JP5440370B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to an galvannealed steel sheet and a method for producing the same. More specifically, the present invention relates to an alloyed hot-dip galvanized steel sheet having a high yield ratio and a method for producing the same, and in particular, a collision-resistant property such as a car body of an automobile. The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet suitable for applications in which properties are indispensable and a method for producing the same.

  In recent years, in order to protect the global environment, there has been a demand for improved fuel economy of automobiles, and steel sheets having a tensile strength of 780 MPa or more, particularly parts that require corrosion resistance, in order to reduce the weight of the vehicle body and ensure the safety of passengers. On the other hand, the need for high-strength galvannealed steel sheets is increasing.

  In a high-strength steel sheet applied to automotive parts, not only various workability required at the time of part molding, such as press formability and weldability, but also strength characteristics other than tensile strength must be satisfied. For example, in a collision-resistant component for protecting a cabin at the time of a collision at the front portion, an impact transmitted to the cabin is mitigated by deformation of the component. For this reason, a steel plate having an ideal crushing behavior from the viewpoint of impact relaxation at the time of collision, specifically, a high yield strength is required. That is, when the tensile strength is the same, a part made of a steel plate having a high yield ratio has excellent impact characteristics.

  Generally, it is difficult to increase the yield strength compared to increasing the tensile strength. This is because a steel sheet having a tensile strength of 780 MPa or more has a structure in which ferrite is a parent phase and is strengthened by a hard phase such as martensite, and the generation of the hard phase introduces movable dislocations into the soft ferrite. This is due to the low stress required for collective motion of dislocations. However, it is possible to simultaneously increase the tensile strength and the yield strength by controlling the structure of the steel sheet.

  For example, Non-Patent Document 1 shows that the balance between the tensile strength and the yield strength of the steel sheet is different depending on the structure control method, precipitation strengthened steel sheet (steel sheet reinforced with fine precipitates), bainite steel sheet (steel sheet reinforced with bainite structure), It discloses the knowledge that martensitic steel sheets (steel sheets with a martensitic single phase structure) have high yield strength and high yield ratio. Non-Patent Document 2 discloses an experimental result that the yield ratio rises with an increase in the volume ratio of martensite in a ferrite and martensite composite steel sheet having the same tensile strength. Non-Patent Document 3 discloses a steel plate having a high yield ratio that achieves a tensile strength of 780 MPa or more and a yield strength of 700 MPa or more by actively utilizing precipitation strengthening.

  However, most of the conventional knowledge does not consider the manufacturing process of the galvannealed steel sheet. Non-Patent Document 1 relates to a cold-rolled steel sheet manufactured by a heat treatment different from the hot-dip galvanizing line, Non-Patent Document 2 relates to a steel sheet manufactured by quenching and tempering, and Non-Patent Document 3 Further, the present invention relates to a hot-rolled steel sheet manufactured by a heat treatment in which microalloys such as Ti and Mo that contribute to precipitation strengthening are once dissolved.

  In the manufacturing process of high strength alloyed hot dip galvanized steel sheet, the recrystallization annealing temperature is 750 to 950 ° C., and the conditions for solid solution of the amount of microalloy necessary to achieve a tensile strength of 780 MPa or more, specifically Is much lower than 1220 ° C. corresponding to the lower limit of the slab heating temperature. Therefore, it is difficult to increase the tensile strength of the alloyed hot-dip galvanized steel sheet only by dispersion strengthening of precipitates composed of microalloys as in the hot-rolled steel sheet described above.

  Moreover, when cooling from recrystallization annealing, the alloying process of immersing in a hot dip galvanizing bath of about 455 degreeC and then reheating to 460-600 degreeC after immersion is performed. That is, in this manufacturing process, cooling is temporarily interrupted in the bainite transformation temperature region, and the steel sheet is heat-treated in a state close to constant temperature. Therefore, when manufacturing a high-strength galvannealed steel sheet, the steel sheet tends to have a structure containing bainite. However, since the time required from the cooling stop to the alloying process is short, only a part of the steel becomes bainite and the remainder becomes austenite or martensite. These austenite and martensite have an extremely high C (carbon) concentration due to the partial formation of bainite, so that they remain annealed or have an extremely hard structure in the early stage of deformation leading to yield strength. Become. When such a hard structure is partially mixed, movable dislocations are introduced into a soft region in steel, yield strength is not increased, and yield ratio is lowered. That is, it has been extremely difficult to produce a steel plate having a high yield ratio that does not contain austenite or hard martensite under the heat treatment conditions for producing a high-strength galvannealed steel plate. Then, the structure control method which devised the combination of a chemical composition and thermomechanical processing was examined with respect to the strength characteristic in an alloyed hot dip galvanized steel sheet, especially the yield ratio.

