JP6958459B2 - Fused Zn-Al-Mg alloy plated steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a molten Zn-Al-Mg alloy plated steel sheet and a method for producing the same.

In recent years, there has been increasing interest in environmental issues, and steel sheets used for various processed products such as automobile members are required to have high strength and weight reduction by thinning. Further, the steel sheet is required to have a high yield strength from the viewpoint of preventing plastic deformation during use. Further, when various deformation styles of processing are applied to a steel sheet, the steel sheet is required to have high workability (for example, bending workability) in addition to strength. Therefore, many inventions have been made regarding steel sheets obtained by precisely controlling the metal structure by combining the addition of expensive alloying elements and complicated heat treatment.

As a technique for obtaining a steel sheet as described above, a hot-dip galvanized steel sheet having a specified chemical composition and a specified metal structure phase such as the area ratio of the retained austenite phase in order to improve bending workability. And a method for producing the same (see, for example, Patent Documents 1 to 3).

JP-A-2009-270126 (published on November 19, 2009) Japanese Unexamined Patent Publication No. 2013-117042 (published on June 13, 2013) Japanese Unexamined Patent Publication No. 2015-193897 (published on November 5, 2015)

However, all of the above-mentioned hot-dip galvanized steel sheets can be obtained by increasing the strength of the steel sheet by transformation strengthening and by utilizing retained austenite to achieve both high strength and workability. In order to obtain the above-mentioned steel sheet, it is necessary to add a large amount of expensive alloying elements such as Si and Mn to the steel sheet, so that the above-mentioned production method has a problem that the production cost is high. Further, in the transformation strengthening, it is very difficult to stably secure good bendability in the steel sheet due to the large difference in strength between the hard phase and the soft phase.

One aspect of the present invention is to reduce the manufacturing cost and to realize a steel sheet having high yield strength and stable and good bending workability and a method for manufacturing the steel sheet.

In order to solve the above problems, the fused Zn-Al-Mg alloy plated steel sheet according to one aspect of the present invention is a fused Zn-Al-Mg alloy having a fused Zn-Al-Mg alloy plating layer on the surface of the material steel sheet. In the plated steel sheet, the material steel sheet has C of 0.005% or more and 0.08% or less, Si of 0.8% or less, Mn of 0.1% or more and 1.8% or less, and P in mass%. 0.05% or less, S 0.005% or less, Ti 0.02% or more and 0.2% or less, B 0.0005% or more and 0.01% or less, and Al 0.1% or less. In addition, Fe and unavoidable impurities are contained as a balance, and the equivalent ratio of the Ti to the C represented by the following formula (1) in the material steel sheet is 0.4 or more and 1.5 or less, and the material steel sheet is , The metallographic structure in the cross section parallel to the rolling direction contains a phase consisting of one or both of a ferrite phase and a bainitic ferrite phase as a main phase, and further contains a martensite phase and a cementite phase, and the martensite phase and the cementite phase. The above-mentioned Ti-containing alloy having a total ratio of 3% or less in terms of area ratio and a maximum particle size of 20 nm or less is dispersed and precipitated. The hot-dip Zn-Al-Mg alloy-plated steel sheet has a yield strength of 600 N / mm ² or more, and the ratio of the yield strength to the tensile strength is 80% or more. However, the content (mass%) of the element in the material steel sheet is substituted in the place of the element symbol of the following formula (1).

Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)

According to the above aspect of the present invention, it is possible to realize a steel sheet having high yield strength and stable and good bending workability while reducing the manufacturing cost and a method for manufacturing the steel sheet.

It is a figure which shows typically the appearance of the boss welding material used for the welding test in the evaluation of an Example and a comparative example. It is a figure which shows typically the structure of the joint body of the test piece and the restraint plate used for the welding test in the evaluation of an Example and a comparative example.

The composition and manufacturing method of the material steel sheet of the molten Zn-Al-Mg alloy-plated steel sheet according to the present embodiment will be described below. In addition, "%" means "mass%".

[Component composition of material steel sheet]
The material steel sheet has C of 0.005% or more and 0.08% or less, Si of 0.8% or less, Mn of 0.5% or more and 1.8% or less, P of 0.05% or less, and S of 0. .005% or less, Ti 0.02% or more and 0.2% or less, B 0.0005% or more and 0.01% or less, and Al 0.1% or less, and the balance of Fe and unavoidable impurities Include as.

Carbon (C) is an element that forms carbides containing Ti (described later) and is finely precipitated in bainitic ferrite (described later) or in the ferrite structure (described later) to increase the strength. If the C content is less than 0.005%, it may not be possible to obtain a yield strength ^{of 600 N / mm 2 or more in the molten Zn-Al-Mg alloy plated steel sheet.} On the other hand, when the C content exceeds 0.08%, the bending workability is lowered due to the coarsening of the precipitate and the formation of the hard second phase (for example, martensite) and cementite. The C content of the material steel sheet in this embodiment is 0.005% or more and 0.08% or less. Further, from the viewpoint of obtaining desired strength and bending workability, the C content is preferably 0.01% or more and 0.06% or less, and more preferably 0.01% or more and 0.04% or less. be.

