JP2018119207A - MOLTEN Zn-Al-Mg-BASED PLATED STEEL SHEET EXCELLENT IN BURRING PROPERTY AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength molten Zn-Al-Mg-based plated steel sheet excellent in burring property and excellent in surface appearance of a plated surface.SOLUTION: The high-strength molten Zn-Al-Mg-based steel sheet has a chemical composition containing, by mass%, C: 0.005 to 0.080%, Si:0 to 0.80%, Mn:0.10 to 1.80%, Ti:0.020 to 0.200%, B:0.0005 to 0.010%, P:0.005 to 0.050%, S:0.0005 to 0.020%, Nb:0 to 0.100%, V:0 to 0.100%, Al:0.010 to 0.100%, and the balance Fe with inevitable impurities, provided that (Ti/48)/(C/12) is 0.40 to 2.00, has a molten Zn-Al-Mg-based plating layer on the surface of a base material steel sheet having a bainitic ferrite phase, an area ratio of cementite of 3% or less and a dislocation density of not more than 1×10cm/cm, and has a tensile strength in a direction perpendicular to a rolling direction of 400 MPa or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength molten Zn—Al—Mg-based plated steel sheet excellent in burring properties (hole expansibility) and a method for producing the same.

  In recent years, steel sheets used for automobiles and building materials have been required to have high strength for the purpose of weight reduction and resource saving. Moreover, the steel plate used for these uses is required to have excellent characteristics that can withstand various processes such as press working and stretch flange working. “Burring property” is an example of a characteristic for evaluating such work-resistant formability. The burring property is a characteristic indicating to what extent a hole formed by punching or the like can be formed without a crack in the subsequent forming process.

  On the other hand, hot-dip Zn—Al—Mg-based plating is known as a surface treatment that exhibits excellent corrosion resistance in applications such as automobiles and building materials, and has recently been widely applied together with conventional hot-dip galvanizing.

  Patent Document 1 describes a high-strength steel plate that is excellent in burring properties (hole expandability). In order to improve hole expansibility, it is effective to suppress the formation of cementite. In Patent Document 1, by reducing Mn, which is an austenite former, and expanding the α region, carbide (TiC) precipitation during cooling from the completion of hot rolling finish rolling to the start of winding is promoted, thereby enhancing precipitation strengthening. At the same time, the formation of cementite is suppressed, and the burring property (hole expanding property) is improved (paragraph 0007 of Patent Document 1). However, according to the study by the present inventors, when a hot-rolled steel sheet obtained by the method of Patent Document 1 is used as a raw material to produce a molten Zn—Al—Mg-based steel sheet, the precipitation of carbide (TiC) is high. It was found that the particle size becomes coarse because it starts with, and it is not easy to obtain high strength.

  In Patent Document 2, plating is performed by defining a rolling reduction ratio in a light rolling process in a process of sequentially performing hot rolling, pickling, light rolling treatment, annealing in a continuous hot dipping plating line, and hot-dip Zn-Al-Mg plating. A method for producing a hot-dip Zn—Al—Mg-based plated steel sheet having both surface appearance and burring properties is disclosed. However, when the cooling rate from the completion of finish rolling in the hot rolling process to winding is excessively reduced, there is a problem that the burring property is reduced due to an increase in the amount of cementite. On the other hand, when the cooling rate is excessively increased, it has been clarified that the burring property may be deteriorated due to an increase in TS accompanying an increase in the dislocation density.

JP 10-287949 A Japanese Patent Laid-Open No. 2006-160499

Koichi Nakajima, 4 others, “Method of evaluating dislocation density using X-ray diffraction”, Materials and Processes (Japan Iron and Steel Institute), Vol. 17 (2004), p. 396-399

  An object of this invention is to provide the high intensity | strength molten Zn-Al-Mg type plated steel plate which is high intensity | strength, exhibits the outstanding burring property stably, and is excellent also in the surface appearance of plating.