  For example, Patent Document 1 actively adds Ti and Nb to a steel sheet, further limits the addition amount of quenching elements such as Mn and Cr, and has a two-phase region (two-phase coexistence temperature range of ferrite and austenite). A steel sheet having a tensile strength of 780 MPa or more and a high yield ratio is disclosed by annealing and making a microstructure of ferrite and pearlite with a refined structure. However, when steel added with Ti is annealed in a two-phase region, the dependence of tensile strength on the annealing temperature increases. That is, it becomes difficult to stably secure desired strength characteristics, and such a technique is not suitable for mass production.

  Patent Document 2 has a tensile strength of 780 MPa or more by positively adding Mo, Ni and Cu to a steel sheet, optimizing the annealing temperature condition, increasing the solid solution N, and making the structure fine. A high ratio steel sheet is disclosed. However, since a large amount of expensive Mo, Ni, or Cu is added, the manufacturing cost becomes extremely high. Furthermore, since the steel plate which has a tensile strength of 780 MPa or more is manufactured by two annealing processes, the productivity of a line is inhibited.

  Patent Document 3 adds Ti, Nb, Mo and B to a steel sheet, further controls the amount thereof, optimizes the annealing temperature condition, promotes bainite transformation, and the ratio of bainite or bainitic ferrite in the structure. Is disclosed, which has a tensile strength of 780 MPa or more and a high yield ratio. However, since a large amount of Si is added, non-plated portions are likely to occur in the steel sheet, making it difficult to stably secure corrosion resistance.

  Patent Document 4 adds B to a steel sheet, limits the amount of Mn added, optimizes the hot rolling coiling temperature, annealing temperature conditions, etc., uniformly disperses unrecrystallized ferrite, and martensite and bainite in the structure. The steel sheet having a tensile strength of 780 MPa or more and a high yield ratio is disclosed. However, since non-recrystallized ferrite is included, the toughness deteriorates, so such a technique is not suitable for manufacturing parts that require impact properties. Furthermore, in the case of a structure containing non-recrystallized material, the tensile strength dependence of the tensile strength is extremely large in the annealing temperature region where it remains, and it becomes difficult to stably secure desired strength characteristics.

  Patent Document 5 actively adds Ti, Mo and V to a steel sheet, optimizes the hot rolling coiling temperature and annealing temperature conditions, etc., and makes the structure mainly composed of ferrite reinforced with fine carbonitride. Discloses a steel sheet having a tensile strength of 980 MPa or more and a high yield ratio. However, since a large amount of expensive V is added, the manufacturing cost becomes extremely high. Furthermore, in the case of a structure strengthened with V-based precipitates, it is difficult to stably precipitate them, and it is presumed that the tensile strength is highly dependent on the annealing temperature, and the desired strength characteristics are stably secured. It becomes difficult.

JP-A-10-273754 JP 2001-303180 A JP 2005-105367 A JP 2008-156680 A JP 2008-174802 A

High Ten Handbook, Japan Iron and Steel Institute, (2008), p. 46 ISIJ International, 44 (2004), no. 3, p. 603-609 ISIJ International, 44 (2004), no. 11, p. 1945-1951

  As described above, an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having a high yield ratio and a tensile strength of 780 MPa or more, which has been difficult to produce by conventional techniques, and a method for producing the same. It is. In the steel sheet of the present invention, the target value of the yield ratio is 75% or more. In the present invention, the “yield ratio” is a value obtained by dividing the yield point by the tensile strength when the yield point appears, and by 0.2% proof stress by the tensile strength when the yield point does not appear. Value.

  The present invention suppresses the formation of martensite and austenite with high C concentration, which lowers the yield ratio and yield strength, and further increases the tensile strength, so that a larger amount of Ti is appropriate. By adding steel to the range, and further controlling the C and Mn amounts to a specific range and finding the optimum production conditions for the chemical composition, the tensile strength is 780 MPa or more and alloying and melting with a high yield ratio. Based on the knowledge that galvanized steel sheets can be obtained. In the prior art, it has been difficult to stably manufacture such a steel plate.

