JP6801496B2 - High-strength molten Zn-Al-Mg-based plated steel sheet with excellent bending workability and its manufacturing method - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention is a molten Zn-Al having excellent bending workability with a tensile strength of 780 MPa or more, which is suitable for a member or a steel pipe material which is mainly used by being bent in an application requiring high corrosion resistance. -Mg-based plated steel sheet.

In recent years, there has been increasing interest in environmental issues, and various processed products, including automobile parts, are required to be lighter by increasing strength and thinning. Further, when various deformation styles such as press working and stretch flange machining are performed, the material steel sheet is required to have ductility and high hole widening property in addition to strength. Therefore, many inventions have been made in which the metal structure is precisely controlled by combining the addition of an expensive alloy element and a complicated heat treatment. Further, in these inventions, a high-strength rust-preventive steel sheet is often required from the viewpoint of extending the life and omitting post-plating.

Patent Documents 1 to 3 disclose high-strength cold-rolled steel sheets, plated steel sheets, and methods for producing the same, which are excellent in bending workability. However, in each case, the strength is increased by strengthening the transformation, and the retained austenite is utilized to achieve both high strength and workability, and it is necessary to add a large amount of expensive alloying elements such as Si and Mn. Therefore, the manufacturing cost is high. Further, in the transformation strengthening, it is very difficult to stably secure good bendability due to the large difference in strength between the hard phase and the soft phase.

Patent Document 4 discloses a cold-rolled steel sheet that does not utilize martensite or retained austenite, has a highly proportional source that utilizes fine precipitates and dislocation strengthening based on a ferrite structure, and is excellent in bending workability. However, it was found that the C content was high and the level of bendability was not always sufficient. The present inventors strengthen precipitation using fine precipitates such as Ti based on a ferrite or bainite structure without using martensite or retained austenite, and suppress the precipitation of coarse hard second phase and cementite. Patent Document 5 discloses a hot-rolled plated steel plate having improved hole expandability, which is an index of high strength and local ductility. However, in Patent Document 5, although very good bending workability can be obtained, sufficient strength cannot always be obtained.

JP-A-2009-270126 Japanese Unexamined Patent Publication No. 2013-117042 Japanese Unexamined Patent Publication No. 2015-193897 JP 2015-147959 International Publication No. 2015/093596

In view of the above-mentioned problems, the present invention is a molten Zn-Al-Mg system having a tensile strength of 780 MPa or more, suppressing an excessive increase in manufacturing cost, and simultaneously improving strength and bending workability, and having excellent corrosion resistance. An object of the present invention is to provide a plated steel sheet and a method for producing the same.

As a result of diligent studies, the present inventors have found that a plated steel sheet having the following structure can solve the above problems.

Specifically, in the present invention, the material steel sheet is based on mass%, C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0. 05% or less, S: 0.005% or less, N: 0.001 to 0.005%, Ti: 0.02 to 0.2%, B: 0.0005 to 0.010%, Al: 0.005 It contains ~ 0.1% or less, the balance consists of Fe and unavoidable impurities, the Ti / C equivalent ratio represented by the following equation (1) is 0.4 to 1.5, and the dislocation density is 1. The main phase is either a single phase of a bainitic ferrite phase or a ferrite phase, which is 8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , or a phase composed of a bainitic ferrite phase and a ferrite phase. Moreover, carbides containing Ti having an area ratio of the hard second phase and cementite of 3% or less and an average particle diameter of 20 nm or less are dispersed and precipitated, and the total elongation T.I. Provided is a high-strength molten Zn-Al-Mg-based galvanized steel sheet having an El of 7% to 15% and a tensile strength of 780 to 1100 MPa and excellent bending workability.

Further, in the relationship between Ti and C, it is a condition that the Ti / C equivalent ratio represented by the following equation (1) is controlled to 0.4 to 1.5.
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 formula (1).

The material steel sheet may further contain one or more of Nb: 0.1% or less and V: 0.1% or less in mass%.

Further, the plating composition is, for example, in mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, B: 0 to 0. It consists of 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and unavoidable impurities.

