JP2009102715A - High-strength hot-dip galvanized steel sheet superior in workability and impact resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet which has a high balance between TS and EI, and is superior in formability for an extension flange and in impact resistance as well, and to provide a manufacturing method therefor. <P>SOLUTION: The high-strength hot-dip galvanized steel sheet superior in workability and impact resistance has a component composition comprising, by mass%, 0.05 to 0.3% C, 0.01 to 2.5% Si, 0.5 to 3.5% Mn, 0.003 to 0.100% P, 0.02% or less S, 0.010 to 1.5% Al, furthermore, at least one element selected from Ti, Nb and V in a total amount of 0.01 to 0.2%, and the balance Fe with unavoidable impurities; and has a microstructure containing, by an area rate, ferrite of 20 to 87%, martensite and retained austenite in a total amount of 3 to 10%, tempered martensite of 10 to 60%, and a second phase that includes the martensite, retained austenite and tempered martensite, and that has an average crystal grain size of 3 μm or smaller. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance used for members such as automobiles and electrical products, and a method for producing the same.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, the movement to increase the strength and thickness of the steel plate, which is the body material, and to reduce the weight of the vehicle body has become active. In addition, since the increase in strength of such body materials also leads to an improvement in safety at the time of automobile collision, the application of high-strength steel sheets to body materials is being actively promoted. However, in general, increasing the strength of a steel sheet causes a decrease in the ductility of the steel sheet, that is, a decrease in workability. Therefore, a hot dip galvanized steel sheet having both high strength and high workability and excellent corrosion resistance is desired. ing.

  In response to these demands, high strength molten zinc of composite structure type, such as DP (Dual Phase) steel composed of ferrite and martensite and TRIP (Transformation Induced Plasticity) steel using transformation induced plasticity of retained austenite, has been developed so far. Plated steel sheets have been developed. For example, Patent Document 1 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 is Fe and inevitable impurities, and (Mn%) / (C%) ≧ 15 and (Si%) / (C%) ≧ 4 are satisfied. A high-strength galvannealed steel sheet with good workability containing 3 to 20% martensite and retained austenite has been proposed. However, such a high-strength hot-dip galvanized steel sheet of the composite structure type has a problem that it has a high elongation El obtained by uniaxial tension, but is inferior in stretch flangeability required for hole expansion processing.

Therefore, as a high-strength hot-dip galvanized steel sheet with excellent stretch flangeability, Patent Document 2 includes mass%, C: 0.02 to 0.30%, Si: 1.50% or less, Mn: 0.60 to 3.0%, P: 0.20%. Below, S: 0.05% or less, Al: 0.01 to 0.10%, the balance of Fe and inevitable impurities steel, hot rolled above the Ac ₃ transformation point, pickled, cold rolled, continuously annealed molten zinc In the plating line, it is heated and maintained above the recrystallization temperature and above the Ac ₁ transformation point, and then rapidly cooled below the Ms point until reaching the molten zinc bath to partially or fully martensite in the steel sheet. High strength hot dip galvanized steel sheet with excellent stretch flangeability that is generated and then heated to at least the hot dip zinc bath temperature and the alloying furnace temperature at a temperature equal to or higher than the Ms point to generate partially or fully tempered martensite A manufacturing method is disclosed.
JP 11-279691 A JP-A-6-93340 `` Iron and steel '', 83 (1997) p748

  However, in the high-strength hot-dip galvanized steel sheet described in Patent Document 2, excellent stretch flangeability is obtained, but not only the product of tensile strength TS and El obtained by uniaxial tension, that is, the TS-El balance is low, There is a problem that it is inferior in impact resistance required for safety in a car collision.

  An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a high TS-El balance, excellent stretch flangeability, and excellent impact resistance, and a method for producing the same.

