JP5141232B2 - High-strength hot-dip galvanized steel sheet with excellent formability and manufacturing method thereof - Google Patents

Description

  The present invention is a high-strength hot-dip galvanized steel sheet excellent in formability suitable mainly for structural members of automobiles, in particular, having a tensile strength TS of 780 MPa or more and excellent ductility such as hole expansibility and bendability. The present invention relates to a high-strength hot-dip galvanized steel sheet and a method for producing the same.

  In recent years, for the purpose of ensuring the safety of passengers in the event of a collision and improving fuel efficiency by reducing the weight of the vehicle body, the application of high-strength steel sheets with a TS of 780 MPa or more and a thin plate thickness has been actively promoted. In particular, recently, the application of high strength steel sheets with extremely high strength having TS of 980 MPa class and 1180 MPa class has been studied.

  However, in general, increasing the strength of a steel sheet leads to a decrease in ductility such as hole expandability and bendability of the steel sheet, leading to a decrease in formability. Therefore, it has both high strength and excellent formability, as well as corrosion resistance. In addition, a hot dip galvanized steel sheet is also desired.

In response to such a request, for example, in Patent Document 1, in mass%, C: 0.04 to 0.1%, Si: 0.4 to 2.0%, Mn: 1.5 to 3.0%, B: 0.0005 to 0.005%, P ≦ Contains 0.1%, 4N <Ti ≦ 0.05%, Nb ≦ 0.1%, the balance is Fe and inevitable impurities steel plate surface layer with alloyed galvanized layer, Fe% in alloyed hot dip galvanized layer is A high-strength galvannealed steel sheet having an excellent formability and plating adhesion of TS800 MPa or higher, which is 5 to 25% and the steel sheet structure is a mixed structure of a ferrite phase and a martensite phase, has been proposed. Patent Document 2 includes mass%, C: 0.05 to 0.15%, Si: 0.3 to 1.5%, Mn: 1.5 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5%, N: 0.0060% or less, the balance being Fe and inevitable impurities, further satisfying (Mn%) / (C%) ≧ 15 and (Si%) / (C%) ≧ 4, and in volume ratio in the ferrite phase A high-strength galvannealed steel sheet with good formability containing 3-20% martensite phase and retained austenite phase has been proposed. Patent Document 3 includes mass%, C: 0.04 to 0.14%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%, P: 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.5%, Contains Ti: 0.005 to 0.1%, N: 0.006% or less, further satisfies (Ti%) / (S%) ≧ 5, consists of the balance Fe and inevitable impurities, the volume of martensite phase and residual austenite phase When the total ratio is 6% or more and the volume fraction α% of the hard phase structure of the martensite phase, residual austenite phase and bainite phase is α ≦ 50000 × {(Ti%) / 48+ (Nb%) / A low-yield-ratio high-strength plated steel sheet excellent in hole expansibility of 93+ (Mo%) / 96+ (V%) / 51} has been proposed. Patent Document 4 contains, in mass%, C: 0.001 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.01 to 3%, Al: 0.001 to 4%, and the balance Fe and inevitable impurities. Is a hot dip galvanized steel sheet having a plating layer consisting of Al: 0.001 to 0.5%, Mn: 0.001 to 2%, and the balance Zn and unavoidable impurities in mass%, and the Si content of the steel : X mass%, Mn content of steel: Y mass%, Al content of steel: Z mass%, Al content of plating layer: A mass%, Mn content of plating layer: B mass% is 0 ≦ 3- (X + Y / 10 + Z / 3) -12.5 × (AB) is satisfied, the microstructure of the steel sheet is 70-97% ferrite main phase by volume ratio and its average grain size is 20 μm or less, High-strength molten zinc with excellent plating adhesion and ductility during molding, consisting of an austenite phase and / or martensite phase with a volume ratio of 3-30% as the second phase, and the average particle size of the second phase being 10 μm or less Plated steel sheets have been proposed.
JP 9-13147 A JP 11-279691 A JP 2002-69574 A JP 2003-55751 A

  However, in the high-strength hot-dip galvanized steel sheets described in Patent Documents 1 to 4, excellent hole expandability and bendability cannot always be obtained.