Further, as a method for producing the molten Zn-Al-Mg-based plated steel sheet, hot rolling, pickling, cold rolling, annealing in a continuous hot-dip plating line, and molten Zn-Al- on a material steel sheet having the above composition. The process of sequentially performing Mg-based plating is performed, the winding temperature in hot rolling is 500 ° C to 650 ° C, the cold rolling rate is 30% to 60%, and the annealing temperature in the continuous hot-dip plating line is 550 ° C to 750 ° C. And.

INDUSTRIAL APPLICABILITY The present invention can provide a molten Zn-Al-Mg-based galvanized steel sheet having a sufficient manufacturing cost, sufficient strength, and excellent bending workability, and a manufacturing method thereof. In particular, the molten Zn-Al-Mg-based plated steel sheet of the present invention has a tip R: 1.0 mm, can be bent by 135 °, and has excellent workability.

It is a perspective view explaining the shape of the boss welding test material. It is sectional drawing explaining the procedure for manufacturing a boss welding test material.

Hereinafter, the components, metallographic structure, and manufacturing method of the present invention will be described in detail. “%” In the steel composition and plating composition means “mass%” unless otherwise specified.

<C: 0.01 to 0.08%>
C is an element that forms a carbide containing Ti and is finely precipitated in bainitic ferrite or a ferrite structure, which is effective for increasing the strength. If the C content is less than 0.01%, it is difficult to obtain a strength of 780 MPa or more, and if it is added in excess of 0.08%, bending workability is caused by coarsening of the precipitate and formation of the hard second phase and cementite. Decreases. Further, it is preferably 0.01 to 0.06%, more preferably 0.01 to 0.04%.

<Si: 0.8% or less>
Si is an element effective for strengthening solid solution. However, if it is added in excess, an oxide is formed on the surface of the steel sheet during heating in the hot-dip galvanizing line, which hinders the plating property and increases the manufacturing cost. Therefore, the upper limit of the addition amount is set to 0.8%. Further, it is preferably 0.4% or less, more preferably 0.2% or less.

<Mn: 0.5 to 1.8%>
Mn is an element effective for increasing the strength. If it is less than 0.5%, it is difficult to obtain a strength of 780 MPa or more, and if it is added in excess of 1.8%, segregation is likely to occur and the bending workability is lowered. In addition, the manufacturing cost will increase. Therefore, the upper limit of the addition amount is set to 1.8%. Further, it is preferably 1.0 to 1.7%, more preferably 1.0 to 1.5%.

<P: 0.05% or less>
P is an element effective for strengthening the solid solution, but if it is added in an amount exceeding 0.05%, segregation is likely to occur and the bending workability is lowered. Therefore, the upper limit of the addition amount is set to 0.05%. Further, it is preferably 0.03% or less, more preferably 0.02% or less. The content of P does not include 0.

<S: 0.005% or less>
S forms sulfide with Mn and deteriorates local ductility including 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%. Further, 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.

<N: 0.001 to 0.005%>
When N remains as a solid solution N in steel, BN is generated, which leads to a decrease in the amount of B effective for embrittlement cracking resistance of molten metal. As a result of the examination, the N content is limited to 0.005% or less, but usually there is no problem even if N of about 0.001% is present. The range of N content is preferably 0.001 to 0.004%.

<Ti: 0.02-0.2%>
Ti is an element that binds to C and precipitates as a fine carbide of Ti, which is effective in increasing the strength and suppressing the precipitation of cementite. Further, since Ti has a high affinity with N and N in steel is fixed as TiN, adding Ti is extremely effective in securing the amount of B that enhances the embrittlement cracking resistance of the molten metal. In order to obtain these effects sufficiently, it is necessary to add 0.02% or more. On the other hand, even if it is added in excess of 0.2%, the effect is saturated and the manufacturing cost is increased. Therefore, it is limited to the range of 0.02 to 0.20%. The Ti content is preferably 0.05 to 0.20%, more preferably 0.08 to 0.20%.

<B: 0.0005 to 0.010%>
B is an element that segregates at the grain boundaries to increase the atomic bond force and is effective in suppressing embrittlement cracking of the molten metal. Further, B is an element that segregates at the grain boundaries to suppress transformation and is effective for increasing the strength through the bainitic ferrite structure. If it is less than 0.0005%, these effects are not obtained, and even if it is added in excess of 0.01%, the effect is saturated and the manufacturing cost is increased. Therefore, the addition range is limited to 0.0005% to 0.010%.

