JP2009149937A - High-strength hot-dip galvanized steel sheet having excellent formability, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet having ≥780 MPa tensile strength TS and having excellent pore expandability and bending properties, and to provide a production method therefor. <P>SOLUTION: The high-strength hot-dip galvanized steel sheet having excellent formability, has the component composed of, by mass, 0.03-0.15% C, 0.8-2.5% Si, 1.0-3.0% Mn, 0.001-0.05% P, 0.0001-0.01% S, 0.001-0.1% Al, 0.0005-0.01% N, 0.1-2.0% Cr and the balance Fe with inevitable impurities, and has a micro-structure wherein ≥50% ferritic phase and ≥10% martensitic phase are included in an area ratio and the above ferritic phase is composed of polygonal ferritic phase and bainitic-ferritic phase, the area ratio of the bainitic-ferritic phase to the ferritic phase is 20-80%, and the average grain diameter of the martensitic phase is ≤10 μm. <P>COPYRIGHT: (C)2009,JPO&INPIT

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) After optimizing the component composition, in terms of area ratio, it contains 50% or more ferrite phase and 10% or more martensite phase, and the ferrite phase consists of polygonal ferrite phase and bainitic ferrite phase. By forming a microstructure with an area ratio of bainitic ferrite phase of 20 to 80% and an average particle size of martensite phase of 10 μm or less, TS of 780 MPa or more and excellent hole expandability and bendability Can be achieved.

ii) During the annealing, the microstructure is 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 + Ac ₃ transformation point) / 2 to Ac ₃ transformation point. 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%, Cr: 0.1-2.0%, the balance is composed of Fe and inevitable impurities, and the area ratio is 50 The ferrite phase comprises a polygonal ferrite phase and a bainitic ferrite phase, and the area ratio of the bainitic ferrite phase in the ferrite phase is 20%. Provided is 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.

  The high-strength hot-dip galvanized steel sheet of the present invention is further selected by mass% from B: 0.0003 to 0.003%, Ti: 0.005 to 0.1%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%. It is preferable that at least one element is contained. Furthermore, it is preferable that Ca: 0.001 to 0.005% is contained by mass%.

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

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 + Ac ₃ transformation point) / 2~Ac 10~500s soaking in a temperature range of ₃ transformation point, 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 particular, during the cooling process, the formation of bainitic ferrite phase is promoted, and at the same time, solid solution C is discharged into the austenite phase to stabilize the austenite phase, thereby suppressing the formation of pearlite phase and bainite phase. Promote generation. 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. Furthermore, Si solidifies and strengthens the ferrite phase to reduce the hardness difference between the ferrite and martensite phases, suppresses the formation of cracks at the interface, improves local deformability, and expands and bends. It contributes to the improvement. 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, and not only the production stability is inhibited, but 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 aging resistance of steel. In particular, when the N content exceeds 0.01%, the deterioration of aging resistance becomes significant. 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%.

Cr: 0.1-2.0%
Like Si, Cr is an extremely important element in the present invention, and has the effect of increasing the amount of the second phase during annealing, and suppresses the decrease in the amount of the second phase caused by the increase of the transformation point due to the addition of Si. To do. In addition, Cr is necessary for securing the desired amount of bainitic ferrite phase because the second phase during annealing, that is, the untransformed austenite phase, transforms into the bainitic ferrite phase in the subsequent cooling process. . At the same time, Cr suppresses the formation of pearlite and bainite phases during the cooling process and effectively acts to form bainitic ferrite phases, and also stabilizes the austenite phase and promotes the formation of martensite phase. Have 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, suppresses the formation of cracks at the interface, and locally deforms. Improve the performance and contribute to the improvement of hole expansibility and bendability. 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, B: 0.0003 to 0.003%, Ti: 0.005 to 0.1%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0% It is preferable to contain seed elements and Ca: 0.001 to 0.005%.

B: 0.0003-0.003%
By coexisting with Cr, the effect of Cr described above, that is, during annealing, suppresses the formation of polygonal ferrite phase and decreases the stability of austenite phase, and the formation of bainitic ferrite phase during the cooling process. Plays a role in facilitating the effect. 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%.

