JP4924203B2 - High-strength galvannealed steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same. More particularly, the present invention relates to a high-strength galvannealed steel sheet excellent in hole expansibility, and a method for producing the same, which are suitable for applications such as press forming such as the body of an automobile, and particularly, stretch flange forming, which has been difficult in the past.

  In recent years, in order to protect the global environment, there has been a demand for improving the fuel efficiency of automobiles. In order to reduce the weight of the vehicle body and ensure the safety of passengers, high strength steel sheets with a tensile strength of 540 MPa or more, particularly considering rust prevention In materials, there is an increasing need for high-strength galvannealed steel sheets.

  However, high strength is not sufficient for the above applications, and the steel sheet must satisfy the performance required at the time of forming a part, such as during pressing or welding. For example, considering the part molding process, high ductility and excellent hole expansibility are required. In particular, with respect to local deformation such as stretch flange deformation, molding defects cannot be avoided only by adjusting the mold, so that excellent hole expandability at a mild steel level is required while having high strength.

  By the way, in general, high-strength steel sheets use ferrite as a parent phase and use a hard phase such as martensite and bainite, so it is easy to generate microvoids that are the starting point of ductile fracture at the heterogeneous interface between ferrite and hard phase. The hole expandability was insufficient. Therefore, many researches and developments have been made to improve the hole expandability of high-strength steel sheets, and a structure control method is being established.

  For example, Non-Patent Document 1 below discloses a finding that the hole expandability is improved as the strength difference between ferrite and martensite, which is a hard phase, becomes smaller in a composite structure of ferrite and martensite.

Non-Patent Document 2 below discloses a ferritic single-phase steel that does not use the hard phase itself that causes the strength difference and has excellent hole expandability.
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 by quenching and tempering process, and Non-Patent Document 2 relates to a hot-rolled steel sheet that makes the most of precipitation strengthening.

  The manufacturing process of the alloyed hot-dip galvanized steel sheet has a temperature history in which it is immersed in a hot-dip galvanizing bath at 400 ° C. or higher when cooled from the recrystallization temperature and then reheated after immersion for the purpose of alloying. There are features. That is, in this manufacturing process, since the cooling is temporarily interrupted at 400 ° C. or higher, it can be said that the bainite transformation is essentially easy to proceed when a high-strength steel sheet is manufactured.

  When bainite is generated, not only coarse cementite but also C (carbon) is concentrated in austenite, so that a structure containing island martensite and massive austenite is easily obtained. Coarse cementite, island martensite, and massive austenite are all hard phases with very high strength and insufficient ductility, making it difficult to improve the hole expandability of high strength hot dip galvanized steel sheets. .

  However, there is a conventional technology for high-strength hot-dip galvanized steel sheets with improved hole expansibility by advanced structure control and optimization of additive components. For example, Patent Document 1 discloses a technique for generating a martensite single-phase structure by positively adding Mo to a steel sheet to achieve both high strength and excellent hole expansibility. However, when a martensite single-phase structure is used, ductility is poor and molding defects occur.

  Patent Document 2 discloses a technique for forming a bainite or bainitic ferrite-based structure by positively adding Nb and Mo to a steel sheet to achieve both high strength and excellent hole expansibility. . However, when a bainite or bainitic ferrite-based structure is used, the ductility is insufficient and molding defects tend to occur.

Therefore, in order to ensure ductility, a technique for improving the hole expansibility of a structure mainly composed of ferrite is required. For example, Patent Document 3 actively adds Ti and Nb to a steel sheet, anneals in a two-phase region, generates a fine ferrite-based structure, and re-anneals in a temperature range of 500 ° C. to Ac ₁ point. Thus, in the manufacturing process of the alloyed hot-dip galvanized steel sheet, a technique is disclosed in which hard island martensite and massive austenite are decomposed to combine high strength, high ductility, and excellent hole expansibility. However, since it is premised on the two-time annealing, not only the manufacturing cost is increased, but if the steel with positive addition of Ti and Nb is annealed in the two-phase region, unrecrystallized may remain, which is stable. It is difficult to ensure ductility.

  Patent Document 4 positively adds Mn and B to a steel sheet, and appropriately controls the amount of B, thereby suppressing the generation of microvoids even in a composite structure containing ferrite and hard martensite. A steel sheet having both high strength, high ductility, and excellent hole expansibility is disclosed. However, it is very difficult to control the amount of B in steel, and when B is added in the composite structure, the tensile strength changes remarkably depending on the amount of B, so that forming defects are likely to occur.