<Al: 0.005-0.1% or less>
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.

<V: 1.0% or less, Nb: 0.1% or less, one or more types>
Nb and V prevent coarsening of γ grains during heating and hot spreading, and are effective for refining ferrite grains. Further, like Ti, it forms a composite carbide containing C and contributes to an increase in strength. Therefore, one or more of these elements can be contained as needed.

<Ti / C equivalent ratio: 0.4 to 1.5>
The Ti / C equivalent ratio is an important value for improving bending workability. The Ti / C equivalent ratio is defined by Eq. (1).
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 formula (1).

When the Ti / C equivalent ratio is less than 0.4, the amount of the hard second phase and cementite increases, so that the bending workability decreases. On the other hand, even if the Ti / C equivalent ratio exceeds 1.5 and is manually added, the effect is saturated and the manufacturing cost is increased. Therefore, it is limited to the range of 0.4 to 1.5.

<Tensile strength>
The present invention relates to a high-strength steel sheet used for structural members for construction, automobile parts, and the like, and is intended for a steel sheet having a tensile strength of 780 MPa or more. However, if the tensile strength exceeds 1100 MPa, cracks occur at 135 ° bending. Therefore, the range of tensile strength is defined in the range of 780 to 1100 MPa.

<Metal structure>
The microstructure of the high-strength molten Zn-Al-Mg-based galvanized steel sheet according to the present invention is a bainitic ferrite phase having a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ^2. A single phase of the ferrite phase or a phase containing a bainitic ferrite phase and a ferrite phase is used as the main phase, and Ti having an area ratio of the hard second phase and cementite of 3% or less and an average particle diameter of 20 nm or less is used. Carbides contained are dispersed and precipitated. These will be described below.

Either single phase of bainitic ferrite or ferrite having a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² or either single phase of bainitic ferrite phase or ferrite phase or The main phase is a bainitic ferrite phase and a phase containing a ferrite phase, and the area ratio of the hard second phase and cementite is 3% or less, which achieves both a tensile strength of 780 MPa or more and good bending workability. Because.
That is, by forming a ferrite and / or bainite structure having a hard second phase and cementite area ratio of 3% or less, good bending workability without cracking can be obtained by 135 ° bending at a tip R: 1.0 mm. By setting the dislocation density to 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , it is possible to secure a tensile strength of 780 MPa or more. Cementite is prone to microcracks at the interface with the ferrite phase or bainite phase during bending, and becomes the starting point of bending cracks, so that bending workability is greatly reduced. Since the area ratio can be up to 3%, the upper limit is set to 3% or less.
The "main phase" means the remaining phase excluding the hard second phase and cementite in the metal structure of the steel sheet of the present invention.

The reason why the average particle size of the carbide containing Ti is set to 20 nm or less is that the carbide containing Ti precipitates during hot rolling, and the strength is increased by the precipitation strengthening action. Further, fine precipitation is effective for improving bending workability. As a result of various studies, it is extremely effective that the average particle size of the carbides dispersed in the bainitic ferrite and / or the ferrite phase is 20 nm or less, and if it exceeds 20 nm, good bending workability cannot be obtained. The carbide containing Ti also includes carbides such as Nb and V.

-Manufacturing method The high-strength hot-dip Zn-Al-Mg-based plated steel plate with excellent workability is, for example, hot-rolled, pickled, cold-rolled, or continuously hot-dip galvanized on a steel material whose composition has been adjusted (continuously cast slab, etc.) It can be manufactured by a process of sequentially performing annealing on a line and hot-dip Zn-Al-Mg plating. Hereinafter, the manufacturing conditions in that case will be illustrated.

A steel slab satisfying the above composition is heated at a heating temperature of 1150 to 1300 ° C., hot rolled at a finishing temperature of 850 to 950 ° C., and then wound at the following winding temperature. After that, a hot-rolled steel strip is obtained at the following winding temperature. Further, after pickling this steel strip, it is cold-rolled under the following conditions and subjected to a plating process on a continuous hot-dip galvanizing line.

