

The alloyed hot-dip galvanized steel sheet is usually produced as follows. The steel sheet is heated in a preheating furnace before hot dipping, annealed in a reducing atmosphere of H ₂ + N ₂ with a dew point adjusted to −20 ° C. or lower so as not to cause non-plating, then cooled to around the plating bath temperature, and then Is hot dip galvanized. Then, an alloyed hot-dip galvanized steel sheet is manufactured by heating the hot-dip galvanized steel sheet at a material temperature of 480 to 600 ° C. for 3 to 30 seconds to form a Fe—Zn alloy plating phase in a heat treatment furnace. To do.

However, when the alloyed hot-dip galvanized steel sheet is pressed, if the plating surface layer has a soft alloy phase (ζ phase) with a relatively low Fe content, the adhesion phenomenon between the plating surface layer and the mold surface may occur. Since the slidability between the mold surface and the steel plate is inferior, plating peeling (flaking) or press cracking of the steel plate may occur. On the other hand, when the Fe content in the plating layer is high, a hard Γ, Γ ₁ , δ _1c phase is formed in the vicinity of the interface between the steel plate and the plating layer, so the alloyed hot-dip galvanized steel plate is pressed. In this case, powdering (powdering) of the plating layer is likely to occur. When this phenomenon occurs, a peeling piece adheres to the mold, and a pressing flaw occurs.

In order to solve such problems, for alloyed hot-dip galvanized steel sheets based on mild steel sheets, the coating layer of the coating may be made of a δ ₁ phase-based alloy phase with a relatively balanced hardness. Proposed.

For example, Patent Document 6 proposes an alloyed hot-dip galvanized steel sheet having a weight per unit area of 45 to 90 g / m ² / one side and excellent in powdering resistance and flaking resistance. Here, the Fe content in the plating layer is controlled to 8 to 12% by mass, and the Al content is controlled to 0.05 to 0.25% by mass so that the η and ζ phases do not exist in the plating layer of the coating. The Γ phase of the alloy layer at the interface between the base material and the plating layer is 1.0 μm or less.

  Patent Document 7 discloses that the Al concentration in the plating bath is set to 0 with respect to the method for producing an alloyed hot-dip galvanized steel sheet in which the alloying treatment is performed so that the Fe content in the coating layer of the coating is 8 to 12%. .Control the temperature of the steel plate, which is the base material, to increase as the Al concentration in the bath increases, and manage the temperature on the high-frequency induction heating furnace exit side within an appropriate range. Has proposed to produce an alloyed hot-dip galvanized steel sheet excellent in powdering resistance and flaking resistance.

  However, the alloyed hot-dip galvanized steel sheet described in Patent Document 6 and the alloyed hot-dip galvanized steel sheet obtained by the manufacturing method described in Patent Document 7 are alloyed hot-dip galvanized steel sheets using mild steel plates as base materials. However, the effect of improving the powdering resistance at the time of press forming is not limited even when applied to an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet as a base material. It turned out that it was hardly recognized. In addition, in order to increase the adhesion at the interface between the base material and the plating layer, in the case of an ultra-low carbon steel plate, by increasing the Al concentration in the plating bath, alloying within the grain and grain boundaries of the base steel plate The method of increasing the unevenness at the interface between the steel sheet and the galvannealed alloy layer can be adopted by increasing the speed difference. In addition, a similar technique should be adopted for a high-tensile steel plate based on ultra-low carbon steel. However, in the case of a steel plate with a high C content such that the tensile strength is 540 MPa or more like the steel of the present invention, even if the Al concentration in the plating bath is increased by the same method, the steel plate and the alloyed hot dip galvanized layer It was found that the adhesion strength of the steel was lower than that of the low-strength steel plate.

  Furthermore, in consideration of the part forming process, it is preferably a high-strength galvannealed steel sheet or a high-strength galvannealed steel sheet that can handle stretch flange forming. In stretch flange molding, it is not possible to avoid molding defects associated with the adjustment of the mold alone. Therefore, it is preferable to have excellent stretch flangeability at a mild steel level while being high strength, specifically, excellent hole expandability.

  In general, high-strength steel sheets use ferrite as the parent phase and use a hard phase such as martensite or bainite to increase the strength, so microvoids that start ductile fracture are generated at the interface between the ferrite and hard phase. It is easy and the hole expandability is insufficient. Therefore, many researches and developments have been made to improve the hole expandability of high-strength steel sheets, and a structure control technique is being established.

  For example, Non-Patent Document 1 discloses that in the composite structure of ferrite and martensite, the hole expandability is improved as the strength difference between ferrite and martensite which is a hard phase becomes smaller. Non-Patent Document 2 discloses a ferrite single-phase steel having excellent hole expansibility without using a hard phase itself that causes a strength difference.

  However, most of these findings do not consider the manufacturing process of the galvannealed steel sheet. Non-Patent Document 1 relates to a cold-rolled steel sheet by quenching and tempering process, and Non-Patent Document 2 relates to a hot-rolled steel sheet that can make maximum use of precipitation strengthening, both of which relate to an alloyed hot-dip galvanized steel sheet. is not.

  For example, the manufacturing process of the alloyed hot dip galvanized steel sheet is characterized by being immersed in a hot dip galvanizing bath at 400 ° C. or higher when cooled from the recrystallization temperature, and then reheated after the immersion for the purpose of alloying. It is in the temperature history. In this manufacturing process, since cooling is temporarily interrupted at 400 ° C. or higher, when manufacturing a high-strength steel sheet, the bainite transformation essentially proceeds easily.

  When bainite is generated, not only coarse cementite but also C is concentrated in austenite, so that a heterogeneous structure including island martensite and massive austenite is easily obtained. The tendency becomes remarkable in the manufacturing process of the galvannealed steel sheet in which cooling is interrupted at a higher temperature. Coarse cementite, island-like martensite, and massive austenite are all hard phases with very high strength, insufficient ductility, and promote non-uniform deformation, so holes in high-strength galvannealed steel sheets It was difficult to improve the spreadability.

  Patent Document 8 discloses a technique for generating a structure mainly composed of bainite or bainitic ferrite by positively adding Nb or Mo to a steel sheet so as to achieve both high strength and excellent hole expansibility. . However, when the structure is mainly composed of bainite or bainitic ferrite, it becomes difficult to work harden, and molding defects due to constriction tend to occur.

  In Patent Document 9, by actively adding Mn and B and appropriately controlling the amount of B, it is possible to suppress the generation of microvoids, while being a composite structure containing ferrite and hard martensite, and high strength. A steel sheet having high ductility and excellent hole expansibility is disclosed. However, control of the amount of B in steel is extremely difficult, and when adding B in the composite structure, the tensile strength changes significantly depending on the amount of B, so that not only molding defects are likely to occur sporadically through the mass production process, Since Mn is actively added, there is a concern that the bendability becomes insufficient.