Silicon (Si) is an element effective for solid solution strengthening of the material steel sheet. However, if the Si content is excessive, oxides are formed on the surface of the material steel sheet during heating in the continuous hot-dip plating step (described later), which hinders the plating property of the material steel sheet and increases the manufacturing cost. Since it is easy to invite, the upper limit of the addition amount is set to 0.8%. The Si content of the material steel sheet in this embodiment is 0.8% or less. From the viewpoint of improving the plating property and reducing the manufacturing cost, it is preferably 0.4% or less, more preferably 0.2% or less.

Manganese (Mn) is an element effective for increasing the strength of the material steel sheet. If the Mn content is less than 0.5%, it may be difficult to obtain a yield strength ^{of 600 N / mm 2 or more in the molten Zn—Al—Mg alloy plated steel sheet.} On the other hand, if the Mn content exceeds 1.8%, segregation is likely to occur in the material steel sheet, and as a result, the bending workability may decrease. Further, if the Mn content exceeds 1.8%, the manufacturing cost will increase. The Mn content of the material steel sheet in this embodiment is 0.5% or more and 1.8% or less. Further, from the viewpoint of obtaining desired strength and bending workability, the Mn content is preferably 1.0% or more and 1.8% or less, and more preferably 1.0% or more and 1.5% or less. be.

Phosphorus (P) is an element effective for solid solution strengthening of the material steel sheet, but if the P content exceeds 0.05%, segregation is likely to occur in the material steel sheet, and the bending workability may decrease. .. Therefore, the upper limit of the amount of P added is set to 0.05%. The P content of the material steel sheet in this embodiment is 0.05% or less. Further, from the viewpoint of obtaining a desired bending workability, it is preferably 0.03% or less, more preferably 0.02% or less. The content of P does not include 0.

Sulfur (S) forms sulfide with Mn and deteriorates local ductility such as bending workability. Therefore, S is an element that should be reduced as much as possible, but since 0.005% is acceptable, the upper limit of the content is limited to 0.005%. The S content of the material steel sheet in this embodiment is 0.005% or less. Further, from the viewpoint of obtaining appropriate ductility, it is preferably 0.003% or less, more preferably 0.002% or less. Note that S is an unavoidable impurity, and its content does not include 0.

Titanium (Ti) is an element that binds to C and precipitates as a fine carbide of Ti, which is effective in increasing the strength of the material steel sheet and suppressing the precipitation of cementite. Further, since Ti has a high affinity with N and N in the material steel plate is fixed as TiN, adding Ti is effective in securing the amount of solid solution B as described above. In order to obtain these effects sufficiently, it is necessary that the material steel sheet contains 0.02% or more of Ti. On the other hand, when the Ti content exceeds 0.2%, the effect is saturated and the manufacturing cost is increased. Therefore, the Ti content is limited to the range of 0.02% or more and 0.20% or less. The Ti content of the material steel sheet in this embodiment is 0.02% or more and 0.20% or less.

Boron (B) is an element that segregates at grain boundaries to increase the interatomic bonding force and is effective in suppressing embrittlement cracking of molten metal. Further, B is an element effective for increasing the strength of the material steel sheet because it segregates at the grain boundaries to suppress ferrite transformation and pearlite transformation, and it becomes easy to obtain a bainitic ferrite structure. If the B content is less than 0.0005%, these effects cannot be expected so much, and even if the B content is added in excess of 0.01%, the effects are saturated and the manufacturing cost is increased. Therefore, the B content of the material steel sheet in the present embodiment is 0.0005% or more and 0.01% or less.

Aluminum (Al) is added as a deoxidizing material during steelmaking. In order to obtain the effect, it is necessary to add 0.005% or more. On the other hand, even if it is added in excess of 0.1%, the effect is saturated and the manufacturing cost is rather increased. The Al content of the material steel sheet in this embodiment is 0.1% or less.

Iron (Fe) is the main metal component of the material steel sheet. Iron constitutes the balance of the material steel sheet, excluding the unavoidable impurities of various components.

The unavoidable impurities are trace components contained in the material steel sheet and trace components unavoidably contained as contamination in the manufacturing process thereof, which are derived from the raw material of the material steel sheet. Inevitable impurities include, for example, N, Cu, Ni, Cr and the like as typical examples in addition to P and S described above. Basically, it contains only a small amount and does not have a great influence on the characteristics of the raw steel sheet, but if it contains unavoidable impurities in excess of 0.5% in total, the bending workability may be deteriorated. Therefore, the content of unavoidable impurities is preferably less than 0.5%.

Further, among the unavoidable impurities, when nitrogen (N) remains as solid solution nitrogen in the material steel sheet, BN is generated, which leads to a decrease in the amount of solid solution B effective for suppressing the embrittlement cracking of the molten metal. As a result of the examination, the content of N in the raw steel sheet is preferably limited to 0.005% or less.

The material steel sheet may further contain components other than the above as long as the effects of the present embodiment can be obtained. Examples of such other components include niobium and vanadium.

Niobium (Nb) and vanadium (V) are effective for refining ferrite grains. Further, it forms a composite carbide containing C in the same manner as Ti, and contributes to an increase in yield strength of the molten Zn—Al—Mg alloy plated steel sheet. Therefore, it is preferable to contain one or more of these elements in the material steel sheet, if necessary. The Nb content and V content of the material steel sheet in this embodiment are 0.1% or less. Therefore, it is preferable that the material steel sheet in the present embodiment further contains one or both of Nb of 0.1% or less and V of 0.1% or less from the viewpoint of high strength.