Patent Document 5 combines high strength, high ductility, and excellent hole expansibility by actively adding Mn and Ti to a steel sheet and appropriately controlling not only the amount of Mn and Ti but also the amount of C. A steel sheet is disclosed. However, in a composite structure that actively uses a transformed structure, the structure changes significantly due to annealing and cooling conditions, and as a result, not only the tensile strength but also the hole expandability changes significantly. It is difficult to provide a steel plate having high strength, high ductility, and excellent hole expansibility, that is, excellent material stability.
ISIJ International, vol.44 (2004), No.3, p.603-609 ISIJ International, vol.44 (2004), No.11, p.1945-1951 JP 2004-315882 A JP 2003-193190 A Japanese Patent Laid-Open No. 2004-211126 JP 2004-211140 A JP 2005-220417 A

  An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having a tensile strength of 540 MPa or more and excellent in all of hole expansibility, ductility and material stability, and a method for producing the same, which has been difficult in the past. .

  In the steel plate of the present invention, the target value of hole expandability (hole expansion rate <HER> described later) is 50% or more, and the target value of ductility is TS × EL value obtained by a tensile test of 12000 MPa ·% or more. Yes, the target value of material stability is that the upper and lower limit range (ΔTS) of tensile strength is 100 MPa or less.

  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 cross member, which is a representative part of an automobile reinforcing member, including adjustment of the shape and mold, desired ductility, It is preferable that the tensile strength is 590 MPa or more and the hole expansibility is 80% or more while achieving 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.

  The present invention balances the chemical composition so that the performance can be stably maintained in mass production while suppressing the formation of coarse cementite, island martensite, and massive austenite that deteriorate the hole expandability, By applying optimum manufacturing conditions, a desired structure can be obtained, and a high-strength galvannealed steel sheet with excellent hole expansibility, ductility, and material stability can be obtained without reducing the strength level. Was found and completed. Conventionally, it has been difficult to provide a steel sheet that simultaneously satisfies such characteristics in the hot dip galvanizing manufacturing process.

Here, the present invention is as follows.
(1) By mass%, C: 0.03 to 0.10%, Si: 0.005 to 0.2%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: It contains 0.01% or less, sol.Al: 0.01 to 0.1%, N: 0.01% or less, and Ti: 0.5% or less, Nb: 0.5% or less Or about 2 types, it contains in the range which satisfy | fills the following inequality, the remainder is made into Fe and an impurity, the area ratio of a ferrite is 60% or more, the area ratio of a retained austenite is 3.0% or less, and the average particle diameter of a ferrite is 1. A high-strength alloyed molten zinc having a steel structure containing 0 to 6.0 μm of precipitates having a particle size of 1 to 10 nm in ferrite of 100 / μm 2 or more and a tensile strength of 540 MPa or more Plated steel:
Ti + Nb / 2> 0.05.

  (2) The chemical composition is selected from the group consisting of, by mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. 1 type or 2 types or more are contained.

(3) A method for producing a high-strength galvannealed steel sheet comprising the following steps (A) to (C):
(A) A steel material having the steel composition described in (1) or (2) above is heated to 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. Hot rolling process to hot rolled steel sheet by hot rolling;
(B) a cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; and
(C) After annealing the cold-rolled steel sheet at a temperature range of 750 ° C. to Ac ₃ transformation point at 1 ° C./second or more, annealing is performed for 5 to 200 seconds in the temperature range of Ac ₃ transformation point to 950 ° C. After the application, the average cooling rate from 750 ° C. to 600 ° C. is 1 to 50 ° C./second, and the cooling is performed to the temperature range of [Zinc plating bath temperature−20 ° C.] to [Zinc plating bath temperature + 100 ° C.] Continuous annealing-alloying hot dip galvanizing process in which alloying treatment is performed at 430 to 600 ° C. after holding for 30 seconds to 1000 seconds including plating immersion in the same temperature range.