In Patent Document 10, Mn and Ti are positively added to a steel sheet, and not only the amount of Mn and Ti but also the amount of C is appropriately controlled, thereby providing high strength, high ductility, and excellent hole expandability. A combined steel sheet is disclosed. However, in a composite structure that actively uses a transformed structure, the structure changes significantly due to annealing conditions and cooling conditions, so that not only the tensile strength but also the hole expandability changes remarkably depending on the manufacturing conditions. For this reason, it is not only difficult to stably produce a steel plate having high strength, high ductility and excellent hole expandability through a mass production process, that is, to provide a steel plate with excellent material stability. This invention is also concerned that the bendability becomes insufficient due to the positive addition of Mn.
Japanese Patent Laid-Open No. 62-13533 Japanese Patent Laid-Open No. 2004-211126 JP 2004-250774 A JP 2002-161336 A JP-A-4-191322 JP-A-1-68456 JP-A-4-276053 JP 2003-193190 A JP 2004-211140 A JP 2005-220417 A ISIJ Int. , Vol. 44 (2004), no. 3, p. 603-609 ISIJ Int. , Vol. 44 (2004), no. 11, p. 1945-1951

  It is an object of the present invention to provide an automotive member, particularly an automotive reinforcing member considering rust prevention, but has been difficult to produce in the past, has a tensile strength of 540 MPa or more, and has good plating adhesion. It is an object to provide a high-strength galvannealed steel sheet excellent in improved bendability and a method for producing them.

  In this specification, “excellent bendability” means that close contact bending is possible in the U-bend test with a tip angle of 180 °, and no irregularities appear on the surface of the bending ridge line after processing by visual observation. To do. Therefore, unless otherwise specified, the bendability in this specification is evaluated by observing such physical properties and actual members.

In addition, in order to apply the steel plate of the present invention to a part having a higher strength and a complicated shape, such as a front side member, which is a typical example of a reinforcing member for an automobile that requires relatively mild stretch flange molding, While achieving the desired bendability, the target value of hole expandability (hole expansion rate <HER> described later) is 50% or more, and the target value of ductility is TS (tensile strength) x El value obtained by a tensile test. The (total elongation) is preferably 12000 MPa ·% or more, and the target value of the material stability is preferably such that the upper and lower limit range (ΔTS) of the tensile strength is 100 MPa or less.
Furthermore, in order to apply the steel plate of the present invention to a part having a higher strength and a very complicated shape, such as a cross member, which is a typical example of a reinforcing member for automobiles, including adjustment of a shape and a mold, it is desirable. It is preferable that the tensile strength is 590 MPa or more and the hole expansion ratio is 80% or more while achieving the bendability, ductility, and material stability. In order to manufacture a cross member having a higher strength and a complicated shape under a highly flexible design without relying on the adjustment of the shape and mold, the tensile strength is 590 MPa or more, and the hole expandability is 80 More preferably, the TS × El value is 14000 MPa ·% or more and ΔTS is 60 MPa or less.

  As a result of studies by the present inventors to solve the above-mentioned problems, the bendability is improved by suppressing the formation of a heterogeneous structure due to solidification segregation, and in particular, a minute amount generated after bending forming that cannot be solved conventionally. By optimizing the chemical composition so as to suppress even irregularities and applying the optimum manufacturing conditions therefor, the structure has a uniform and fine ferrite with an average particle diameter of 1.0 to 6.0 μm Thus, it was found that an alloyed hot-dip galvanized steel sheet having excellent bendability can be provided without lowering the strength level.

  However, in the case of a steel sheet having a fine structure as described above, the grain interface area on the steel sheet surface is extremely increased. For this reason, the conventional technique for improving plating adhesion, which increases the difference in alloying speed between grain boundaries and grains by increasing the Al concentration in the plating bath, is applied to high-strength steel sheets with a grain size of 6 μm or less. It turned out not to be possible.

  As a result of studying a method for delaying the intragranular alloying rate, the present inventors have found that it is extremely effective to contain 0.02 to 0.50% Si. In the case of a conventional structure having a grain size of more than 6.0 μm, if Si is added at the above content, the intragranular area having a slow alloying rate increases too much, so that it takes a long time for alloying, and Non-plating also occurs. For this reason, it has been difficult to produce an alloyed hot-dip galvanized steel sheet in a continuous hot-dip galvanizing line using a steel sheet containing Si at the above-described content as an original plate. However, as in the present invention, when 0.02 to 0.50% of Si is contained in a fine high-strength steel sheet having a grain size of 6.0 μm or less, the alloying speed is high and the wettability is also good. Completed based on the knowledge that the grain boundary part and the inside of the grain with moderately controlled alloying speed can be properly balanced, and the adhesion between the steel sheet and the alloyed hot dip galvanizing can be improved. It is.

  The present invention further provides a balanced chemical composition that suppresses the formation of coarse cementite, island martensite, and even massive austenite, which degrades hole expansibility, and applies optimum manufacturing conditions to the bending. It has been completed based on the knowledge that it can provide a high-strength galvannealed steel sheet that has excellent properties, further hole expandability, ductility, and material stability. It has been difficult to provide a steel sheet having these characteristics at the same time even by a conventional manufacturing process of a galvannealed steel sheet.

The present invention is an alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, the steel sheet being in mass%, C: 0.03 to 0.12%, Si: 0.02 -0.50%, Mn: 2.0-4.0%, P: 0.1% or less, S: 0.01% or less, sol. One or two of Al: 0.01 to 1.0% and N: 0.01% or less, and Ti: 0.50% or less and Nb: 0.50% or less are represented by the following formula (1 ), The balance is a chemical composition consisting of Fe and impurities, the area ratio of ferrite is 60% or more, and the average grain size of ferrite is 1.0 to 6.0 μm. The alloyed hot-dip galvanized layer contains, by mass%, Fe: 8 to 15% and Al: 0.08 to 0.50%, and the balance is made of Zn and impurities, and the alloyed molten The galvanized steel sheet has a tensile strength of 540 MPa or more, can be tightly bent in a U-bend test with a tip angle of 180 °, and has no unevenness on the surface of the bent ridge line after processing by visual observation. Alloyed molten zinc Steel plate.
Ti + Nb / 2 ≧ 0.03 (1)

  The alloyed hot-dip galvanized steel sheet according to the present invention has a chemical composition of mass% instead of a part of Fe, and further Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01 % Or less and Zr: It is preferable to contain one or more selected from the group consisting of 0.01% or less.

  In the galvannealed steel sheet according to the present invention, the area ratio of retained austenite in the steel structure is preferably 3.0% or less. In this case, the hole expansion rate measured by the method prescribed in JFST1001 is preferably 50% or more, and the value of tensile strength × elongation is more preferably 12000 MPa ·% or more.