The equivalent ratio of the Ti expressed by the following equation (1) to the C in the material steel sheet is 0.4 or more and 1.5 or less. The material steel sheet having the above equivalent ratio has good bending workability. The Ti / C equivalent ratio is defined by the following equation (1).
Ti / C equivalent ratio = (Ti / 48) / (C / 12) ... (1)
The element content (mass%) in the material steel sheet is substituted in place of the element symbol in the above formula (1). In order to obtain a steel sheet having good bending workability, the content of Ti and C is controlled in the material steel sheet in the present embodiment so that the Ti / C equivalent ratio is 0.4 or more and 1.5 or less. Will be done. If the Ti / C equivalent ratio is less than 0.4, the amount of martensite and the amount of cementite increase, so that the bendability of the steel sheet decreases. On the other hand, when the Ti / C equivalent ratio exceeds 1.5, the effect is saturated and the manufacturing cost is increased.

[Strength of hot-dip Zn-Al-Mg alloy-plated steel sheet]
The yield strength of the molten Zn-Al-Mg alloy plated steel sheet according to one aspect of the present invention ^{is defined to be 600 N / mm 2} or more, and the ratio of the yield strength to the tensile strength is specified to be 80% or more. In order to satisfy the numerical value of the ratio of the yield strength to the tensile strength described above, the manufacturing method described later is used. The "ratio of the above-mentioned yield strength to the tensile strength" is calculated by the following equation (2).

Yield strength / tensile strength x 100 ... (2)
The above-mentioned yield strength and the ratio of the yield strength to the tensile strength are controlled within the chemical composition range described above, and the cold rolling step in the manufacturing method described later and the continuous hot-dip plating step including annealing are appropriate conditions. It can be realized by doing with.

As a method for measuring the yield strength described above, JIS Z2241 (metal material tensile test method) can be mentioned. Further, as a means for measuring the bending workability, JIS Z2248 (metal material bending test method) can be mentioned.

The difference (CH25) between the plate thickness at a position 25 mm from the edge of the material steel plate in the plate width direction and the plate thickness at the center of the plate width is ± 50 μm or less, which is the thickness of the plated steel sheet in the plate thickness direction. It is preferable from the viewpoint of increasing uniformity. The "center portion of the plate width" is a portion inside the position 25 mm from the end in the plate width direction, and preferably a portion including the center in the plate width direction. The plate thickness can be measured and adjusted by a known method.

[Metal structure of material steel plate]
The material steel plate according to the embodiment of the present invention contains a phase in which the metal structure is composed of one or both of a ferrite phase and a bainitic ferrite phase as a main phase in a cross section parallel to the rolling direction, and a martensite phase and a cementite phase. Including further. Here, the total ratio of the martensite phase and the cementite phase is 3% or less in terms of area ratio.

During bending, the interface between the ferrite phase or bainitic ferrite phase and the martensite or cementite phase tends to be the starting point of cracks. Therefore, according to the above configuration, the starting points of cracks are reduced, resulting in stability. Good bending workability can be obtained.

Further, in the material steel sheet, carbides containing Ti having a maximum particle size of 20 nm or less are dispersed and precipitated. According to the above configuration, the material steel sheet has high yield strength and good bending workability by strengthening the particle dispersion based on the Orowan mechanism.

The carbide containing Ti also includes a carbide further containing Nb or V in addition to Ti.

Further, the maximum particle size of the carbide containing Ti is preferably 10 nm or less from the viewpoint of yield strength. As a method for measuring the maximum particle size, for example, a thin film prepared from each of a plurality of molten Zn-Al-Mg alloy plated steel sheets is observed using a transmission electron microscope (TEM), and 30 or more Ti-containing carbides are contained. The particle size (major axis) of the carbide in a certain region contained therein may be measured, and the average value thereof may be used as the average particle size of the Ti-containing carbide. Here, if the maximum particle size is less than 5 nm, the Orowan mechanism becomes difficult to work. Therefore, the maximum particle size of the carbide containing Ti is more preferably 5 nm or more.

The following procedure is given as an example of calculating the area ratio of the total of the martensite phase and the cementite phase on the surface of the material steel sheet. (I) The cut out molten Zn-Al-Mg alloy plated steel plate sample is mirror-polished in a cross section parallel to the rolling direction. (Ii) The cross section is etched with a nital reagent or the like, and then the cross section is scanned. Observe using an electron microscope (SEM) (iii) The ratio of the area of the region where the martensite phase and the cementite phase exist in the field to the area of each field is calculated for each field and the calculated value. The average value of is calculated and used as the above area ratio.

[Fused Zn-Al-Mg alloy plating layer]
The hot-dip Zn-Al-Mg alloy-plated steel sheet according to one aspect of the present invention has a hot-dip Zn-Al-Mg alloy-plated layer on its surface. The component composition of the plating layer will be described below. The "melt plating bath liquid" described later is a liquid in which each metal described later is melted, in which the plating layer is stored in a tank (pot) used in the continuous melt plating step (described later). means.