  The present invention relates to an alloyed hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet. The steel sheet has a composition of C: 0.065% or more and 0.12% or less (in this specification, unless otherwise specified). "%" Means "mass%"), Si: 0.001% or more and 0.2% or less, Mn: more than 2.0%, 2.7% or less, P: 0.1% or less , S: 0.01% or less, sol. Al: 0.001% or more and 0.25% or less, Ti: 0.12 or more and 0.30% or less, N: 0.01% or less and O: 0.01% or less An alloyed hot-dip galvanized steel sheet having a steel structure in which the area ratio of austenite is 3.0% or less and a mechanical property in which a tensile strength is 780 MPa or more.

In the galvannealed steel sheet according to the present invention, it is preferable that the chemical composition further contains Nb: 0.2% or less by mass%.
In the alloyed hot-dip galvanized steel sheet according to the present invention, the chemical composition is mass%, Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1 % Or less and V: It is preferable to further contain one or more selected from the group consisting of 0.1% or less.

In the galvannealed steel sheet according to the present invention, the chemical composition is mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01. % Or less and Bi: It is preferable to further contain one or more selected from the group consisting of 0.01% or less.
In the galvannealed steel sheet according to the present invention, it is preferable that the chemical composition further contains B: 0.002% or less in terms of mass%.

From another viewpoint, the present invention is a method for producing a hot dip galvanized steel sheet, comprising the following steps (A) to (C).
(A) A hot rolling process in which a steel sheet having the chemical composition of the hot dip galvanized steel sheet according to the present invention is subjected to hot rolling at a rolling start temperature of 1220 ° C. or higher and 1300 ° C. or lower to obtain a hot rolled steel sheet;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to recrystallization annealing in a temperature range of Ac _{3 to} 950 ° C. And then cooled to a temperature range of [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less, and then held for 500 seconds or less including the time of immersion in the plating bath in the temperature range. A continuous annealing-alloying hot dip galvanizing process in which alloying is performed in a temperature range of 460 ° C. or higher and 600 ° C. or lower and [zinc plating bath temperature + 40 ° C.] or higher.

  According to the present invention, it is possible to mass-produce galvannealed steel sheets having a tensile strength of 780 MPa or more and a high yield ratio. The alloyed hot-dip galvanized steel sheet according to the present invention can be widely used industrially, particularly in the automobile field.

1. Chemical Composition The reason why the chemical composition of the galvannealed steel sheet according to the present invention is specified as described above will be described.

(C: 0.065% to 0.12%)
C is an element contributing to strength improvement, and is contained by 0.065% or more in order to make the steel sheet have a tensile strength of 780 MPa or more. However, when C is contained exceeding 0.12%, a hard structure comes to be mixed and the yield ratio decreases. For this reason, C content shall be 0.065% or more and 0.12% or less. In addition, in order to acquire the said effect more reliably, it is preferable that C content is 0.075% or more and 0.105% or less.

(Si: 0.001% to 0.2%)
Si is an element contributing to strength improvement, and in the present invention, Si is contained in an amount of 0.001% or more. However, when Si is contained exceeding 0.2%, a non-plating part will generate | occur | produce in a steel plate and corrosion resistance will deteriorate. For this reason, Si content shall be 0.001% or more and 0.2% or less. Preferably, the Si content is 0.05% or more and 0.15% or less. By controlling the Si amount in this way, the adhesion of plating is improved, and powdering and flaking occur due to press molding. Can be prevented.

(Mn: more than 2.0% and 2.7% or less)
Mn is an element that significantly contributes to strength improvement, and is contained in excess of 2.0% in order to make the tensile strength of the steel plate 780 MPa or more. In addition, although Mn delays the bainite transformation, the balance of C amount, Ti amount and Nb amount, and optimization of hot dip galvanizing conditions sufficiently suppress the formation of martensite even in steels containing a relatively large amount of Mn. The However, when Mn is contained exceeding 2.7%, martensite is generated and the yield ratio is lowered. For this reason, the Mn content is more than 2.0% and not more than 2.7%. In addition, in order to acquire the said effect more reliably, it is preferable that Mn content exceeds 2.0% and is 2.6% or less.