From another viewpoint, the present invention is a method for producing an alloyed hot-dip galvanized steel sheet having excellent bendability, comprising the following steps (A) to (C) .
(A) A casting process in which the molten steel having the above chemical composition is cooled to satisfy the following formula (2) to form a steel ingot;
(B) The steel ingot is subjected to hot rolling at a rolling start temperature: 1050 ° C. to 1300 ° C., a finishing temperature: 800 ° C. to 950 ° C., and a winding temperature: 450 to 750 ° C. to obtain a hot rolled steel sheet, A rolling step in which the total reduction ratio from the steel ingot is rolled to 98.80% or more to obtain a base plate for the alloyed hot dip galvanized steel sheet; and (C) reduction to the base plate for the alloyed hot dip galvanized steel sheet In the atmosphere, reduction annealing is carried out in the temperature range of Ac _{3 to} 950 ° C. for 5 to 200 seconds, and then the average cooling rate from 750 ° C. to 600 ° C. is set to 1 to 50 ° C./second [Zinc plating bath temperature −20 C.] to [Zinc plating bath temperature + 100 ° C.], and subsequently held in the temperature range for 10 to 1000 seconds, and a hot dip galvanizing bath containing 0.08 to 0.20% by mass of Al. After immersion in steel plate surface An annealing-alloying hot dip galvanizing process in which the amount of zinc deposited is controlled, and then alloyed in the temperature range of 460 to 540 ° C.
ΔT _D / V ≧ 20 (2)
Here, ΔT _D (° C.) is a temperature interval where the δ phase appears in the equilibrium diagram, and V (° C./sec) is a solidification calculated from the dendrite secondary arm interval at a depth of 10 mm from the steel ingot surface. Indicates the cooling rate.

  In these galvannealed steel sheet manufacturing methods according to the present invention, the casting process is performed by continuous casting. During the casting, the molten steel in the mold of the continuous casting machine is stirred by a moving magnetic field. It is desirable to apply it because the performance can be stably maintained by mass production.

  According to the present invention, a high-strength galvannealed steel sheet having a strength of 540 MPa or more, improved plating adhesion, and excellent bendability can be mass-produced. In addition, according to the present invention, mass production of a high-strength galvannealed steel sheet having a strength of 540 MPa or more and excellent not only in bendability and plating adhesion but also in hole expansibility, ductility, and material stability. Can do. Ultimately manufactures high-strength galvannealed steel sheets with higher strength of 590 MPa and higher, excellent bendability and plating adhesion, hole expansibility, ductility, and material stability. It is also possible to do. The alloyed hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

  Hereinafter, the best mode for carrying out the present invention will be described in detail. In the present specification, “%” indicating the content of the steel component, the content of the plating layer component, and the concentration of the plating bath component is mass% with respect to the entire steel, and with respect to the entire plating layer, respectively, unless otherwise specified. The mass% and the mass% with respect to the whole plating bath are meant.

1. Chemical Composition of Steel Sheet First, the reason for defining the chemical composition of the steel sheet according to the present invention as described above will be described.
(C: 0.03% to 0.12%)
C is contained by 0.03% or more in order to ensure the strength of the steel. However, excessive content narrows the stable region of the δ phase, which not only makes it difficult to cool under the conditions satisfying the formula (2) during casting, but also acts on the formation of retained austenite, and bendability In order to deteriorate the hole expandability, the C content is set to 0.12% or less. If the C content is less than 0.04% and the Mn content is less than 2.3%, as described later, it is difficult to ensure a tensile strength of 590 MPa or more. On the other hand, when the C content exceeds 0.06%, the C content in the retained austenite increases, and the hole expandability tends to decrease. For this reason, when especially outstanding hole expansibility is calculated | required, it is preferable to make C content into 0.04% or more and 0.06% or less.

(Si: 0.02% to 0.50%)
Si promotes the diffusion of Fe from the steel plate grain boundary into the coating layer of the coating during the alloying process, but suppresses the diffusion of Fe from within the grain into the plating layer. By increasing the difference in alloying speed with the inside, and by changing the plating thickness between the grain boundary part and the inner grain part due to this speed difference, the unevenness at the interface between the base material and the plating layer is increased as a result. It is an important element that increases the interfacial adhesion strength between the base steel plate and the plating layer.

If the Si content is less than 0.02%, the effect of improving the interfacial adhesion strength is not sufficient, so the Si content is set to 0.02% or more. Preferably it is 0.04% or more. On the other hand, when the Si content exceeds 0.50%, the alloying speed is remarkably reduced, so that it is necessary to lengthen the alloying treatment time, leading to a decrease in productivity and an increase in equipment length. Increasing the alloying treatment temperature in order to shorten the alloying treatment time results in a decrease in operability or a decrease in the interfacial adhesion strength. For this reason, Si content shall be 0.50% or less. Preferably it is 0.30% or less. Incidentally, Si is to narrow the preferred annealing temperature range is significantly elevated Ac ₃ point of the steel, from the viewpoint of operability in the annealing step, the Si content is more preferably set to 0.20% or less.

(Mn: 2.0% to 4.0%)
Mn improves the strength by solid solution strengthening and also has the effect of lowering the Ac ₃ point of the steel to widen the suitable annealing temperature range, so the Mn content is 2.0% or more. However, excessive content not only promotes a heterogeneous structure due to segregation and degrades bendability, but also suppresses ferrite transformation and degrades ductility, so the Mn content is 4.0% or less. . As described above, even if the C content is 0.04% or more, if the Mn content is less than 2.3%, it becomes difficult to ensure a tensile strength of 590 MPa or more. When high tensile strength is required, the Mn content is preferably 2.3% to 4.0%.

(P: 0.1% or less)
Although P is generally contained as an impurity, it is also a solid solution strengthening element and is effective for strengthening the steel sheet. Therefore, P may be contained in the present invention. However, excessive content deteriorates the adhesion and weldability of the plating, so the P content is 0.1% or less. For the purpose of solid solution strengthening, the P content is preferably 0.005% or more.

(S: 0.01% or less)
S is contained as an impurity, and its content is preferably as low as possible from the viewpoints of bendability, hole expansibility, and weldability. Therefore, the S content is 0.01% or less. Preferably it is 0.005% or less.

(Sol.Al: 0.01% or more and 1.0% or less)
Al has a function of deoxidizing steel and is effective in improving the yield of carbonitride-forming elements such as Ti. The Al content is 0.01% or more.

However, when the content is excessive, the oxide inclusions increase, so that the surface properties and weldability deteriorate and the cost increases. Al content shall be 1.0% or less. Incidentally, Al is to narrow the preferred annealing temperature range significantly raise the Ac ₃ point of the steel, from the viewpoint of production easiness in the annealing process, sol. The Al content is preferably 0.1% or less. Furthermore, it is preferably 0.02% or more and 0.08% or less.

(N: 0.01% or less)
In general, N is inevitably contained, but in the present invention, Ti-based, Nb-based, or Ti-Nb composite-based nitrides or carbonitrides are formed in the steel plate to increase the strength of the steel plate. Since it is effective, it may be contained positively. However, if it is excessively contained, coarse TiN is formed and the bendability and hole expandability are deteriorated, so the N content is set to 0.01% or less. When it contains for the purpose of raising the intensity | strength of a steel plate, it is preferable that N content shall be 0.0005% or more.

(Ti: 0.50% or less and / or Nb: 0.50% or less and Ti + Nb / 2 ≧ 0.03)
Ti and Nb contain one kind alone or in combination of two kinds to form carbides, nitrides or carbonitrides, and contribute to increasing the strength of the steel sheet. Further, when the above-described C content is combined with annealing conditions as will be described later, it has the effect of promoting ferrite transformation, is effective in improving the ductility of the steel sheet, and remarkably refines the crystal grain size. It has an effect and is effective in improving the bendability of the steel sheet. In order to exhibit such an effect, one or two of Ti and Nb are contained, and the value of Ti + Nb / 2 is set to 0.03 or more. These elements are further effective in suppressing the formation of retained austenite and improving the hole expansibility of the steel sheet. In order to improve the hole expandability, the value of Ti + Nb / 2 is preferably 0.05 or more, and more preferably 0.1 or more.

