TW201404898A - High-strength hot-dip zinc-plated steel sheet having excellent formability and shape freezability and manufacturing method therefor - Google Patents

Abstract

A high-strength hot-dip zinc-plated steel sheet having excellent formability and shape freezability and a manufacturing method therefor are provided, wherein the zinc-plated steel sheet has a tensile strength (TS) equal to or more than 1180 MPa, a total elongation (EL) equal to or more than 14%, a hole expanding ratio ( λ ) equal to or more than 30% and a yield ratio (YR) equal to or less than 70%. The high-strength hot dip zinc-plated steel sheet having excellent formability and shape freezability is characterized in the following. It has a componential composition including, by mass%, 0.10% to 0.35% C, 0.5% to 3.0% Si, 1.5% to 4.0% Mn, 0.100% or less P, 0.02% or less S and 0.010% to 0.5% Al, and the rest includes Fe and inevitable impurities. A microstructure includes, by area ratio, 0% to 5% polygonal ferrite, 5% or more bainitic ferrite, 5% to 20% martensite, 30% to 60% tempered martensite and 5% to 20% retained austenite. An average grain size of prior austenite is 15 μ m or less.

Description

High-strength molten zinc-plated steel sheet excellent in formability and shape freezing property and method for producing same

The present invention relates to a high-strength hot-dip galvanized steel sheet which is excellent in moldability and shape freezeability, which is suitable for use as a steel sheet for automobiles, and a method for producing the same.

In recent years, the improvement of fuel consumption of automobiles has become an important issue in terms of preserving the global environment. Therefore, the vehicle body material is made thinner by the increase in strength, and the fuel consumption is increased by the weight reduction of the vehicle body itself. For a steel sheet which is molded into a product by press working or bending processing like an automobile part, it is required to maintain high strength and formability to be processed. In Patent Document 1, high strength and high workability are simultaneously achieved by effectively utilizing the tempered granules and the residual austenite. However, generally, as the strength of the steel sheet becomes higher, there is a problem that the spring back after processing becomes large and the shape freezeability is lowered. In Patent Document 1, no research has been made on the shape freezeability, and there is room for improvement. On the other hand, in Patent Document 2, by using a structure containing ferrite, toughened iron, and a Worth field having a low C concentration, a low yield ratio (YR) and a shape freeze property are obtained. Excellent steel plate. However, regarding the stretch flangeability, it is difficult to say that it has sufficient workability. In Patent Document 3, high strength and high ductility are coexisted by effectively utilizing the tempered granules, the toughened iron, and the residual Worth field, but the shape freezeability is not mentioned. Further, the absolute value of the stretch flangeability is not necessarily high, and there is room for improvement.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2009-209450

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2010-126808

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2010-90475

The present invention advantageously solves the problems of the prior art described above, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet which is suitable as a raw material for automobile parts and has a tensile strength (TS), and a method for producing the same. When it is 1180 MPa or more, the total elongation (EL) is 14% or more, the hole expansion ratio (λ) is 30% or more, and the drop ratio (YR) is 70% or less, and the formability and the shape freeze property are excellent. Further, the ratio of fall (YR) is expressed by YR (%) = (YS / TS) × 100 in terms of the ratio of the drop strength (YS) to the tensile strength (TS).

In order to achieve the above-mentioned problems, the inventors of the present invention have intensively studied the high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property, and have repeatedly studied the composition and microstructure of the steel sheet. As a result, the following has been found.

By appropriately adjusting the alloying elements, it is set to contain 0% to 5% of polygonal ferrites, 5% or more of tough ferrites (bainitic ferrites), 5% to 20% by area ratio. % of 麻田散体, 30%~60% of tempered 麻田散体 and 5%~20% of residual Worth field structure, and the average particle size of the original prior austenite is set to 15 μm or less It can combine high strength with high formability and high shape freezing.

The reason for the improvement of the shape freezing property by dispersing the granules in the main body of the tempering Matian body is not necessarily clear, but it is generally considered to be the original. This is because, after cooling after plating or after alloying, the Worth body in contact with the tempered Matian bulk body undergoes metamorphosis in the field and introduces mobile dislocation into the tempered mass of the field, thereby YR Reduced. In addition, the reason why the λ is increased by the fineness of the original Woustian body particles is not clear, but it is presumed that the reason is that the average particle diameter of the microstructure after annealing is made fine by the original Woustian body particles. When the flange is formed, the propagation path of the crack increases and the connection of the crack is suppressed.