(P: 0.1% or less)
P is an element contained as an impurity, but may also be positively incorporated because it is also an element contributing to strength improvement. However, when P is contained exceeding 0.1%, the weldability is remarkably deteriorated. Therefore, the P content is 0.1% or less. Preferably, the P content is 0.005% or more and 0.025% or less. By controlling the P content in this way, the steel sheet can be strengthened more reliably and plating defects such as powdering can be prevented. It becomes possible to do.

(S: 0.01% or less)
S is an element inevitably contained as an impurity, and is an element that significantly deteriorates press formability. For this reason, S content shall be 0.01% or less. In addition, a moldability improves, so that the content is low, Preferably, it is 0.005% or less. More preferably, it is 0.0015% or less.

(Sol.Al: 0.001% to 0.25%)
Al is an element that deoxidizes steel and improves the yield of carbonitride-forming elements such as Ti. In order to suppress the formation of Ti-based, Nb-based, or Ti—Nb composite-based oxides, sol. The Al content is 0.001% or more. However, the sol. When Al is contained, a non-plated portion is generated in the steel sheet, and the corrosion resistance is deteriorated. For this reason, sol. Al content shall be 0.001% or more and 0.25% or less. In addition, Preferably it is more than 0.02% and 0.2% or less, More preferably, it is more than 0.02% and 0.08% or less.

(N: 0.01% or less)
N is an element inevitably contained as an impurity, and is an element that significantly deteriorates press formability. For this reason, N content shall be 0.01% or less. In addition, a moldability improves, so that the content is low, Preferably, it is 0.005% or less. More preferably, it is 0.004% or less.

(O: 0.01% or less)
O is unavoidably contained as an impurity, and is an element that significantly deteriorates press formability. For this reason, the O content is set to 0.01% or less. In addition, a moldability improves, so that the content is low, Preferably, it is 0.005% or less. More preferably, it is 0.002% or less.

(Ti: 0.12% to 0.30%)
Ti is an element that forms fine carbides, nitrides, or carbonitrides and contributes significantly to improving the strength. Further, as described above, by balancing the amount of C and the amount of Mn and further combining annealing conditions, hot dip galvanizing, and alloying treatment conditions as will be described later, martensite becomes difficult to form, and the tensile strength is 780 MPa. Nevertheless, a high yield ratio is also achieved. In order to express such an effect, 0.12% or more of Ti is contained. However, if Ti is contained in excess of 0.30%, coarse carbides are formed and the tensile strength is lowered, and the carbides become a dislocation source, the yield strength is significantly lowered, and the yield ratio is also lowered. For this reason, Ti content shall be 0.12% or more and 0.30% or less. In addition, in order to acquire the said effect more reliably, it is preferable that Ti content is 0.14% or more and 0.24% or less.

(Nb: 0.2% or less)
Nb is an element that contributes to strength improvement, and is an optional element that can be contained as necessary. However, when Nb is contained exceeding 0.2%, the properties of the cut surface deteriorate, and the press formability deteriorates. For this reason, it is preferable to contain Nb by the said quantity. In addition, in order to acquire the said effect more reliably, it is preferable to contain Nb 0.01% or more.

(Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1% or less, and V: 0.1% or less, or one selected from the group consisting of 2 types or more)
Cr, Mo, Cu, Ni and V are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. However, even if each content exceeds 0.1%, the effect is saturated and the manufacturing cost is increased. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Cr, Mo, Cu, Ni, and V by the said quantity. In addition, 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, Zr: 0.01% or less, and Bi: 0.01% or less, or one selected from the group consisting of 2 types or more)
Ca, Mg, REM, Zr, and Bi are all elements that improve press formability such as bendability and hole expansibility without lowering the yield ratio, and can be included as needed. It is an element. However, if each content exceeds 0.01%, the surface properties deteriorate. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Ca, Mg, REM, Zr, and Bi with the said quantity. In addition, in order to acquire the said effect more reliably, it is preferable to contain any element 0.0005% or more.
Here, REM refers to 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: 0.002% or less)
B is an element that contributes to strength improvement, and is an optional element that can be contained as necessary. However, even if it contains B exceeding 0.002%, the said effect is saturated. For this reason, it is preferable that B content shall be 0.002% or less. In addition, in order to acquire the said effect more reliably, it is preferable to make it contain 0.0002% or more.