  The present inventors have a high TS-El balance, specifically TS × El ≧ 19000 MPa ·%, excellent stretch flangeability, specifically, a hole expansion ratio λ ≧ 50% described later, and impact resistance characteristics. As a result of intensive studies on a high-strength hot-dip galvanized steel sheet having a ratio AE / TS ≧ 0.063 of absorbed energy AE and TS, which will be described later, the following was found.

  i) After optimizing the component composition, in terms of area ratio, it contains 20 to 87% ferrite, 3 to 10% total martensite and retained austenite, 10 to 60% tempered martensite, martensite and retained austenite By using a microstructure in which the average crystal grain size of the second phase consisting of tempered martensite is 3 μm or less, not only excellent stretch flangeability but also high TS-El balance and excellent impact resistance can be achieved.

ii) These microstructures have a temperature range from 500 ° C to Ac ₁ transformation point during annealing at a temperature rising rate of 10 ° C / s or more, and temperatures from Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C). After heating to a region and holding for 10 s or more to produce fine austenite by transformation, forced cooling to a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C), then reheating, It can be obtained by hot dip galvanizing. Here, the Ms point is a temperature at which martensitic transformation of austenite starts, and can be obtained from a change in the coefficient of linear expansion of steel during cooling.

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, S: 0.02 % Or less, Al: 0.010 to 1.5%, further containing at least one element selected from Ti, Nb and V in a total of 0.01 to 0.2%, the balance has a component composition consisting of Fe and inevitable impurities, and The second phase is composed of 20 to 87% ferrite, 3 to 10% total martensite and retained austenite, and 10 to 60% tempered martensite, and consists of the martensite, retained austenite and tempered martensite. Provided is a high-strength hot-dip galvanized steel sheet having a microstructure with an average crystal grain size of 3 μm or less and excellent workability and impact resistance.

  In the high-strength hot-dip galvanized steel sheet of the present invention, at least 1% selected from Cr: 0.005-2.00%, Mo: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00%. It is preferable that a seed element is contained. Furthermore, it is more preferable that at least one element selected from B: 0.0002 to 0.005%, Ca: 0.001 to 0.005%, and REM: 0.001 to 0.005% by mass% is contained.

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

High-strength hot-dip galvanized steel sheet of the present invention, for example, a slab having the above component composition, hot rolling, subjected to cold rolling and cold-rolled steel sheet, the cold-rolled steel sheet, 500 ° C. to Ac ₁ transformation point Is heated at an average temperature increase rate of 10 ° C / s or more, heated to the temperature range of Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C) and held for 10s or more, then 10 ° C / s Cool at a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at the above average cooling rate, reheat to a temperature range of 350 to 600 ° C, and perform annealing under the condition of holding for 1 to 600 s. Then, it can be manufactured by a manufacturing method for applying hot dip galvanizing.

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

  The present invention makes it possible to produce a high-strength hot-dip galvanized steel sheet having a high TS-El balance, excellent stretch flangeability, and excellent impact resistance. By applying the high-strength hot-dip galvanized steel sheet of the present invention to an automobile body, not only weight reduction and corrosion resistance of an automobile can be improved, but also safety at the time of collision can be improved.

  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: 0.05-0.3%
C is an element that stabilizes austenite, and is an element that is necessary for generating a second phase such as martensite other than ferrite to raise TS and improve the TS-El balance. If the C content is less than 0.05%, it is difficult to secure a second phase other than ferrite, and the TS-El balance is lowered. On the other hand, if the C content exceeds 0.3%, the weldability deteriorates. Therefore, the C content is 0.05 to 0.3%, preferably 0.08 to 0.15%.

Si: 0.01-2.5%
Si is an effective element for improving the TS-El balance by solid-solution strengthening steel. In order to obtain such an effect, the Si amount needs to be 0.01% or more. On the other hand, if the amount of Si exceeds 2.5%, the El decreases, the surface properties, and the weldability deteriorate. Therefore, the Si content is 0.01 to 2.5%, preferably 0.7 to 2.0%.

Mn: 0.5-3.5%
Mn is an element that is effective in strengthening steel and promotes the formation of a second phase such as martensite. In order to obtain such effects, the Mn content needs to be 0.5% or more. On the other hand, if the amount of Mn exceeds 3.5%, the ductile deterioration of ferrite due to excessive increase of the second phase or solid solution strengthening becomes remarkable, and the workability deteriorates. Therefore, the Mn content is 0.5 to 3.5%, preferably 1.5 to 3.0%.

P: 0.003-0.100%
P is an element effective for strengthening steel. In order to obtain such an effect, the P amount needs to be 0.003% or more. On the other hand, if the P content exceeds 0.100%, the steel is embrittled by grain boundary segregation, and the impact resistance is deteriorated. Therefore, the P content is 0.003 to 0.100%.