  An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more and excellent in hole expansibility and bendability and a method for producing the same.

  The present inventors have conducted extensive studies on a high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more and excellent hole expansibility and bendability, and found the following.

  i) In order to improve ductility such as hole expandability and bendability, it is necessary to have a composite structure in which the ferrite phase and martensite phase are uniformly and finely dispersed and the hardness difference between the ferrite phase and martensite phase is not significantly increased. It is effective.

  ii) After optimizing the component composition, the area ratio includes a ferrite phase of 50% or more and a martensite phase of 10% or more, the particle size in the ferrite phase is 15 μm or less, and the aspect ratio is 2.0 or less. By using a microstructure in which the area ratio of the ferrite grains is 70% or more and the average grain size of the martensite phase is 10 μm or less, a TS of 780 MPa or more and excellent hole expansibility and bendability can be achieved.

iii) During annealing, these microstructures are heated to a temperature range above the Ac ₁ transformation point at an average heating rate of 5 ° C./s or more, and (Ac ₁ transformation point + 50) to (Ac ₃ transformation point + 25) ° C. In the temperature range of 10 to 500 s, cooled to a temperature range of 550 ° C. or less at an average cooling rate of 3 to 30 ° C./s, and then hot dip galvanized.

  The present invention has been made based on such findings, and in mass%, C: 0.03 to 0.15%, Si: 0.8 to 2.5%, Mn: 1.0 to 3.0%, P: 0.001 to 0.05%, S: 0.0001 -0.01%, Al: 0.001-0.1%, N: 0.0005-0.01%, Nb: 0.005-0.05%, Cr: 0.1-2.0%, with the balance being composed of Fe and inevitable impurities, In the area ratio, the ferrite phase containing 50% or more ferrite phase and 10% or more martensite phase, the grain size in the ferrite phase is 15μm or less, and the area ratio of ferrite grains having an aspect ratio of 2.0 or less is 70% or more. And providing a high-strength hot-dip galvanized steel sheet excellent in formability having a microstructure in which the average particle size of the martensite phase is 10 μm or less.

  In the high-strength hot-dip galvanized steel sheet of the present invention, in mass%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, at least one element selected from Ti: 0.005 to 0.1%, V: It is preferable that at least one element selected from 0.005 to 0.1% and B: 0.0003 to 0.003% are contained.

  In the high-strength hot-dip galvanized steel sheet of the present invention, galvanization can be alloyed galvanization.

The high-strength hot-dip galvanized steel sheet of the present invention, for example, heats a steel sheet having the above component composition to a temperature range equal to or higher than the Ac ₁ transformation point at an average heating rate of 5 ° C./s or more (Ac ₁ transformation point + 50) ~ (10~500s soaking in a temperature range of Ac ₃ transformation point +25) ° C., after annealing under conditions of cooling to a temperature range of 550 ° C. or less at an average cooling rate of 3 to 30 ° C. / s, hot dip galvanized It can manufacture by the method of giving.

  In the method for producing a high-strength hot-dip galvanized steel sheet according to the present invention, it is preferable to perform hot-dip galvanization after heat treatment in a temperature range of 350 to 550 ° C. for 20 to 150 seconds after cooling during annealing. Furthermore, after hot dip galvanization, galvanization can also be alloyed in the temperature range of 450-550 degreeC.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having a TS of 780 MPa or more and excellent in hole expansibility and bendability can be produced. By applying the high-strength hot-dip galvanized steel sheet of the present invention to automobile structural members, it is possible to further improve occupant safety and improve fuel efficiency by significantly reducing the weight of the vehicle body.

  Details of the present invention will be described below. “%” Representing the content of component elements means “% by mass” unless otherwise specified.

1) Component composition
C: 0.03-0.15%
C is an important element for strengthening steel, has high solid solution strengthening ability, and is an indispensable element for adjusting the area ratio and hardness when utilizing structure strengthening by martensite phase. . When the amount of C is less than 0.03%, it becomes difficult to obtain a martensite phase having a required area ratio, and the martensite phase does not harden, so that sufficient strength cannot be obtained. On the other hand, if the amount of C exceeds 0.15%, weldability deteriorates, and formation of a segregation layer causes a decrease in formability. Therefore, the C content is 0.03 to 0.15%.

