TW201313916A - Galvanized steel sheet and method for manufacturing the same - Google Patents

Abstract

A high intensity galvanized steel sheet excellent in processability and having a tensile strength of 400 MPa or more, and a method for manufacturing the same are provided. A texture of the steel sheet includes a ferrite phase having an area ratio of 50% or more and an average particle diameter of 15 μ m or less, a pearlite phase having an area ratio of 10% to 30% and an average particle diameter of 10 μ m or less, and a bainite phase having an area ratio of 3% to 10% and an average particle diameter of 5 μ m or less, wherein an area ratio of an existing cementite phase in particles of the ferrite phase is 10% or less. In a manufacturing process, a hot rolled sheet or a cold rolled sheet is heated to a temperature region of 650 DEG C or more by an average heating speed of 10 DEG C/s or more. The sheet is maintained 10 seconds or more at a temperature of (Ac3+5) DEG C or more, and then cooled to a temperature region of 300 DEG C or less by an average cooling speed of 10 DEG C/s to 200 DEG C/s. The sheet is maintained 30 to 300 seconds at the temperature region of 300 DEG C or less, and then a galvanized process is performed.

Description

Hot dip galvanized steel sheet and manufacturing method thereof

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent workability such as a tensile strength of 440 MPa or more suitable for use in an automobile part or the like, and a method for producing the same.

In recent years, attempts have been made to reduce the exhaust gas such as CO ₂ from the viewpoint of protecting the global environment. In the automobile industry, measures for reducing the amount of exhaust gas are realized by increasing the fuel efficiency by reducing the weight of the vehicle body.

One of the methods for reducing the weight of the vehicle body is a method of reducing the thickness of the steel sheet by increasing the strength of the steel sheet used in the automobile. In addition, the steel sheets used around the bottom plate require high-strength thinning and rust-preventing properties, and the application of high-strength hot-dip galvanized steel sheets is studied. As the steel sheet is increased in strength, the ductility is lowered, so that a steel sheet having both high strength and ductility is required. In addition, the parts around the bottom plate are mostly formed into a complicated shape, and at the same time, ductility and stretch flangeability are required.

For such a request, for example, in Patent Document 1, a method for producing a high-strength hot-dip galvanized steel sheet having a low drop-to-volt ratio and a balance between strength and ductility and stretch flangeability reveals the following method: on a continuous hot dip galvanizing line. The quenching zone after soaking in heat is cooled at a specific speed and maintained in a specific temperature zone, thereby causing a phase change of the toughened iron, quenching after hot dip galvanizing and alloying, and causing the field to be quenched. The iron produces a phase change, so that the steel sheet structure forms a three-phase composite structure of ferrite + toughened iron + 麻田散铁.

Patent Document 2, a method for producing a high-strength steel sheet excellent in workability The following method is disclosed: after annealing and soaking, the average cooling rate from 650 ° C to the hot dip zinc bath or 450 ° C is specified, before hot dip galvanizing or after hot dip galvanizing, at 300 ° C ~ 450 The temperature zone between °C is maintained for a specific period of time, whereby residual Worthite iron is formed in the steel sheet structure, and a high-strength steel sheet excellent in balance between strength and ductility is produced.

Further, as a bending workability (λ: (λ:) which is evaluated by a hole expanding test in which a hole is drilled by a taper punch having a tensile strength of 440 MPa to 1500 MPa and a tip end of 60° until cracks occur around the hole. (Pore Expansion Ratio)) A method for producing a high-strength hot-dip galvanized steel sheet, and Patent Document 3 discloses a method of adjusting a component composition to an appropriate range, and providing a reheating step after the galvanizing step, followed by After the crystallization annealing step and before the reheating step, cooling is performed at a specific cooling rate, whereby the steel sheet is tempered to form 麻田散铁.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Special Publication No. 5-43779

[Patent Document 2] Japanese Patent Laid-Open No. Hei 4-26744

[Patent Document 3] Japanese Patent Laid-Open No. Hei 6-108152

As described above, as the steel sheet is increased in strength and the ductility is lowered, it is necessary to have a steel sheet having high strength and ductility. In addition, the parts around the bottom plate are mostly formed into a complicated shape, and at the same time, ductility and stretch flangeability are required. The strength level of high-strength steel sheets for automobiles is under development with a tensile strength of 590 MPa or higher. The strength level of steel sheets used as structural steel sheets around the bottom plate can be 440 MPa (440 MPa to 490 MPa). At the same time as the strength level, the steel sheet having excellent ductility and stretch flangeability is required in order to be processed into a shape required for members around the bottom plate.

However, Patent Document 1 discloses that the steel sheet structure is formed into a three-phase composite structure of ferrite + toughened iron + 麻田散铁, and has a low drop ratio and excellent balance between strength and ductility and stretch flangeability. The manufacturing method of the high-strength hot-dip galvanized steel sheet, but the introduction of the granulated iron in the steel sheet structure, the strength level exceeds 490 MPa, and the balance of strength and ductility and the stretch flangeability of the 440 MPa grade are not Make any consideration.

Further, in Patent Document 2, a method of producing a high-strength steel sheet having a balance of strength and ductility in the formation of a Worstian iron in a steel sheet structure is disclosed, but the phase transition induction of the residual Worstian iron is effectively utilized. Since plasticity TRIP (Transformation Induced Plasticity) steel is excellent in ductility, it has a problem that the stretch flangeability is poor.