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

(Area ratio of retained austenite: 3.0% or less)
The alloyed hot-dip galvanized steel sheet according to the present invention having the chemical composition described above has a structure mainly composed of ferrite, bainitic ferrite and bainite, and preferably contains no martensite and retained austenite as much as possible. Among them, austenite is the hardest structure as it is annealed or at the initial stage of deformation leading to yield strength, and the yield ratio decreases as the area ratio of retained austenite increases. Therefore, the retained austenite is 3.0% or less (including 0%) in the fraction evaluated by the area ratio. The total area ratio of martensite and retained austenite is preferably 20% or less. In the galvannealed steel sheet according to the present invention, it is difficult to clearly distinguish between ferrite, bainitic ferrite, and bainite, and thus it is difficult to define each area ratio. The total area ratio of ferrite, bainitic ferrite and bainite is preferably 80% or more.

3. Manufacturing Method Next, a preferable manufacturing method of the galvannealed steel sheet according to the present invention will be described.
It is preferable that the molten steel having the above-described chemical composition is melted by a 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, casting may be performed by an ingot casting method, a thin slab casting method, or the like. This steel material is hot-rolled to obtain a hot-rolled steel sheet. Hot rolling does not cool the cast steel material to room temperature, inserts it into a heating furnace in the form of a hot piece, direct feed rolling to roll after heating, or direct rolling to roll immediately after holding a little heat Alternatively, the steel material may be once cooled and then reheated and rolled.

(Starting temperature of hot rolling: 1220 ° C or higher and 1300 ° C or lower)
Due to the dispersion of fine precipitates such as Ti and Nb, the tensile strength and yield strength of the galvannealed steel sheet according to the present invention are enhanced. In order to make the tensile strength of the steel sheet 780 MPa or more and increase the yield ratio without dispersing coarse precipitates, it is necessary to once dissolve Ti and Nb before hot rolling. For this reason, heating temperature before hot rolling shall be 1220 degreeC or more. However, if it heats exceeding 1300 degreeC, the internal oxidation of a steel raw material will be accelerated | stimulated and surface properties will deteriorate remarkably. For this reason, the heating temperature of a steel raw material shall be 1220 degreeC or more and 1300 degrees C or less. In other words, the hot rolling start temperature is not less than 1220 ° C and not more than 1300 ° C. In addition, Preferably, the starting temperature of hot rolling is 1240 degreeC or more and 1280 degrees C or less, By limiting temperature in this way, it becomes possible to acquire the said effect more reliably. Moreover, it is preferable to hold | maintain a steel raw material in a temperature range of 1220 degreeC or more for 30 minutes or more before starting hot rolling. By holding the steel material at a high temperature in this manner, the non-uniform structure due to solidification segregation of Mn is eliminated, and press formability such as bendability is improved. However, even if it is kept for more than 180 minutes, the above effect is saturated and the manufacturing cost only becomes high, so it is preferable to keep it for 180 minutes or less.

(Finishing rolling temperature: 800 ° C or higher and 1000 ° C or lower)
In order to reduce the deformation resistance during hot rolling and make the operation easier, the finish rolling temperature is preferably set to 800 ° C. or higher. However, when finish rolling is performed at a temperature exceeding 1000 ° C., scale wrinkles are easily generated, and the surface properties are significantly deteriorated. For this reason, it is preferable that finish rolling temperature shall be 800 degreeC or more and 1000 degrees C or less. More preferably, it is 850 degreeC or more and 950 degrees C or less.

(Hot rolled coiling temperature: 750 ° C or less)
In order to suppress oxidation of the steel sheet and ensure good surface properties, it is preferable to set the hot rolling coiling temperature to 750 ° C. or lower. However, if it winds below 200 degreeC, a hard bainite and a martensite will produce | generate and it will become difficult to cold-roll after that. For this reason, it is preferable to make hot rolling coiling temperature into 200 degreeC or more and 750 degrees C or less. More preferably, it is 500 degreeC or more and 630 degrees C or less.

  In the hot rolling process, in order to suppress fluctuations in characteristics, it is preferable to achieve uniform temperature of the entire length of the rough bar by performing induction heating or the like on the rough bar before the finish rolling after the rough rolling.