Si: 0.8-2.5%
Si is an extremely important element in the present invention, and promotes ferrite transformation during annealing and discharges solute C from the ferrite phase to the austenite phase to clean the ferrite phase and improve the ductility. A martensite phase is generated even in a hot dip galvanizing line, where rapid cooling is difficult to stabilize the phase, facilitating complex organization. In addition, Si dissolved in the ferrite phase promotes work hardening and increases ductility, and also improves the bendability by improving the strain propagation at the site where the strain is concentrated. In addition, Si solidifies and strengthens the ferrite phase to reduce the hardness difference between the ferrite phase and the martensite phase, suppresses the occurrence of cracks at the interface, improves local deformability, and improves hole expansibility. Contribute. In order to obtain such an effect, the Si amount needs to be 0.8% or more. On the other hand, when the Si content exceeds 2.5%, the transformation point is remarkably increased, not only the production stability is inhibited, but also an abnormal structure develops and the moldability is lowered. Therefore, the Si content is 0.8 to 2.5%.

Mn: 1.0-3.0%
Mn is effective for preventing hot embrittlement of steel and ensuring strength, and improves hardenability and facilitates the formation of a composite structure. In order to obtain such an effect, the Mn content needs to be 1.0% or more. On the other hand, when the amount of Mn exceeds 3.0%, the moldability is deteriorated. Therefore, the Mn content is 1.0 to 3.0%.

P: 0.001 ~ 0.05%
P is an element that can be added according to the desired strength, and is also an element effective for complex organization in order to promote ferrite transformation. In order to obtain such effects, the P amount needs to be 0.001% or more. On the other hand, if the amount of P exceeds 0.05%, weldability is deteriorated and, when galvanizing is alloyed, the alloying speed is lowered and the quality of galvanizing is impaired. Therefore, the P content is 0.001 to 0.05%.

S: 0.0001 ~ 0.01%
S segregates at the grain boundary and embrittles the steel during hot working, and also exists as a sulfide and reduces local deformability, so the amount is 0.01% or less, preferably 0.003% or less, more preferably It is necessary to make it 0.001% or less. However, due to production technology constraints, the S content needs to be 0.0001% or more. Therefore, the S content is 0.0001 to 0.01%, preferably 0.0001 to 0.003%, more preferably 0.0001 to 0.001%.

Al: 0.001 to 0.1%
Al is an element effective in generating ferrite and improving the strength-ductility balance. In order to obtain such an effect, the Al content needs to be 0.001% or more. On the other hand, when the Al content exceeds 0.1%, the surface properties are deteriorated. Therefore, the Al amount is 0.001 to 0.1%.

N: 0.0005-0.01%
N is an element that degrades the material due to room temperature aging. In particular, when the N content exceeds 0.01%, the degree becomes remarkable. The smaller the amount, the better. However, the amount of N needs to be 0.0005% or more due to restrictions on production technology. Therefore, the N content is 0.0005 to 0.01%.

Nb: 0.005-0.05%
Nb, like Si, is an extremely important element in the present invention, and suppresses recrystallization during annealing, so that the austenite transformation in an unrecrystallized state is promoted, and the austenite phase is extremely finely dispersed during subsequent soaking. In the subsequent cooling, a composite structure in which the ferrite phase and the martensite phase are uniformly and finely dispersed is formed. This uniform and fine composite structure suppresses the propagation of cracks generated during plastic deformation to enhance local deformability and improve hole expansibility, and at the same time enhances strain propagation and improves bendability. In order to obtain such effects, the Nb amount needs to be 0.005% or more. On the other hand, if the Nb content exceeds 0.05%, the effect of strengthening the precipitation of the ferrite phase becomes large, resulting in a decrease in ductility. Therefore, the Nb content is 0.005 to 0.05%.