Further, Patent Document 3 discloses a method in which the steel sheet is tempered to form a granulated iron and has both a hole expanding property and a high strength. However, the tensile strength is 600 MPa or more, and the expansion is 440 MPa. The hole characteristics are not taken into account.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot-dip galvanized steel sheet having a tensile strength of 440 MPa (440 MPa to 490 MPa level), high workability, particularly ductility and stretch flangeability. And its manufacturing method.

The inventors of the present invention performed the viewpoints based on the composition of the steel sheet and the metal structure. Force research. As a result, it has been found that it is extremely important to adjust the composition of the components to an appropriate range and to appropriately control the metal structure. Further, it has been found that a ferrite phase having an area ratio of 50% or more and an average particle diameter of 15 μm or less, an area ratio of 10% to 30%, and an average particle diameter of 10 μm or less are formed, and a toughened iron phase having an area ratio of 3% to 10% and an average particle diameter of 5 μm or less, and a metal structure in which the area ratio of the Xueming carbon iron phase existing in the grain of the ferrite phase is 10% or less The tensile strength is 440 MPa or more, and it has both workability (ductility and stretch flangeability).

Further, in order to obtain a metal structure having high ductility, a two-phase composite structure of a ferrite phase and a granulated iron phase is preferred. However, the two-phase composite structure cannot obtain a high stretch flangeability (hole expandability) due to a large difference in hardness between the ferrite phase and the maitian iron phase.

On the other hand, the inventors of the present invention have a tensile strength of 440 MPa or more in a composite structure having a ferrite phase, a ferrite phase, and a toughened iron phase by defining the steel sheet composition and the metal structure as described above. And can have both ductility and stretch flangeability. In other words, by specifying the area ratio and the average particle diameter of the ferrite phase as the metal structure, the strength and the ductility are ensured, and the area ratio of the toughened iron phase as the second phase is appropriately controlled to ensure the strength. In order to alleviate the difference in hardness between the ferrite phase and the toughened iron phase and to secure the desired strength, a ferrite phase is introduced to form a composite structure of a ferrite phase, a ferrite phase, and a tough iron phase. Then, by appropriately controlling the average particle diameter of the iron phase and the toughened iron phase, the area ratio of the ferroalloy phase, and the area ratio of the stellite carbon phase in the ferrite phase, it is possible to ensure a high ratio. Extends the flangeability and achieves high ductility.

The present invention has been completed based on the above findings, and its characteristics are as follows.

(1) A hot dip galvanized steel sheet comprising, by mass%, C: 0.100% to 0.200%, Si: 0.50% or less, Mn: 0.60% or less, P: 0.100% or less, and S: 0.0100% or less, Al: 0.010% to 0.100%, N: 0.0100% or less; the rest contains Fe and unavoidable impurities; the structure of the steel sheet has an area ratio of 50% to 87% and an average particle diameter of 15 μm or less. a ferrite phase, a tough iron phase having an area ratio of 10% to 30% and an average particle diameter of 10 μm or less, an area ratio of 3% to 10%, and an average particle diameter of 5 μm or less, The area ratio of the Xueming carbon iron phase existing in the grain of the ferrite phase is 10% or less.

(2) The hot-dip galvanized steel sheet according to the above aspect, wherein the component composition of the steel sheet further comprises, in mass%, from: Cr: 0.05% to 0.80%, V: 0.005% to 0.100%, Mo: at least one element selected from the group consisting of 0.005% to 0.500%, Cu: 0.01% to 0.10%, Ni: 0.01% to 0.10%, and B: 0.0003% to 0.2000%.

(3) The hot-dip galvanized steel sheet according to (1), wherein the composition of the steel sheet further comprises, in mass%, from: Ca: 0.001% to 0.005%, and rare earth metal (REM): 0.001% to 0.005% At least one of the elements in the group.

(4) The hot-dip galvanized steel sheet according to (1), wherein the composition of the steel sheet further comprises, in mass%, selected from the group consisting of Cr: 0.05% to 0.80%, V: 0.005% to 0.100%, and Mo: 0.005. %~0.500%, Cu: 0.01%~0.10%, Ni: 0.01%~0.10%, B: 0.0003%~0.2000%, at least one element in the group, and from Ca: 0.001% to 0.005%, REM : 0.001% to 0.005% of at least one element in the group.

(5) The hot-dip galvanized steel sheet according to (1), wherein the hot-dip galvanized steel sheet is an alloyed hot-dip galvanized steel sheet.

(6) The hot-dip galvanized steel sheet according to (1), wherein the hot-dip galvanized steel sheet has a galvanized layer, and the galvanized layer is an alloyed hot-dip galvanized layer.

(7) The hot-dip galvanized steel sheet according to (6), wherein the alloyed zinc-plated layer has a Fe content of 7% to 15%.

(8) The hot-dip galvanized steel sheet according to (1), wherein the hot-dip galvanized steel sheet is a hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more.

(9) The hot-dip galvanized steel sheet according to (8), wherein the tensile strength is 440 MPa to 490 MPa.

(10) The hot-dip galvanized steel sheet according to (1), wherein the hot-dip galvanized steel sheet has an elongation of 37% or more.

(11) The hot-dip galvanized steel sheet according to (1), wherein the hot-dip galvanized steel sheet has a stretch flangeability of 70% or more.

(12) The hot-dip galvanized steel sheet according to (1), wherein the Si content is 0.01% to 0.50%.

(13) The hot-dip galvanized steel sheet according to (1), wherein the Mn content is from 0.10% to 0.60%.