In order to achieve the above object, the present invention discloses the following invention.
[1] By mass%, C: 0.005 to 0.080%, Si: 0 to 0.80%, Mn: 0.10 to 1.80%, Ti: 0.020 to 0.200%, B And a chemical composition having a Ti / C equivalent ratio defined by the following formula (1) of 0.40 to 2.00, consisting of 0.0005 to 0.010%, the balance Fe and inevitable impurities, A molten Zn—Al—Mg-based plating layer is formed on the surface of a base steel sheet having a metal structure in which the area of cementite is 3% or less, the main phase of which is a single phase of nitrite ferrite or a structure of both bainitic ferrite and ferrite. In the burring property, the tensile strength in the direction perpendicular to the rolling is 400 MPa or more, and the product TS × λ of the tensile strength TS (MPa) and the hole expansion ratio λ (%) is 40000 MPa ·% or more. Excellent high-strength molten Zn-Al-Mg-based plated steel sheet.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the value of the content in steel represented by mass% of each element is substituted for the element symbol on the right side of the formula (1).
[2] The chemical composition of the base steel sheet is further mass%, P: 0.005 to 0.050%, S: 0.0005 to 0.020%, Nb: 0 to 0.100%, V: 0. The hot-dip Zn—Al—Mg-based plated steel sheet according to the above [1], which satisfies the content of ˜0.100% and Al: 0.010 to 0.100%.
[3] By mass%, C: 0.005 to 0.080%, Si: 0 to 0.80%, Mn: 0.10 to 1.80%, Ti: 0.020 to 0.200%, B : 0.0005 to 0.010%, P: 0.005 to 0.050%, S: 0.0005 to 0.020%, Nb: 0 to 0.100%, V: 0 to 0.100%, Al: 0.0010 to 0.100%, balance Fe and inevitable impurities, having a chemical composition having a Ti / C equivalent ratio defined by the following formula (1) of 0.40 to 2.00, The main phase is a bainitic ferrite single phase or a structure of both bainitic ferrite and ferrite, and has a cementite area ratio of 3% or less, and a dislocation density of 1 × 10 ¹⁴ cm / cm ³ or less. It has a molten Zn-Al-Mg plating layer on the surface of a certain base steel plate, and has a tensile strength of 400M in the direction perpendicular to the rolling direction. High-strength hot-dip Zn-Al-Mg plated steel sheet having excellent burring properties is more than a.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the value of the content in steel represented by mass% of each element is substituted for the element symbol on the right side of the formula (1).
[4] The molten Zn—Al—Mg-based plating layer is, by mass, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Si: 0 to 2.0%, Any of the above [1] to [3] having a composition comprising Ti: 0 to 0.10%, B: 0 to 0.05%, Fe: 0 to 2.0%, the balance Zn and inevitable impurities The hot-dip Zn-Al-Mg-based plated steel sheet described.
[5] The average cooling rate R (° C./s) from the completion of finish rolling to the start of winding after the hot rolling is performed under the condition that the finish rolling delivery temperature is 830 to 940 ° C. Cooling to be satisfied, obtaining a hot-rolled steel sheet at a coiling temperature of 500 to 650 ° C.,
Pickling the hot-rolled steel sheet,
A step of performing light rolling at a rolling reduction of 1.5 to 8.0% after the pickling,
A step of performing hot-dip Zn-Al-Mg-based plating after annealing at 550 to 750 ° C in a reducing atmosphere after the cold rolling;
The manufacturing method of the hot-dip Zn-Al-Mg system plated steel plate in any one of said [1]-[4] which has these.
R ≧ −6 × [Ti / C equivalent ratio] +18 (2)
However, the value of the Ti / C equivalent ratio determined by the above equation (1) is substituted into the [Ti / C equivalent ratio] position on the right side of the equation (2).
[6] In the step of obtaining the hot-rolled steel sheet, the molten Zn—Al—Mg system according to the above [5], wherein the cooling is performed so that the average cooling rate R (° C./s) further satisfies the following formula (3): Manufacturing method of plated steel sheet.
R <50 (3)

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength fusion | melting Zn-Al-Mg type plated steel plate excellent in the surface external appearance with favorable burring property and very few plating defects, such as non-plating, can be provided.

The SEM photograph which illustrates the metal structure of No. 8 (invention example). The SEM photograph which illustrates the metal structure of No. 16 (comparative example). The SEM photograph which illustrates the metal structure of No. 20 (comparative example). The photograph which illustrates the appearance of the plating surface where there is no unplating. The photograph which illustrates the external appearance of the plating surface where non-plating exists.

  Hereinafter, “%” in the steel composition and plating composition means “mass%” unless otherwise specified.

[Steel composition of base steel sheet]
C forms a Ti-based carbide (mainly TiC) and finely precipitates in a bainitic ferrite phase that is a matrix or other ferrite phase (hereinafter simply referred to as a ferrite phase). It is an element that plays a role of ensuring strength. In order to reduce the weight of the member in applications such as automobile members and building materials, a strength level such that the tensile strength in the direction perpendicular to the rolling direction is 400 MPa or more is desired. For that purpose, the C content needs to be 0.005% or more, and more preferably 0.010% or more. However, if the C content exceeds 0.080%, coarsening of precipitates and formation of cementite (Fe ₃ C) occur, and the burring property tends to decrease. Moreover, it is necessary to adjust C content so that the below-mentioned Ti / C equivalent ratio may be set to 0.40-2.00.