Al contained in the plating layer improves the corrosion resistance of the plated steel sheet. Further, by containing Al in the hot-dip plating bath liquid, there is also an effect of suppressing the generation of Mg oxide-based dross. In order to sufficiently obtain these effects, the Al content contained in the plating layer needs to be 3.0% or more, and more preferably 4.0% or more. On the other hand, when the Al content exceeds 22.0%, the Fe—Al alloy layer grows remarkably at the interface between the plating layer and the material steel sheet, and the adhesion of the plating deteriorates. In order to ensure excellent plating adhesion, the Al content is preferably 15.0% or less, and more preferably 10.0% or less.

Mg contained in the plating layer exhibits an action of remarkably enhancing the corrosion resistance of the plated steel sheet by forming a uniform corrosion product on the surface of the plating layer. In order to fully exert its action, the Mg content of the plating layer needs to be 0.05% or more, and it is desirable to secure 2.0% or more. On the other hand, if the Mg content exceeds 10.0%, Mg oxide-based dross is likely to occur. In order to obtain a higher quality plating layer, the Mg content is preferably 5.0% or less, and more preferably 4.0% or less.

When one or both of Ti and B are contained in the hot-dip plating bath liquid, the formation and growth of _{Zn 11} Mg _{2-phase, which gives a speckled appearance defect, is suppressed in the hot-dip Zn—Al—Mg alloy-plated steel sheet.} Even if Ti and B are contained alone, the _{effect of suppressing the Zn 11} Mg _2- phase is produced, but from the viewpoint of greatly relaxing the degree of freedom of production conditions, it is desirable to contain Ti and B in combination. In order to sufficiently obtain these effects, it is effective that the Ti content of the plating layer is 0.0005% or more and the B content is 0.0001% or more. However, if the Ti content is too high, Ti—Al-based precipitates are formed in the plating layer, causing irregularities in the plating layer and impairing the appearance. Therefore, when Ti is added to the hot-dip plating bath liquid, the content range needs to be 0.10% or less, and more preferably 0.01% or less. On the other hand, if the B content is too large, Al-B-based or Ti-B-based precipitates are formed and coarsened in the plating layer, causing unevenness and impairing the appearance. Therefore, when B is added to the hot-dip plating bath liquid, the content range needs to be 0.05% or less, and more preferably 0.005% or less.

When Si is contained in the hot-dip plating bath liquid, the growth of the Fe-Al alloy layer is suppressed, and the workability of the hot-dip Zn-Al-Mg alloy plated steel sheet is improved. Further, Si contained in the plating layer is an element effective in preventing the plating layer from changing to black and maintaining the glossiness of the surface. In order to fully bring out the action of Si, it is effective to set the Si content of the hot-dip plating solution to 0.005% or more. However, if Si is added in excess, the amount of dross contained in the hot-dip plating bath liquid increases. Therefore, when Si is contained in the plating bath liquid, the content is set to 2.0% or less.

A certain amount of Fe and unavoidable impurities are mixed into the hot-dip plating bath from the above-mentioned material steel plate and pot constituent members. In molten Zn-Al-Mg alloy plating, Fe contained in the plating bath is allowed to be contained up to about 2.0%. In addition to the above-mentioned elements, for example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr or Mn may be mixed in the plating bath solution. good. In this case, their total content is preferably 1% or less.

As is clear from the above description, the preferable composition of the molten Zn-Al-Mg alloy plating layer is, for example, 3.0% or more and 22.0% or less for Al and 0.05% or more and 10.0% for Mg. Below, Ti is 0.1% or less, B is 0.0005% or more and 0.01% or less, Si is 0 or more and 2.0% or less, and Fe is 2.0% or less, and the balance is an unavoidable impurity. Is.

The composition in the hot-dip plating bath liquid is reflected almost as it is in the composition of the plating layer coated on the hot-dip Zn-Al-Mg alloy-plated steel sheet.

The molten Zn-Al-Mg alloy plated steel sheet can be preferably produced by the following production method. Hereinafter, a method for manufacturing the molten Zn-Al-Mg alloy plated steel sheet will be described.

[Manufacturing method of molten Zn-Al-Mg alloy plated steel sheet]
The method for producing the hot-dip Zn-Al-Mg alloy-plated steel sheet according to the present embodiment may be the same as that for the normal hot-dip plated steel strip, and ironmaking (blast furnace), steelmaking, continuous casting, hot rolling, It is manufactured through a continuous hot-dip plating process after each process of pickling and cold rolling. Each of the above-mentioned series of steps will be described in detail below.

<Steelmaking process / continuous casting process>
First, in the steelmaking process, a molten steel having a predetermined chemical composition can be obtained by removing impurities from the pig iron obtained in the blast furnace by a converter and vacuum degassing and adjusting the composition. After that, steel slabs are manufactured by continuous casting.

<Hot rolling process>
The hot rolling step heats the slab to a predetermined temperature. The temperature at which the slab is heated is preferably 1100 ° C. or higher in order to sufficiently dissolve the additive element.

The heated slab is rolled to a predetermined plate thickness through a rough rolling step and a finish rolling step. It is preferable that the finish rolling end temperature ends at the Ar3 transformation point or higher. This is because if the number of points is less than 3 points, the plate thickness fluctuates greatly and good plate thickness accuracy cannot be obtained.