(14) The hot-dip galvanized steel sheet according to (1), wherein the P content is 0.003% to 0.100%.

(15) A method for producing a hot-dip galvanized steel sheet, comprising: preparing a steel material, the composition of the steel material comprising, by mass%: C: 0.100% to 0.200%, Si: 0.50% or less, Mn: 0.60% Hereinafter, P: 0.100% or less, S: 0.0100% or less, Al: 0.010% to 0.100%, N: 0.0100% or less, and the balance containing Fe and unavoidable impurities; heating the steel material; at the ferrite metamorphic temperature (Ar ₃ point) hot rolling at the finish rolling finishing temperature or higher; coiling the heat-expanding plate at a temperature of 600 ° C or lower; pickling the hot-drained plate; heating to 650 ° C at an average heating rate of 10 ° C / s or more The above temperature zone; maintained at a temperature above (Wosefield iron metamorphic temperature (Ac ₃ ) + 5) ° C for more than 10 seconds; at an average cooling rate of 10 ° C / s to 200 ° C / s to 100 ° C ~ 300 Temperature zone of °C; maintained in the above temperature range of 100 ° C ~ 300 ° C for 30 seconds ~ 300 seconds; hot dip galvanizing.

(16) The method for producing a hot-dip galvanized steel sheet according to (15), wherein the pickled hot-rolled sheet is further subjected to cold rolling.

(17) The method for producing a hot-dip galvanized steel sheet according to (16), wherein the cold rolling has a reduction ratio of 40% to 85%.

(18) The method for producing a hot-dip galvanized steel sheet according to (15), wherein after the hot dip galvanizing, the alloying treatment is further performed.

(19) The method for producing a hot-dip galvanized steel sheet according to (18), wherein the alloying treatment is an alloying treatment of heating the steel sheet to 450 ° C to 600 ° C.

Further, in the present invention, the high strength means that the tensile strength TS is 440 MPa or more. In the present invention, in particular, a hot-dip galvanized steel sheet having a tensile strength of 440 MPa to 490 MPa and excellent workability can be provided. Further, the high-strength hot-dip galvanized steel sheet according to the present invention includes any one of a cold-rolled steel sheet and a heat-expanded steel sheet as a galvanized base steel sheet, and may include a plated steel sheet which is not subjected to alloying treatment after hot-dip galvanizing treatment ( Hereinafter, it is also referred to as GI) or any of a plated steel sheet (hereinafter also referred to as GA) subjected to alloying treatment.

According to the present invention, a high-strength hot-dip galvanized steel sheet having excellent workability with a tensile strength of 440 MPa or more can be obtained. Further, in the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet having an alloy composition such as Mn which is reduced in alloy cost and which is inexpensive and which has improved ductility and stretch flangeability.

Since the high-strength hot-dip galvanized steel sheet according to the present invention is excellent in ductility and stretch flangeability, for example, it can be used for an automobile structural member to achieve fuel efficiency improvement due to weight reduction of the vehicle body, and industrial use value is particularly high. Big.

Hereinafter, the present invention will be specifically described. In the following description, the composition and structure of the steel or the steel sheet means the composition and structure of only the steel or steel sheet except for the plating layer of the hot-dip galvanized steel sheet. In the following description, the unit of the content of each element of the steel component composition is “% by mass”, and unless otherwise specified, it is represented by “%”.

First, the chemical composition of the steel sheet which is the most important requirement in the present invention will be described.

C: 0.100%~0.200%

C is an element necessary for ensuring the desired strength and combining the structure to improve strength and ductility. Therefore, C is preferably 0.100% or more. On the other hand, when C is added in excess of 0.200%, the strength rises remarkably, and desired workability cannot be obtained. Therefore, C is preferably in the range of 0.100% to 0.200%.

Si: 0.50% or less

Si is a ferrite phase generating element and is an effective element for strengthening steel. However, if the amount added exceeds 0.50%, the strength is remarkably increased, and the desired workability cannot be obtained. Therefore, it is preferable that Si is 0.50% or less. Further, when Si is 0.01% or more, the strengthening of steel is effective, and therefore Si is more preferably 0.01% or more.

Mn: 0.60% or less

Similarly to C, Mn is an element necessary for securing the desired strength, stabilizes the iron phase of the Worth, and promotes the formation of the second phase of the toughened iron. However, when the Mn is excessively added in excess of 0.60%, the area ratio of the second phase structure becomes too large, and the ductility is lowered. Therefore, Mn is preferably 0.60% or less. Further, when Mn is 0.10% or more, the strengthening of steel is effective, and therefore Mn is more preferably 0.10% or more.

P: 0.100% or less

P is an element which is effective for strengthening steel. When the amount added exceeds 0.100%, embrittlement is caused by segregation at grain boundaries, and impact resistance is deteriorated. Therefore, P is preferably 0.100% or less. Further, when P is 0.003% or more, the strengthening of steel is effective, and therefore P is more preferably 0.003% or more.

S: 0.0100% or less

S forms non-metallic inclusions such as MnS, and in the hole expanding test, the end faces of the holes are easily broken during the punching hole processing, and the hole expandability is lowered. Preferably, S is extremely low, and S is preferably 0.0100% or less. Further, in terms of manufacturing cost, S is set to be 0.0100% or less. More preferably, S is more preferably 0.0070% or less.

Al: 0.010%~0.100%

Al is added in an amount of 0.010% or more for deacidification of steel. On the other hand, when Al exceeds 0.100%, the surface appearance after plating is remarkably deteriorated, so Al is preferably in the range of 0.010% to 0.100%.