The hot-rolled steel sheet obtained by the hot rolling process is descaled by a conventional method such as pickling, and then cold-rolled to obtain a cold-rolled steel sheet. In this case, the total rolling reduction in hot rolling and cold rolling is preferably 95% or more. Here, the total rolling reduction is calculated by the following equation.
Total rolling reduction (%) = {1− (thickness of cold-rolled steel sheet) / (thickness of slab used for hot rolling)} × 100

  When the total rolling reduction is increased, the Mn segregation band distributed in the sheet thickness direction becomes thinner, and the press formability such as bendability and hole expandability is improved. In addition, in order to make the steel structure after continuous annealing uniform, it is preferable that the total rolling reduction of cold rolling is 30% or more. In order to ensure the flatness of the steel sheet, it is preferable to correct the shape by rolling at a reduction rate of 5% or less before or after pickling. Moreover, by performing such mild rolling before pickling, pickling performance is improved, removal of surface concentrating elements is promoted, and surface properties are improved.

The cold-rolled steel sheet obtained by the hot rolling process and the cold rolling process is subjected to recrystallization annealing in a temperature range of Ac ₃ points or more and 950 ° C., and then [zinc plating bath temperature −20 ° C.] or more [zinc [Plating bath temperature + 100 ° C.] After cooling to a temperature range of not more than 500 ° C., including at the time of plating bath immersion, the temperature range is 460 ° C. or more and 600 ° C. or less, and [Zinc plating bath temperature + 40 ° C. An alloying treatment is performed in the above temperature range to obtain an alloyed hot-dip galvanized steel sheet. These steel plates are preferably manufactured by annealing, hot dip galvanizing, alloying treatment in a continuous hot dip galvanizing line.

(Recrystallization annealing temperature: Ac ₃ points or more and 950 degrees C or less)
As described above, when steel containing a large amount of Ti or Nb is annealed in the two-phase region, unrecrystallized ferrite remains, and not only the toughness is deteriorated, but also the tensile strength depends on the annealing temperature. For this reason, annealing temperature shall be Ac ₃ points or more. However, if the annealing temperature exceeds 950 ° C., the annealing furnace is rapidly damaged, and its repair is required, and the productivity is deteriorated. For this reason, recrystallization annealing temperature shall be Ac ₃ points or more and 950 degrees C or less. In order to stably ensure good surface properties, it is preferable recrystallization annealing temperature is below 900 ° C. ₃ point or more Ac. Moreover, it is preferable to hold for 10 seconds or more at Ac ₃ points or more. By controlling the annealing time in this way, it becomes easy to stably ensure good press formability. However, even if it is held for more than 300 seconds, the above effect is saturated and the manufacturing cost is increased.

Moreover, in order to improve the wettability and alloying processability of plating, it is preferable that the dew point during annealing is −40 ° C. or higher.
After recrystallization annealing, the steel sheet is cooled in the process of being immersed in a galvanizing bath. In this case, the average cooling rate is from 1 ° C./second to 50 ° C./second from the highest temperature to 700 ° C., and then from 3 ° C./second to 50 ° C./second from 700 ° C. to the cooling stop temperature. It is preferable. By cooling to 700 ° C. at 1 ° C./second or more and 50 ° C./second or less, it becomes possible to easily adjust the area ratio of ferrite, bainitic ferrite and bainite, and is contained in the galvannealed steel sheet. It becomes easy to control the amount of retained austenite. On the other hand, by cooling from 700 ° C. to the cooling stop temperature at 3 ° C./second or more, it becomes possible to suppress pearlite transformation leading to strength reduction. Further, when cooling to the cooling stop temperature at more than 50 ° C./second, it is necessary to significantly modify the continuous hot dip galvanizing equipment and the manufacturing cost is remarkably increased.

(Cooling stop temperature: [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less)
The cooling stop temperature is set to [zinc plating bath temperature −20 ° C.] or higher in order to reduce the heat removal upon entering the plating bath and facilitate the operation. However, if the cooling of the steel sheet is stopped exceeding [zinc plating bath temperature + 100 ° C.], the temperature change of the plating bath becomes remarkable, and the operation becomes difficult. For this reason, the cooling stop temperature is set to [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less. The hot dip galvanizing treatment follows a conventional method of immersing the annealed steel sheet in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower.

(Holding time of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less: 500 seconds or less, but also includes plating immersion.)
In order to suppress softening of ferrite, bainitic ferrite and bainite and to secure a desired tensile strength, the holding time of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less is plating immersion. 500 seconds or less including time. Preferably, the holding time is 10 seconds or longer. By giving such a holding time, the coating amount of the steel sheet can be adjusted, and good corrosion resistance can be secured stably.