  According to the present invention, a high-strength galvannealed steel sheet having a strength of 540 MPa or more and excellent in all of hole expansibility, ductility, and material stability can be mass-produced. Furthermore, it is also possible to produce a high-tensile galvannealed steel sheet having a strength of 590 MPa or higher, which is higher strength, and further excellent hole expansibility, ductility, and material stability. The alloyed hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

The reason why the steel composition of the steel sheet according to the present invention is defined as described above will be described. In this specification, “%” defining the chemical composition of steel is “mass%”.
C: 0.03 to 0.10%
Since C (carbon) is an element necessary for ensuring the strength of the steel, its content is set to 0.03% or more. However, excessive addition acts on the formation of retained austenite and degrades the hole expandability, so the content is made 0.10% or less. In addition, if the content is less than 0.04% and the content of Mn is less than 2.3%, as described later, it becomes difficult to obtain a tensile strength of 590 MPa or more. On the other hand, if it exceeds 0.06%, the amount of C in the retained austenite increases, and the hole expandability cannot be further improved. For this reason, it is preferably 0.04 to 0.06%.

Si: 0.005 to 0.20%
Since Si is an element that improves strength by solid solution strengthening and promotes ferrite transformation to improve ductility and the like, its content is set to 0.005% or more. However, excessive addition degrades the wettability and adhesion of the plating, so the content is made 0.20% or less.

Mn: 2.0-4.0%
Mn is an element that has the effect of improving the strength by solid solution strengthening, lowering the Ac ₃ point of the steel, and expanding the preferred annealing temperature range, so its content is made 2.0% or more. However, excessive addition suppresses ferrite transformation and degrades ductility, so the content is made 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 is difficult to obtain a tensile strength of 590 MPa or more. For this reason, it is preferably 2.3 to 4.0%.

P: 0.1% or less P is an element that is generally contained as an impurity, but in the present invention, it is a solid solution strengthening element and is effective for strengthening a steel sheet. % Or more is preferable. On the other hand, since the adhesiveness and weldability of plating are deteriorated, the content is made 0.1% or less.

S: 0.01% or less S is an element contained as an impurity in steel, and is preferably as low as possible from the viewpoints of workability, hole expansibility, and weldability. Therefore, the content is made 0.01% or less. Preferably it is 0.005% or less.

Al: 0.01 to 0.1%
Al is an element added to deoxidize steel, and is an element that effectively acts to improve the yield of carbonitride-forming elements such as Ti. Therefore, its content is 0.01% or more. To do. However, excessive addition causes an increase in oxide inclusions, resulting in deterioration of surface properties and moldability and high cost, so the content is made 0.1% or less. Preferably it is 0.02 to 0.08%.

N: 0.01% or less N is an element which is inevitably contained in general, but in the present invention, Ti, Nb, or Ti—Nb composite nitride or carbonitride in the steel sheet Is effective in increasing the strength of the steel sheet, and therefore the content is preferably 0.0005% or more. On the other hand, excessive addition forms coarse TiN and degrades hole expansibility, so the content is made 0.01% or less.

Ti: 0.50% or less, Nb: 0.50% or less, Ti + Nb / 2 ≧ 0.05%
Ti and Nb are contained in one or two kinds, and are elements effective in increasing the strength of the steel sheet by forming carbides, nitrides, or carbonitrides. Further, when the amount of C as described above is combined with annealing conditions as described later, it has an effect of promoting ferrite transformation, is an element effective for improving the ductility of the steel sheet, and the crystal grain size is extremely fine. It is an element that has an effect of reducing the generation of retained austenite and is effective in improving the hole expansibility of the steel sheet. In order to express such an effect, at least one or two of Ti and Nb are contained, and the content of Ti + Nb / 2 is 0.05% or more. This value is preferably 0.1% or more.

  Further, when 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, the material stability is ensured. However, even if added excessively, the effect is saturated and the cost is increased, so each content is 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, the material stability and hole expansibility are further improved. Since it is also an effective element that can be produced, it is preferable that the P _EQ value defined below exceeds 0.8. P _EQ is the ratio of the total mole fraction (A) of Ti and Nb not fixed to N and S to the mole fraction (B) of C.

P _EQ = 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, and Zr are optional addition components, and appropriate amounts thereof are added. Therefore, one element or two or more elements are added so that the total amount becomes 0.001% or more because it is an element that contributes to inclusion control, particularly fine dispersion, and further increases the hole expandability. Is preferred. However, excessive addition deteriorates ductility, so the total content is set to 0.01% or less.