N: 0.0100% or less

When N is contained in a normal steel in an amount of 0.0100% or less, the effects of the present invention are not impaired. Therefore, N is preferably 0.0100% or less.

The rest contains Fe and inevitable impurities

The above component is a basic composition, but in addition to the above-described basic composition, the present invention may further contain at least one element selected from the group consisting of Cr, V, Mo, Cu, Ni, and B.

The inclusion is selected from the group consisting of Cr: 0.05% to 0.80%, V: 0.005% to 0.100%, Mo: 0.005% to 0.500%, Cu: 0.01% to 0.10%, Ni: 0.01% to 0.10%, and B: 0.0003% to 0.2000%. At least one element in the group

Cr and V can be added in order to improve the hardenability of steel and to increase the strength. Mo is an element which is effective for hardening strengthening of steel, and can be added in order to achieve high strength. Cu and Ni are elements which contribute to strength and can be added for strengthening of steel. B has an effect of suppressing the formation of ferrite from the iron grain boundary of the Vostian, and therefore can be added as needed. The lower limit of each element is the minimum amount at which the desired effect can be obtained, and the upper limit is the effect achieved. The amount of saturation. According to the above, when adding, it is preferable that Cr is 0.05% to 0.80%, V is 0.005% to 0.100%, Mo is 0.005% to 0.500%, Cu is 0.01% to 0.10%, and Ni is 0.01%. 0.10%, B is 0.0003%~0.2000%.

Containing at least one element selected from the group consisting of Ca: 0.001% to 0.005%, rare earth metal (REM): 0.001% to 0.005%

Ca and REM can be added in order to make the sulfide shape spherical and improve the stretch flangeability. The lower limit of each element is the minimum amount at which the desired effect can be obtained, and the upper limit is the amount at which the effect reaches saturation. From the above, when added, it is preferable that Ca is 0.001% to 0.005% and REM is 0.001% to 0.005%.

Next, the reason for limiting the structure of the high-strength hot-dip galvanized steel sheet having excellent tensile properties of 440 MPa or more in the present invention will be described.

Area ratio of ferrite phase: 50%~87%

In order to ensure high ductility, the area ratio of the ferrite phase is preferably 50% or more. More preferably 55% or more. In order to secure the minimum amount of the following ferrite phase, tough iron phase, and swarf carbon iron phase, the area ratio of the ferrite phase is preferably 87% or less.

Average particle size of ferrite phase: 15 μm or less

In order to ensure tensile strength and high ductility, the average particle diameter is preferably 15 μm or less. On the other hand, when the average particle diameter is 5 μm or more, since the specific tensile strength is ensured and the high ductility is obtained, the average particle diameter is more preferably 5 μm or more.

Area ratio of Borne iron phase: 10%~30%

In order to secure the strength and to alleviate the difference in hardness between the ferrite phase and the toughened iron phase, a high stretch flange property is obtained, and the area ratio of the ferrite phase is desirably 10% or more. On the other hand, in order not to excessively increase the strength and obtain desired workability, the area ratio of the ferrite phase is preferably 30% or less.

Average particle size of the Borne iron phase: 10 μm or less

When the average particle diameter is 10 μm or less, good ductility can be obtained, and therefore the average particle diameter is preferably 10 μm or less. When the average particle diameter exceeds 10 μm, the ductility is lowered. In order to obtain better ductility, the average particle diameter is more preferably 8 μm or less. On the other hand, when the average particle diameter is 3 μm or more, the effect of relaxing the hardness difference between the ferrite phase and the toughened iron phase is exhibited, and therefore the average particle diameter is more preferably 3 μm or more.

Area ratio of toughened iron phase: 3%~10%

In order to secure the desired strength, the area ratio of the toughened iron phase is desirably 3% or more. On the other hand, in order not to excessively increase the strength and obtain desired workability, the area ratio of the toughened iron phase is desirably 10% or less.

Average particle size of toughened iron phase: 5 μm or less

In order to secure the stretch flangeability, the average particle diameter is preferably 5 μm or less. More preferably, it is 3 μm or less. On the other hand, when the average particle diameter is 1 μm or more, the specific strength is ensured and the effect of obtaining high stretch flangeability is obtained. Therefore, the average particle diameter is more preferably μm 1 or more.

Area ratio of the sulphur-carbon iron phase present in the granules of the ferrite phase: 10% or less

In order to obtain good stretch flangeability, the area ratio of the swarf carbon iron phase present in the granules of the ferrite phase is preferably 10% or less.

Further, the ferrite phase, the ferrite phase, the toughened iron phase, and the structure other than the Schönming carbon iron phase present in the grain of the ferrite phase may include the residual Worth iron phase. At this time, from the viewpoint of ensuring good stretch flangeability, the area ratio of the remaining Worth iron phase is preferably 1% or less.

The metal structure was ground at a quarter-section of the plate thickness section parallel to the rolling direction of the steel sheet, and then etched by a 3% nitric acid etching solution, and observed by a scanning electron microscope (SEM) at a magnification of 2000 times. The field of view is analyzed by image analysis processing using image analysis software "Image Pro Plus ver. 4.0" manufactured by Media Cybernetics, for example, and the area ratio of each phase can be obtained. At this time, the stellite carbon phase existing in the granules of the ferrite phase, the ferrite phase, the toughened iron phase, and the ferrite phase is discriminated on the digital image by image analysis, and the graph is performed. The area ratio of each phase in each measurement field of view is obtained as the processing. These values may be averaged (for example, 10 fields of view) as the area ratio of each phase. Of course, the method of calculating the area ratio is not limited to this method, and may include a method of visual observation in the past.