(Alloying temperature: 460 ° C. or higher and 600 ° C. or lower and [zinc plating bath temperature + 40 ° C.] or higher)
In order to suppress the occurrence of non-alloying treatment and improve the corrosion resistance, the alloying treatment temperature after immersion in the plating bath is set to 460 ° C. or higher. Furthermore, in order to decompose the austenite to the steel plate having the chemical composition according to the present invention, to reduce the retained austenite to 3.0% or less in terms of area ratio, and to increase the yield ratio of the steel plate, the alloying treatment after immersion in the plating bath The temperature is [zinc plating bath temperature + 40 ° C.] or higher. However, when alloying is performed at a temperature exceeding 600 ° C., ferrite, bainitic ferrite and bainite are softened, and the tensile strength is remarkably lowered. For this reason, the alloying treatment temperature is set to 460 ° C. or higher and 600 ° C. or lower and [zinc plating bath temperature + 40 ° C.] or higher. Preferably, the alloying treatment temperature is 490 ° C. or more and 560 ° C. or less, and the degree of alloying (Fe content of the plating layer) is 8 mass% or more and 13 mass% or less by controlling the temperature in this way. It becomes easy to improve the adhesion of plating.

  In order not only to suppress the occurrence of yield point elongation, but also to prevent seizure and galling during pressing, it is preferable to perform temper rolling at an elongation of 0.05% or more and 1% or less after continuous hot dip galvanizing treatment.

Moreover, in order to improve the wettability and alloying processability of plating, the steel plate before annealing may be plated with one or more of Ni, Cu, Co, and Fe.
By the production method, an alloyed hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and a high yield ratio can be produced.

  Thus, by devising the chemical composition of steel and optimizing the conditions for continuous annealing after hot rolling and cold rolling, hot dip galvanizing and alloying treatment, the tensile strength is 780 MPa or more, and the yield ratio is A high, ie, 0.75 or higher, galvannealed steel sheet and a method for producing the same are provided.

Steel having the chemical composition shown in Table 1 was melted in a converter, and a 245 mm thick slab was cast by continuous casting.
Ac ₃ shown in Table 1 was obtained by regression analysis of literature values and experimental values, and was obtained from the following equation.

Ac ₃ = 910-203 × (C ^1/2 ) −15.2 × Ni + 44.7 × Si + 104 × V + 31.5 × Mo-30 × Mn-11 × Cr-20 × Cu + 700 × P + 400 × Al

  The obtained slab was hot rolled under the conditions shown in Table 2 to produce a 2.6 mm thick hot rolled steel sheet. The obtained hot-rolled steel sheet was pickled and cold-rolled to produce a 1.2 mm-thick cold-rolled steel sheet.

  The obtained cold-rolled steel sheet was heated to 700 ° C. at a heating rate of 10 ° C./second, heated from 700 ° C. to the annealing temperature shown in Table 3, which was the highest temperature, at a heating rate of 3 ° C./second, The temperature was maintained for the time shown in Table 3 and annealed. Cooling is performed at an average cooling rate of 8 ° C./second from the annealing temperature to the cooling stop temperature. Further, in order to simulate the thermal history during the hot dip galvanizing process, the cooling stop temperature is maintained for the time shown in Table 3, and an assumed plating bath The temperature is cooled to 455 ° C. over 5 seconds, held at that temperature for 10 seconds, further heated to the alloying treatment temperature shown in Table 3 over 5 seconds, held at that temperature for 10 seconds, and then brought to room temperature for 10 seconds. It cooled at the cooling rate of (degreeC / sec), and produced the annealing cold-rolled steel plate. The holding time of the cooling stop temperature is the total of the holding time at the cooling stop temperature, the time for cooling to the plating bath temperature, and the time for holding at the plating bath temperature.

  The annealed cold-rolled steel sheet produced in this example is not hot-dip galvanized, but receives the same thermal history as the alloyed hot-dip galvanized steel sheet, so the mechanical properties of the steel sheet are alloys with the same thermal history. It is substantially the same as the galvannealed steel sheet.

  The obtained annealed cold-rolled steel sheet was analyzed for structure using an optical microscope or an electron microscope, and further using an X-ray diffraction method, and mechanical properties were evaluated by a tensile test.