The microstructure is obtained by heating to an average heating rate of 5 ° C / s or more to 500 ° C ~ Ac ₁ point during annealing, heating to Ac ₃ points -20 ° C ~ 1000 ° C and maintaining 10 After sec to 1000 sec, it is cooled from 750 °C to an Ms point of -80 °C to Ms point -30 °C at an average cooling rate of 15 °C/s or more, and then heated to 350 °C to 500 °C for 10 seconds. ~600 seconds.

The present invention has been made based on such findings and provides the following invention.

(1) A high-strength hot-dip galvanized steel sheet excellent in moldability and shape freezeability, which comprises 0.10% to 0.35% of C, 0.5% to 3.0% of Si, and 1.5% by mass%. 4.0% Mn, 0.100% or less of P, 0.02% or less of S, and 0.010% to 0.5% of Al, and the remainder contains Fe and unavoidable impurities; and the microstructure is 0% by area ratio. ~5% polygonal ferrite, 5% or more toughened iron ferrite, 5% to 20% of Ma Tian bulk, 30% to 60% of tempered Matian bulk and 5% to 20% of residual Worthian The original Worth field has an average particle size of 15 μm or less.

(2) High strength excellent in formability and shape freezing property as described in (1) The hot-dip galvanized steel sheet further contains, in mass%, 0.005% to 2.00% of Cr, 0.005% to 2.00% of Mo, 0.005% to 2.00% of V, 0.005% to 2.00% of Ni, and 0.005%. ~ 2.00% of at least one element in Cu.

(3) The high-strength hot-dip galvanized steel sheet having excellent moldability and shape-freezing property as described in (1) or (2) further contains, in mass%, Ti and 0.01% from 0.01% to 0.20%. 0.20% of at least one element of Nb.

(4) The high-strength hot-dip galvanized steel sheet excellent in moldability and shape-freezing property according to any one of (1) to (3) further contains 0.0005% to 0.0050% of B by mass%.

(5) The high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property according to any one of (1) to (4) further containing, in mass%, Ca selected from 0.001% to 0.005%. And at least one element of 0.001% to 0.005% of REM.

(6) A high-strength hot-dip galvanized steel sheet excellent in formability and shape freezeability according to any one of (1) to (5), wherein galvanizing is alloying galvanization.

(7) A method for producing a high-strength hot-dip galvanized steel sheet having excellent formability and shape freezeability, characterized in that the billet having the composition of any one of (1) to (5) is heated Rolling or further cold rolling, and then, when performing continuous annealing, heating to 500 ° C ~ Ac ₁ point at an average heating rate of 5 ° C / s or more, heating to a temperature range of Ac ₃ -20 ° C ~ 1000 ° C After holding for 10 seconds to 1000 seconds, it is cooled from 750 ° C to an Ms point of -80 ° C to Ms point of -30 ° C at an average cooling rate of 15 ° C / s or more, and then heated to 350 ° C to 500 ° C and After holding for 10 seconds to 600 seconds, hot-dip galvanizing or further plating alloying treatment is performed.

According to the present invention, it is possible to obtain a tensile strength (TS) of 1180 MPa or more, a total elongation (EL) of 14% or more, a hole expansion ratio (λ) of 30% or more, and a fall ratio (YR) of 70% or less. A high-strength hot-dip galvanized steel sheet excellent in formability and shape freezeability.

The invention is described in detail below. In addition, "%" indicating the content of the component element means "% by mass" unless otherwise specified.

1) Composition

C: 0.10%~0.35%

C is an element necessary for generating a low-temperature metamorphic phase such as a granulated loose body or a tempered granule to increase the TS. When the amount of C is less than 0.10%, it is difficult to ensure a tempered granule body having an area ratio of 30% or more and a mash ore of 5% or more. On the other hand, when the amount of C exceeds 0.35%, EL or spot weldability deteriorates. Therefore, the amount of C is set to be 0.10% to 0.35%, preferably 0.15% to 0.3%.

Si: 0.5%~3.0%

Si is an element which effectively solidifies steel and strengthens the TS-EL balance or generates a residual Worth field. In order to obtain such an effect, the amount of Si must be set to 0.5% or more. On the other hand, when Si exceeds 3.0%, the EL is lowered, and the surface properties and weldability are deteriorated. Therefore, the amount of Si is set to 0.5 to 3.0%, preferably 0.9% to 2.0%.