Cr: 0.1-2.0%
Cr, like Si and Nb, is an extremely important element in the present invention, and promotes the distribution of C from the ferrite phase to the austenite phase during annealing so that the austenite phase is stabilized, and the ferrite phase and martensite. It facilitates complex organization of phases and significantly delays the formation of pearlite and bainite phases during subsequent cooling. In addition, Cr softens the martensite phase by cooling and alloying after cooling, reduces the hardness difference between the ferrite phase and the martensite phase, and suppresses the occurrence of cracks at the interface, thereby local deformation. Performance and contribute to the improvement of hole expansion. In particular, by overlapping with the effect of suppressing crack propagation by the uniform and fine composite structure formed by adding Nb as described above, the hole expandability can be remarkably improved. In order to obtain such an effect, the Cr amount needs to be 0.1% or more. On the other hand, if the amount of Cr exceeds 2.0%, Cr carbides are excessively generated and ductility is reduced. Therefore, the Cr content is 0.1 to 2.0%.

  The balance is Fe and inevitable impurities, but for the following reasons, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, at least one element selected from Ti, 0.005 to 0.1%, V: 0.005 to It is preferable that at least one element selected from 0.1% or B: 0.0003 to 0.003% is contained.

Mo: 0.01-1.0%, Ni: 0.01-2.0%
Mo and Ni not only play a role as solid solution strengthening elements, but also stabilize the austenite phase in the cooling process during annealing, facilitating complex organization. In order to obtain such an effect, the Mo content and the Ni content must each be 0.01% or more. On the other hand, when the Mo content exceeds 1.0% and the Ni content exceeds 2.0%, the plating property, formability, and spot weldability deteriorate. Therefore, the Mo amount is 0.01 to 1.0%, and the Ni amount is 0.01 to 2.0%.

Ti: 0.005-0.1%, V: 0.005-0.1%
Ti and V contribute to the improvement of strength and toughness by forming precipitates with C, S, and N. In order to obtain such effects, the Ti content and the V content must each be 0.005% or more. On the other hand, when the Ti content and V content exceed 0.1%, precipitation strengthening works excessively, leading to a decrease in ductility. Therefore, the Ti amount and the V amount are each 0.005 to 0.1%.

B: 0.0003-0.003%
B is an element that suppresses the formation of ferrite from the austenite grain boundaries and improves the hardenability, and promotes the composite organization. In order to obtain such an effect, the B content needs to be 0.0003% or more. On the other hand, if the amount of B exceeds 0.003%, ductility is reduced. Therefore, the B amount is set to 0.0003 to 0.003%.

2) Microstructure Area ratio of ferrite phase: 50% or more The high-strength hot-dip galvanized steel sheet according to the present invention is a composite structure in which a hard martensite phase is uniformly and finely dispersed in a soft ferrite phase rich in ductility. However, to ensure sufficient ductility, a ferrite phase with an area ratio of 50% or more is required.

Area ratio of ferrite grains with a particle size of 15 μm or less and an aspect ratio of 2.0 or less in the ferrite phase: 70% or more Even if the area ratio of the ferrite phase is secured, as described above, the ferrite phase and the martensite phase are If it is not uniformly and finely dispersed, the effect of improving the hole expansibility and bendability is reduced by suppressing the propagation of cracks or increasing the propagation of strain and suppressing its local concentration. Therefore, it is necessary to make the crystal grains of the ferrite phase fine. Further, when the crystal grains are expanded, the propagation of cracks is promoted and the hole expandability is lowered, so that the aspect ratio needs to be reduced. In order to obtain excellent hole expansibility and bendability, it is necessary to make the area ratio of ferrite grains having a grain size in the ferrite phase of 15 μm or less and an aspect ratio of 2.0 or less 70% or more.

Martensite phase area ratio: 10% or more, Martensite phase average particle size: 10 μm or less
In order to secure TS of 780 MPa or more, the area ratio of the martensite phase needs to be 10% or more. In addition, as described above, when the martensite phase is uniformly and finely dispersed in the ferrite phase, the size of cracks generated at the interface between the martensite phase and the ferrite phase becomes minute, and the occurrence frequency is suppressed, The martensite phase itself becomes an obstacle to the propagation of cracks, and the ductility such as hole expandability is improved. Furthermore, the uniformly and finely dispersed martensite phase becomes a source of dislocations and contributes to the improvement of bendability by increasing the propagation of strain. In order to obtain such effects, the average particle size of the martensite phase needs to be 10 μm or less.