S: 0.02% or less
Since S exists as inclusions such as MnS and degrades impact resistance and weldability, the amount is preferably reduced as much as possible. However, in terms of manufacturing cost, the S amount is 0.02% or less.

Al: 0.010-1.5%
Al is an element effective in generating ferrite and improving the TS-El balance. In order to obtain such an effect, the Al content needs to be 0.010% or more. On the other hand, if the Al content exceeds 1.5%, the risk of slab cracking during continuous casting increases. Therefore, the Al content is 0.010 to 1.5%.

At least one selected from Ti, Nb and V: 0.01-0.2% in total
Ti, Nb, and V are elements effective for refining the steel structure by precipitating as TiC, NbC, and VC, respectively. In order to obtain such an effect, the content of at least one element selected from Ti, Nb and V needs to be 0.01% or more in total. On the other hand, when the content of at least one element selected from Ti, Nb, and V exceeds 0.2% in total, precipitates become excessive and ductility is reduced. Therefore, the content of at least one element selected from Ti, Nb and V is set to 0.01 to 0.2% in total.

  The balance is Fe and inevitable impurities, but for the following reasons, Cr: 0.005-2.00%, Mo: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00%, B: 0.0002-0.005% Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% are preferably contained.

Cr, Mo, Ni, Cu: 0.005-2.00% each
Cr, Mo, Ni, and Cu are effective elements for suppressing the formation of pearlite during cooling from the heating temperature during annealing, and promoting the formation of martensite and strengthening the steel. In order to obtain such an effect, the content of at least one element selected from Cr, Mo, Ni, and Cu needs to be 0.005%. On the other hand, when the content of each element of Cr, Mo, Ni, and Cu exceeds 2.00%, the effect is saturated and the cost is increased. Therefore, the contents of Cr, Mo, Ni, and Cu are each 0.005 to 2.00%.

B: 0.0002-0.005%
B is an element effective in suppressing the formation of ferrite from the austenite grain boundary and increasing the strength by generating a second phase such as martensite. In order to obtain such effects, the B content needs to be 0.0002% or more. On the other hand, when the amount of B exceeds 0.005%, the effect is saturated and the cost is increased. Therefore, the B amount is 0.0002 to 0.005%.

Ca, REM: 0.001 to 0.005% each
Ca and REM are both effective elements for improving workability by controlling the morphology of sulfides. In order to obtain such an effect, the content of at least one element selected from Ca and REM must be 0.001% or more. On the other hand, if the content of each element of Ca and REM exceeds 0.005%, the cleanliness of steel may be adversely affected. Therefore, the Ca and REM contents are 0.001 to 0.005%, respectively.

2) Microstructure Area ratio of ferrite: 20-87%
Ferrite improves TS-El balance. In order to satisfy TS × El ≧ 19000 MPa ·%, the area ratio of ferrite needs to be 20% or more, preferably 50% or more. Note that the total area ratio of martensite and retained austenite is 3% or more in total and the area ratio of tempered martensite is 10% or more, so the upper limit of the area ratio of ferrite is 87%.

Martensite and retained austenite area ratio: 3-10% in total
Martensite and retained austenite not only contribute to strengthening the steel, but also improve the TS-El balance. In order to obtain such an effect, the total area ratio of martensite and retained austenite needs to be 3% or more. However, when the area ratio of martensite and retained austenite exceeds 10% in total, stretch flangeability deteriorates. Therefore, the total area ratio of martensite and retained austenite is 3 to 10%.

Tempered martensite area ratio: 10-60%
Tempered martensite has less adverse effects on stretch flangeability than martensite and retained austenite before tempering, so it is effective in increasing strength while maintaining excellent stretch flangeability of λ ≧ 50%. Two phases. In order to obtain such an effect, the area ratio of tempered martensite needs to be 10% or more. However, when the area ratio of tempered martensite exceeds 60%, TS × El ≧ 19000 MPa ·% cannot be obtained. Therefore, the area ratio of tempered martensite is 10 to 60%.

Average grain size of the second phase composed of martensite, retained austenite, and tempered martensite: 3 μm or less The presence of the second phase composed of martensite, retained austenite, and tempered martensite effectively acts to improve impact resistance. In particular, when the average crystal grain size of the second phase is 3 μm or less, AE / TS ≧ 0.063 can be achieved. Therefore, the average crystal grain size of the second phase composed of martensite, retained austenite, and tempered martensite is 3 μm or less.