Mn: 1.5%~4.0%

Mn is an element that effectively strengthens steel and promotes the formation of low-temperature metamorphic phases such as granules. In order to obtain such an effect, the amount of Mn must be set to 1.5% or more. On the other hand, when the amount of Mn exceeds 4.0%, deterioration of EL becomes remarkable, and workability is lowered. Therefore, the amount of Mn is set to be 1.5% to 4.0%, preferably 2.0% to 3.5%.

P: 0.100% or less

Since P deteriorates the steel due to grain boundary segregation and deteriorates weldability, it is preferable to reduce the amount as much as possible. However, the amount of P is set to be 0.100% or less in terms of manufacturing cost and the like.

S: 0.02% or less

S is in the form of inclusions such as MnS, and the weldability is deteriorated. Therefore, it is preferable to reduce the amount as much as possible. However, in terms of manufacturing cost, the amount of S is set to 0.02% or less.

Al: 0.010%~0.5%

Al functions as a deoxidizer, and is preferably added in the deoxidation step. In order to obtain such an effect, the amount of Al must be set to 0.010% or more. On the other hand, when the amount of Al exceeds 0.5%, the risk of breakage of the material during continuous casting becomes high. Therefore, the amount of Al is set to be 0.010% to 0.5%.

The remainder is Fe and unavoidable impurities, and may contain one or more of the following elements as necessary.

It is selected from the group consisting of 0.005% to 2.00% of Cr, 0.005% to 2.00% of Mo, 0.005% to 2.00% of V, 0.005% to 2.00% of Ni, and at least one of 0.005% to 2.00% of Cu.

Cr, Mo, V, Ni, and Cu are elements that efficiently form low-temperature metamorphic phases such as granules of the field. In order to obtain such an effect, the content of at least one element selected from the group consisting of Cr, Mo, V, Ni, and Cu must be set to 0.005%. On the other hand, when the content of each of Cr, Mo, V, Ni, and Cu exceeds 2.00%, the effect is saturated, resulting in an increase in cost. Therefore, the contents of Cr, Mo, V, Ni, and Cu are set to be 0.005% to 2.00%, respectively.

Further, at least one selected from the group consisting of 0.01% to 020% of Ti and 0.01% to 0.20% of Nb may be further contained.

Ti and Nb are elements which form a carbonitride efficiently and increase the strength of the steel by precipitation strengthening. In order to obtain such an effect, the content of Ti and Nb must be set to 0.01% or more. On the other hand, when the content of Ti and Nb exceeds 0.20%, the effect of increasing the strength is saturated, and the EL is lowered. Therefore, the content of Ti and Nb is set to be 0.01% to 0.20%.

In addition, it may further contain 0.0005% to 0.0050% of B.

B is an element which effectively suppresses the formation of ferrite from the grain boundary of the Worthfield and generates a low-temperature metamorphic phase. In order to obtain such an effect, the amount of B must be set to 0.0005% or more. On the other hand, if the amount of B exceeds 0.0050%, the effect is saturated, resulting in an increase in cost. Therefore, the amount of B is set to 0.0005% to 0.0050%.

Further, at least one selected from the group consisting of 0.001% to 0.005% of Ca and 0.001% to 0.005% of rare earth metal (Rare Earth Metals, REM) may be further contained.

Both Ca and REM are elements which are effective in improving workability by controlling the form of sulfide. In order to achieve this effect, it must be selected from Ca, REM The content of at least one of the elements is set to 0.001% or more. On the other hand, if the content of each of Ca and REM exceeds 0.005%, the cleanliness of the steel may be adversely affected, and the desired characteristics may not be obtained. Therefore, the content of Ca and REM is set to be 0.001% to 0.005%.

2) Micro organization

Area ratio of polygonal ferrite: 0%~5%

When the area ratio of the polygonal ferrite exceeds 5%, it is difficult to have a TS of 1180 MPa or more and a hole expansion ratio of 30% or more. Therefore, the area ratio of the polygonal ferrite is set to be 0% to 5%.