  In addition to the ferrite phase and martensite phase, even if the retained austenite phase, pearlite phase, and bainite phase are included within a total area ratio of 20% or less, the effects of the present invention are not impaired.

  Here, the area ratio of the ferrite phase and the martensite phase is the ratio of the area of each phase in the observation area, and the area ratio of the ferrite grains having a grain size of 15 μm or less and an aspect ratio of 2.0 or less. Is the ratio of the area of ferrite grains in which the particle size occupies 15 μm or less and the aspect ratio is 2.0 or less in the area of the ferrite phase. The area ratio, grain size, and aspect ratio of each of these phases were determined by polishing 10 mm of cross section parallel to the rolling direction of the steel sheet, corroding with 3% nital, and observing 10 fields at a magnification of 2000 times with a scanning electron microscope (SEM). And obtained using commercially available image processing software.

3) Manufacturing conditions The high-strength hot-dip galvanized steel sheet of the present invention, for example, heats a steel sheet having the above component composition to a temperature range equal to or higher than the Ac ₁ transformation point at an average heating rate of 5 ° C./s or higher. After annealing under conditions that soak for 10 to 500 s in the temperature range of ₁ transformation point +50) to (Ac ₃ transformation point +25) ° C and cool to a temperature range of 550 ° C or less at an average cooling rate of 3 to 30 ° C / s It can be manufactured by a method of applying hot dip galvanizing.

Heating conditions for annealing: Heating to a temperature range above the Ac ₁ transformation point at an average heating rate of 5 ° C / s or higher
By heating to the temperature range above the Ac ₁ transformation point at an average heating rate of 5 ° C / s or more, the non-recrystallized ferrite phase is transformed into the austenite phase, and in the subsequent soaking and cooling processes, uniform and fine Since a composite structure of ferrite phase and martensite phase is obtained, hole expandability and bendability are improved. If the average heating rate is less than 5 ° C / s and the heating temperature is less than the Ac ₁ transformation point, a ferrite phase with a low aspect ratio that has been layered in the coarse rolling direction during recrystallization will form, and in the subsequent soaking and cooling processes A uniform and fine composite structure cannot be obtained.

Soaking conditions for annealing: (Ac ₁ transformation point +50) to (Ac ₃ transformation point +25) 10 to 500 s soaking in a temperature range of ° C To obtain a uniform and fine composite structure as described above, during annealing It is necessary to sufficiently generate the recrystallized ferrite phase and to stabilize the austenite phase by concentrating C into the austenite phase. If the soaking temperature is less than (Ac ₁ transformation point +50) ° C or if the soaking time is less than 10 s, the processed structure remains in the ferrite phase and recovers, so that the aspect ratio decreases and the solid solution dissolves. Since the distribution of C is delayed, a soft ferrite phase rich in ductility cannot be obtained, and hole expansibility and bendability deteriorate. Furthermore, stabilization of the austenite phase becomes insufficient, martensitic transformation is suppressed, and high strength cannot be achieved. On the other hand, if the soaking temperature exceeds (Ac ₃ transformation point +25) ° C or the soaking time exceeds 500 s, the austenite phase becomes coarse, and a uniform and fine composite structure cannot be obtained in the subsequent cooling process. Hole expandability and bendability are reduced.

Cooling conditions for annealing: After cooling soaking from the soaking temperature to a temperature range of 550 ° C or less at an average cooling rate of 3 to 30 ° C / s, 550 ° C at an average cooling rate of 3 to 30 ° C / s from the soaking temperature It is necessary to cool to the following temperature range (cooling stop temperature), but if the average cooling rate is less than 3 ° C / s, a large amount of pearlite phase or bainite phase is generated to suppress the formation of martensite phase. Therefore, sufficient strength and hole expansibility cannot be obtained, and if the average cooling rate exceeds 30 ° C / s, the generation of a sufficient amount of ferrite phase is suppressed, or the hardness difference between the ferrite phase and the martensite phase is reduced. This is because the size is remarkably increased and the hole expandability is lowered. In order to avoid the formation region of the pearlite phase and the bainite phase and to secure the necessary amount of martensite phase, it is necessary to cool to a stop temperature of 550 ° C. or lower at such an average cooling rate.