  Si is an element effective for solid solution strengthening, and can be contained as necessary. For example, it is more effective to secure a Si content of 0.05% or more. However, when the Si content is increased, an oxide is easily formed on the surface of the steel sheet during annealing in the continuous hot dipping line, which becomes a factor that impairs the plateability. The Si content is limited to 0.80% or less.

Mn is an element effective for increasing the strength. In order to obtain a strength level in which the tensile strength in the direction perpendicular to the rolling is, for example, 400 MPa or more, the Mn content needs to be 0.10% or more. It is more effective to secure a Mn content of 0.45% or more, or even 0.55% or more. Moreover, securing a sufficient Mn content is effective in preventing an excessive increase in the A ₃ transformation point. In this case, even if the hot rolling temperature and the annealing temperature are performed at a slightly high temperature, the coarsening of the Ti-based carbide can be suppressed, and the degree of freedom of manufacturing conditions for obtaining good burring properties is expanded. However, if Mn is contained in a large amount, the burlinkability is lowered due to segregation of Mn in the steel. As a result of various studies, the Mn content is limited to 1.80% or less.

  P is an element effective for solid solution strengthening. In addition, it segregates at the grain boundaries and effectively acts to suppress molten metal embrittlement cracking during welding. In order to obtain these effects sufficiently, a P content of 0.005% or more is secured here. However, when P is contained in a large amount, segregation occurs excessively and the bar-linkability decreases. As a result of various studies, the P content is limited to 0.050% or less.

  S easily forms sulfides with Ti and Mn, and these sulfides reduce the burring properties of the steel sheet. As a result of various studies, S needs to be made 0.020% or less. However, excessive desulfurization increases the steelmaking load, so here the S content should be 0.0005% or more.

  Ti combines with C to form fine Ti-based carbides (mainly TiC), contributing to high strength. Moreover, since Ti has a high affinity with N, fixing N in the steel as TiN is also effective in suppressing consumption of added B as BN. In order to sufficiently obtain these effects, Ti content of 0.020% or more is necessary. However, a large amount of Ti causes a decrease in workability. Here, the Ti content is limited to 0.200% or less. Further, it is necessary to adjust the Ti content so that the Ti / C equivalent ratio described later is 0.40 to 2.00.

  Nb and V are elements that prevent the austenite crystal grains from coarsening during slab heating and hot rolling, and are effective for refining ferrite crystal grains after cooling. Moreover, like Ti, it forms a compound with C and contributes to an increase in strength. For this reason, 1 or more types of Nb and V can be contained as needed. When Nb is contained, it is more effective to make the Nb content 0.010% or more. When V is contained, it is more effective to set the V content to 0.010% or more. However, excessive Nb content and V content are both uneconomical. Here, the Nb content is limited to 0.100% or less, and the V content is limited to 0.100% or less.

  B is an element having an action of suppressing the austenite-ferrite transformation of steel. Due to this action, the precipitation temperature of the Ti-based carbide is lowered, which is extremely advantageous for the refinement of the Ti-based carbide. B is also effective in improving the resistance to molten metal embrittlement cracking. As a result of various studies, in order to obtain these effects sufficiently, it is necessary to ensure that the B content is 0.0005% or more. It is more effective to set it to 0.0025% or more. However, if a large amount of B is contained, the workability deteriorates due to the formation of borides. Here, the B content is limited to 0.010% or less.

  Al is added as a deoxidizer during steelmaking. In order to obtain the action, it is desired that Al content is 0.010% or more. However, a large amount of Al is a factor that causes a decrease in workability. The Al content is limited to 0.100% or less.

The Ti / C equivalent ratio defined by the following formula (1) corresponds to the molar ratio of Ti and C contained in the steel.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
Here, the value of the content in steel represented by mass% of each element is substituted for the element symbol on the right side of the formula (1).

  The Ti / C equivalent ratio is an important index that affects burring properties and ductility. According to the study by the present inventors, the abundance of cementite is very small (for example, the area ratio) by applying a steel whose Ti / C equivalent ratio is adjusted to 0.40 or more to a production method described later. It was found that the steel sheet can be stably obtained. The ability to stably reduce the amount of cementite is extremely important for stably improving burring properties. On the other hand, when the Ti / C equivalent ratio is excessive, the ductility decreases due to an increase in the dislocation density. As a result of various studies, the Ti / C equivalent ratio is limited to 2.00 or less. More preferably, it is 1.50% or less.

[Metal structure]
In the hot-dip Zn—Al—Mg-based plated steel sheet according to the present invention, the matrix (metal substrate) of the base steel sheet is preferably composed of a ferrite phase. Here, the ferrite phase may be constituted by either or any one of “bainitic ferrite phase” and “other ferrite phases”. In particular, the bainitic ferrite phase has a low dislocation density and is an advantageous phase for improving burring properties and ductility. Therefore, it is more preferable that the bainitic ferrite phase is mixed in the ferrite phase.