Ti: 0.005-0.1%
Ti forms precipitates with C, S, and N and contributes effectively to the improvement of strength and toughness. Further, when B is added, since N is precipitated as TiN, the precipitation of BN is suppressed, and the effect of B is effectively exhibited. In order to obtain such an effect, the Ti amount needs to be 0.005% or more. On the other hand, if the Ti content exceeds 0.1%, precipitation strengthening works excessively, leading to a decrease in ductility. Therefore, the Ti content is 0.005 to 0.1%.

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%.

Ca: 0.001 to 0.005%
Ca precipitates S as CaS, suppresses the generation of MnS that promotes the generation and propagation of cracks, and has the effect of improving hole expansibility and bendability. In order to obtain such an effect, the Ca content needs to be 0.001% or more. On the other hand, when the Ca content exceeds 0.005%, the effect is saturated. Therefore, the Ca content is 0.001 to 0.005%.

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

Area ratio of bainitic ferrite phase in ferrite phase: 20-80%
Even if the area ratio of the ferrite phase is ensured, if the ferrite phase consists entirely of a polygonal ferrite phase, the hardness difference between the ferrite phase and the martensite phase is large. It is not possible to obtain excellent hole expansibility and bendability by promoting the propagation of. Therefore, in the present invention, the ferrite phase is higher than the polygonal ferrite phase and has a higher dislocation density and more solid solution elements such as Mn, so it is harder than the polygonal ferrite phase but softer and higher than the martensite phase. As a composite phase composed of a ductile bainitic ferrite phase, the hardness difference between the ferrite phase and the martensite phase is reduced to improve hole expansibility and bendability. In order to obtain such an effect, the area ratio of the bainitic ferrite phase in the ferrite phase needs to be 20% or more. On the other hand, if the area ratio of the bainitic ferrite phase exceeds 80%, sufficient ductility cannot be secured.

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, when the martensite phase is finely dispersed in the ferrite phase, the size of cracks generated at the interface between the martensite phase and the ferrite phase becomes minute, the frequency of occurrence is suppressed, and the martensite phase itself is cracked. It becomes an obstacle to propagation, and ductility such as hole expandability is improved. Further, the finely dispersed martensite phase becomes a source of dislocations and enhances strain propagation. In order to obtain such an effect, it is necessary to secure an area ratio of 10% or more and to make the average particle size of the martensite phase 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 to the observation area, and the area ratio of the bainitic ferrite phase is the bainitic ratio of the area of the ferrite phase. It is the ratio of the area of the ferrite phase. The area ratio of each phase and the average grain size of the martensite phase are 10% at 2000 times magnification by SEM (scanning electron microscope) after corroding the plate thickness section parallel to the rolling direction of the steel plate and corroding with 3% nital. The field of view was observed 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. ₁ transformation point + Ac ₃ transformation point) / 2~Ac 10~500s soaking in a temperature range of ₃ transformation point, 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 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 a temperature range above the Ac ₁ transformation point at an average heating rate of 5 ° C / s or more, the resulting ferrite phase and austenite phase can be finely and uniformly dispersed. Nal ferrite phase, bainitic ferrite phase and martensite phase can be uniformly dispersed, and hole expandability and bendability can be improved. When the average heating rate is less than 5 ° C / s and the heating temperature is less than the Ac ₁ transformation point, a coarse ferrite phase is formed, and in the subsequent soaking and cooling processes, the polygonal ferrite phase, bainitic ferrite phase and martensite The phase cannot be dispersed uniformly.