The steel strip rolled to a desired thickness in the finish rolling step is rolled up using a take-up device. At this time, the winding temperature is 500 ° C. or higher and 650 ° C. or lower. If the winding temperature is less than 500 ° C., the amount of carbides containing Ti deposited is insufficient, and as a result, the strength of the molten Zn—Al—Mg alloy-plated steel sheet may decrease. On the other hand, when the winding temperature exceeds 650 ° C., the Ti-containing carbides may be coarsened, and as a result, the strength and bending workability of the finally obtained molten Zn-Al-Mg alloy-plated steel sheet may be lowered. be.

In the pickling step, the scale adhering to the surface of the hot-rolled steel strip obtained by the hot-rolling treatment is removed by the pickling treatment.

In the cold rolling step, cold rolling is performed on the steel strip after the pickling treatment at a cold rolling ratio of 4% or more and 60% or less.

By controlling the cold rolling ratio to 4% or ^{more, a molten Zn-Al-Mg alloy-plated steel sheet having a yield strength of 600 N / mm 2} and a ratio of the yield strength to the tensile strength of 80% or more can be obtained. Be done. Further, by controlling the cold rolling ratio to 60% or less, a molten Zn-Al-Mg alloy plated steel sheet having good bending workability can be obtained. If the cold rolling ratio is less than 4%, the yield strength may be less than ^{600 N / mm 2.} On the other hand, when the cold rolling ratio exceeds 60%, the ductility of the molten Zn-Al-Mg alloy-plated steel sheet is greatly reduced, and as a result, the bending workability of the molten Zn-Al-Mg alloy-plated steel sheet is deteriorated. May be done.

The annealing temperature in the continuous annealing step in the continuous melting Zn-Al-Mg plating step is 550 ° C. or higher and 750 ° C. or lower. If the annealing temperature is less than 550 ° C., the surface of the material steel sheet cannot be sufficiently reduced, which may cause plating defects. Further, when annealing is performed at a temperature exceeding 750 ° C., the metal structure of the material steel sheet is recrystallized and softened, so that a yield strength of 600 MPa or more cannot be obtained.

The annealed material steel sheet is passed through a molten Zn-Al-Mg plating bath adjusted to a predetermined component to obtain a molten Zn-Al-Mg plated steel sheet having a desired adhesion amount.

After cooling to room temperature, the plated steel sheet may be temper-rolled or passed through a tension leveler in order to suppress yield elongation and correct the shape. The temper rolling and tension leveler may be applied by an in-line device installed after the continuous plating line or by an offline device. If the amount of strain applied by temper rolling or tension leveler is too large, the workability of the product deteriorates. Therefore, it is desirable that the amount of strain applied is 2% or less in terms of elongation.

(summary)
(1) The molten Zn-Al-Mg alloy plated steel sheet according to one aspect of the present invention is a molten Zn-Al-Mg alloy plated steel sheet having a molten Zn-Al-Mg alloy plating layer on the surface of the material steel sheet. The steel plate has C of 0.005% or more and 0.08% or less, Si of 0.8% or less, Mn of 0.1% or more and 1.8% or less, P of 0.05% or less, and S in mass%. 0.005% or less, Ti 0.02% or more and 0.2% or less, B 0.0005% or more and 0.01% or less, and Al 0.1% or less, and Fe and unavoidable impurities The equivalent ratio of the Ti to the C, which is included as the balance and is represented by the following equation (1) in the material steel plate, is 0.4 or more and 1.5 or less, and the material steel plate has a cross section parallel to the rolling direction. , The metal structure contains a phase consisting of one or both of a ferrite phase and a bainitic ferrite phase as a main phase, and further contains a martensite phase and a cementite phase, and the total ratio of the martensite phase and the cementite phase is the area ratio. On the surface of the material steel sheet, the carbides containing Ti having a maximum particle size of 20 nm or less are dispersed and precipitated, and the yield strength is 600 N / mm ² or more. Moreover, the ratio of the yield strength to the tensile strength is 80% or more.

Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the content (mass%) of the element in the material steel sheet is substituted in place of the element symbol in the above formula (1).

According to the above configuration, the component composition and the dispersion mode are such that the plated steel plate sufficiently has a desired yield strength and good bending workability. Further, by defining the ratio of the total of the martensite phase and the cementite phase on the surface of the material steel plate, the ferrite phase or the bainitic ferrite phase and the martensite or the cementite phase at the interface during bending are specified. The origin of cracks can be reduced, and as a result, bending workability can be improved. Further, a high value of 600 N / mm ² or more and a ratio of the yield strength to the tensile strength of 80% or more can be obtained. Therefore, a hot-dip Zn-Al-Mg alloy-plated steel sheet having higher yield strength and bending workability is realized.

(2) Further, in the plated steel sheet according to one aspect of the present invention, even if the material steel sheet further contains one or both of Nb of 0.1% or less and V of 0.1% or less in mass%. good.

According to the above configuration, it is even more effective from the viewpoint of increasing the strength.

(3) Further, in the plated steel sheet according to one aspect of the present invention, the content of N among the unavoidable impurities in the material steel sheet is preferably 0.005% or less in mass%.

According to the above configuration, it is more effective from the viewpoint of suppressing the melt-resistant metal embrittlement cracking in the material steel sheet.

(4) Further, the composition of the molten Zn-Al-Mg alloy plating layer contained in the plated steel plate according to one aspect of the present invention is 3.0% or more and 22.0% or less for Al and 0.05% or more for Mg. 10.0% or less, Ti 0.1% or less, B 0.0005% or more and 0.01% or less, Si 0 or more and 2.0% or less, and Fe 2.0% or less, and the balance May be an unavoidable impurity.