Since cementite in the base steel sheet tends to be a starting point of cracking in the hole expanding process, it is desirable that the amount of cementite is small. For example, it is preferable that the abundance of cementite is a metal structure having an area ratio of 3.0% or less. The measurement of the area ratio of cementite can be determined by observing a field of view randomly set for a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. At that time, the total area of the observation visual field is set to 0.05 mm ² or more.

[Dislocation density]
According to the researches of the inventors, in order to stably improve the burring property (hole expansibility) of a high-strength steel sheet annealed in a hot dipping line, it is extremely effective to control the dislocation density of the steel sheet to be low. I found out that Even in the case of a hot-dip Zn-Al-Mg plated steel sheet having a high strength level, when the dislocation density is controlled to be low, excellent burring properties are maintained, and the tensile strength TS (MPa) and the hole expansion ratio λ ( %)), A high strength plated steel sheet having a high value of TS × λ and excellent in “strength-burring balance” can be realized. As a result of various studies, when the dislocation density determined based on the half-value width of the X-ray diffraction peak is reduced to 1 × 10 ¹⁴ cm / cm ³ or less, TS has a high strength of 400 MPa or more while having a tensile strength of 400 MPa or more. An excellent “strength-burring balance” with a value of xλ of, for example, 55000 MPa ·% or more can be stably realized. The lower limit of the dislocation density is not particularly limited as long as a high strength having a tensile strength of 400 MPa or more is obtained. For example, a good “strength-burring property balance” can be obtained at 1 × 10 ¹³ cm / cm ³ or more. Have confirmed. The dislocation density can be determined by the following method.

(How to find dislocation density)
The dislocation density is determined by measuring an X-ray diffraction pattern by a diffractometer method for a base material (a steel plate portion excluding a plating layer) constituting the plated steel plate, and (110) plane, (211) plane, and (220) plane. It can be determined by measuring the half width β (°) of each X-ray diffraction peak. Specifically, using the “Williamson-Hall method” shown in Non-Patent Document 1, it can be obtained as follows. From the diffraction angle θ (°) and the half-value width β (°) for the (110) plane, the (211) plane, and the (220) plane, sin θ / λ (nm ⁻¹ ) and β cos θ / λ (nm ⁻¹ ) Are plotted on Cartesian coordinates, an approximate straight line is obtained by the least square method using these plots, and the strain ε and crystallite size D (nm) shown in the following equation (5) are calculated from the slope and intercept of the straight line. Determine. λ is the wavelength (nm) of X-rays.
βcos θ / λ = 0.9 / D + 2εsin θ / λ (5)
By substituting the strain ε and Burgers vector b (= 25 nm) into the following equation (6), the dislocation density ρ (nm ⁻² = cm / cm ³ ) is obtained.
ρ = 14.4ε ² b ⁻² (6)

  The metal structure and dislocation density as described above can be realized by adopting the above steel composition and applying the manufacturing process described later.

[Hot rolling process]
A hot-rolled steel sheet can be manufactured using a general hot rolling facility. First, a steel slab (for example, a continuously cast slab) having the above chemical composition is placed in a slab heating furnace and heated. The slab heating temperature may be set in the range of 1150 to 1300 ° C., and the holding time in the above temperature range may be set in the range of 60 to 180 minutes. The heated slab is taken out of the furnace and subjected to hot rolling in multiple passes. At that time, the finish rolling delivery temperature is set to a range of 830 to 940 ° C. The finish rolling exit temperature can be grasped by measuring the surface temperature of the steel sheet immediately after leaving the final rolling roll of the finishing hot rolling mill. In the case of steel having the above chemical composition, when the finish rolling outlet temperature is lower than 830 ° C., at least in the final hot rolling pass, rolling is performed in a two-phase temperature range. In this case, the ferrite structure of the hot-rolled steel sheet exhibits a form in which crystal grains are spread unevenly, and the burring property is lowered. On the other hand, when the finish rolling exit temperature exceeds 940 ° C., the crystal grains are likely to be coarsened, and the strength may be lowered.

After hot rolling, cooling is performed so that the average cooling rate R (° C./s) from the completion of finish rolling to the start of winding satisfies the following expression (2).
R ≧ −6 × [Ti / C equivalent ratio] +18 (2)
Here, the value of the Ti / C equivalent ratio determined by the above equation (1) is substituted into the [Ti / C equivalent ratio] on the right side of the equation (2).