  If the C content is not more than the upper limit of the present invention, the value of Ti + Nb / 2 is within the range of the present invention, and the annealing temperature is not less than the lower limit of the present invention, material stability is ensured. However, even if added excessively, the effect is saturated and the cost increases, so the respective contents are set to 0.50% or less.

  Ti and Nb remarkably accelerate the ferrite transformation during cooling after annealing, suppress the formation of a hard transformation phase, and if the content satisfies the following formula, further preferable levels of material stability and hole expandability are achieved. Since it can be ensured, the PEQ value defined below is preferably 0.8 or more. The PEQ value is a ratio of the total mole fraction (A) of Ti and Nb not fixed to N and S to the mole fraction (B) of C.

PEQ = A / B
A = {(Ti + Nb / 2)-(48/14) N- (48/32) S} / 48
B = C / 12

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less)
Ca, Mg, REM (rare earth element) and Zr are all elements that can be arbitrarily contained, and by containing an appropriate amount of these, it contributes to inclusion control, particularly fine dispersion, and hole expandability. Can be further improved. Therefore, it is preferable to contain one or more of these elements so that the total amount is 0.001% or more. However, excessive content deteriorates ductility, so the content of each element is 0.01% or less.

  The balance other than the components described above is Fe and impurities.

2. Next, the reason why the steel structure of the steel sheet according to the present invention is specified as described above will be described.

(Area ratio of ferrite: 60% or more)
The steel structure of the steel sheet according to the present invention is a fraction (hereinafter abbreviated as “area ratio”) evaluated by the area ratio of each structure when observed by the observation method described in the examples of the present specification. The ratio of ferrite is 60% or more. By including ferrite in an area ratio of 60% or more, it is possible to ensure a high tensile strength of 540 MPa or more while ensuring good bendability, and further hole expandability and ductility. For this reason, the area ratio of a ferrite shall be 60% or more.

(Average diameter of ferrite: 1.0 μm or more and 6.0 μm or less)
In the steel structure of the steel sheet according to the present invention, the average grain size of ferrite is 1.0 μm or more and 6.0 μm or less. By setting the average grain size of ferrite to 6.0 μm or less, the bendability and hole expansibility are improved. However, when the average particle size is less than 1.0 μm, it becomes difficult to work harden, and bendability and ductility deteriorate. For this reason, the average particle diameter of a ferrite shall be 1.0 micrometer or more and 6.0 micrometers or less.

  Furthermore, in order to obtain excellent hole expansibility, the steel structure of the steel sheet is the fraction evaluated by the above-mentioned area ratio, and the ratio of retained austenite is 3.0% or less (including the case of 0%). It is preferable. Residual austenite undergoes processing-induced transformation in punching and further hole expansion after punching, and becomes extremely hard martensite. Formation of hard martensite leads to concentration of strain and generation of microcracks, which adversely affects local ductility. Therefore, the area ratio of retained austenite is preferably as low as possible from the viewpoint of hole expansibility. For this reason, it is preferable to make the area ratio of a retained austenite 3.0% or less.

  If the C concentration in the austenite is low, the hardness of the martensite can be reduced even with the processing-induced transformation, so that it is difficult to adversely affect the local ductility. Therefore, in order to achieve a more favorable level of hole expansibility, it is effective to reduce the C content in the retained austenite to 0.6% or less by optimizing the chemical composition and continuous annealing.

3. Next, the reason for prescribing the chemical composition of the plating layer to be the coating of the steel sheet according to the present invention as described above will be described.

(Fe: 8% to 15%)
If the Fe content in the alloyed hot-dip galvanized layer to be the coating is less than 8%, a soft part is likely to be formed on the surface layer portion of the plated layer after the alloying treatment, and the slidability is lowered and the coating layer of the coating is reduced. Flake-like peeling due to peeling from the interface with the base steel sheet increases. Therefore, the Fe content is 8% or more. Preferably it is 9.5% or more. On the other hand, if the Fe content exceeds 15%, when the steel sheet is subjected to bending, the amount of powdering peeling increases due to compression deformation of the galvannealed layer inside the bent portion. For this reason, Fe content shall be 15% or less. Preferably it is 14% or less.

(Al: 0.08% to 0.50%)
If the Al content in the alloyed hot-dip galvanized layer to be a coating is less than 0.08%, the effect of suppressing the development of the alloy layer in the plating bath becomes insufficient, and it becomes difficult to control the plating adhesion amount. Therefore, the Al content is 0.08% or more. Preferably it is 0.20% or more, More preferably, it is 0.25% or more. On the other hand, if the Al content exceeds 0.50%, the alloying speed is significantly reduced, and at the normal line speed, the alloying temperature has to be higher than 540 ° C. in order to realize the Fe content. In some cases, as will be described later, it becomes difficult to set the interfacial adhesion strength between the steel sheet and the galvannealed layer to 20 MPa or more. Therefore, the Al content is 0.50% or less. Preferably it is 0.45% or less, More preferably, it is 0.40% or less.

  In addition, Si, Mn, P, S, Ti, Nb, Cr, Mo, V, B, Ca, REM, etc. are incorporated into the alloyed hot-dip galvanized layer as the coating from the base material during the alloying process. However, there is no problem because the plating quality is not adversely affected as long as it is within the range that can be incorporated in the plating layer when the hot dipping and alloying treatment is performed under normal conditions. The normal plating conditions here are, as will be described later, a plating bath temperature of 400 ° C. to 500 ° C., a steel plate intrusion temperature of 400 ° C. to 500 ° C., and an alloying temperature of 460 to 540 ° C. The balance of the chemical composition of the plating film is substantially Zn.

4). Production Method The hot-dip galvanized steel sheet according to the present invention may be produced by any production method as long as it has the chemical composition and structure described above for the steel sheet and the chemical composition described above for the plating layer. However, by employing the following manufacturing method, it is possible to more efficiently and stably manufacture the hot-dip galvanized steel sheet according to the present invention.

(A) Casting process (ΔT _D / V ≧ 20)
Molten steel having the above-described chemical composition is melted and cast by a well-known conventional melting method such as a converter or an electric furnace. As the casting method, it is preferable to adopt the continuous casting method from the viewpoint of productivity. An ingot casting method, a thin slab casting method, or the like may be employed.

When cooling the above-mentioned molten steel, the solidification cooling rate converted from the dendrite secondary arm interval at a depth of 10 mm from the surface of the steel ingot is V (° C./second), and the temperature at which the δ phase appears in the equilibrium diagram When the section is ΔT _D (° C.), V and ΔT _D are in a range satisfying the following formula (2).