<Take-up temperature for hot rolling from 500 ° C to 650 ° C>
If the winding temperature is less than 500 ° C., the amount of carbides containing Ti is insufficiently precipitated and the strength is lowered. On the other hand, when the winding temperature exceeds 650 ° C., the carbide containing Ti is coarsened, and the strength and bending workability are lowered.

<Cold rolling rate: 30-60%>
After hot rolling, a continuous pickling line is passed through the plate to remove surface scale, and cold rolling is performed. At that time, if the rolling ratio of cold rolling is less than 30%, the dislocation density after annealing in the continuous hot dip galvanizing line becomes less than 1.8 × 10 ¹⁴ / m ^2, and the tensile strength of 780 MPa or more may not be obtained. is there. On the other hand, if it exceeds 60%, the dislocation density exceeds 5.7 × 10 ¹⁴ / m ² and the decrease in ductility becomes large, and the bending workability may deteriorate. Therefore, the cold rolling ratio is preferably in the range of 30% to 50% or less.

<Annealing temperature in continuous hot dip galvanizing line: 550 to 750 ° C>
If the annealing temperature is less than 550 ° C., the surface of the steel sheet is not sufficiently reduced and the plating property is deteriorated. On the other hand, when the annealing temperature exceeds 750 ° C., recrystallization occurs and the dislocation density becomes less than 1.8 × 10 ¹⁴ / m ² , which causes a decrease in strength. That is, the present invention is characterized in that it is annealed at a temperature equal to or lower than recrystallization annealing to maintain a high dislocation density, and the metal structure of the base metal is based on the structure at the time after the completion of hot spreading. ..

<Fused Zn-Al-Mg plating>
In the present invention, a known method of molten Zn-Al-Mg plating can be applied.
Al in the plating layer has an action of improving the corrosion resistance of the plated steel sheet. In addition, the inclusion of Al in the plating bath also has the effect of suppressing the generation of Mg oxide-based dross. In order to sufficiently obtain these effects, the Al content of the hot dip galvanizing 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 plating adhesion 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 in the plating layer has an action of generating a uniform corrosion product on the surface of the plating layer and remarkably enhancing the corrosion resistance of the plated steel sheet. In order to fully exert its action, the Mg content of hot dip galvanizing 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%, the harmful effect that Mg oxide-based dross is likely to occur becomes large. 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 Ti and B are contained in the hot-dip galvanizing bath, the formation and growth of Zn ₁₁ Mg _two- phase, which gives a speckled appearance defect in the hot-dip Zn-Al-Mg-based plated steel sheet, is suppressed. Even if Ti and B are contained alone, the effect of suppressing the Zn ₁₁ Mg _two- phase is produced, but it is desirable that Ti and B are contained in a composite manner in order to greatly reduce the degree of freedom in the production conditions. In order to sufficiently obtain these effects, it is effective that the Ti content of the hot dip galvanizing 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, and unevenness called “butsu” is generated in the plating layer, which impairs the appearance. Therefore, when Ti is added to the plating bath, 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, and irregularities called "butsu" are also generated, which impairs the appearance. Therefore, when B is added to the plating bath, 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 galvanizing bath, the growth of the Fe—Al alloy layer is suppressed, and the workability of the hot-dip Zn—Al—Mg-based plated steel sheet is improved. Further, Si in the plating layer is effective in preventing the blackening of the plating layer 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 hot dip galvanizing to 0.005% or more. However, if Si is added in excess, the amount of dross in the hot-dip galvanizing bath increases. Therefore, when Si is contained in the plating bath, the content is set to 2.0% or less.

A certain amount of Fe is mixed in the hot-dip galvanizing bath from the material steel plate and the pot constituent members. In Zn-Al-Mg-based plating, Fe in the plating bath is allowed to be contained up to about 2.0%. For example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr, and Mn may be mixed in the plating bath as other elements, but the total content thereof. Is preferably 1% by mass or less. The hot-dip galvanized bath composition is reflected in the plating layer composition of the hot-dip galvanized steel sheet almost as it is.

[Example 1]
Each steel whose composition is shown in Table 1 is melted, the slab is heated to 1250 ° C., hot-rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 520 to 680 ° C., and hot-rolled with a plate thickness of 2.6 mm. Obtained a steel strip. The winding temperature of each hot-rolled steel strip is shown in Table 2.