Area ratio of tough iron ferrite: 5% or more

The toughened iron metamorphism effectively concentrates the C in the Worth field to stabilize the Worth field, thereby ensuring an effective increase in the residual Worth field of the EL. In order to obtain this effect, the area ratio of the tough iron ferrite must be set to 5% or more. On the other hand, if the area ratio exceeds 60%, it is difficult to obtain the desired mash field and the residual Worth field, and therefore the area ratio of the tough iron ferrite is preferably set to 5% to 60%.

Area ratio of Ma Tian's bulk: 5~20%

Ma Tian's bulk effectively increases TS. In addition, the YR is effectively reduced.

In order to obtain such an effect, the area ratio of the Ma Tian bulk body must be 5% or more. On the other hand, if it exceeds 20%, the decrease in EL or the hole expansion ratio becomes remarkable. Therefore, the area ratio of Ma Tian's bulk is set at 5% to 20%.

Area ratio of tempered Matian bulk: 30%~60%

If the area ratio of the tempered granules is less than 30%, it is difficult to have a TS of 1180 MPa or more and a hole expansion ratio of 30% or more. On the other hand, if it is When the product ratio exceeds 60%, the rise of YR becomes remarkable and the shape freezeability is lowered. Therefore, the area ratio of the tempered Matian bulk is set at 30% to 60%. Further, the tempered granules in the present invention have a Vickers hardness of 250 or more.

Area ratio of residual Worth field: 5%~20%

The residual Worth field effectively increases the EL. In order to obtain such an effect, the area ratio of the residual Worth field body must be set to 5% or more. Therefore, if the area ratio exceeds 20%, the decrease in the hole expansion ratio becomes remarkable. Therefore, the area ratio of the residual Worth field is set to 5% to 20%.

The original Worth field has an average particle size of 15 μm or less.

The refinement of the original Worthite body particles effectively increases λ. In order to obtain such an effect, the average particle size of the original Worth field body must be set to 15 μm or less. Therefore, the average particle size of the original Worth field is set to be 15 μm or less. Although the lower limit is not particularly specified, if it is too small, YR may rise, so it is preferably 5 μm or more.

In addition, the corrugated body may be contained as a polygonal ferrite, a tough iron ferrite, a granulated loose body, a tempered granule, or a phase other than the residual Worth, but as long as the conditions of the above microstructure are satisfied, Achieve the purpose of the invention.

Here, the area ratio of the polygonal ferrite, the tough iron ferrite, the Ma Tian bulk, and the tempered Ma Tian bulk refers to the proportion of the area of each phase in the observation area, polygonal ferrite, 麻田散体, The area ratio of the tough iron ferrite and the tempered Matian bulk was determined by the method shown below. After grinding the plate thickness profile of the steel plate, it was etched with 3% nitric acid etching solution (Nital), and the scanning electron microscope (Scanning) was used for the plate thickness 1/4 position. Electron Microscope (SEM) photographed three fields of view at a magnification of 1500 times, and used Image-Pro manufactured by Media Cybernetics to differentiate the object tissues coated with each field of view to determine the area occupied by the object tissue in the field of view. The average value of the area ratio of each field of view is obtained as the area ratio of the target tissue. Further, regarding the area ratio of the residual Worth field, the steel sheet was polished to a thickness of 1/4 position, and then further polished by chemical polishing to 0.1 mm, and the obtained surface was measured by Kα ray using Mo by an X-ray diffraction apparatus. The integral strength of the (200) plane, the (220) plane, the (311) plane of fcc iron, the (200) plane, the (211) plane, and the (220) plane of bcc iron, and the residual Worstian body is obtained from the integral strength. The ratio is taken as the area ratio of the residual Worth field. In addition, regarding the average particle diameter of the original Worth field, the plate thickness profile of the steel plate was polished, and then etched with a 3% nitric acid etching solution, and 1200 times the 1/4 position of the plate thickness by SEM (scanning electron microscope) The magnification was observed, and the average area was obtained by dividing the total value of the area of the tissue surrounded by the original Worstian grain boundary of the field of view by the number of squares, and the square root was taken as the average particle diameter.