The balance other than the components described above is Fe and impurities.
Next, the reason for limiting the steel structure of the high-strength cold-rolled steel sheet of the present invention will be described.
The high-strength cold-rolled steel sheet of the present invention having the above composition has ferrite as a main phase, ferrite as an area ratio of 60% or more, retained austenite as an area ratio of 3.0% or less, and the average grain size of the ferrite is 1. The steel structure has 0 to 6.0 μm, and further contains 100 precipitates / μm ² or more of precipitates having a particle size of 1 to 10 nm in the ferrite.

[Area ratio of ferrite: 60% or more]
The steel structure of the steel sheet according to the present invention is a fraction evaluated by area ratio, and the ferrite ratio is 60% or more. By including ferrite in an area ratio of 60% or more, it becomes possible to ensure a tensile strength of 540 MPa or more while achieving both hole expandability and ductility. For this reason, the area ratio of a ferrite shall be 60% or more.

[Area ratio of retained austenite: 3.0% or less (including 0%)]
The steel structure of the steel sheet according to the present invention is a fraction evaluated by area ratio, and the ratio of retained austenite is 3.0% or less. Residual austenite undergoes processing-induced transformation in punching and further hole expansion after punching, and becomes extremely hard martensite. Generation of hard martensite leads to concentration of strain and generation of microcracks, which adversely affects local ductility. Accordingly, the area ratio of retained austenite is preferably as low as possible from the viewpoint of hole expansibility. For this reason, the area ratio of a retained austenite shall be 3.0% or less.

  If the C concentration in the austenite is low, the hardness of the martensite can be reduced even after the process-induced transformation, so that the local ductility is less likely to be adversely affected. It is advantageous to reduce the C content in the retained austenite to 0.6% or less by optimizing the composition and continuous annealing. At this time, when the area ratio of the retained austenite is 0%, the amount of C in the retained austenite cannot be measured, but since hard martensite is not generated by the processing-induced transformation, the hole expandability can be further improved. .

[Average diameter of ferrite: 1.0 to 6.0 μm]
The steel structure of the steel sheet according to the present invention has a ferrite average particle size of 1.0 to 6.0 μm. By making the average particle diameter of ferrite 6.0 μm or less, the hole expandability can be improved. However, if the average particle size is less than 1.0 μm, the moldability deteriorates. For this reason, the average particle diameter of a ferrite shall be 1.0-6.0 micrometers.

[Precipitates in ferrite: particle size of 1 to 10 nm including 100 particles / μm ² or more]
The steel structure of the steel sheet according to the present invention contains precipitates having a particle size of 1 to 10 nm at a density of 100 / μm ² or more in the ferrite grains. When the number of precipitates having a particle size of 1 to 10 nm is less than 100 / μm ² , the amount of strengthening becomes small and the desired strength cannot be obtained. Since this precipitate is a carbonitride of Ti and / or Nb, it is necessary to appropriately control hot rolling conditions and annealing conditions described later together with containing desired Ti and / or Nb.

  In the present invention, the area ratio of ferrite and the average particle diameter, the area ratio of retained austenite, the particle diameter and density of precipitates in ferrite are as follows. It will be easily understood that the composition, hot rolling conditions, continuous annealing conditions, and the like can be appropriately adjusted to be within the scope of the present invention.

Next, the reason for limitation of the manufacturing method of the high-strength galvannealed steel sheet of the present invention will be described.
It is desirable that molten steel having the above steel composition is melted by a generally known melting method such as a converter or an electric furnace, and used as a steel material such as a slab by a continuous casting method. In place of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be employed. This steel material is hot rolled to obtain a hot rolled steel sheet. In hot rolling, cast steel material is not cooled to room temperature but directly fed into a heating furnace while being heated and heated and then rolled, or directly after a little heat retention Rolling may be performed, or the steel material may be once cooled and then reheated for rolling. At this time, it is preferable to equalize the entire length of the rough bar after the rough rolling and before the finish rolling by induction heating or the like because the characteristic variation can be suppressed.

[Rolling start temperature of steel material: 1050-1300 ° C.]
In the case of reheating, it is necessary to re-dissolve TiC or NbC in order not to deteriorate the hole expandability. In the present invention, such an effect is recognized by heating to 1050 ° C. or higher, but when heated to over 1300 ° C., not only the effect is saturated but also the scale loss increases. For this reason, the reheating temperature of a steel raw material shall be 1050 to 1300 degreeC. In other words, the hot rolling start temperature is 1050 ° C to 1300 ° C.