According to the above configuration, the composition is such that the processability is more excellent. Therefore, a molten Zn-Al-Mg alloy plated steel sheet having more excellent bending workability is realized.

(5) Further, in the plated steel sheet according to one aspect of the present invention, the difference (CH25) between the plate thickness at a position 25 mm from the edge of the material steel plate in the plate width direction and the plate thickness at the center of the plate width is ± 50 μm. It may be as follows.

According to the above configuration, since the plate thickness accuracy is good, the yield is improved and the cost at the time of processing is reduced. Therefore, a hot-dip Zn-Al-Mg alloy-plated steel sheet that further reduces the manufacturing cost is realized.

(6) Further, the method for producing a steel sheet according to one aspect of the present invention is the method for producing a molten Zn-Al-Mg alloy-plated steel sheet according to any one of (1) to (5) above. A hot rolling process in which a slab having the above composition contained in the material steel sheet is hot-rolled, and a pickling step in which the hot-rolled steel strip obtained by performing the hot-rolling treatment is pickled. , The cold rolling step of cold rolling the steel strip after the pickling treatment, and the melting of the steel strip obtained after the cold rolling treatment in-line after the steel strip containing the composition according to claim 3. The winding temperature at the time of winding the steel in the hot rolling step includes a continuous hot-dip plating step of applying a Zn-Al-Mg alloy plating layer to the surface of the steel strip, and the winding temperature is 500 ° C. or higher and 650 ° C. or lower. In the cold rolling step, cold rolling is performed on the steel strip after the pickling treatment at a cold rolling ratio of 4% or more and 60% or less, and the quenching temperature, which is the temperature in the furnace used in the continuous annealing treatment, is 550. The temperature is 750 ° C or higher.

According to the above configuration, in the hot rolling step, the steel strip is wound at the above winding temperature, and the cold rolling step and the continuous hot-dip plating step including annealing are performed under appropriate conditions to achieve 600 N / mm ² or more. A molten Zn-Al-Mg alloy plated steel sheet having a yield strength and a ratio of the yield strength to the tensile strength of 80% can be obtained.

Each of the steels A to W whose composition is shown in Table 1 is melted, the slab is heated to 1250 ° C., and then hot-rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 520 to 680 ° C. to obtain a plate thickness of 2. A 6 mm hot-rolled steel strip was obtained. The compositions of steels A to W are shown in Tables 1 and 2.

The hot-rolled steel strip was wound at a predetermined winding temperature T1. Next, the hot-rolled steel strip is pickled and cold-rolled at 4%, 30% and 50% cold rolling ratios, and then in a continuous hot-dip plating line at 500 to 790 ° C. in a hydrogen-nitrogen mixed gas. Annealing was carried out at an annealing temperature of T2, and the material was cooled to about 420 ° C. at an average cooling rate of 5 ° C./sec to obtain material steel sheets (plated original plates) 1 to 24. Then, while the surface of the material steel sheet is not exposed to the atmosphere, it is immersed in a molten Zn-Al-Mg-based plating bath having the following plating bath composition, then pulled up, and the amount of plating adhered per side is about 90 g / by the gas wiping method. Fused Zn-Al-Mg-based plated steel sheets 1 to 24 adjusted to m ^{2 were obtained.} The plating bath temperature was about 410 ° C.

[Plating bath composition (% by mass)]
Al: 6.0%, Mg: 3.0%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: Remaining [Ti-containing carbides Average particle size]
A thin film prepared from the collected molten Zn-Al-Mg-based plated steel plate sample was observed with a transmission electron microscope (TEM), and the particle diameter (major axis) of the carbide in a certain region containing 30 or more Ti-containing carbides. Was measured, and the average value was taken as the average particle size R1 of the Ti-containing carbide.

[Metallic structure of cross section]
A cross section parallel to the rolling direction of the sample cut out from the sample of the collected molten Zn-Al-Mg-based plated steel sheet is polished, etched with a nital reagent and observed by SEM, and the metal in the cross section of the steel sheet is observed from the observed structure. Determined the chief of the organization.

[Total area ratio of martensite phase and cementite phase]
The area ratio Sms of the hard second phase (martensite phase) and cementite phase is determined by polishing the cross section parallel to the rolling direction of the sample cut out from the sample of the collected molten Zn-Al-Mg galvanized steel sheet and using a nital reagent. It was etched, observed in SEM, and calculated from the observed structure by image analysis.

For each of the molten Zn-Al-Mg-based plated steel sheets 1 to 24, the take-up temperature T1 in hot rolling under the manufacturing conditions, the annealing temperature T2 in the plating line, the main phase of the metal structure in the cross section, and the carbide in the cross section. Table 3 shows the average particle size R1 and the total area ratio Sms of the martensite phase and the cementite phase in the cross section. In Table 3, "BF" represents a bainitic ferrite phase, "F" represents a ferrite phase, and "P" represents a pearlite phase.

[Tensile characteristics]
Using JIS No. 5 test pieces collected so that the longitudinal direction of the sample of the collected molten Zn-Al-Mg-based plated steel sheet is perpendicular to the rolling direction of the material steel sheet, the yield strength YS and tensile strength are in accordance with JIS Z2241. TS and total elongation T.El were determined.