If ΔT _D / V is less than 20, the cooling rate is too high, and microsegregation that is difficult to be eliminated by the process described later is formed during solidification, and unevenness is likely to occur after bending. The bendability deteriorates. Therefore, the temperature interval [Delta] T _D solidification cooling rate V and δ-phase appears in a range satisfying equation (2).
ΔT _D / V ≧ 20 (2)

  At this time, it is preferable to stir the molten steel with a moving magnetic field in the casting mold of the continuous casting machine because the microsegregation is suppressed, and further improvement in bendability can be expected. Moreover, since generation | occurrence | production of the surface defect of a galvannealed steel plate is suppressed, it is preferable. Here, the stirring of the molten steel by the moving magnetic field is preferably performed so that the flow rate at a position where the distance from the mold is 20 mm is 10 cm / second or more and 100 cm / second or less. If the flow rate is less than 10 cm /, the microsegregation suppressing action and the surface defect suppressing action may not be sufficient, and if it exceeds 100 cm / second, inclusions may increase due to the entrainment of powder, which may promote the generation of surface defects. is there. The flow velocity can be measured using, for example, a Karman vortex flow meter.

(B) Rolling process The steel ingot obtained in this way is hot-rolled to obtain a hot-rolled steel sheet. In hot rolling, cast steel ingots are not cooled to room temperature but directly fed into a heating furnace while being heated and heated and then rolled directly, 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 characteristic fluctuations can be further suppressed.

(I) Rolling start temperature of hot rolling: 1050 ° C. or higher and 1300 ° C. or lower When reheating a steel ingot, it is effective to re-dissolve TiC or NbC in order to prevent deterioration of bendability and hole expandability. is there. Such an effect is recognized by heating the steel material once cooled to 1050 ° C. or higher. However, heating above 1300 ° C. not only saturates the above effect but also increases scale loss. For this reason, the reheating temperature of a steel ingot shall be 1050 degreeC or more and 1300 degrees C or less. In other words, the hot rolling start temperature is 1050 ° C. or higher and 1300 ° C. or lower.

  Moreover, in order to perform the said solid solution reliably by reheating, it is preferable to make this heating time into 10 minutes or more, and in order to suppress an excessive scale loss, it is preferable to set it as 3 hours or less. More preferably, it is 30 minutes or more and 2 hours or less.

  Of course, when performing direct feed rolling or direct rolling, as long as TiC or NbC is dissolved, rolling may be started as it is, but also in this case, the rolling start temperature is preferably 1050 ° C. or higher and 1300 ° C. or lower. is there.

(Ii) Finishing temperature: 800 ° C. or higher and 950 ° C. or lower The hot rolling finishing temperature is 800 ° C. or higher and 950 ° C. or lower.
If the finishing temperature is less than 800 ° C., deformation resistance becomes excessive and rolling becomes difficult. On the other hand, when the finishing temperature exceeds 950 ° C., the precipitates become coarse, and it becomes difficult not only to secure the target strength in the final product, but also the scale formation on the steel sheet surface becomes remarkable, and the coiling temperature is controlled. It becomes difficult to do.

(Iii) Winding temperature: 450 degreeC or more and 750 degrees C or less Winding temperature shall be 450 degreeC or more and 750 degrees C or less.
When the coiling temperature is less than 450 ° C., hard bainite and martensite are generated, and it is difficult to perform cold rolling thereafter. On the other hand, when the coiling temperature exceeds 750 ° C., the precipitate becomes coarse, and it becomes difficult not only to secure the desired strength in the final product, but also the surface scale develops, and the subsequent pickling Scale removal becomes difficult, and the appearance of the galvannealed steel sheet deteriorates.

  The hot-rolled steel plate may be cold-rolled after being pickled by a normal method to obtain a cold-rolled steel plate. In addition, it is advantageous in securing flatness if the shape is corrected by performing mild rolling of about 0% to 5% before or after pickling. In addition, the mild rolling improves pickling properties and promotes removal of surface concentrating elements, thereby improving the adhesion of hot dipping.

(Iv) Total rolling reduction of rolling process: 98.80% or more The total rolling reduction of both hot rolling and cold rolling in the rolling process is 98.80% or more. If the total rolling reduction is less than 98.80%, the effect of remaining microsegregation tends to appear, and the bendability deteriorates. If the total rolling reduction in hot rolling is 98.80% or more, it is not necessary to perform cold rolling. In addition, in order to refine | miniaturize the structure | tissue of the steel plate after continuous annealing, it is preferable to perform cold rolling and to make the reduction rate into 30% or more.

(C) Annealing-alloying hot-dip galvanizing step According to the present invention, the base plate for the alloyed hot-dip galvanized steel sheet obtained in this way has a temperature in the range of Ac _{3 to} 950 ° C in a reducing atmosphere. The temperature range of [Zinc plating bath temperature−20 ° C.] to [Zinc plating bath temperature + 100 ° C.] is applied, with reduction annealing held for up to 200 seconds, followed by an average cooling rate from 750 ° C. to 600 ° C. of 1-50 ° C./second And then kept in the above temperature range for 10 to 1000 seconds, immersed in a hot dip galvanizing bath containing 0.08 to 0.20 mass% Al, and then alloyed at 460 to 540 ° C. . These annealing heat treatment and hot dip galvanizing treatment are preferably performed in a continuous hot dip galvanizing line. Hereinafter, the case where this process is performed in a continuous hot dip galvanizing line will be described as an example.

(I) Reduction annealing Rolling oil and iron powder are adhering to the original sheet for alloyed hot-dip galvanized steel sheet. Therefore, from the viewpoint of improving the plating appearance, etc., the steel sheet after cold rolling may be cleaned by placing it in an alkaline degreasing tank and degreasing with alkali. Then, reduction annealing is performed by raising a steel plate to the temperature described below in a reducing atmosphere containing hydrogen.

  In the present invention, since a large amount of Ti and / or Nb is contained, recrystallization of the processed ferrite is remarkably suppressed. Therefore, processing strain remains up to the austenite region during heating, and the phase transformation to austenite is significantly promoted. Therefore, a desired structure is obtained under the following annealing conditions.

(Ii) Annealing temperature: Ac ₃ points to 950 ° C. Temperature range Ac ₃ points to 950 ° C.
When the annealing temperature is less than Ac ₃ points, non-recrystallized remains, a uniform structure cannot be obtained, bending properties, ductility and hole expansibility are lowered, and good material stability can be secured. It becomes difficult. On the other hand, if the annealing temperature exceeds 950 ° C., the precipitates become coarse and fine precipitates cannot be obtained, making it difficult to secure the desired strength in the final product, and the continuous annealing furnace is likely to be damaged. , Mass production is not possible. In order to refine the structure of the steel sheet after the continuous annealing, the heating rate in the temperature range from 750 ° C. to Ac ₃ points is set to 2 ° C./second or more and 10 ° C./second when heating to the annealing temperature. It is preferable to set it to 2 seconds or less.

(Iii) Annealing time: 5 seconds or more and 200 seconds or less It anneals by hold | maintaining for 5 seconds or more and 200 seconds or less at the annealing temperature mentioned above. If the annealing time is less than 5 seconds, the transformation from processed ferrite to austenite does not proceed sufficiently, so that unrecrystallized remains, a uniform structure cannot be obtained, and bendability, ductility, and hole expansibility are obtained. descend. On the other hand, if the annealing time exceeds 200 seconds, the structure becomes coarse due to grain growth, and the hole expandability deteriorates. From the viewpoint of productivity, the annealing time is preferably within 120 seconds.