[Test method]
(Area ratio of retained austenite)
Each annealed cold-rolled steel sheet is subjected to chemical polishing to reduce the thickness by ¼, X-ray diffraction is applied to the surface after chemical polishing, the obtained profile is analyzed, and the area ratio of residual austenite is calculated. did.

(Total area ratio of martensite and austenite)
Test specimens were collected from each annealed cold rolled 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 in the direction perpendicular to the rolling direction were photographed with an optical microscope or electron microscope, and the total of martensite and austenite was analyzed by image analysis. The area ratio was measured. Thus, the average value of the data of the plurality of total area ratios obtained for one test material was defined as the total area ratio of martensite and austenite of the test material.

(mechanical nature)
A JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction, and the tensile strength and yield strength (yield point or 0.2% yield strength) were measured.

  The evaluation results 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 Nos. 1, 4, 5, 7, 9, 11, 14, 17, and 19 are steel plates of the present invention that satisfy all the conditions of the present invention. 2, 3, 6, 8, 10, 12, 13, 15, 16, and 18 are comparative steel plates that do not satisfy at least one of the conditions of the present invention.

  Specimen No. The steel sheets of Examples 1, 4, 5, 7, 9, 11, 14, 17 and 19 have an area ratio of retained austenite of 3.0% or less, a tensile strength of 780 MPa or more, and a yield ratio of It is a high (0.75 or more) steel plate.

  On the other hand, steel plate No. of the comparative example. 2 and 15 are production conditions (retention time and alloying treatment temperature in a temperature range of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less, respectively, out of the scope of the present invention, The desired tensile strength cannot be obtained. Steel plate No. 3 and 6 have chemical compositions (C content and Mn content, respectively) that are out of the scope of the present invention, and a desired tensile strength cannot be obtained. Steel plate No. Nos. 8 and 18 have chemical compositions (both Ti contents) that are out of the scope of the present invention, and not only a desired tensile strength cannot be obtained, but also the yield ratio is low. Steel plate No. No. 10 has manufacturing conditions (slab heating temperature, that is, hot rolling start temperature) that are out of the scope of the present invention, and not only a desired tensile strength cannot be obtained, but also the yield ratio is low. Steel plate No. No. 12 has manufacturing conditions (alloying temperature) that are out of the scope of the present invention, and the area ratio of retained austenite is high and the yield ratio is low. Steel plate No. 13 and 16 have chemical compositions (Mn content and C content, respectively) that are out of the scope of the present invention, and the yield ratio is low.

Claims (6)

  In the alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, the steel sheet is in mass%, C: 0.065% to 0.12%, Si: 0.001% to 0.00. 2% or less, Mn: more than 2.0%, 2.7% or less, P: 0.1% or less, S: 0.01% or less, sol. Having a chemical composition containing Al: 0.001% or more and 0.25% or less, Ti: 0.12% or more and 0.30% or less, N: 0.01% or less, and O: 0.01% or less, An alloyed hot-dip galvanized steel sheet having a steel structure in which the area ratio of retained austenite is 3.0% or less and having a tensile strength of 780 MPa or more.   The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition further contains, by mass%, Nb: 0.2% or less.   The chemical composition is, in mass%, Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1% or less, and V: 0.1% or less. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, further comprising one or more selected from the group.   The chemical composition is, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less, and Bi: 0.01% or less. The galvannealed steel sheet according to any one of claims 1 to 3, further comprising one or more selected from the group.   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the chemical composition further contains, by mass%, B: 0.002% or less. A method for producing an alloyed hot-dip galvanized steel sheet, comprising the following steps (A) to (C):
(A) A hot rolling step in which a steel material having the chemical composition according to any one of claims 1 to 5 is subjected to hot rolling at a rolling start temperature of 1220 ° C or higher and 1300 ° C or lower to form a hot-rolled steel plate. ;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to recrystallization annealing in a temperature range of Ac ₃ points or less and 950 ° C. or less. And then cooled to a temperature range of [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less, and then held for 500 seconds or less including the time of immersion in the plating bath in the temperature range. A continuous annealing-alloying hot dip galvanizing process in which alloying is performed in a temperature range of 460 ° C. or higher and 600 ° C. or lower and [zinc plating bath temperature + 40 ° C.] or higher.