  After annealing, hot dip galvanization is performed under normal conditions, but it is preferable to perform the following heat treatment before that.

Heat treatment conditions after annealing: 20 to 150 s in the temperature range of 350 to 550 ° C
After annealing, heat treatment for 20 to 150 s in a temperature range of 350 to 550 ° C softens the martensite phase, so the hardness difference from the ferrite phase becomes smaller, and the hole expandability and bendability can be further improved. . These effects are small when the heat treatment temperature is less than 350 ° C. or when the heat treatment time is less than 20 s. On the other hand, when the heat treatment temperature exceeds 550 ° C. or the heat treatment time exceeds 150 s, the hardness of the martensite phase is remarkably reduced, and a TS of 780 MPa or more cannot be obtained.

  Moreover, after annealing, galvanization can be alloyed in a temperature range of 450 to 550 ° C. regardless of whether or not the heat treatment is performed. By alloying in the temperature range of 450 to 550 ° C., the Fe concentration during plating becomes 8 to 12%, and the adhesion of plating and the corrosion resistance after coating are improved. If the temperature is lower than 450 ° C, alloying does not proceed sufficiently, leading to a decrease in sacrificial anticorrosive action and sliding property. If the temperature exceeds 550 ° C, alloying proceeds too much and powdering properties are reduced. A large amount of phases and bainite phases are formed, and it is not possible to increase the strength and improve the hole expansibility.

  The conditions for other production methods are not particularly limited, but the following conditions are preferable.

  The steel sheet before galvanization used for the high-strength hot-dip galvanized steel sheet of the present invention is manufactured by hot-rolling a slab having the above component composition to a desired sheet thickness. From the viewpoint of productivity, the series of treatments such as annealing, pre-galvanizing heat treatment, hot dip galvanizing, and alloying treatment of galvanizing are preferably performed in a continuous hot dip galvanizing line.

  The slab is preferably produced by a continuous casting method in order to prevent macro segregation, but can also be produced by an ingot-making method or a thin slab casting method. When the slab is hot-rolled, the slab is reheated, but in order to prevent an increase in rolling load, the heating temperature is preferably 1150 ° C. or higher. Further, in order to prevent an increase in scale loss and an increase in fuel consumption, the upper limit of the heating temperature is preferably 1300 ° C.

The hot rolling is performed by rough rolling and finish rolling, but the finish rolling is preferably performed at a finishing temperature equal to or higher than the Ar ₃ transformation point in order to prevent deterioration of formability after cold rolling / annealing. Further, in order to prevent the occurrence of non-uniform structure and scale defects due to the coarsening of crystal grains, the finishing temperature is preferably 950 ° C. or lower.

  The steel sheet after hot rolling is preferably wound at a winding temperature of 500 to 650 ° C. in order to precipitate fine Nb carbonitride that suppresses recrystallization during annealing.

  The steel sheet after winding is preferably cold-rolled at a reduction rate of 40% or more in order to promote austenite transformation from the non-recrystallized ferrite phase after removing the scale by pickling.

  For hot dip galvanizing, it is preferable to use a galvanizing bath containing 0.10 to 0.20% of Al. Moreover, after plating, wiping can be performed to adjust the basis weight of plating.