  In addition, pearlite and bainite can also be included as the second phase other than martensite, retained austenite, and tempered martensite, but the area ratio of the above ferrite, martensite, retained austenite, tempered martensite, and the average of the second phase If the crystal grain size is satisfied, the object of the present invention can be achieved. Further, from the viewpoint of stretch flangeability, the area ratio of pearlite is preferably 3% or less.

  Here, the area ratio of ferrite, martensite, retained austenite, and tempered martensite is the ratio of the area of each phase in the observed area, and after corroding the plate thickness section of the steel sheet, it corrodes with 3% nital, The position of the plate thickness 1/4 was observed with a SEM (scanning electron microscope) at a magnification of 1000 to 3000 times, and obtained using commercially available image processing software. Also, the total area of the second phase consisting of martensite, retained austenite, and tempered martensite is divided by the total number of the second phase to obtain the average area per second phase, and the square root is the average of the second phase. The crystal grain size was used.

3) Production conditions The high-strength hot-dip galvanized steel sheet of the present invention is, for example, a slab having the above component composition, hot-rolled and cold-rolled to obtain a cold-rolled steel sheet. After raising the temperature range of the Ac ₁ transformation point at an average temperature increase rate of 10 ° C./s or more, heating to the temperature range of the Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C.) and holding for 10 s or more, Conditions for cooling to a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at an average cooling rate of 10 ° C / s or more, and reheating to a temperature range of 350 to 600 ° C and holding for 1 to 600 s It can be manufactured by applying hot dip galvanizing after annealing.

Temperature rising conditions during annealing: Temperature range from 500 ° C to Ac ₁ transformation point at an average temperature rising rate of 10 ° C / s or more The temperature rising rate during temperature rising annealing consists of martensite, retained austenite, and tempered martensite. This is an important condition for reducing the average crystal grain size of the second phase. In the steel having the component composition of the present invention, recrystallization is suppressed by fine carbides of Ti, Nb, and V, but the temperature range from 500 ° C. to Ac ₁ transformation point is increased at an average temperature increase rate of 10 ° C./s or more. When heated, it is heated to a temperature range above the subsequent Ac ₁ transformation point with almost no recrystallization. Therefore, austenite transformation of unrecrystallized ferrite occurs during heating, and fine austenite is generated, so the average crystal grain size of the second phase after cooling and reheating is 3 μm or less, and excellent AE / TS ≧ 0.063 Impact resistance is obtained. On the other hand, if the average rate of temperature rise in the temperature range from 500 ° C to Ac ₁ transformation point is less than 10 ° C / s, recrystallization occurs in the temperature range from 500 ° C to Ac ₁ transformation point during temperature rise, and some recrystallized ferrite forms. Since the austenite transformation occurs after the grain growth, the austenite cannot be refined and the average crystal grain size of the second phase cannot be reduced to 3 μm or less. Therefore, it is necessary to raise the temperature range from 500 ° C. to Ac ₁ transformation point at an average temperature increase rate of 10 ° C./s or more, preferably 20 ° C./s or more.

Heating conditions during annealing: When the heating temperature during holding annealing is less than Ac ₁ transformation point or holding time is less than 10 s in the temperature range from Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C), austenite is generated Does not occur or becomes insufficient, and subsequent cooling cannot secure a sufficient amount of the second phase such as martensite. On the other hand, if the heating temperature exceeds (Ac ₃ transformation point + 30 ° C.), the austenite grains grow remarkably and the austenite cannot be refined. Further, the growth of austenite grains suppresses the formation of ferrite during cooling, and ferrite with an area ratio of 20% or more cannot be obtained. Therefore, the heating during annealing needs to be performed under the condition of holding for 10 s or more in the temperature range from the Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C.). The holding time is preferably 300 s or less from the viewpoint of suppressing austenite coarsening and energy cost.