Annealing soaking conditions: to obtain (Ac ₁ transformation point + Ac ₃ transformation point) / 2~Ac microstructure of 10~500s soaking the present invention as described above at a temperature range of ₃ transformation point at the time of annealing It is necessary to suppress the formation of the polygonal ferrite phase, reduce the stability of the austenite phase, and facilitate the formation of the bainitic ferrite phase during the cooling process. If the soaking temperature is less than (Ac ₁ transformation point + Ac ₃ transformation point) / 2, the amount of polygonal ferrite phase increases, and an appropriate amount, that is, 20-80% bainitic in terms of the area ratio in the ferrite phase. The ferrite phase cannot be secured, and the hole expandability and bendability are reduced. On the other hand, when the soaking temperature exceeds the Ac ₃ transformation point, the amount of polygonal ferrite phase decreases and ductility decreases. On the other hand, if the soaking time is less than 10 s, the amount of the polygonal ferrite phase decreases, and an unrecrystallized structure remains and the moldability deteriorates. If the soaking time exceeds 500 s, the amount of polygonal ferrite phase increases or the distribution of solute C becomes excessive, and an appropriate amount of bainitic ferrite phase cannot be obtained in the subsequent cooling process. 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). This is because when the average cooling rate is less than 3 ° C / s, the polygonal ferrite phase is generated or grows, and an appropriate amount of bainitic ferrite phase is formed. When the average cooling rate exceeds 30 ° C./s, the formation of the bainitic ferrite phase is suppressed, the hard phase is generated at a low temperature, and the moldability is deteriorated. Note that it is necessary to cool to a stop temperature of 550 ° C. or less at such an average cooling rate in order to avoid the pearlite and bainite formation regions and secure the necessary amount of martensite.

  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 the temperature range of 350 to 550 ° C softens the martensite phase or generates a small amount of bainite phase, so the hardness difference between these phases and the ferrite phase becomes smaller. In addition, 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. from the viewpoint of preventing scale defects and ensuring good shape.

  The steel sheet after winding is preferably cold-rolled at a reduction rate of 40% or more in order to efficiently generate a polygonal ferrite phase after removing the scale by pickling or the like.

  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 j 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 plates No. 1 to 18 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, the martensite phase, the bainitic ferrite phase, and the average particle diameter of the martensite phase 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. All of the galvanized steel sheets of the present invention have excellent hole expansibility and bendability with a TS of 780 MPa or more, a hole expansion ratio λ of 40% 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 (9)

  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- It contains 0.01%, Cr: 0.1-2.0%, the balance is composed of Fe and unavoidable impurities, and includes an area ratio of 50% or more ferrite phase and 10% or more martensite phase. The ferrite phase is composed of a polygonal ferrite phase and a bainitic ferrite phase, the area ratio of the bainitic ferrite phase in the ferrite phase is 20 to 80%, and the average particle size of the martensite phase is A high-strength hot-dip galvanized steel sheet with a microstructure that is 10 μm or less and excellent formability.   2. The high-strength hot-dip galvanized steel sheet having excellent formability according to claim 1, further comprising B: 0.0003 to 0.003% by mass%.   The high-strength hot-dip galvanized steel sheet having excellent formability according to claim 1 or 2, further comprising, by mass%, Ti: 0.005 to 0.1%.   The high-strength molten zinc excellent in formability according to any one of claims 1 to 3, further comprising at least one element selected from Mo: 0.01 to 1.0% and Ni: 0.01 to 2.0% by mass%. Plated steel sheet.   5. The high-strength hot-dip galvanized steel sheet having excellent formability according to any one of claims 1 to 4, further comprising Ca: 0.001 to 0.005% by mass%.   6. The high-strength hot-dip galvanized steel sheet with excellent formability according to claim 1, wherein the galvanizing is alloyed galvanizing. The steel sheet having the component composition according to any one of claims 1 to 5 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, (Ac ₁ transformation point + Ac ₃ transformation point). ) Formability of hot-dip galvanizing after annealing at a temperature range of 550 ° C or less at an average cooling rate of 3-30 ° C / s, soaking for 10-500 s in the temperature range of) / 2-Ac ₃ transformation point For producing high-strength hot-dip galvanized steel sheets with excellent resistance.   8. The method for producing a high-strength hot-dip galvanized steel sheet with excellent formability according to claim 7, wherein after the cooling at the time of annealing, the hot-dip galvanizing is performed after performing a heat treatment for 20 to 150 seconds in a temperature range of 350 to 550 ° C.   9. The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to claim 7 or 8, wherein after galvanizing, galvanizing alloying treatment is performed in a temperature range of 450 to 550 ° C.