The average particle diameter of the ferrite phase, the ferrite phase, and the toughened iron phase can be obtained, for example, in the following manner. Similarly to the above, the 1/4 position of the plate thickness section parallel to the rolling direction of the steel sheet was polished, and then etched by a 3% nitric acid etching solution, and the ferrite phase and the ferrite phase were observed by a SEM at a magnification of 1000 times. 10 fields of view, the toughened iron phase was observed by SEM at a magnification of 5000 times, and the image was analyzed using the image analysis software described above, and the ferrite phase was obtained in each field of view. The area of the Borne iron phase, the toughened iron phase, the number of ferrite particles, the ferrite particles, and the toughened iron particles, and calculate each The area of a ferrite grain, a wave of iron particles, and a toughened iron particle, and the approximate circle diameter corresponding to the area of each ferrite grain, the wave iron particle, and the tough iron particle is calculated at all 10 These values are calculated in each field of view, and the values are averaged, and may be used as the average particle diameter of the ferrite phase, the ferrite phase, and the toughened iron phase. Of course, the calculation method of the average particle diameter is not limited to this method, and a method including a conventional visual method may be included.

Next, a method of producing the hot dip galvanized steel sheet of the present invention will be described.

The molten steel having the above-described composition can be dissolved by a melting method such as a converter, and can be used as a steel material (slab) by a casting method such as a continuous casting method.

Next, hot rolling is performed, that is, using the obtained steel material, heating and rolling are performed, and it is set as the heat extension board. In this case, it is preferable that the hot rolling is performed at a temperature at which the finishing temperature of the finish rolling is equal to or higher than the ferrite transformation temperature (Ar ₃ point), and is taken up at a temperature of 600 ° C or lower.

End temperature of finish rolling: Ar ₃ points or more

When the finishing temperature of the finish rolling is less than Ar ₃ point, a ferrite phase is formed in the surface layer portion of the steel sheet, and the microstructure in the thickness direction is not formed due to the coarsening of the ferrite phase due to the processing strain. Uniformity, the average particle size of the ferrite phase cannot be controlled to 15 μm or less in the structure after cold rolling or continuous hot dip galvanizing. Therefore, it is preferable that the finishing temperature of finish rolling is set to Ar ₃ or more. Further, the Ar ₃ point can be calculated according to the following formula (1), but the actually measured temperature can also be used.

Ar ₃ = 910-310 × [C] - 80 × [Mn] + 0.35 × (t - 0.8)...... (1)

Here, [M] represents the content (% by mass) of the element M, and t represents the sheet thickness (mm). In addition, a correction term may be introduced according to the element, for example, when Cu, Cr, Ni, and Mo are contained, -20×[Cu], -15×[Cr], and -55× may be added to the right side of the formula (1). Corrections such as [Ni], -80×[Mo].

Coiling temperature: below 600 °C

When the coiling temperature exceeds 600 ° C, the area ratio of the ferroalloy phase increases, and in the steel sheet after the continuous hot dip galvanizing treatment, the area ratio of the ferroalloy phase exceeds 30%, and the strength is excessively increased. . Further, the ferrite particles are easily grown, and the average ferrite grain size cannot be controlled to 15 μm or less in the structure after cold rolling or continuous hot dip galvanizing treatment. Therefore, it is preferable that the coiling temperature is 600 ° C or less. Further, since the shape of the heat extension plate is deteriorated, it is more preferable that the coiling temperature is 200 ° C or higher.

Next, the steel sheet is pickled and further cold rolled as needed.

In the pickling step, the black rust generated on the surface is removed. Further, the pickling conditions are not particularly limited.

Cold rolling reduction rate: 40% or more (better conditions)

In order to achieve a suitable thickness of the steel sheet, the pickled steel sheet may be cold rolled as needed. When the rolling reduction ratio of cold rolling is 40% or more, recrystallization of the ferrite phase is promoted, and the residual of the non-recrystallized ferrite phase is prevented in the structure after the continuous hot dip galvanizing treatment, and further improved. The effect of ductility and stretch flangeability is more preferably 40% or more for cold rolling. On the other hand, if the reduction ratio of the cold rolling is 85% or less, the steel sheet after the continuous hot dip galvanization treatment has a desired metal structure, and therefore The reduction ratio of cold rolling is more preferably 85% or less.

Next, continuous hot dip galvanizing treatment is performed. In this case, it is preferred to heat the steel sheet to a temperature range of 650 ° C to 700 ° C at an average heating rate of 10 ° C / s to 30 ° C / s, and maintain the temperature at (Ac ₃ + 5) ° C or higher for 10 seconds to 600 ° Secondly, it is cooled to a temperature range of 300 ° C or lower at an average cooling rate of 10 ° C / s to 200 ° C / s, and is maintained in a temperature range of 300 ° C or lower for 30 seconds to 300 seconds, and then subjected to hot dip galvanization.

Heating to a temperature range of 650 ° C to 700 ° C at an average heating rate of 10 ° C / s to 30 ° C / s

When the heating temperature zone is 650 ° C or higher, the ferrite recrystallization is promoted, and the area ratio of the ferrite phase in the steel sheet after the continuous hot dip galvanizing treatment is 50% or more, thereby ensuring strength and improving ductility. And thus better. When the average heating rate is 10 ° C / s to 30 ° C / s, no longer furnace or a large amount of energy is consumed, and the production efficiency can be improved at low cost.