  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 direct rolling or direct rolling is performed, as long as TiC and NbC are dissolved, rolling may be started as it is. In this case, the rolling start temperature is preferably 1050 to 1300 ° C.

[Finish temperature: 800-950 ° C]
The finishing temperature of hot rolling is set to a range of 800 ° C to 950 ° C. When the finishing temperature is less than 800 ° C., the deformation resistance during rolling is large and the operation is impossible. On the other hand, when it exceeds 950 ° C., the precipitates become coarse and it becomes difficult to ensure the strength after continuous annealing.

[Winding temperature: 450-750 ° C]
The coiling temperature is in the range of 450 to 750 ° C. When the coiling temperature is less than 450 ° C., hard bainite and martensite are generated, and subsequent cold rolling becomes difficult. On the other hand, if the coiling temperature exceeds 750 ° C., the precipitates become coarse and fine precipitates cannot be obtained, and it becomes difficult to ensure the strength after continuous annealing. Preferably it is 550-650 degreeC.

  The hot-rolled steel sheet is pickled by a normal method and then cold-rolled to obtain a cold-rolled steel sheet. In order to refine the structure of the steel sheet after the continuous annealing, it is preferable that the rolling reduction of the cold rolling is 30% or more. In addition, before or after pickling, it is advantageous in terms of ensuring flatness if mild rolling of about 0 to 5% is performed and the shape is corrected. In addition, the mild rolling improves pickling properties, promotes removal of surface concentrating elements, and has an effect of improving the adhesion of hot dipping.

According to the present invention, the cold-rolled steel sheet thus obtained is heated at a temperature range of 750 ° C. to Ac ₃ transformation point at 1 ° C./second or more, so that the temperature range of Ac ₃ transformation point to 950 ° C. After annealing for 5 to 200 seconds, the average cooling rate from 750 ° C. to 600 ° C. is 1 to 50 ° C./second, and [zinc plating bath temperature −20 ° C.] to [zinc plating bath temperature + 100 ° C.]. After cooling to a temperature range, hot dip galvanizing is performed. The annealing heat treatment and hot dip galvanizing treatment by soaking for 5 to 200 seconds in the temperature range of Ac ₃ transformation point to 950 ° C. 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.

[Temperature increase rate from 750 ° C. to Ac ₃ transformation point: 1 ° C./second or more]
When heating to the annealing temperature, the temperature increase rate from 750 ° C. to Ac ₃ transformation point is set to 1 ° C./second or more. If the rate of temperature increase at this time is less than 1 ° C./sec, non-uniform grain growth due to Mn segregation occurs during temperature increase, a uniform structure cannot be obtained, and ductility decreases. The rate of temperature rise in the temperature region higher than the Ac ₃ transformation point is not particularly limited. A preferable temperature rising rate is 2 to 10 ° C./second.

  In the present invention, since a large amount of Ti and / or Nb is added, 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 achieved under the following annealing conditions.

[Annealing temperature: the temperature range of the Ac ₃ transformation point ~950 ℃]
Annealing is performed in a temperature range of Ac ₃ transformation point to 950 ° C. When the annealing temperature is less than the Ac ₃ transformation point, unrecrystallized remains, a uniform structure cannot be obtained, ductility is lowered, and it is difficult to ensure material stability. On the other hand, if it exceeds 950 ° C., the precipitates become coarse and fine precipitates cannot be obtained, and it becomes difficult to ensure the strength after continuous annealing.

[Annealing time: 5 to 200 seconds]
Annealing is performed by holding at the annealing temperature for 5 to 200 seconds. If the soaking time is less than 5 seconds, the transformation from processed ferrite to austenite is not sufficient, so that unrecrystallized remains, a uniform structure cannot be obtained, and ductility and hole expansibility deteriorate. On the other hand, if it exceeds 200 seconds, the structure becomes coarse due to grain growth, and the hole expansibility decreases. However, from the viewpoint of productivity, it is desirable to be within 120 seconds.