3) Manufacturing conditions

The high-strength hot-dip galvanized steel sheet of the present invention is produced in the following manner. First, the billet having the above composition is subjected to hot rolling, pickling, or further cold rolling. Then, in continuous annealing, heating to 500 ° C ~ Ac ₁ point at an average heating rate of 5 ° C / s or more, heating to Ac ₃ ° -20 ° C ~ 1000 ° C temperature range and maintaining 10 seconds ~ 1000 seconds It is cooled from 750 ° C to an Ms point of -80 ° C to Ms point of -30 ° C at an average cooling rate of 15 ° C / s or more. Further, after the steel sheet is heated to 350 ° C to 500 ° C for 10 seconds to 600 seconds, hot-dip galvanizing or further plating alloying treatment is performed. The details will be described below.

The steel having the above composition is melted to obtain a billet, and the billet is hot rolled, then cooled and taken up. When the coiling temperature after hot rolling exceeds 650 ° C, black stains are formed and the plating property is lowered. On the other hand, if the coiling temperature after hot rolling is less than 400 ° C, the shape of the hot rolled sheet deteriorates. Therefore, the coiling temperature after hot rolling is preferably set to 400 ° C to 650 ° C.

Then, it is preferred to subject the hot-rolled sheet to pickling to remove the scale of the surface layer of the hot-rolled sheet. The pickling step is not particularly limited, and a conventional method can also be used. The hot rolled sheet after pickling is subjected to cold rolling as needed. The cold rolling step is not particularly limited, and a conventional method can also be used. The hot-rolled sheet after pickling or the cold-rolled sheet after cold rolling is continuously annealed under the following conditions.

Average heating rate up to 500 ° C ~ Ac ₁ point: 5 ° C / s or more

When the average heating rate up to 500 ° C to Ac ₁ point is less than 5 ° C / s, the Worstian body becomes coarse due to recrystallization, and the microstructure of the present invention cannot be obtained. Therefore, the average heating rate up to 500 ° C to Ac ₁ point is set to 5 ° C / s or more.

Heat to a temperature range of Ac ₃ -20 ° C ~ 1000 ° C and keep the heat for 10 seconds ~ 1000 seconds

When the soaking temperature is kept lower than Ac ₃ to -20 ° C, the formation of the Worth field is insufficient, and the microstructure of the present invention cannot be obtained. On the other hand, when the soaking temperature is maintained at a temperature exceeding 1000 ° C, the Worth field body becomes coarse, and the constituent phase after the annealing becomes coarse, and the toughness and the like are lowered. Therefore, the soaking temperature is set to Ac ₃ points -20 ° C to 1000 ° C. When the soaking time is less than 10 seconds, the formation of the Worth field is insufficient, and the microstructure of the present invention cannot be obtained. In addition, if the soaking time is more than 1000 seconds, the cost increases. Therefore, the soaking hold time is set to 10 seconds to 1000 seconds.

Cooling from 750 ° C to an Ms point of -80 ° C ~ Ms point -30 ° C at an average cooling rate of 15 ° C / s or more

If the average cooling rate from 750 ° C to a temperature range of -80 ° C to Ms point -30 ° C is less than 15 ° C / s, a large amount of ferrite is formed during cooling, and the microstructure of the present invention cannot be obtained. . Therefore, the average cooling rate is set to 15 ° C / s or more.

Cooling stop temperature: Ms point -80 ° C ~ Ms point -30 ° C

If it is cooled to the temperature at which it reaches the cooling temperature, a part of the Worth field is metamorphosed into the Matian bulk. After the reheating or the plating alloying treatment, the Ma Tian bulk becomes the tempered Ma Tian bulk, and the untransformed Worth field becomes a residue. Vostian or Ma Tian bulk or toughened iron. At this time, if the cooling reaching temperature exceeds the Ms point by -30 ° C, the amount of the tempered mass in the field is insufficient. If the temperature is less than the Ms point of -80 ° C, the untransformed Worth field is significantly reduced, and the tempered granules are increased, so The microtissue of the invention is obtained. Therefore, the cooling arrival temperature is set to an Ms point of -80 ° C to Ms point of -30 ° C.

Reheating temperature: 350 ° C ~ 500 ° C

After cooling to the temperature at which the cooling reaches the temperature, and then heating to a temperature range of 350 ° C to 500 ° C, the granulated granules of the granules formed during the tempering are tempered to become tempered granules, and the C-concentration is carried out in the untransformed Worth field. It became stable as a residual Worth field. In addition, the toughened iron metamorphosis is carried out, and the C self-transformed iron ferrite diffuses to make the untransformed Worth field more stable. If again When the heating temperature is less than 350 ° C, the toughened iron metamorphic state is a toughened iron containing carbides, so C is not concentrated in the untransformed Worth field, and the stability as a residual Worth field is insufficient. . On the other hand, if it exceeds 500 ° C, the untransformed Worth field tends to form carbides or undergo wave-transformation, and the microstructure of the present invention cannot be obtained. Therefore, the reheating temperature is set to 350 ° C to 500 ° C. It is preferably from 380 ° C to 480 ° C.