(Iv) Average cooling rate from 750 ° C. to 600 ° C .: 1 ° C./second or more and 50 ° C./second or less After cooling, the average cooling rate from 750 ° C. to 600 ° C. is 1 ° C./second or more and 50 ° C./second. The following. The reason for defining the average cooling rate in the temperature range from 750 ° C. to 600 ° C. is that when a large amount of Ti and / or Nb is contained, austenite easily transforms into ferrite in the above temperature range. This is because by controlling the cooling rate, the properties of the ferrite that is the main phase of the structure can be controlled, and the strength, bendability and ductility can be controlled. When the average cooling rate is less than 1 ° C./second, the structure becomes coarse due to grain growth, nonuniform deformation is promoted, and bendability and hole expansibility are lowered. On the other hand, if the average cooling rate exceeds 50 ° C./second, soft ferrite cannot be obtained and ductility is deteriorated. In addition, nonuniform deformation is promoted and bendability is also deteriorated. In addition, as described above, by setting the PEQ value to be greater than 0.8, even if heat treatment is performed in a wide cooling rate range assumed for such operation, it is easy to suppress the fluctuation in tensile strength. Good material stability can be ensured.

(V) Cooling stop temperature: (Zinc plating bath temperature−20 ° C.) or more (Zinc plating bath temperature + 100 ° C.) or less In the present invention, the cooling stop temperature is set to [Zinc plating bath temperature−20 ° C.] or more [Zinc plating bath temperature + 100. [° C] The following temperature range. When the cooling stop temperature is less than [zinc plating bath temperature −20 ° C.], the heat removal during the subsequent immersion in the hot dip galvanizing bath is large, and the operation becomes difficult. On the other hand, when the cooling stop temperature is higher than [zinc plating bath temperature + 100 ° C.], the operation is difficult, and coarse cementite is generated and the hole expansibility is lowered.
Hot dip galvanization is performed by immersing the annealed steel sheet in a hot dip galvanizing bath at 400 ° C. or higher and 500 ° C. or lower according to a conventional method.

(Vi) (zinc plating bath temperature −20 ° C.) or more (zinc plating bath temperature + 100 ° C.) holding time: 10 seconds or more and 1000 seconds or less After the cooling is stopped in the above-described temperature range, Hold for 10 seconds to 1000 seconds.
If the holding time is less than 10 seconds, the austenite is not sufficiently decomposed and residual austenite is generated, and the hole expandability deteriorates. Accordingly, the holding time is 10 seconds or more. Preferably it is 20 seconds or more. On the other hand, if the holding time exceeds 1000 seconds, coarse cementite is generated and the hole expansibility deteriorates. Therefore, the holding time is within 1000 seconds. From the viewpoint of productivity, it is preferably within 200 seconds.
The holding time includes a time required for a dipping process in a hot-dip galvanizing bath and a hot-dip galvanizing adhesion amount controlling process, which will be described later.

(Vii) Al concentration in hot dip galvanizing bath: 0.08 to 0.20%
When the Al concentration in the hot dip galvanizing bath is less than 0.08%, an excessive Fe—Zn interface alloy layer is already formed in the hot dip galvanizing bath before the alloying treatment. For this reason, it becomes difficult to control the amount of adhesion. Therefore, the Al concentration in the hot dip galvanizing bath is set to 0.08% or more. Preferably it is 0.09% or more.

  On the other hand, if the Al concentration in the hot dip galvanizing bath exceeds 0.20%, the concentration of Al in the plating film proceeds excessively, resulting in a decrease in the alloying rate. In order to achieve the above, the alloying treatment temperature may be forced to exceed 540 ° C., and as will be described later, it becomes difficult to set the interfacial adhesion strength between the steel sheet and the galvannealed layer to 20 MPa or more. Therefore, the Al concentration in the hot dip galvanizing bath is 0.20% or less. Preferably it is 0.15% or less.

  About immersion time, if it is less than 5 second, a performance and operativity will not be inhibited especially. About other plating conditions, the range generally employ | adopted may be sufficient, and if a plating bath temperature is 400-500 degreeC and an intrusion board temperature is the range of 400-500 degreeC, there will be no problem in particular. Even if 0.1% or less of Fe, Pb, Cd, Cr, Ni, W, Ti, Mg and Si, which are inevitable elements, are contained as components other than Al in the plating bath, this performance is affected. Absent.

After the immersion step, the adhesion amount of plating is controlled by a known method such as gas wiping. The adhesion amount may be in the range of 25 to 70 g / m ² per side generally used as a product.
Furthermore, the hole expandability is improved by alloying at 460 ° C. or higher and 540 ° C. or lower after immersion in the hot dip galvanizing bath.

(Viii) Alloying treatment temperature: 460 ° C. or more and 540 ° C. or less If the alloying treatment temperature is less than 460 ° C., coarse crystals of ζ phase are likely to be formed on the surface layer portion of the alloyed hot dip galvanized layer. The Fe content in the layer may be less than 8%. Therefore, the alloying treatment temperature is set to 460 ° C. or higher. Preferably it is 470 degreeC or more, More preferably, it is 480 degreeC or more.

  On the other hand, when the alloying treatment temperature exceeds 540 ° C., the diffusion of Fe from the inside of the steel sheet grains into the plating layer of the coating becomes active, and the effect of containing Si in the steel sheet described above, that is, the grain boundary of the steel sheet By promoting the diffusion of Fe into the plating layer of the coating and suppressing the diffusion of Fe from within the grains into the plating layer, the unevenness at the interface between the base material and the plating layer is increased, The effect of increasing the interfacial adhesion strength between the steel plate and the plating layer is reduced. As a result, it becomes difficult to obtain the interfacial adhesion strength between the target steel sheet and the galvannealed layer. Therefore, the alloying temperature is set to 540 ° C. or lower. Preferably it is 520 degrees C or less. As a heating means in the alloying treatment, any means such as radiant heating, high frequency induction heating, energization heating and the like may be used.

  When the alloying treatment temperature is set to [zinc plating bath temperature + 40 ° C.] or higher, even if the C content exceeds 0.06%, the decomposition of austenite is promoted to reduce the area ratio of residual austenite. The hole expansibility can be further improved. For this reason, the alloying treatment temperature is preferably 460 ° C. or higher and 540 ° C. or lower and [zinc plating bath temperature + 40 ° C.] or higher.

  After the alloying treatment, it is preferable to further perform temper rolling in the range of the elongation of 0.05% to 1%. The temper rolling can suppress the yield point elongation and can prevent seizure and galling during pressing.

  The product surface after plating may be untreated, but may be subjected to post-treatment such as known chromic acid treatment, phosphate treatment, and resin film coating. In addition, rust preventive oil may be applied, and the rust preventive oil used for the application may be a commercially available general one, but it is highly lubricious and contains an extreme pressure additive such as S or Ca. Rust oil may be applied.

5. Evaluation Criteria As described above, by adjusting the chemical composition, casting, rolling, and optimizing the subsequent annealing-hot galvanizing conditions, the ferrite has an area ratio of 60% or more, and the average particle diameter of the ferrite is 1.0 μm or more and 6.0 μm. The following steel structure can be obtained, high strength of tensile strength of 540 MPa or higher, alloyed hot-dip galvanized steel sheet with good bendability and plating adhesion, and hole expansibility, ductility, and material stability are also good. An alloyed hot-dip galvanized steel sheet is also obtained. The evaluation criteria judged as good for each characteristic are described below.