[Average cooling rate from 750 ° C. to 600 ° C .: 1 to 50 ° C./second]
For cooling after annealing, the average cooling rate from 750 ° C. to 600 ° C. is 1 to 50 ° C./second. The reason why the average cooling rate is defined in the temperature range from 750 ° C. to 600 ° C. is that when a large amount of Ti and / or Nb is added, austenite easily transforms into ferrite in that temperature range, and the 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 and ductility can be controlled. When the average cooling rate is less than 1 ° C./second, the structure grows coarse due to grain growth, and the hole expandability decreases. On the other hand, if it exceeds 50 ° C./second, ductility deteriorates because soft ferrite cannot be obtained. As described above, by setting the P _EQ value to more than 0.8, even if heat treatment is performed in such a wide cooling rate range that is assumed for operation, the tensile strength is unlikely to fluctuate and good material stability is achieved. Is secured.

[Cooling stop temperature: (Zinc plating bath temperature −20 ° C.) to (Zinc plating bath temperature + 100 ° C.)]
In the present invention, the cooling stop temperature is a temperature range from [zinc plating bath temperature −20 ° C.] to [zinc plating bath temperature + 100 ° C.]. When the cooling stop temperature is less than [zinc plating bath temperature −20 ° C.], heat removal at the time of entering the plating bath is large, and operation is not possible. On the other hand, if 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 expandability is lowered. In hot dip galvanizing, an annealed hot-rolled steel sheet is immersed in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower according to a conventional method.

[Holding time of (zinc plating bath temperature −20 ° C.) to (zinc plating bath temperature + 100 ° C.): 30 to 1000 seconds, but also included during plating immersion]
After stopping the cooling in the above temperature range, the same temperature range is continuously maintained for 30 to 1000 seconds including the plating immersion time. When the holding time is less than 30 seconds, austenite is not sufficiently decomposed, residual austenite is generated, and hole expansibility is deteriorated. On the other hand, if the holding time exceeds 1000 seconds, coarse cementite is generated and the hole expandability deteriorates. However, from the viewpoint of productivity, it is desirable to be within 300 seconds.

[Alloying temperature: 430 to 600 ° C.]
In the present invention, alloying is performed at 430 to 600 ° C. after immersion in the plating bath. When the alloying treatment temperature is less than 430 ° C., unalloyed treatment occurs, and the surface properties of the steel sheet deteriorate. On the other hand, when the alloying temperature exceeds 600 ° C., coarse cementite is generated, and the hole expansibility deteriorates. Note that when the alloying treatment temperature is set to [zinc plating bath temperature + 40 ° C.] or higher, the decomposition of austenite is promoted even in the case where the C content exceeds 0.06%, and the amount of C in the retained austenite is reduced. Since it can be lowered and the hole expandability can be further improved, the alloying temperature is preferably set to 430 to 600 ° C. and [zinc plating bath temperature + 40 ° C.] or higher.

  Particularly preferable alloying treatment conditions are a temperature of 500 to 530 ° C. and a treatment time of 10 to 60 seconds. Thereby, the degree of alloying (Fe content of the plating layer) is preferably about 8 to 13%.

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

Thus, by adjusting steel composition, continuous annealing after hot rolling and cold rolling and optimization of galvanizing conditions for alloying, ferrite is 60% or more and retained austenite is 3.0% or less. A steel structure containing ferrite having an average particle diameter of 1.0 to 6.0 μm and precipitates having a particle diameter of 1 to 10 nm in ferrite of 100 pieces / μm ² or more can be obtained, and a tensile strength of 540 MPa or more is high. Thus, an alloyed hot-dip galvanized steel sheet having good hole expansibility, ductility, and material stability can be obtained.

  Regarding the hole expandability, the hole expandability is assumed to be good when the hole expansion ratio (HER) measured by the method defined in JFST1001 is 50% or more. 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. Regarding the ductility, it is assumed that the ductility is good when the TS × EL value obtained by the tensile test is 12000 MPa ·% or more. If this value is 14000 MPA ·% or more, the ductility is better. The material stability is assumed to be favorable when the upper and lower limit range (ΔTS) of the tensile strength is 100 MPa or less, and better when this value is 60 MPa or less.