Retention time at reheating temperature: 10 seconds to 600 seconds

When the holding time is less than 10 seconds, the formation of toughened iron becomes insufficient, and if it exceeds 600 seconds, the untransformed Worth field tends to form carbides or undergo wave-like transformation, and the microstructure of the present invention cannot be obtained. Therefore, the hold time is set to 10 seconds to 600 seconds.

The hot-dip galvanizing treatment is preferably carried out by immersing the steel sheet obtained by the above operation in a galvanizing bath of 440 ° C or more and 500 ° C or less, and then adjusting the amount of plating adhesion by gas wiping or the like. When the galvanization alloy is further alloyed, it is preferably alloyed in a temperature range of 460 ° C or more and 550 ° C or less for 1 second or longer and 40 seconds or shorter. Preferably, the galvanizing bath uses a galvanizing bath having an Al content of 0.08% to 0.18%.

The steel sheet subjected to the hot-dip galvanizing treatment may be subjected to temper rolling to correct the shape or adjust the surface roughness. Further, various coating treatments such as resin or grease coating can be carried out.

The conditions of other production methods are not particularly limited, but are preferably carried out under the following conditions.

In order to prevent segregation, the material is preferably produced by a continuous casting method, and may be produced by a bulking method or a thin casting method. Billet When the material is hot-rolled, the billet may be temporarily cooled to room temperature, and then heated and hot-rolled, or may be placed in a heating furnace without hot-rolling the billet to room temperature. Alternatively, an energy-saving process can be applied, that is, hot rolling is performed immediately after a slight heat retention. When the billet is heated, it is preferably heated to 1100 ° C or higher in order to dissolve the carbide or prevent an increase in the rolling load. Further, in order to prevent an increase in scale loss, the heating temperature of the billet is preferably set to 1300 ° C or lower.

When the billet is hot-rolled, the thick rod after the rough rolling can be heated from the viewpoint of preventing trouble at the time of rolling, even if the heating temperature of the billet is lowered. Further, a so-called continuous rolling press can be applied, that is, the thick rods are joined to each other and the finish rolling is continuously performed. The finish rolling may increase the anisotropy and lower the workability after cold rolling and annealing. Therefore, it is preferably carried out at a finishing temperature of at least the Ar ₃ transformation point. Further, in order to reduce the rolling load or to uniformize the shape and material, it is preferable to apply a lubricating rolling pressure having a friction coefficient of 0.10 to 0.25 over the entire path or a part of the path of the finish rolling.

After the coiled steel sheet is removed by pickling or the like, the hot-rolled sheet is annealed under the above conditions, or the hot-rolled sheet is cold-rolled, and then annealed under the above conditions to carry out hot-dip galvanizing. In the case of performing cold rolling, it is preferred to set the cold rolling ratio to 40% or more. Further, in order to reduce the rolling load at the time of cold rolling, the rolled steel sheet may be subjected to hot-rolled sheet annealing.

[Example]