Hold at temperatures above (Ac ₃ +5) °C for more than 10 seconds

By setting the annealing (holding) temperature to (Ac ₃ + 5) ° C or more and setting the annealing (holding) time to 10 seconds or more, the snow-capped carbon iron can be sufficiently melted during annealing, and the Wolsfield can be sufficiently formed. The iron phase ensures a sufficient amount of the second phase (the ferromagnetic phase, the toughened iron phase) during annealing and cooling, and sufficient strength can be obtained. Further, the area ratio of the sulphur-carbon iron phase existing in the granules of the ferrite phase can be suppressed to 10% or less, and good ductility can be obtained. The upper limit of the annealing (holding) temperature and the annealing (holding) time is not particularly limited, and sufficient effects can be obtained at a temperature of 1000 ° C or less and 600 seconds or less, and it is preferable to separately anneal in order not to cause an increase in cost. (Holding) The temperature was set to 1000 ° C or lower, and the annealing (holding) time was set to 600 seconds or less.

In addition, the Wosefield iron metamorphic temperature (Ac ₃ point) can be calculated according to the following formula (2), and the actually measured temperature can also be used.

Here, [M] represents the content (% by mass) of the element M, Indicates the square root of the C content (% by mass).

Cooling to a temperature range below 300 ° C at an average cooling rate of 10 ° C / s to 200 ° C / s

The average cooling rate condition is one of the important requirements in the present invention. By quenching to a temperature zone below 300 ° C at a specific average cooling rate, the area ratio of the stellite carbon iron phase present in the granules of the ferrite phase can be controlled, and the ferritic phase and the toughened iron can be controlled. The area ratio of the phase. When the average cooling rate is 10 ° C / s or more, the area ratio of the stellite carbon iron phase existing in the granule of the ferrite phase can be suppressed to 10% or less, and the average ferrite particle diameter can be 15 Below μm, the average particle diameter of the ferrite is 10 μm or less, so that sufficient strength can be secured and good ductility can be obtained. When the average cooling rate is 200 ° C / s or less, the ferrite phase is sufficiently precipitated, and the ferrite phase or the toughened iron phase is not excessively precipitated, and good ductility can be obtained with appropriate strength. In addition, in terms of obtaining a good shape of the steel sheet, it is preferable to The average cooling rate is 200 ° C / s or less. In addition, when the cooling completion temperature is 100° C. or higher, the cooling end temperature is more preferably 100° C. or more in order to suppress deterioration of the shape of the steel sheet due to uneven cooling.

Hold in the temperature range below 300 ° C for 30 seconds to 300 seconds

The maintenance of this temperature zone is one of the important requirements in the present invention. By setting the holding temperature to 300 ° C or lower, the toughened iron can be phase-transformed at a sufficient speed, and the steel sheet after the continuous hot dip galvanizing treatment can be obtained by maintaining the temperature in the temperature range for 30 seconds to 300 seconds. The area ratio of the toughened iron phase is 3% or more, so that sufficient strength can be ensured, and the area ratio of the Schönming carbon iron phase existing in the grain of the ferrite phase can be suppressed to 10% or less, and The average particle size of the Borne iron can be set to 10 μm or less, so that good ductility can be obtained. On the other hand, when the holding time is less than 30 seconds, the toughened iron phase transformation is not performed, and the area ratio of the toughened iron phase of the steel sheet after the continuous hot dip galvanizing treatment is not more than 3%, and it is difficult to ensure the strength. . When the holding time exceeds 300 seconds, the area ratio of the toughened iron phase becomes excessive, and the average hardness of the toughened iron exceeds 5 μm, so the hardness difference between the ferrite phase and the toughened iron phase becomes large, and the extended flange Reduced sex. Further, at this time, the area ratio of the sulphur-carbon iron phase existing in the granules of the ferrite phase exceeds 10%, and the ductility is lowered. On the other hand, when the temperature zone is maintained for 30 seconds to 300 seconds, sufficient strength can be obtained, and good ductility and stretch flangeability can be obtained.

Next, hot dip galvanizing treatment is performed. Or if the galvanized layer is further alloyed as needed, it is cooled to room temperature.

When the alloying treatment is performed by hot dip galvanizing treatment, it is preferred to heat the steel sheet to 450 ° C or higher and 600 ° C after hot dip galvanizing treatment. The alloying treatment is carried out as follows, and the Fe content of the plating layer is 7% to 15%. When the Fe content of the plating layer is in the range of 7% or more, unevenness in alloying can be prevented and the peeling property can be improved, and when the Fe content of the plating layer is 15% or less, the plating peeling resistance is improved. Therefore, it is more desirable that the Fe content of the plating layer is 7% to 15%.

According to the above, the hot-dip galvanized steel sheet having the high strength of the tensile strength of 440 MPa or more and excellent in workability can be obtained.

Further, in the heat treatment in the production method of the present invention, if the temperature is within the above temperature range, the holding temperature does not need to be fixed, and when the cooling rate changes during the cooling process, if the cooling rate is within the range of the predetermined cooling rate, there is no problem. . Further, in the heat treatment, if the desired thermal history is satisfied, the object of the present invention is not impaired regardless of the type of equipment used for the heat treatment. Further, quenching and temper rolling is performed for correcting the shape, which is also included in the scope of the present invention. In the present invention, it is assumed that the steel material is produced by the usual steps of steel making, casting, and heat extension, and the case where a part or all of the heat-expanding step is omitted by thin-plate casting or the like is also included in the scope of the present invention. Inside. Further, in the present invention, even if various high-strength hot-dip galvanized steel sheets are subjected to various surface treatments such as chemical conversion treatment, the effects of the present invention are not impaired.