The cooling rate from the completion of hot rolling finish rolling to the start of winding greatly affects the amount of cementite produced. The higher the cooling rate is slow, actually austenite between the finish rolling completion until the start of winding - temperature ferrite transformation occurs, the austenite at equilibrium - so closer to ferrite transformation temperature (A ₃ points), ferrite phase A large amount is generated at high temperatures. Since the diffusion rate of C is higher in the ferrite phase than in the austenite phase, the more ferrite phase is generated at a higher temperature, the more the precipitation of cementite is promoted. Cementite causes a reduction in burring properties.

  The amount of cementite produced is also affected by the chemical composition of the steel. Specifically, the amount of cementite generated increases as the Ti / C equivalent ratio represented by the above formula (1) decreases. According to the detailed examination by the inventors, the lower limit of the average cooling rate from the completion of hot rolling finish rolling to the start of winding is set according to the Ti / C equivalent ratio so that the average cooling rate is equal to or higher than the lower limit. It was found that by controlling the cooling conditions, the amount of cementite in the steel sheet can be suppressed to a range (3.0% by volume or less) in which no problem occurs in burring properties. The index representing the lower limit is the right side of equation (2).

On the other hand, when the average cooling rate R (° C./s) from the completion of hot rolling finish rolling to the start of winding becomes excessive, an acicular metal structure is formed and the dislocation density increases. In that case, it was found that even when the annealing in the hot dipping line was finished, a structure state having a high dislocation density was maintained, which was likely to cause a reduction in burring properties. As a result of detailed examination, by controlling the average cooling rate R from the completion of hot rolling finish rolling to the start of winding to less than 50 ° C./s, the dislocation density of the base steel sheet is 1 × 10 ¹⁴ cm / cm ³ or less. A certain high-strength molten Zn—Al—Mg-based plated steel sheet can be obtained, and a reduction in burring properties due to an increase in dislocation density is avoided. Therefore, an excellent “strength that the tensile strength TS is 400 MPa or more and the value of TS × λ represented by the product of the tensile strength TS (MPa) and the hole expansion ratio λ (%) is, for example, 55000 MPa ·% or more. In order to stably realize “-burring balance”, the average cooling rate R (° C./s) from the completion of hot rolling finish rolling to the start of winding satisfies the above formula (2), and the following (3 It is extremely important to cool so as to satisfy the formula (1).
R <50 (3)

  The coiling temperature after hot rolling is in the range of 500 to 650 ° C. This coiling temperature is important for precipitating a large amount of fine Ti carbide (mainly TiC). The fine precipitation of the Ti-based carbide is achieved by utilizing a time zone during which the hot-rolled steel sheet wound in a coil shape is naturally cooled. When the coiling temperature is less than 500 ° C., the amount of Ti-based carbide deposited is insufficient and the strength improvement tends to be insufficient. More preferably, the temperature is set to 550 ° C. or higher. When the coiling temperature exceeds 650 ° C., the Ti-based carbides are coarsened and the burring property is liable to be lowered. In addition, the strength tends to decrease.

  As described above, hot rolling has a microstructure in which a Ti-based carbide is sufficiently finely dispersed in a matrix composed of a ferrite phase, cementite is extremely small, for example, 3.0% by volume or less, and the dislocation density is kept low. A steel plate can be obtained. What is necessary is just to adjust the plate | board thickness of a hot-rolled steel plate in the range of 2.0-5.0 mm, for example.

[Pickling process]
The hot-rolled steel sheet obtained as described above is pickled by a conventional method to remove the oxide scale on the surface.

[Light rolling process]
The steel plate after the pickling is subjected to cold rolling under a light pressure at a rolling rate of 1.5 to 8.0%. This light rolling is different from ordinary cold rolling mainly aimed at reducing the sheet thickness, and is mainly intended to desorb foreign matter called smut that may be present on the surface of the steel sheet after pickling. Is what you do. The smut is a fine powdery black substance which is supposed to be generated in a portion where pickling is excessive. If the smut remains on the surface and is subjected to hot dipping, “non-plating” tends to occur in that portion. Therefore, when producing a hot-dip plated steel sheet using a hot-rolled steel sheet as a plating original sheet, the occurrence of non-plating due to smut tends to be a problem. When hot-rolled steel sheet having the above chemical composition is used as a plating base plate and hot-dip Zn-Al-Mg-based plating is applied, smut can be effectively detached by performing cold rolling under light pressure after pickling. It was confirmed that the occurrence of non-plating can be remarkably suppressed.