Cooling conditions during annealing: After cooling and heating to a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at an average cooling rate of 10 ° C / s or more from the heating temperature, 10 ° C / s from the heating temperature It is necessary to cool at the above average cooling rate. However, if the average cooling rate is less than 10 ° C / s, a large amount of pearlite is generated, and the required amount of tempered martensite, martensite and retained austenite are obtained. It is because it is not possible. The upper limit of the cooling rate is not particularly specified, but it is difficult to control the cooling to the cooling stop temperature range from (Ms point -100 ° C) to (Ms point -200 ° C), although the steel plate shape deteriorates, It is preferable to be 200 ° C./s or less. The cooling stop temperature is one of the most important conditions in the present invention for controlling the amount of martensite, retained austenite, and tempered martensite generated during subsequent reheating, hot dip galvanizing, and alloying treatment of the plating phase. is there. That is, the amount of martensite and untransformed austenite is determined when cooling is stopped, and in the subsequent heat treatment, martensite becomes tempered martensite, untransformed austenite becomes martensite or retained austenite, and the strength of the steel, TS-El It affects the balance and stretch flangeability. When the cooling stop temperature exceeds (Ms point -100 ° C), the martensitic transformation becomes insufficient, the amount of untransformed austenite increases, and finally the area ratio of martensite and residual austenite becomes 10% in total. Exceeding, stretch flangeability is reduced. On the other hand, when the cooling stop temperature is less than (Ms point -200 ° C), most of the austenite undergoes martensitic transformation, the amount of untransformed austenite decreases, and finally the total area ratio of martensite and residual austenite is 3 Less than% and TS-El balance deteriorates. Therefore, cooling during annealing needs to be performed under a cooling condition in a temperature range of (Ms point −100 ° C.) to (Ms point −200 ° C.) at an average cooling rate of 10 ° C./s or more from the heating temperature.

Reheating conditions during annealing: Hold for 1 to 600 s in the temperature range of 350 to 600 ° C
After cooling to the temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) with an average cooling rate of 10 ° C / s or more, reheat the sample for 1s or more in the temperature range of 350 to 600 ° C. Thus, the martensite generated during cooling is tempered to produce tempered martensite with an area ratio of 10 to 60%, and high strength can be achieved while maintaining excellent stretch flangeability. If the reheating temperature is less than 350 ° C. or the holding time is less than 1 s, the area ratio of tempered martensite is less than 10%, and stretch flangeability deteriorates. In addition, when the reheating temperature exceeds 600 ° C. or the holding time exceeds 600 s, the untransformed austenite generated during cooling is transformed into pearlite or bainite, and finally the total area ratio of martensite and residual austenite is 3 Less than% and TS-El balance deteriorates. Therefore, it is necessary to perform reheating at the time of annealing in the temperature range of 350 to 600 ° C. for 1 to 600 s.

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

  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. To hot-roll the slab, the slab may be cooled to room temperature and then re-heated for hot rolling, or the slab may be charged in a heating furnace without being cooled to room temperature. Can also be done. Alternatively, an energy saving process in which hot rolling is performed immediately after performing a slight heat retention can also be applied. When heating the slab, it is preferable to heat to 1100 ° C. or higher in order to dissolve carbides and prevent an increase in rolling load. In order to prevent an increase in scale loss, the heating temperature of the slab is preferably 1300 ° C. or lower.

When hot rolling a slab, the rough bar after rough rolling can be heated from the viewpoint of securing the rolling temperature. Moreover, what is called a continuous rolling process which joins rough bars and performs finish rolling continuously can be applied. Finish rolling is performed at a finishing temperature equal to or higher than the Ar ₃ transformation point in order to prevent the formation of a band structure that causes a decrease in workability after cold rolling / annealing and an increase in anisotropy. Further, in order to reduce the rolling load and make the shape and material uniform, it is preferable to perform lubrication rolling with a friction coefficient of 0.10 to 0.25 in all passes or a part of the finishing rolling.

  The steel sheet after hot rolling is preferably wound at a winding temperature of 450 to 700 ° C. from the viewpoint of temperature control and prevention of decarburization.

  The steel sheet after winding is removed by scale pickling or the like, then cold-rolled preferably at a rolling reduction of 40% or more, annealed under the above conditions, and hot dip galvanized.

  In hot dip galvanization, if the plating is not alloyed, the amount of Al is 0.12-0.22%, or if the plating is alloyed, the steel plate is immersed in a 440-500 ° C plating bath containing the amount of Al 0.08-0.18%. , Adjust the amount of plating by gas wiping. When alloying the plating, an alloying treatment is further performed at 450 to 600 ° C. for 1 to 30 seconds.