[Example]

Hereinafter, the present invention will be specifically described based on examples.

A steel material (plate) having the composition shown in Table 1 was used as a starting material. These steel materials were heated to the heating temperatures shown in Tables 2 and 3, and then hot rolled and pickled by the conditions shown in Tables 2 and 3, followed by Cold rolling, continuous hot dip galvanizing treatment. The plate thickness before cold rolling is different for each steel plate number. A part of the steel sheet (steel plate number 5) was not subjected to cold rolling. Next, except for a part, the alloying treatment is performed after the continuous hot dip galvanizing treatment.

Further, hot dip galvanizing treatment was carried out by a continuous hot dip galvanizing treatment apparatus through a Zn bath containing Al at 460 ° C, GA was a Zn bath containing 0.14% by mass of Al, and GI was using Zn containing 0.18 mass % of Al. bath. The amount of adhesion is adjusted by gas injection, and the GA is alloyed.

The hot-dip galvanized steel sheets (GA and GI) obtained in the above manner were evaluated for the structure observation, the tensile properties, and the stretch flangeability (expansion test). The measurement method is shown below.

(1) Organizational observation

The plate thickness section parallel to the rolling direction of the steel sheet was ground at a quarter position, and then etched by a 3% nitric acid etching solution, and 10 fields of view were observed by a scanning electron microscope (SEM) at a magnification of 2000 times. The area ratio of each phase was obtained by analyzing using the image analysis processing of the image analysis software "Image Pro Plus ver. 4.0" manufactured by Media Cybernetics. That is, the image of the ferrite phase, the ferrite phase, the toughened iron phase, and the ferrite phase in the grain of the ferrite phase is discriminated by image analysis, and image processing is performed. The area ratio of each phase in each measurement field of view was determined. These values were averaged (10 fields of view) as the area ratio of each phase. The average particle diameter of the ferrite phase, the ferrite phase, and the toughened iron phase was determined as follows. In the same manner as above, the 1/4 position of the plate thickness section parallel to the rolling direction of the steel sheet is polished, and then etched by a 3% nitric acid etching solution, and the ferrite phase and the wave come. The iron phase was observed by SEM at a magnification of 1000 times, and the toughened iron phase was observed by SEM at a magnification of 5000 times, and the image was analyzed using the image analysis software described above. Determine the area of the ferrite phase, the ferromagnetic phase, the toughened iron phase, the number of ferrite particles, the ferrite particles, and the toughened iron particles in each field of view, and calculate each ferrite grain. , the area of the Bora iron particles, the toughened iron particles, calculate the diameter of the approximate circle corresponding to the area of each ferrite grain, the wave iron particle, and the tough iron particle, and calculate these values in all 10 fields of view. The values are averaged to obtain the average particle diameter of the ferrite phase, the ferrite phase, and the toughened iron phase.

(2) Tensile properties

A JIS No. 5 tensile test piece was taken from the rolling direction of the obtained steel sheet, and a tensile test (JIS Z2241 (2011)) was carried out. The tensile test was carried out until the fracture, and the tensile strength and elongation at break (ductility) were determined. The evaluation criteria of the ductility were judged to be particularly excellent in ductility when the elongation at break was 37.0% or more.

(3) extended flangeability

The stretch flangeability is implemented in accordance with the Japan Iron and Steel Federation Standard (JFS) T1001 (1996). The obtained steel plate was cut into 100 mm × 100 mm, and a hole having a diameter of 10 mm (d ₀ ) was punched by press working at a gap of 12%, and then pressed at a blanking force of 9 ton using an inner diameter of 75 mm. A 60° cone puncher was pressed into the hole, and the hole diameter d _b at the time when the thickness of the hole was penetrated through the crack was measured, and the ultimate hole expansion ratio defined by the following formula was obtained: λ (%), according to the limit The value of the hole expansion ratio was evaluated for the stretch flangeability. When the stretch flangeability is 70% or more, a steel sheet excellent in stretch flangeability is evaluated.

λ = 100 × (d _{b -} d ₀ ) / d ₀ ... (3)

The results and conditions obtained above are shown in Table 2 and Table 3.

According to Tables 2 and 3, the steel sheet has a ferrite phase having an area ratio of 50% or more and an average particle diameter of 15 μm or less, an area ratio of 10% to 30%, and an average particle diameter of 10 μm or less. And a toughened iron phase having an area ratio of 3% to 10% and an average particle diameter of 5 μm or less, wherein the area ratio of the Xueming carbon iron phase existing in the grain of the ferrite phase is 10% or less. In the example, the ductility and the stretch flangeability are high.

On the other hand, in the comparative example, any one of ductility and stretch flangeability was low. In particular, in the comparative examples in which the composition of the components is not appropriate, the area ratio and the average particle diameter of the ferrite phase, the area ratio and the average particle diameter of the ferroalloy phase, the area ratio and the average particle diameter of the toughened iron phase, and The area ratio of the sulphur-carbon iron phase present in the granules of the ferrite phase is appropriate, and the ductility and the stretch flangeability are not improved.