If the rolling reduction is less than 1.5%, the pressure applied to the smut is small, so that the smut may not be sufficiently crushed and detached from the surface of the steel sheet, and the occurrence of non-plating due to the smut cannot be effectively prevented. When the rolling reduction exceeds 8.0%, the work hardening of the steel sheet occurs, and the burring property decreases. This light reduction rolling may be performed using a temper rolling apparatus provided in the pickling line. Usually, rolling in one pass is sufficient, but multiple passes can also be used. In the case of a plurality of passes, it is desirable to carry out at least one pass of reduction at a reduction rate of 1.5 to 8.0% and a total rolling rate of 8.0% or less. In addition, the rolling reduction in a certain rolling pass is determined by the following equation (4).
Reduction ratio (%) = (t ₀ −t ₁ ) / t ₀ × 100 (4)
Here, t ₀ is the plate thickness (mm) before the rolling pass, and t ₁ is the plate thickness (mm) after the rolling pass.

[Annealing and hot dipping process]
The hot-rolled steel sheet that has been subjected to the above-described light rolling is passed through a hot dipping line equipped with an annealing facility that can be annealed in a reducing atmosphere to produce a hot-dip Zn—Al—Mg-based plated steel sheet. The annealing atmosphere is preferably a reducing atmosphere containing hydrogen. For example, a hydrogen-nitrogen mixed gas having a hydrogen content of 25 to 35% by volume is mentioned. Ammonia decomposition gas may be used.

  If the annealing temperature is too low, the reduction (activation) of the steel sheet surface becomes insufficient, and plating defects such as non-plating are likely to occur. Here, the annealing temperature needs to be 550 ° C. or higher. It is more effective to set the temperature higher than 670 ° C. (for example, 675 ° C. or higher). However, when the annealing temperature is increased, the Ti-based carbide is coarsened, and burring properties and strength are lowered. As a result of various studies, the annealing temperature is limited to 750 ° C. or less. As for the annealing time, after the surface temperature of the steel sheet reaches a predetermined annealing temperature, the cooling is started immediately or the holding time at that temperature may be set within 60 seconds to start the cooling.

After finishing the above-described reduction annealing, a method of immersing in a molten Zn—Al—Mg plating bath is applied in accordance with a conventional method so that the steel sheet surface is not exposed to the atmosphere. The hot dipping bath is in mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.10%, B : 0 to 0.05%, Fe: 0 to 2.0%, the balance is preferably set within the composition range consisting of Zn and inevitable impurities. The plating bath composition is almost directly reflected in the plating composition of the plated steel sheet. The plating adhesion amount may be, for example, 30 to 300 g / m ² per one side of the steel plate. The final plate thickness of the plated steel plate is, for example, 1.9 to 4.9 mm.

After melting each steel shown in Table 1 and heating the slab to 1250 ° C., it was hot-rolled at the finish rolling exit temperature, average cooling rate R, and coiling temperature shown in Table 2, and the thickness 2 A 0.6 mm hot rolled steel strip was obtained. The average cooling rate R means the average cooling rate from the completion of finish rolling to the start of winding. After pickling the hot-rolled steel strip, 1-pass light rolling was performed at a rolling reduction shown in Table 2 using a temper rolling mill installed at the latter stage of the pickling line. The indication of a rolling reduction of 0% means that this light rolling is not performed. Thereafter, each steel strip was subjected to reduction annealing at a temperature shown in Table 2 in a hydrogen-nitrogen mixed gas atmosphere having a hydrogen content of about 30% by volume in a continuous hot dipping line, and an average cooling rate of 5 ° C./s. Cool to about 420 ° C, soak the steel plate surface in a molten Zn-Al-Mg plating bath without touching the atmosphere, pull it up, and adjust the coating weight to 90 g / m ² per side by gas wiping. Thus, a molten Zn—Al—Mg-based plated steel sheet was obtained. The annealing time (the holding time after the steel sheet surface reached the predetermined annealing temperature) was about 50 seconds. The plating bath composition was as follows (common to each example).
(Plating bath composition)
In mass%, Al: 6.0%, Mg: 3.0%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, balance Zn.

The following was investigated by using the obtained molten Zn—Al—Mg-based plated steel sheet as a test steel sheet.
(Cementite area ratio)
After polishing the cross section (L cross section) parallel to the rolling direction and thickness direction of the sample taken from the test steel plate, etching with Picral reagent, observation of the structure with SEM, and the area of cementite by image analysis of the observed image The rate was determined.

(Dislocation density)
A sample taken from the test steel plate was mechanically polished to a thickness of 1/4 position, and then subjected to chemical polishing to prepare a sample having a surface free from the influence of mechanical polishing strain. The X-ray diffraction pattern of the surface of the sample was measured with an X-ray diffractometer (manufactured by Rigaku Corporation; RINT2500), and the half width of the X-ray diffraction peaks on the (110) plane, (211) plane, and (220) plane Asked. The measurement conditions are Co-Kα ray (λ = 0.17889 nm), tube voltage 40 kV, tube current 100 mA, measurement range 51 to 54 ° ((110) surface), 98 to 102 ° ((211) surface), 122 to It was set to 126 ° ((220) plane). Based on the measurement data of the half width, the dislocation density was determined by the method according to the above-mentioned “How to determine the dislocation density”.