  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. The obtained slab was hot-rolled under the conditions shown in Table 2. The obtained hot-rolled steel sheet was pickled and cold-rolled at the cold pressure rate shown in Table 2 to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was heated to 700 ° C. at a heating rate of 10 ° C./second, and was heated from 700 ° C. to the annealing temperature under the conditions shown in Table 2 for annealing. Cooling is performed from the annealing temperature to the cooling stop temperature at the cooling rate shown in Table 2, and thereafter, the cooling stop temperature for the time shown in Table 2 is maintained so as to simulate the thermal history during the alloying hot dip galvanizing process. Cool to the assumed plating bath temperature of 460 ° C. over 5 seconds, hold at the temperature of 460 ° C. for 10 seconds, then heat to the alloying treatment temperature shown in Table 2 over 5 seconds, and the time shown in Table 2 An annealing cold-rolled steel sheet was produced by performing a heat treatment for alloying. The holding time at 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 structure and mechanical properties of the steel sheet have the same thermal history. It is substantially the same as the alloyed hot-dip galvanized steel sheet.

While measuring the Ac ₃ transformation point of the steel slab having the chemical composition shown in Table 1, the structure of the steel sheet was analyzed by X-ray diffraction, SEM observation, and TEM observation on the obtained galvannealed steel sheet. Tensile tests and hole expansion tests were conducted to evaluate mechanical properties. The results are shown in Table 3.

[Test method]
(Measurement of Ac ₃ transformation point)
The Ac ₃ transformation point of each test steel was measured by analyzing the change in expansion coefficient when heated at a heating rate of 10 ° C./second using a cold-rolled steel plate having the chemical composition shown in Table 1.

(Tissue observation)
Samples were taken 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 image analysis was performed. Were used to measure the fraction of each phase and the particle size of each phase. 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 concentration in austenite)
Each annealed cold and dark steel sheet was subjected to chemical polishing to reduce the thickness by 0.3 mm, the surface after chemical polishing was subjected to X-ray diffraction, the obtained profile was analyzed, and the residual austenite area ratio (residual γ in the table) (Area ratio) amount and C concentration in residual austenite (residual γ C amount) were calculated.

(Particle size and density of precipitates)
For the measurement of precipitate particle size and density, an electron microscope replica method was adopted, five fields of view were photographed at a magnification of 100,000 times for each steel sheet sample, calculated as a circle-converted particle size, and a particle size of 1 to 10 nm. The total number of precipitates was measured, and the number was divided by the area of the field of view to calculate the density.

(mechanical nature)
JIS No. 5 tensile test specimens were taken from the direction perpendicular to the rolling direction, and the yield strength (YS), tensile strength (TS), and elongation (EL) were measured. The hole expansion rate (HER) was measured by the method prescribed in JFST1001.

(Material stability)
In operation, it is difficult to control the cooling rate in the continuous annealing process. Therefore, the cold-rolled steel sheet obtained as described above is heated to 700 ° C. at a heating rate of 10 ° C./second, heated from 700 ° C. to the annealing temperature under the conditions shown in Table 2, and the conditions shown in Table 2 Annealed with. 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 under the conditions shown in Table 2 so as to simulate the heat treatment at the time of manufacturing the alloy hot-dip galvanized steel sheet. And annealing and alloying heat treatment were performed to produce an annealed cold-rolled steel sheet. That is, for each of the test steels, four conditions of annealed cold-rolled steel sheets were produced with only the cooling rate varied, and the difference between the maximum and minimum values of TS was ΔTS, which was used as an index of material stability.

The steel sheet of the present invention contains 60% or more of ferrite in area% and less than 3.0% of retained austenite, the average grain size of ferrite is 1.0 to 6.0 μm, and the ferrite has a grain size of 1 to It has a structure containing 100 nm / μm ² or more of 10 nm precipitates. As a result, despite having a high tensile strength of 540 MPa or higher, the TS × EL value is 12000 MPa ·% or higher, the HER is 50% or higher, and has excellent formability in hole expansibility and material stability. ΔTS is 100 MPa or less.

  On the other hand, the steel plate No. 2 of the comparative example has a manufacturing condition that is out of the scope of the present invention, and a desired retained austenite area ratio cannot be obtained. Steel plate No. 4 has manufacturing conditions outside the scope of the present invention, and a uniform structure cannot be obtained. Therefore, ductility and hole expansibility are poor. Steel plate No. 5 has manufacturing conditions that are out of the scope of the present invention, and coarse carbides are generated, so the hole expandability is poor. Steel plate No. 6 is not ductile because the manufacturing conditions are out of the scope of the present invention and the desired ferrite area ratio cannot be obtained. Steel plates No. 7 and 20 have chemical components that are out of the scope of the present invention, and a desired strength cannot be obtained. Steel plate No. 9 has poor ductility because the manufacturing conditions are out of the scope of the present invention and a uniform structure cannot be obtained.