[Bending test]
Samples of 20 × 50 mm in the direction perpendicular to the rolling direction were taken from the hot-dip Zn—Al—Mg-based plated steel sheets 1 to 24 with a cold rolling ratio of 30% and 50%, and subjected to a 135 ° bending test. That is, bending is performed with a pressing force of 20 kN using a V-shaped punch and a die with a tip R1.0 mm and a tip angle of 45 ° so that the rolling direction is the axis of bending at the center of the sample sample in the longitudinal direction. The presence or absence of cracks on the outer surface of the tip of the bent portion was evaluated by ○ ×. "○" indicates that no crack has occurred, and "x" indicates that a crack has occurred.

[Evaluation of molten metal embrittlement crackability]
The embrittlement characteristics of the molten metal were evaluated by performing a welding test according to the following procedure.

A sample of 100 mm × 75 mm is cut out from a plated steel sheet having a cold rolling ratio of 30% and 50% in the molten Zn-Al-Mg-based plated steel sheets 1 to 24, and this is used to evaluate the maximum crack depth due to the embrittlement of the molten metal. It was used as a test piece. In the welding test, "boss welding" was performed to prepare the boss welded material having the appearance shown in FIG. 1, and the cross section of the welded portion was observed to investigate the occurrence of cracks. That is, a boss (protrusion) 1 made of steel bar (SS400 material specified in JIS) having a diameter of 20 mm and a length of 25 mm is vertically erected at the center of the plate surface of the test piece 3, and this boss 1 is arc-welded to the test piece 3. Welded at. YGW12 is used as the welding wire, and the welding bead 6 goes around the boss once from the welding start point, and even after the welding start point is passed, the welding is further advanced and the welding start point is passed to form the overlapping portion 8 of the welding bead. Welding was finished at that point. Welding conditions are 190A, 23V, welding speed 0.3m / min, shield gas: Ar-20vol. % CO ₂ and shield gas flow rate: 20 L / min.

At the time of welding, as shown in FIG. 2, a test piece 3 to which the test piece 3 was previously joined to the restraint plate 4 was used. For the joint, first prepare a restraint plate 4 (SS400 material specified in JIS) having a thickness of 120 mm × 95 mm × 4 mm, place the test piece 3 in the center of the plate surface, and then cover the entire circumference of the test piece 3. It is welded to the restraint plate 4. The above-mentioned boss welded material is produced by fixing this joint (test piece 3 and restraint plate 4) on a horizontal laboratory table 5 with a clamp 2 and performing boss welding in this state.

After boss welding, the joint body of the boss 1 / test piece 3 / restraint plate 4 is cut at the cut surface 9 passing through the central axis of the boss 1 and passing through the overlapping portion 8 of the beads, and the cut surface 9 is microscopically formed. Observation was performed, and the maximum crack depth observed in the test piece 3 was measured, and this was defined as the maximum crack depth of the base metal. This crack corresponds to a molten metal embrittlement crack. As the crackability (LMEC resistance) of the molten metal embrittlement property, those having a maximum base metal crack depth of 0.1 mm or less were evaluated as acceptable, and those exceeding 0.1 mm were evaluated as rejected. "○" indicates pass and "x" indicates failure.

The evaluation results are shown in Tables 4 and 5.

In the molten Zn-Al-Mg galvanized steel sheets 1 to 15 which are within the scope of the present invention, the high-strength molten Zn- which has a yield strength of 600 MPa or more and is capable of 135 ° bending with a tip bending R of 1.0 mm under any condition. An Al-Mg-based galvanized steel sheet can be obtained.

However, since the molten Zn-Al-Mg-based plated steel sheet 16 has a large amount of C and the molten Zn-Al-Mg-based plated steel sheet 17 has a low Ti amount and a low Ti / C ratio, the hard second phase + cementite area The rate Sms is high and the bending workability is inferior.

Since the molten Zn-Al-Mg-based plated steel sheet 18 has a large amount of Mn, it is inferior in bending workability due to the precipitation of sulfide. Since the molten Zn-Al-Mg-based plated steel sheet 19 has a low B content, sufficient yield strength is not obtained, and the LMEC resistance is inferior. Since the molten Zn-Al-Mg-based plated steel sheet 20 has a large amount of P, it is inferior in bending workability.

The molten Zn-Al-Mg-based plated steel sheet 21 has a low C amount, and therefore has a high Ti / C ratio, so that a sufficient yield strength cannot be obtained. Since the molten Zn-Al-Mg-based plated steel sheet 22 has a low Mn amount, sufficient yield strength cannot be obtained. Since the hot-rolled winding temperature T1 of the molten Zn-Al-Mg-based plated steel sheet 23 is high, the particle size of Ti carbide is large and the bending workability is inferior. The hot-dip Zn-Al-Mg-based plated steel sheet 24 is inferior in yield strength because the annealing temperature at the plating line is too high and the dislocation density is low.