The steel having the composition shown in Table 1 was melted by a converter, and a slab was produced by continuous casting (in Table 1, N is an unavoidable impurity). These slabs were heated to 1,200 ° C, and then subjected to rough rolling and finish rolling, and coiled at a coiling temperature of 400 ° C to 650 ° C to obtain a hot rolled sheet having a thickness of 2.3 mm. Then, a part of the mixture was subjected to batch treatment at a temperature of 600 ° C and a heat treatment time of 5 hours, and after pickling, cold rolling was carried out to a thickness of 1.4 mm to produce a cold-rolled steel sheet and annealed. For the other part, the steel sheet which was hot rolled to a thickness of 2.3 mm was pickled and directly annealed. The annealing was carried out under the conditions shown in Table 2 and Table 3 by continuous hot-dip galvanizing line, and immersed in a plating bath at 460 ° C to form a plating amount of adhesion amount of 35 g/m ² to 45 g/m ² . The material was cooled at a cooling rate of 10 ° C / s to prepare a hot-dip galvanized steel sheet 1 to a hot-dip galvanized steel sheet 29 . The other part was further plated and alloyed at 525 ° C after plating, and cooled at a cooling rate of 10 ° C / s to prepare a alloyed hot-dip galvanized steel sheet. Then, the obtained plated steel sheet was subjected to the above-described method to measure the area ratio of the polygonal ferrite, the tough iron ferrite, the Ma Tian bulk, the tempered Ma Tian bulk, the area ratio of the residual Worth field, and the original Worth field. The average particle size. 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 strain rate of 10 ^-3 . Further, a test piece of 150 mm × 150 mm was taken, and an average hole expansion ratio (%) was obtained by performing a three-hole expansion test in accordance with JFST 1001 (Japan Iron and Steel Federation Standard, 2008), and the stretch flangeability was evaluated. The results are shown in Tables 4 and 5.

In the present invention, YR is 70% or less, and it is confirmed that it has a high shape freezeability. Further, TS was 1180 MPa or more, EL was 14% or more, and λ was 30% or more, and it was confirmed that it had high strength and formability. Therefore, according to the example of the present invention, a hot-dip galvanized steel sheet having excellent shape freezeability can be obtained, and an excellent effect of contributing to weight reduction of an automobile and contributing to high performance of an automobile body can be obtained.

According to the present invention, it is possible to obtain a tensile strength (TS) of 1180 MPa or more, a total elongation (EL) of 14% or more, a hole expansion ratio (λ) of 30% or more, and a fall ratio (YR) of 70% or less. A high-strength hot-dip galvanized steel sheet excellent in formability and shape freezeability. When the high-strength hot-dip galvanized steel sheet of the present invention is used for automotive parts, it contributes to the weight reduction of automobiles and contributes to the improvement of the performance of automobile bodies.

Claims (7)

A high-strength hot-dip galvanized steel sheet excellent in moldability and shape freezeability, comprising 0.1% to 0.35% of C, 0.5% to 3.0% of Si, and 1.5% to 4.0% by mass% Mn, 0.100% or less of P, 0.02% or less of S, and 0.010% to 0.5% of Al, and the remainder contains Fe and unavoidable impurities; and the microstructure is 0% to 5% by area ratio. Polygonal ferrite, more than 5% of toughened iron ferrite, 5% to 20% of Ma Tian bulk, 30% to 60% of tempered Ma Tian bulk and 5% to 20% of residual Worth field; The original Worth field has an average particle size of 15 μm or less. The high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property as described in claim 1 further contains, in mass%, 0.005% to 2.00% of Cr, 0.005% to 2.00% of Mo, 0.005% to 2.00% of V, 0.005% to 2.00% of Ni, and at least one element of 0.005% to 2.00% of Cu. The high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property as described in the first or second aspect of the patent application, further contains, in mass%, Ti and 0.01 to 0.20 selected from 0.01% to 0.20%. At least one element of % Nb. The high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property as described in any one of the first to third aspects of the invention is further contained in an amount of 0.0005% to 0.0050% by mass. A high-strength hot-dip galvanized steel sheet excellent in formability and shape freezeability as described in any one of claims 1 to 4, which is of mass The % further contains at least one element selected from the group consisting of 0.001% to 0.005% of Ca and 0.001% to 0.005% of REM. The high-strength hot-dip galvanized steel sheet excellent in formability and shape-freezing property as described in any one of Claims 1 to 5, wherein galvanizing is alloying galvanization. A method for producing a high-strength hot-dip galvanized steel sheet having excellent formability and shape-freezing property, characterized in that the billet having the composition of any one of items 1 to 5 of the patent application is heat-treated Rolling or further cold rolling, and then, when performing continuous annealing, heating to 500 ° C ~ Ac ₁ point at an average heating rate of 5 ° C / s or more, and heating to a temperature range of Ac ₃ points -20 ° C ~ 1000 ° C and After holding for 10 seconds to 1000 seconds, cool from 750 °C at an average cooling rate of 15 ° C / s or more to a temperature range of Ms point -80 ° C ~ Ms point -30 ° C, then heat to 350 ° C ~ 500 ° C and keep 10 After the second to 600 seconds, hot-dip galvanizing or further plating alloying treatment is performed.