The hot-rolled steel strip is pickled and cold-rolled at 30% and 50% cold-rolling rates, and then annealed in a hydrogen-nitrogen mixed gas at 500 to 790 ° C. on a continuous hot-dip plating line to approximately 420. In a molten Zn-Al-Mg-based plating bath having the following plating bath composition, the material steel plate (plating original plate) is obtained by cooling to ℃ at an average cooling rate of 5 ° C./sec, and then the steel plate surface is not exposed to the atmosphere. A molten Zn-Al-Mg-based plated steel sheet was obtained in which the amount of plating adhered to about 90 g / m ² per side was adjusted by a gas wiping method. The plating bath temperature was about 410 ° C. The cold spreading ratio and annealing temperature of each steel are also shown in Table 2.

[Plating bath composition (mass%)]
Al: 6.0%, Mg: 3.0%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: balance

[Average particle size of Ti-containing carbide]
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 size (major axis) of the carbide in a certain region containing 30 or more Ti-containing carbides was observed. Was measured, and the average value was taken as the average particle size of the Ti-containing carbide.

[Dislocation density]
After mechanical polishing from the surface layer of the sample cut out from the collected molten Zn-Al-Mg plated steel sheet sample to 1/4 of the plate thickness, chemical polishing is performed to remove processing strain, and the dislocation density is calculated by X-ray analysis. Obtained by. For X-ray diffraction, Kα1 line of a Co tube is used, the local strain η is obtained from the half-value widths of the three diffraction peaks <110>, <211>, and <220>, and the dislocation density is calculated using the following equation. I calculated.
ρ = 14.4 × η ² / b ²
Here, ρ is the dislocation density and b is the Burgers vector (0.25 nm). The modified Williamson-Hall / Warren-Averbach method was used to calculate the dislocation density. The calculated dislocation densities are also shown in Table 2.

[Area ratio of cementite]
The area ratios of the hard second phase and cementite were observed by polishing the sample cut out from the collected molten Zn-Al-Mg galvanized steel sheet sample in the rolling direction cross section, etching it with a picral reagent, and observing it by SEM. It was calculated from the image analysis. The measured area ratios of the hard second phase and cementite are also shown in Table 2.

[Tensile characteristics]
Using a JIS No. 5 test piece collected so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the material steel sheet, the tensile strength TS and the total elongation T.El were determined in accordance with JISZ2241.

[Bending workability]
A sample of 20 × 50 mm was taken from the molten Zn-Al-Mg-based plated steel sheet in the direction perpendicular to the rolling direction, and this was 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 ○ ×.

[Evaluation of molten metal embrittlement crackability]
The embrittlement characteristics of molten metal were evaluated by performing a welding test according to the following procedure.
A 100 mm × 75 mm sample was cut out from a molten Zn-Al-Mg-based plated steel sheet, and this was used as a test piece for evaluating the maximum crack depth due to embrittlement of the molten metal. 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. In the above-mentioned production of the boss welded material, the joint body (test piece 3 and restraint plate 4) is fixed on a horizontal laboratory table 5 with a clamp 2 and boss welded 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 microscopic. 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. Those with a maximum cracking depth of 0.1 mm or less were evaluated as acceptable, and those with a maximum crack depth of more than 0.1 mm were evaluated as rejected.

No. which is the scope of the present invention. Dislocation densities 1 to 15 are 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , tensile strength is 780 to 1100 MPa, and tip bending R1.0 mm can be bent by 135 °. It is a high-strength molten Zn-Al-Mg-based plated steel sheet.
However, No. In No. 16, the amount of C is large, and in No. 17, the amount of Ti is low and the Ti / C equivalent ratio is low, so that the hard second phase + cementite area ratio is high and the bending workability is inferior. No. Since 18 has a large amount of Mn, it is inferior in bending workability due to precipitation of sulfide. No. Since B is low in No. 19, sufficient tensile strength is not obtained, and LMEC resistance is inferior. No. Since 20 has a large amount of P, it is inferior in bending workability. No. No. 21 has a low amount of C, and sufficient tensile strength cannot be obtained. No. Since the amount of Mn of No. 22 is low, sufficient tensile strength is not obtained. No. In No. 23, since the winding temperature in hot rolling is high, the particle size of Ti carbide is large and the bending workability is inferior. No. In No. 24, the annealing temperature at the plating line is too high, the dislocation density is low, and the tensile strength is inferior.