[Examination of cold rolling rate (cold rolling rate)]
Using the steels A and F in Table 1, the slabs of these steels were heated to 1250 ° C. in the same manner as in the above examples, and then hot-rolled at a finish rolling temperature of 880 ° C. and a take-up temperature (T1) of 590 ° C. , A hot-rolled steel strip having a plate thickness of 2.6 mm was obtained. The obtained hot-rolled steel strip is pickled and cold-rolled at 2%, 4%, 50%, 60% and 70% cold rolling ratios, and then hydrogen-nitrogen mixed in a continuous hot-dip plating line. Annealing was carried out in gas at an annealing temperature (T2) of 620 or 630 ° C., and the material was cooled to about 420 ° C. at an average cooling rate of 5 ° C./sec to obtain material steel sheets 25 to 34 (plated original plate). Then, while the surface of the steel sheet is not exposed to the atmosphere, the steel sheet is immersed in a molten Zn-Al-Mg-based plating bath having the same plating bath composition as in the above example, pulled up, and the amount of plating adhered is determined by a gas wiping method. Fused Zn-Al-Mg-based plated steel sheets 25 to 34 adjusted to about 90 g / m ^{2 per side were obtained.} The plating bath temperature was about 410 ° C.

The tensile properties, bending workability, and melt metal embrittlement cracking resistance (LMEC resistance) of the obtained molten Zn-Al-Mg-based plated steel sheets 25 to 34 were measured and determined in the same manner as in the above-mentioned Examples and the like. bottom. The results are shown in Table 6.

As is clear from Table 6, when the cold rolling ratio is 4 to 60%, a yield strength of 600 MPa or more and good bending workability can be obtained, but when the cold rolling ratio is less than 4%, the yield strength is 600 MPa. It may be lower and the tensile strength may be insufficient. Further, if the cold rolling ratio exceeds 60%, good bending workability may not be obtained.

1 Boss 2 Clamp 3 Test piece 4 Restraint plate 5 Laboratory table 6 Welding bead 7 Welding bead on the entire circumference of the test piece 8 Overlapping part of the welding bead 9 Cut surface

Claims (6)

A molten Zn-Al-Mg alloy plated steel sheet having a molten Zn-Al-Mg alloy plating layer on the surface of the material steel sheet.
In the material steel plate, C is 0.005% or more and 0.08% or less, Si is 0.8% or less, Mn is 0.5% or more and 1.8% or less, P is 0.05% or less in terms of mass%. S is 0.005% or less, Ti is 0.02% or more and 0.2% or less, B is 0.0005% or more and 0.01% or less, and Al is 0.1% or less, and the balance is Fe and Consists of unavoidable impurities
The equivalent ratio of the Ti and C represented by the following formula (1) in the material steel sheet is 0.4 or more and 1.5 or less.
The material steel sheet contains a phase in which the metal structure in a cross section parallel to the rolling direction consists of one or both of a ferrite phase and a bainitic ferrite phase as a main phase, and further contains a martensite phase and a cementite phase, and is a martensite phase. And the total ratio of cementite phase is 3% or less in area ratio, and
The above-mentioned carbide containing Ti having an average particle size of 20 nm or less is dispersed and precipitated.
A hot-dip Zn-Al-Mg alloy-plated steel sheet having a yield strength of 600 N / mm ² or more and a ratio of the yield strength to the tensile strength of 80% or more.
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the content (mass%) of the element in the material steel sheet is substituted in place of the element symbol in the above formula (1). The molten Zn-Al-Mg alloy according to claim 1, wherein the material steel sheet further contains one or both of Nb of 0.1% or less and V of 0.1% or less in mass%. Plated steel plate. The molten Zn-Al-Mg alloy-plated steel sheet according to claim 1 or 2, wherein the content of N among the unavoidable impurities in the material steel sheet is 0.005% or less in mass%. The composition of the molten Zn-Al-Mg alloy plating layer is such that Al is 3.0% or more and 22.0% or less, Mg is 0.05% or more and 10.0% or less, Ti is 0.1% or less, and B is. The claim is characterized in that 0.0005% or more and 0.01% or less, Si is 0 or more and 2.0% or less, Fe is 2.0% or less, and the balance is Zn and unavoidable impurities. The molten Zn-Al-Mg alloy plated steel sheet according to any one of 1 to 3. Any of claims 1 to 4, wherein the difference (CH25) between the plate thickness at a position 25 mm from the edge of the material steel plate in the plate width direction and the plate thickness at the center of the plate width is ± 50 μm or less. The molten Zn-Al-Mg alloy plated steel sheet according to item 1. The method for producing a molten Zn-Al-Mg alloy-plated steel sheet according to any one of claims 1 to 5.
A hot rolling step of performing a hot rolling process on a slab having the above composition contained in the material steel sheet.
A pickling step of pickling a hot-rolled steel strip obtained by performing the hot-rolling treatment.
Cold rolling step of cold rolling the steel strip after the pickling treatment,
After continuous annealing a steel strip obtained after the cold rolling process includes a continuous hot dipping step of subjecting the surface of the steel strip a hot-dip Zn-Al-Mg alloy plating layer having a composition according to claim 4,
When the steel is wound in the hot rolling step, the winding temperature is 500 ° C. or higher and 650 ° C. or lower.
In the cold rolling step, cold rolling is performed on the steel strip after the pickling treatment at a cold rolling rate of 4% or more and 60% or less.
A method for producing a molten Zn—Al—Mg alloy plated steel sheet, wherein the annealing temperature, which is the temperature in the furnace used in the continuous annealing treatment, is 550 ° C. or higher and 750 ° C. or lower.

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