Bendability: As described above, in a U-bend test with a tip angle of 180 °, it is possible to perform close contact bending, and this is good when unevenness does not appear on the surface of the bent ridge line after processing by visual observation.
Plating adhesion: Evaluated by the interfacial adhesion strength described in the following examples, and the strength is 20 MPa or more.

  Hole expandability: A case where the hole expansion ratio (HER) measured by the method specified in JFS T 1001 is 50% or more is considered good. When the HER value is 80% or more, the hole expandability is better, and when it is 100% or more, the hole expandability is even better.

  Ductility: A case where the TS × El value obtained by a tensile test is 12000 MPa ·% or more is judged to be good. If this value is 14000 MPa ·% or more, the ductility is better.

  Material stability: The case where the upper and lower limit range (ΔTS) of the tensile strength is 100 MPa or less is considered good, and the case where this value is 60 MPa or less is made better.

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, and a slab having a thickness of 245 mm was obtained by continuous casting. In order to calculate the average cooling rate during solidification, a part of the obtained slab was cut out. Note that the temperature interval in which the δ phase appears is the equilibrium phase diagram calculation software considering only the amounts of C, Si, Mn, Ti, and Nb that are the main elements excluding trace addition elements in the chemical composition shown in Table 1. Calculation was performed by performing thermodynamic calculation using (THERMO-CALC).

  In Table 1, the content underlined indicates that the content is outside the range of the chemical composition according to the present invention.

  The obtained slab was hot-rolled under the conditions shown in Table 2. The obtained hot-rolled steel sheet was pickled and cold-rolled under the conditions shown in Table 2. The obtained cold-rolled steel sheet was annealed and galvannealed under the conditions shown in Table 2.

  The underlined manufacturing conditions in Table 2 indicate that the conditions are outside the range of the manufacturing conditions according to the present invention.

  The obtained alloyed hot-dip galvanized steel sheet is analyzed by X-ray diffraction, SEM observation, and TEM observation, the structure of the steel sheet is analyzed, a tensile test, a bending test, a hole expansion test are performed, mechanical properties are evaluated, and plating is performed. The characteristics were investigated. The bendability is confirmed not only by cracking but also by the appearance after bending deformation, and the surface of the test piece after adhesion bending (U-bend test with a tip angle of 180 °) is visually observed to show the streak-like contrast due to unevenness. Evaluation was made based on the presence / absence of the sample, and it was determined that the test material without cracks and streak-like contrast was good.

[Test method]
(Calculation of solidification cooling rate)
The cross section of the obtained slab was etched with picric acid, and the five dendritic secondary arm intervals λ (μm) were measured at a depth position of 10 mm from the surface of the steel ingot. From the average value, the solidification cooling rate A (° C./min) at the liquidus temperature to the solidus temperature was calculated.
λ = 710 × A ^−0.39

(Ac ₃ point measurement)
By using a cold-rolled steel sheet having the chemical composition shown in Table 1 and analyzing the change in expansion coefficient when heated at a rate of temperature increase of 10 ° C./second, _three points of Ac of each test steel were measured.

(Tissue observation)
Test specimens were taken from each alloyed hot-dip galvanized steel sheet in the rolling direction and in the direction perpendicular to the rolling direction, and the cross-sectional structure in the rolling direction and the cross-sectional structure perpendicular to the rolling direction were photographed with an optical microscope or an electron microscope. The fraction of each phase and the particle size of each phase were measured by image analysis. The ferrite grain size was measured in accordance with the provisions of the cross line segment method of JISG 0552 for the total thickness of the sheet in the rolling direction cross section and in the cross section perpendicular to the rolling direction, and expressed as an average value thereof.

(Residual austenite area ratio and C content in retained austenite)
Each alloyed hot-dip galvanized steel sheet is subjected to chemical polishing to reduce the thickness by 0.3 mm, the surface after chemical polishing is subjected to X-ray diffraction, the obtained profile is analyzed, and the area ratio of residual austenite and residual The C content in austenite was calculated.

(mechanical nature)
For each galvannealed steel sheet, a JIS No. 5 tensile specimen was taken from the direction perpendicular to the rolling direction, and yield stress (YS), tensile strength (TS), and elongation (El) were measured. The hole expansion rate (HER) was measured by the method prescribed in JFS T 1001.

(Bending test)
From each alloyed hot-dip galvanized steel sheet, a bending test piece (width 40 mm × length 100 mm × sheet thickness 1.2 mm) having a longitudinal direction in the direction perpendicular to the rolling direction was taken so that the bending ridge line was in the rolling direction. At that time, the alloyed hot-dip galvanized steel sheet having a plate thickness of 3.0 mm was ground on the inner side of the bend to obtain a test piece having a plate thickness of 1.2 mm. Adhesion bending (U-bending test with a tip angle of 180 °) was performed to visually confirm the presence or absence of cracks and to visually confirm the presence or absence of surface irregularities. Those with cracks and irregularities were judged to be defective, and those with no cracks were considered good.

(Material stability)
In operation, it is difficult to precisely control the cooling rate in the continuous annealing process. Therefore, the cold-rolled steel sheet not subjected to annealing obtained as described above was heated to 700 ° C. at a temperature rising rate of 10 ° C./second and annealed at the temperature shown in Table 2. Cooling is performed at 1 ° C, 5 ° C, 10 ° C, 50 ° C from the annealing temperature to the cooling stop temperature, and thereafter, at the cooling stop temperature shown in Table 2 so as to simulate the heat treatment at the time of manufacturing the galvannealed steel sheet. An annealing cold-rolled steel sheet was produced by simulating holding, immersion in a plating bath, and alloying heat treatment. That is, for each of the test steels, an annealed cold-rolled steel sheet having four conditions in which only the cooling rate was changed was produced, and the difference between the maximum value and the minimum value of TS was ΔTS, which was used as an index of material stability.

Furthermore, the plating characteristics were investigated as follows.
(Composition analysis of plating layer)
A 25 mmφ sample piece was taken from the sample after alloying treatment, and the plating layer was dissolved with a 10% HCl aqueous solution containing 0.5 vol% inhibitor (trade name: “Ibit 710N” manufactured by Asahi Kagaku). The sample was subjected to composition analysis of the plating layer.

(Measurement of interfacial adhesion strength between steel plate and alloyed hot-dip galvanized layer)
The alloyed sample is cut to 20 mm × 100 mm so that the longitudinal direction is the rolling direction, and a one-pack type epoxy structural adhesive (trade name: E-6993) manufactured by Sunstar Co., Ltd. is bonded. The tension test was carried out in the longitudinal direction under the conditions of using a stacking margin of 12.5 mm, an adhesive film thickness of 200 μm, a baking condition of 180 × 20 minutes, a tensile speed of 5 mm / min, and room temperature. The interfacial adhesion strength in this test is affected by the substrate strength due to the deformation of the base material, but is almost negligible for the base material having a YP of 350 MPa or more as in this case. As a result of the test, it was determined that the plating adhesion was good when the interface adhesion strength was 20 MPa or more, and the plating adhesion was poor when the interface adhesion strength was less than 20 MPa.