  FIG. 1 illustrates SEM photographs of the above-described tissue observation for No. 8 (example of the present invention), FIG. 2 for No. 16 (comparative example), and FIG. Each of the photographs in FIGS. 1 to 3 corresponds to a lateral width (long side length) of 20 μm. The granular phase that appears white is cementite. Although these are steels having the same composition, granular cementite is conspicuous in FIG. 2 compared to FIG. 1, and the area ratio of cementite is large (see Table 3). This is because No. 16 in FIG. 2 employs hot rolling conditions that do not satisfy the formula (2). 3 has an acicular metal structure, and has a higher dislocation density (see Table 3). This is because No. 20 in FIG. 3 employs a hot rolling condition in which the average cooling rate R from the completion of hot rolling finish rolling to the start of winding does not satisfy the above formula (3).

(Tensile properties)
Tensile strength TS and total elongation at break T.El were determined according to JIS Z2241: 2011 using a JIS No. 5 test piece collected so that the longitudinal direction of the test piece was in the direction perpendicular to the rolling direction.

(Burring properties)
A 70 × 70 mm sample was taken from the test steel plate, and a punched hole was formed in the center of the sample using a punch and a die. A punch having a diameter D ₀ of 10.0 mm and a die having a clearance of 12% of the plate thickness was selected. A punch having an apex angle of 60 ° was pushed into the punched hole from the opposite side of the burr to enlarge the initial hole. At that time, the moving speed of the punch was set to 10 mm / min. Stopped punch when cracks in the thickness direction to expand the hole of the steel sheet was penetrated was measured internal diameter D _b of the hole. Then, a hole expansion ratio λ (%) defined by (D _b −D ₀ ) / D ₀ × 100 was obtained. If the value of the product TS x λ of the tensile strength TS (MPa) and hole expansion ratio λ (%) is 40000 MPa ·% or more, it is extremely useful for reducing the weight of the member in applications such as automobile members and building materials. Can be evaluated as having a good balance between strength and burring. Therefore, a sample having a value of TS × λ of 40000 MPa ·% or more was determined to be acceptable. Among them, those having a tensile strength TS of 400 MPa or more and a TS × λ value of 55000 MPa ·% or more can be evaluated as having a particularly excellent “strength-burring balance”.

(Plating surface appearance)
Three 200 mm × 200 mm samples were randomly collected from the cut plates collected from each test steel plate. Both sides of the sample were observed to examine whether “non-plating” exists on each plated surface. 3 sheets for each test steel plate x double-sided = total 6 plating surfaces were examined. Test steel plates with no unplating on all 6 surfaces were ○ (no plating), and one of the 6 surfaces was unplated The existing test steel sheet was evaluated as x (with non-plating), and the one evaluated as ○ was determined to be acceptable.

  In FIG. 4, the external appearance photograph of the sample surface of No. 4 in which non-plating does not exist is illustrated. In FIG. 5, the external appearance photograph of the sample surface of No. 30 in which non-plating exists is illustrated. 4 and 5 each have a width (long side length) of 50 mm. In FIG. 5, “non-plating” is observed in a portion indicated by a broken line. These non-plating results from the smut generated in the pickling process.

The above survey results are shown in Table 3. The dislocation density (cm / cm ³ ) in each example is a value obtained by multiplying the numerical value described in the column of dislocation density in Table 3 by 10 ¹³ . For example, the dislocation density of No. 1 is 5.5 × 10 ¹³ cm / cm ³ , and the dislocation density of No. 18 is 1.2 × 10 ¹⁴ .

  The hot-dip Zn—Al—Mg plated steel sheet of the example of the present invention has a high strength with a tensile strength of 400 MPa or more and is also excellent in “strength-burring balance” represented by TS × λ value.