According to the above results, it has been confirmed that the steel sheet having a specific composition has a ferrite phase having an area ratio of 50% or more and an average particle diameter of 15 μm or less, an area ratio of 10% to 30%, and an average particle diameter of 10 μm or less. The iron phase and the toughened iron phase having an area ratio of 3% to 10% and an average particle diameter of 5 μm, and the area ratio of the Xueming carbon iron phase existing in the grain of the ferrite phase is 10% or less A hot-dip galvanized steel sheet having a high tensile strength of 440 MPa to 490 MPa and good ductility and stretch flangeability can be obtained.

[Industrial availability]

The hot-dip galvanized steel sheet according to the present invention is excellent in strength and workability, and is lightweight and high in strength when used in a periphery of an automobile for forming a complicated shape, and can be used as an advantageous surface-treated steel sheet. .

Claims (19)

A hot dip galvanized steel sheet comprising, by mass%, C: 0.100% to 0.200%, Si: 0.50% or less, Mn: 0.60% or less, P: 0.100% or less, and S: 0.0100% or less. , Al: 0.010%~0.100%, N: 0.0100% or less; the rest contains Fe and unavoidable impurities; the structure of the steel plate has ferrites with an area ratio of 50% to 87% and an average particle diameter of 15 μm or less. a tough iron phase having a phase ratio of 10% to 30% and an average particle diameter of 10 μm or less, an area ratio of 3% to 10%, and an average particle diameter of 5 μm or less; the above ferrite The area ratio of the Xueming carbon iron phase existing in the phase grains is 10% or less. The hot dip galvanized steel sheet according to claim 1, wherein the composition of the steel sheet further comprises, in mass%, from: Cr: 0.05% to 0.80%, V: 0.005% to 0.100%, Mo: at least one element selected from the group consisting of 0.005% to 0.500%, Cu: 0.01% to 0.10%, Ni: 0.01% to 0.10%, and B: 0.0003% to 0.2000%. The hot dip galvanized steel sheet according to claim 1, wherein the composition of the steel sheet further comprises, in mass%, from: Ca: 0.001% to 0.005%, and rare earth metal (REM): 0.001% to 0.005%. At least one of the elements in the group. The hot dip galvanized steel sheet according to claim 1, wherein the composition of the steel sheet further comprises, in mass%, selected from the group consisting of Cr: 0.05% to 0.80%, V: 0.005% to 0.100%, and Mo: 0.005. %~0.500%, Cu: 0.01%~0.10%, Ni: 0.01%~0.10%, B: 0.0003%~0.2000%, at least one element in the group, and from Ca: 0.001% to 0.005%, REM : 0.001% to 0.005% of at least one element in the group. The hot dip galvanized steel sheet according to claim 1, wherein the hot dip galvanized steel sheet is an alloyed hot dip galvanized steel sheet. The hot dip galvanized steel sheet according to claim 1, wherein the hot dip galvanized steel sheet has a galvanized layer, and the galvanized layer is alloyed hot dip coating. Zinc layer. The hot dip galvanized steel sheet according to claim 6, wherein the alloyed zinc plating layer has a Fe content of 7% to 15%. The hot dip galvanized steel sheet according to the first aspect of the invention, wherein the hot dip galvanized steel sheet is a hot dip galvanized steel sheet having a tensile strength of 440 MPa or more. The hot dip galvanized steel sheet according to claim 8, wherein the tensile strength is 440 MPa to 490 MPa. The hot dip galvanized steel sheet according to claim 1, wherein the hot dip galvanized steel sheet has an elongation of 37% or more. The hot dip galvanized steel sheet according to claim 1, wherein the hot dip galvanized steel sheet has an extended flangeability of 70% or more. The hot dip galvanized steel sheet according to claim 1, wherein the Si content is 0.01% to 0.50%. The hot dip galvanized steel sheet according to claim 1, wherein the Mn content is 0.10% to 0.60%. The hot dip galvanized steel sheet according to claim 1, wherein the P content is 0.003% to 0.100%. A method for producing a hot-dip galvanized steel sheet, comprising: preparing a steel material, wherein a composition of the steel material comprises, by mass%: C: 0.100% to 0.200%, Si: 0.50% or less, Mn: 0.60% or less, P : 0.100% or less, S: 0.0100% or less, Al: 0.010% to 0.100%, N: 0.0100% or less, the rest contains Fe and unavoidable impurities; the steel material is heated; at the ferrite metamorphic temperature (Ar ₃ Hot rolling is performed at the finish rolling temperature above; the hot-rolled sheet is taken up at a temperature of 600 ° C or lower; the hot-rolled sheet is pickled; and heated at an average heating rate of 10 ° C/s or more to 650 ° C or higher Temperature zone; maintained at a temperature above (Wustian iron metamorphic temperature (Ac ₃ ) + 5) ° C for more than 10 seconds; cooled to 100 ° C ~ 300 ° C at an average cooling rate of 10 ° C / s ~ 200 ° C / s Temperature zone; maintained in the above temperature range of 100 ° C ~ 300 ° C for 30 seconds ~ 300 seconds; hot dip galvanizing. The method for producing a hot-dip galvanized steel sheet according to the fifteenth aspect of the invention, wherein the pickled hot-rolled sheet is further subjected to cold rolling. The method for producing a hot-dip galvanized steel sheet according to claim 16, wherein the cold rolling has a reduction ratio of 40% to 85%. The method for producing a hot-dip galvanized steel sheet according to claim 15, wherein after the hot dip galvanizing, the alloying treatment is further performed. The method for producing a hot-dip galvanized steel sheet according to claim 18, wherein the alloying treatment is an alloying treatment of heating the steel sheet to 450 ° C to 600 ° C.