  The results are summarized in Table 3. Note that the underlined results in Table 3 indicate that the conditions are outside the scope of the present invention.

Steel plate No. of the example of the present invention. 1-3, 5, 6, 8, 10, 13, 14, 17, 20, and 22 all have a desired chemical composition, and have an area ratio of 60% or more of ferrite, and the average particle diameter of ferrite is It has a steel structure of 1.0 μm or more and 6.0 μm or less, and the plating layer has a desired chemical composition.
As a result, even when the tensile strength is 540 MPa or higher, even when tightly bent, it has excellent bendability that does not generate cracks or irregularities on the surface, and is excellent in plating adhesion.

  Steel plate No. containing less than 3.0% of retained austenite among the steel plates of the invention examples. 1-3, 5, 6, 8, 10, 13, 14, 17, 20, and 22 have a TS × El value of 12000 MPa ·% or more, a HER of 50% or more, in addition to the tensile strength and bendability described above. Material stability is 100 MPa or less in ΔTS, which is a preferable steel plate.

  Further, among them, at least the C content and the Mn content are in the above-described preferable ranges, and no retained austenite is contained, or the area ratio of the retained austenite is 3.0% or less, and the C in the retained austenite Steel plate No. whose concentration is 0.6 mass% or less. 1, 3, 5, 10, 17, 20, and 22 are more preferable steel plates having a tensile strength of 590 MPa or more and a HER of 80% or more.

  Furthermore, among them, the steel plate No. whose PEQ value exceeds 0.8. 3, 5, 10, 17, and 22 are further preferable steel plates in addition to the above-described tensile strength and hole expansibility, and the TS × El value is 14000 MPa ·% or more and the material stability is ΔTS of 60 MPa or less.

  In contrast, the steel plate No. No. 4 has a Mn content exceeding the upper limit of the range defined in the present invention, so that a desired ferrite area ratio cannot be obtained and the bendability is poor.

  Steel plate No. No. 7 has poor plating adhesion because the Si content is below the lower limit of the range defined in the present invention.

  Steel plate No. No. 9 has a value of Ti + Nb / 2 that is lower than the lower limit of the range defined in the present invention.

  Steel plate No. No. 11, because the slab heating temperature (rolling start temperature) is below the lower limit of the range of the present invention, Ti and Nb cannot be re-dissolved, and the bendability is poor.

  Steel plate No. No. 12 has a C content below the lower limit of the range defined in the present invention. In No. 21, since the Mn content is below the lower limit of the range defined in the present invention, any desired strength cannot be obtained.

  Steel plate No. No. 15, the alloying treatment temperature exceeds the upper limit of the range defined in the present invention, so the Fe concentration in the plating layer exceeds the desired value, and the plating adhesion is poor.

  Steel plate No. In No. 16, since the total rolling reduction is less than the lower limit of the range defined in the present invention, a uniform structure cannot be obtained.

  Steel plate No. No. 18 has an annealing temperature that is lower than the lower limit of the range defined in the present invention, so that a uniform structure cannot be obtained sufficiently, cracking occurs during bending molding, and bendability is poor.

Steel plate No. No. 19 exceeds the upper limit of the range defined by the present invention, and ΔT _D / V is lower than the lower limit of the range defined by the present invention, so that cracks occur during bending and irregularities occur during bending. And bendability is bad.

It is explanatory drawing which shows the condition where the fluctuation | variation of the local Mn density | concentration produced by the solidification segregation in the high strength steel plate containing Mn abundantly. It is explanatory drawing which shows the condition where the remarkable unevenness | corrugation which can be observed visually also appeared on the surface of the process part by the heterogeneous structure | tissue corresponding to the fluctuation | variation of a local chemical composition.

Claims (7)

An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The said steel plate is the mass%, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 2.0-4.0%, P: 0.1% or less, S : 0.01% or less, sol. One or two of Al: 0.01 to 1.0% and N: 0.01% or less, and Ti: 0.50% or less and Nb: 0.50% or less are represented by the following formula (1 ), The balance is a chemical composition consisting of Fe and impurities, the area ratio of ferrite is 60% or more, and the average grain size of ferrite is 1.0 to 6.0 μm. Have
The alloyed hot-dip galvanized layer contains, by mass%, Fe: 8 to 15% and Al: 0.08 to 0.50%, and the balance is made of Zn and impurities.
The alloyed hot-dip galvanized steel sheet has a tensile strength of 540 MPa or more and can be bent tightly in a U-bend test with a tip angle of 180 °. Unevenness appears on the surface of the bending ridge line after processing by visual observation. An alloyed hot-dip galvanized steel sheet characterized by not .
Ti + Nb / 2 ≧ 0.03 (1)   The chemical composition is mass% instead of part of Fe, and further Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. The alloyed hot-dip galvanized steel sheet according to claim 1, containing one or more selected from the group consisting of:   The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein an area ratio of retained austenite in the steel structure is 3.0% or less.   The alloyed hot-dip galvanized steel sheet according to claim 3, wherein a hole expansion ratio measured by a method defined in JFS T 1001 is 50% or more.   The alloyed hot-dip galvanized steel sheet according to claim 4, wherein the value of tensile strength × elongation is 12000 MPa ·% or more. A method for producing an alloyed hot-dip galvanized steel sheet having excellent bendability, comprising the following steps (A) to (C):
(A) A casting process in which the molten steel having the chemical composition according to claim 1 or 2 is cooled under conditions satisfying the following formula (2) to form a steel ingot;
(B) The steel ingot is subjected to hot rolling at a rolling start temperature: 1050 ° C. to 1300 ° C., a finishing temperature: 800 ° C. to 950 ° C., and a winding temperature: 450 to 750 ° C. to obtain a hot rolled steel sheet, A rolling step in which the total rolling reduction from the steel ingot is rolled to 98.80% or more to obtain a base plate for the alloyed hot dip galvanized steel sheet; and (C) the base plate for the alloyed hot dip galvanized steel sheet,
In a reducing atmosphere, reduction annealing is performed in a temperature range of Ac _{3 to} 950 ° C. for 5 to 200 seconds, and then the average cooling rate from 750 ° C. to 600 ° C. is set to 1 to 50 ° C./second [Zinc plating bath temperature − 20 [deg.] C. to [Zinc plating bath temperature + 100 [deg.] C.], and subsequently held in the temperature range for 10 to 1000 seconds, and hot dip galvanizing containing 0.08 to 0.20 mass% Al. An annealing-alloying hot dip galvanizing process in which the steel is immersed in a bath and then alloyed in a temperature range of 460 to 540 ° C.
ΔT _D / V ≧ 20 (2)
Here, ΔT _D (° C.) is a temperature interval where the δ phase appears in the equilibrium diagram, and V (° C./sec) is a solidification calculated from the dendrite secondary arm interval at a depth of 10 mm from the steel ingot surface. Indicates the cooling rate.   7. The alloyed hot dip galvanizing according to claim 6, wherein the casting step is performed by continuous casting, and the molten steel in the mold of the continuous casting machine is agitated by a moving magnetic field during the casting. A method of manufacturing a steel sheet.

Images