  The steel sheet after the hot dip galvanization or the steel sheet after the alloying treatment of the plating can be subjected to temper rolling for the purpose of shape correction, adjustment of surface roughness, and the like. Moreover, various coating processes, such as resin and oil-fat coating, can also be given.

Steels A to L having the composition shown in Table 1 were melted in a converter and made into a slab by a continuous casting method, and then hot-rolled to a sheet thickness of 3.0 mm at a finishing temperature of 900 ° C. and 10 ° C./s after rolling. And cooled at a winding temperature of 600 ° C. Next, after pickling, it was cold rolled to a plate thickness of 1.2 mm, and after annealing under the annealing conditions shown in Table 2 by a continuous hot dip galvanizing line, it was immersed in a 460 ° C. plating bath, and the adhesion amount was 35 to 45 g / m. ² plating layers were formed, alloyed at 520 ° C., and cooled at a cooling rate of 10 ° C./sec to prepare plated steel sheets 1 to 30. As shown in Table 2, some of the plated steel sheets were not alloyed. Then, with respect to the obtained plated steel sheet, the area ratio of ferrite, martensite, retained austenite, tempered martensite and the average crystal grain size of the second phase composed of martensite, retained austenite, tempered martensite 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 conducted in accordance with JIS Z 2241 to obtain TS × El. Further, a test piece of 150 mm × 150 mm 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 stretch flangeability was evaluated. Furthermore, according to the method described in Non-Patent Document 1, a specimen having a width of 5 mm and a length of 7 mm in a direction perpendicular to the rolling direction was taken, and a tensile test was performed at a strain rate of 2000 / s. The absorption energy AE was calculated by integrating the stress-true strain curve in the range of 0 to 10% of strain, AE / TS was obtained, and the impact resistance characteristics were evaluated.

  The results are shown in Tables 3 and 4. All of the plated steel sheets according to the present invention have high TS-El balance with TS × El ≧ 19000MPa ·%, excellent stretch flangeability with hole expansion ratio λ ≧ 50%, and impact resistance characteristics with AE / TS ≧ 0.063. It turns out that it is excellent.

Claims (7)

  % By mass, C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, Al: 0.010-1.5%, Ti, Nb and It contains at least one element selected from V in a total of 0.01 to 0.2%, the balance is composed of Fe and inevitable impurities, and the area ratio is 20 to 87% of ferrite, martensite, Processing having a microstructure in which the residual austenite is 3 to 10% in total and tempered martensite is 10 to 60%, and the average grain size of the second phase composed of martensite, residual austenite and tempered martensite is 3 μm or less. High-strength hot-dip galvanized steel sheet with excellent heat resistance and impact resistance.   Furthermore, it contains at least one element selected from Cr: 0.005-2.00%, Mo: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00% by mass%. High-strength hot-dip galvanized steel sheet with excellent workability and impact resistance.   The high-strength hot-dip galvanized steel sheet having excellent workability and impact resistance according to claim 1 or 2, further comprising B: 0.0002 to 0.005% by mass%.   Furthermore, high mass excellent in workability and impact resistance according to any one of claims 1 to 3 containing at least one element selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% in mass%. Strength hot dip galvanized steel sheet.   5. The high-strength hot-dip galvanized steel sheet excellent in workability and impact resistance properties according to claim 1, wherein the galvanizing is alloyed galvanizing. The slab having the component composition according to any one of claims 1 to 4, is subjected to hot rolling and cold rolling to form a cold rolled steel sheet, and the cold rolled steel sheet has a temperature range of 500 ° C to Ac ₁ transformation point. The temperature is increased at an average temperature increase rate of 10 ° C / s or more, heated to the temperature range from Ac ₁ transformation point to (Ac ₃ transformation point + 30 ° C) and held for 10s or more, then average cooling of 10 ° C / s or more Cool to the temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at a speed, reheat to a temperature range of 350-600 ° C and anneal for 1 to 600 s, then melt A method for producing a high-strength hot-dip galvanized steel sheet that is excellent in workability and impact resistance properties for galvanizing.   7. The method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and impact resistance properties according to claim 6, wherein the hot-dip galvanizing is followed by galvanizing alloying treatment.