  On the other hand, in Nos. 15 to 17, since the average cooling rate R from the completion of hot rolling finish rolling to the start of winding deviates from the formula (2) and is slow, the amount of cementite increases and the TS × λ value is low. Nos. 18 to 20 are examples in which the dislocation density of the base steel sheet after hot dipping was high due to the high average cooling rate R from the completion of finish rolling to the start of winding. As seen in these examples, when the dislocation density is high, it is difficult to stably realize an excellent strength-burring balance. No. 21 had high hot rolling finish rolling temperature, so the crystal grains became coarse and the strength was low. Since No. 22 had a low hot rolling finish rolling temperature, the ferrite crystal grains were unevenly expanded and the TS × λ value was low. Since No. 23 had a high winding temperature after hot rolling, it became coarse during the temperature drop of the coil wound with Ti carbide (mainly TiC), and the TS × λ value was lowered. In No. 24, since the coiling temperature after hot rolling was low, the precipitation amount of the Ti-based carbide could not be sufficiently ensured during the temperature decrease of the coil wound, and the strength was low. In No. 25, the reduction annealing temperature in the plating line was high, so the Ti-based carbide was coarsened and the TS × λ value was low. In Nos. 26 and 27, steel with a low Ti / C equivalent ratio was used, so the amount of cementite produced increased from the completion of hot rolling finish rolling to the start of winding, and the TS × λ value was low. In Nos. 28 and 29, work reduction occurred because the rolling reduction in the light rolling after pickling was high, and the TS × λ value was low. Nos. 30 and 31 were not subjected to light rolling after pickling, and thus non-plating due to smut occurred.

Claims (6)

By mass%, C: 0.005-0.080%, Si: 0-0.80%, Mn: 0.10-1.80%, Ti: 0.020-0.200%, B: 0.0. Bainitic ferrite having a chemical composition consisting of 0005 to 0.010%, balance Fe and inevitable impurities, and having a Ti / C equivalent ratio of 0.40 to 2.00 defined by the following formula (1) It has a molten Zn-Al-Mg-based plating layer on the surface of a base steel sheet that has a single phase or a microstructure of both bainitic ferrite and ferrite as a main phase and has a cementite area ratio of 3% or less. The tensile strength TS in the direction perpendicular to the rolling is 400 MPa or more, and the product TS × λ of the tensile strength TS (MPa) and the hole expansion ratio λ (%) is 40,000 MPa ·% or more. High-strength Zn-Al-Mg-based plated steel sheet.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the value of the content in steel represented by mass% of each element is substituted for the element symbol on the right side of the formula (1).   The chemical composition of the base steel sheet is further mass%, P: 0.005 to 0.050%, S: 0.0005 to 0.020%, Nb: 0 to 0.100%, V: 0 to 0.00. The hot-dip Zn—Al—Mg-based plated steel sheet according to claim 1, satisfying a content of 100%, Al: 0.010 to 0.100%. By mass%, C: 0.005-0.080%, Si: 0-0.80%, Mn: 0.10-1.80%, Ti: 0.020-0.200%, B: 0.0. 0005 to 0.010%, P: 0.005 to 0.050%, S: 0.0005 to 0.020%, Nb: 0 to 0.100%, V: 0 to 0.100%, Al: 0 It has a chemical composition having a Ti / C equivalent ratio defined by the following formula (1) of 0.40 to 2.00, consisting of 0.010 to 0.100%, the balance Fe and inevitable impurities, and bainitic A base material having a ferrite single phase or a microstructure of both bainitic ferrite and ferrite as a main phase, a cementite area ratio of 3% or less, and a dislocation density of 1 × 10 ¹⁴ cm / cm ³ or less It has a molten Zn-Al-Mg plating layer on the surface of the steel plate, and the tensile strength in the direction perpendicular to the rolling is 400 MPa or more. High-strength hot-dip Zn-Al-Mg plated steel sheet having excellent burring properties is.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the value of the content in steel represented by mass% of each element is substituted for the element symbol on the right side of the formula (1).   The molten Zn—Al—Mg-based plating layer is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Si: 0 to 2.0%, Ti: 0. The molten Zn according to any one of claims 1 to 3, which has a composition comprising: -0.10%, B: 0-0.05%, Fe: 0-2.0%, the balance Zn and unavoidable impurities. -Al-Mg plated steel sheet. The average cooling rate R (° C./s) from the completion of finish rolling to the start of winding after satisfying the following equation (2) after hot rolling under conditions where the finish rolling exit temperature is 830 to 940 ° C. To obtain a hot-rolled steel sheet at a coiling temperature of 500 to 650 ° C.,
Pickling the hot-rolled steel sheet,
A step of performing light rolling at a rolling reduction of 1.5 to 8.0% after the pickling,
A step of performing hot-dip Zn-Al-Mg-based plating after annealing at 550 to 750 ° C in a reducing atmosphere after the cold rolling;
The manufacturing method of the hot-dip Zn-Al-Mg-based plated steel sheet according to any one of claims 1 to 4.
R ≧ −6 × [Ti / C equivalent ratio] +18 (2)
However, the value of the Ti / C equivalent ratio determined by the above equation (1) is substituted into the [Ti / C equivalent ratio] position on the right side of the equation (2). The manufacturing of the hot-dip Zn-Al-Mg-based plated steel sheet according to claim 5, wherein in the step of obtaining the hot-rolled steel sheet, cooling is performed so that the average cooling rate R (° C / s) further satisfies the following formula (3). Method.
R <50 (3)