Steel Nos. A to l having the composition shown in Table 1 were melted by a converter and made into a slab by a continuous casting method. These slabs were heated to 1200 ° C., hot-rolled at a finishing temperature of 850 to 920 ° C., and wound at a winding temperature of 600 ° C. Next, after pickling, cold rolled to a plate thickness shown in Table 2 at a reduction rate of 50%, and after annealing under the annealing conditions shown in Table 2 by a continuous hot dip galvanizing line, the time shown in Table 2 at 350 to 550 ° C After pre-plating heat treatment, dipped in a 475 ° C zinc plating bath containing 0.13% Al for 3 s to form a galvanized coating with an adhesion amount of 45 g / m ² and alloyed at the temperatures shown in Table 2 The galvanized steel sheets No. 1 to 20 were produced. As shown in Table 2, some galvanized steel sheets were not subjected to pre-plating heat treatment or alloying treatment. And about the obtained galvanized steel plate, the area ratio of the ferrite phase and the martensite phase, the particle size of the ferrite phase, the average particle size of the martensite phase, and the aspect ratio of the ferrite particle size were measured by the above method. Further, a JIS No. 5 tensile test piece was taken in a direction perpendicular to the rolling direction, and a tensile test was performed at a crosshead speed of 20 mm / min in accordance with JIS Z 2241 to measure TS and total elongation El. Further, a 100 mm × 100 mm test piece was collected and subjected to a hole expansion test three times in accordance with JFST 1001 (iron standard) to obtain an average hole expansion ratio λ (%), and the hole expansion property was evaluated. Furthermore, a strip-shaped test piece having a width of 30 mm and a length of 120 mm in the direction perpendicular to the rolling direction was collected, and after smoothing the end so that the surface roughness Ry was 1.6 to 6.3 S, by a push bending method. A bending test was performed at a bending angle of 180 °, and the minimum bending radius at which no cracks or necking occurred was obtained as the limit bending radius.

  The results are shown in Table 3. Each of the galvanized steel sheets of the present invention has excellent hole expansibility and bendability with TS of 780 MPa or more, a hole expansion ratio λ of 25% or more, and a limit bending radius of 1.0 mm or less. It can be seen that TS × El ≧ 18000 MPa ·%, a high strength-ductility balance, and a high-strength hot-dip galvanized steel sheet with excellent formability.

Claims (8)

  In mass%, C: 0.03-0.15%, Si: 0.8-2.5%, Mn: 1.0-3.0%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005-0.01%, Nb: 0.005-0.05%, Cr: 0.1-2. 0% is contained, the balance is composed of Fe and inevitable impurities, the area ratio includes 50% or more ferrite phase and 10% or more martensite phase, and the particle size occupied in the ferrite phase is High-strength melt excellent in formability having a microstructure in which the area ratio of ferrite grains having an aspect ratio of 2.0 or less and an aspect ratio of 2.0 or less is 70% or more, and the average particle size of the martensite phase is 10 μm or less Galvanized steel sheet.   Furthermore, the high excellent in the moldability of Claim 1 containing at least 1 sort (s) of element chosen from Mo: 0.01-1.0% and Ni: 0.01-2.0% by the mass%. Strength hot dip galvanized steel sheet.   Furthermore, it is excellent in the moldability according to claim 1 or 2 containing at least one element selected from Ti: 0.005 to 0.1% and V: 0.005 to 0.1% by mass%. High strength hot dip galvanized steel sheet.   Furthermore, the high intensity | strength hot-dip galvanized steel plate excellent in the moldability in any one of Claim 1 to 3 which contains B: 0.0003-0.003% by mass%.   The high-strength hot-dip galvanized steel sheet excellent in formability according to any one of claims 1 to 4, wherein the galvanizing is alloyed galvanizing. A steel sheet having the component composition according to any one of claims 1 to 4 is heated to a temperature range equal to or higher than the Ac ₁ transformation point at an average heating rate of 5 ° C / s or more, and (Ac ₁ transformation point +50) to (Ac ₃ transformation point +25) By soaking in a temperature range of 10 to 500 s in a temperature range of 10 ° C. and cooling to a temperature range of 550 ° C. or less at an average cooling rate of 3 to 30 ° C./s, by applying hot dip galvanization , The area ratio of ferrite grains containing a ferrite phase of 50% or more and a martensite phase of 10% or more, having a grain size in the ferrite phase of 15 μm or less and an aspect ratio of 2.0 or less is 70% or more. A method for producing a high-strength hot-dip galvanized steel sheet having excellent formability having a microstructure in which the average particle size of the martensite phase is 10 μm or less .   The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the formability of Claim 6 which performs hot-dip galvanization after performing 20-150 second heat processing in the temperature range of 350-550 degreeC after cooling at the time of annealing.   The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to claim 6 or 7, wherein after hot-dip galvanizing, galvanizing alloying treatment is performed in a temperature range of 450 to 550 ° C.