  Steel plate No. 10 has a manufacturing condition that is out of the scope of the present invention, and a desired ferrite particle size cannot be obtained. Steel plates No. 11 and 14 have manufacturing conditions that are out of the scope of the present invention, and a desired fine precipitate cannot be sufficiently obtained, so that a desired strength cannot be obtained. Steel plate No. 13 has manufacturing conditions that are out of the scope of the present invention, and since a uniform structure cannot be obtained sufficiently, not only the ductility and hole expansibility are poor, but also the structure during annealing is not stable. Poor nature. Steel plate No. 17 has poor slab heating because the slab heating temperature (rolling start temperature) is out of the scope of the present invention and Ti and Nb cannot be re-dissolved in the manufacturing conditions. Steel plates No. 18 and 22 have chemical compositions that are out of the scope of the present invention, and a desired retained austenite area ratio cannot be obtained. Therefore, not only the hole expandability but also the material stability is poor. Steel plate No. 19 has a chemical composition that is out of the scope of the present invention, and a desired ferrite area ratio cannot be obtained.

Among the examples of the present invention, at least the Mn content is in the above-mentioned preferable range, no retained austenite is contained, or the area ratio of retained austenite is 3.0% or less, and the C concentration in the retained austenite is 0. Steel plates Nos. 3, 8, 12, 15, and 16 that are .6% by mass or less are preferable high-strength galvannealed steel plates having a tensile strength of 590 MPa or more and a HER of 80% or more. Among them, steel plates Nos. 12, 15, and 16 having a P _EQ value exceeding 0.8 have a TSxEL value of 14000 MPa ·% and a material stability of ΔTS of 60 MPa or less in addition to the tensile strength and hole expansibility described above. Furthermore, a preferable high-strength galvannealed steel sheet is obtained.

Claims (3)

By mass%, C: 0.03 to 0.10%, Si: 0.005 to 0.2%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01 %: Sol.Al: 0.01-0.1%, N: 0.01% or less, and Ti: 0.074% or more and 0.50% or less, or Ti: 0 0.074% or more and 0.50% or less and Nb: 0.55% or less, with the balance being a chemical composition composed of Fe and impurities, the area ratio of ferrite being 60% or more, and the area ratio of residual austenite being 3 0.0% or less, ferrite has an average particle diameter of 1.0 to 6.0 μm, and further has a steel structure containing precipitates having a particle diameter of 1 to 10 nm in ferrite of 100 pieces / μm ² or more, and a tensile strength of 540 MPa. As described above, the hole expansion ratio measured by the method prescribed in JFST1001 is 50% or more, and the value of tensile strength × elongation is 12000 MPa · An alloyed hot-dip galvanized steel sheet characterized by being at least%. The chemical composition, instead of a part of Fe, by mass percent, further Ca: 0.01% or less and Mg: containing one or two species selected from the group consisting of 0.01% or less, wherein Item 5. An alloyed hot-dip galvanized steel sheet according to Item 1. The method for producing an galvannealed steel sheet according to claim 1 or 2, comprising the following steps (A) to (C):
(A) Hot rolling at a rolling start temperature: 1050 ° C. to 1300 ° C., a finishing temperature: 800 ° C. to 950 ° C., and a coiling temperature: 450 to 750 ° C. is applied to the steel material having the chemical composition according to claim 1 or 2. Hot rolling process to give hot-rolled steel sheet;
(B) a cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; and
(C) After annealing the cold rolled steel sheet at a temperature range of 750 ° C. to Ac ₃ transformation point at 1 ° C./second or more, annealing is performed for 5 to 200 seconds in the temperature range of Ac ₃ transformation point to 950 ° C. Then, the obtained cold-rolled annealed steel sheet has an average cooling rate of 1 to 50 ° C./second from 750 ° C. to 600 ° C., and [zinc plating bath temperature −20 ° C.] to [zinc plating bath temperature + 100 ° C. A continuous annealing-alloying hot dip galvanizing step in which the alloying treatment is performed at 430 to 600 ° C. after the temperature is cooled to a temperature range of