[Example 2]
The steels A and F in Table 1 were heated to 1250 ° C. in the same manner as in Example 1, and then hot-rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 590 ° C., and hot-rolled steel having a plate thickness of 2.6 mm. I got a band. The obtained hot-rolled steel strip is pickled and cold-rolled at 10%, 30%, 50%, 60% and 70% cold rolling ratios, and then hydrogen-nitrogen mixed in a continuous hot-dip plating line. It is annealed in gas at 620 and 630 ° C. and cooled to about 420 ° C. at an average cooling rate of 5 ° C./sec to obtain a material steel sheet (plated original plate). Fused Zn-Al-Mg-based plated steel sheet that has the same plating bath composition, is immersed in a molten Zn-Al-Mg-based plating bath, pulled up, and the amount of plating adhesion is adjusted to about 90 g / m ² per side by the gas wiping method. Got The plating bath temperature was about 410 ° C.

The tensile properties, bending workability, and melt metal embrittlement cracking resistance of the obtained plated steel sheet were investigated and summarized in Table 3.
From Table 3, when the cold spreading ratio is 30 to 60%, a tensile strength of 780 to 1100 MPa and good bending workability can be obtained, but when the cold spreading ratio is less than 30%, the tensile strength may be insufficient. .. Further, it can be seen that if the cold spreading 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 (5)

In a plated steel sheet having a molten Zn-Al-Mg based plating layer on the surface of the material steel sheet, the material steel sheet is C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0 in mass%. .5 to 1.8%, P: 0.05% or less, S: 0.005% or less, N: 0.001 to 0.005%, Ti: 0.02 to 0.2%, B: 0. It contains 0005 to 0.010%, Al: 0.005 to 0.1% or less, the balance is Fe and unavoidable impurities, and the Ti / C equivalent ratio represented by the following equation (1) is 0.4. Either a single phase of a bainitic ferrite phase or a ferrite phase or a bainitic ferrite having a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ^2. Carbides containing Ti having an area ratio of the hard second phase and cementite of 3% or less and an average particle diameter of 20 nm or less are dispersed and precipitated with a phase composed of a phase and a ferrite phase as the main phase . A high-strength hot-dip Zn-Al-Mg-based galvanized steel sheet having an El of 7% to 15% and a tensile strength of 780 to 1100 MPa and excellent bending workability.
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 formula (1). The bending according to claim 1, wherein the material steel sheet further contains at least one of Nb: 0.1% or less and V: 0.1% or less in mass%, and has a tensile strength of 780 to 1100 MPa. High-strength molten Zn-Al-Mg-based galvanized steel sheet with excellent workability. The plating composition of the molten Zn-Al-Mg-based galvanized steel sheet is, in mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, The tensile strength according to claim 1 or 2, wherein B: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and unavoidable impurities. A high-strength molten Zn-Al-Mg-based galvanized steel sheet having excellent bending workability of 780 to 1100 MPa. The material steel plate is C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.005% or less, N: 0.001 to 0.005%, Ti: 0.02 to 0.2%, B: 0.0005 to 0.010%, Al: 0.005 to 0.1% or less, and the balance is Hot rolling, pickling, cold rolling, continuous melting on a steel material consisting of Fe and unavoidable impurities and having a Ti / C equivalent ratio of 0.4 to 1.5 represented by the following equation (1). In the process of sequential annealing and hot-dip Zn-Al-Mg-based plating on the plating line, the winding temperature in hot rolling is 500 ° C to 650 ° C, the cold rolling rate is 30% to 60%, and the continuous hot rolling line. A method for producing a high-strength molten Zn-Al-Mg-based plated steel sheet having an annealing temperature of 550 ° C. to 750 ° C. and an excellent bending workability with a tensile strength of 780 to 1100 MPa .
The fused Zn-Al-Mg-based plated steel sheet has a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , whichever is a single phase of a bainitic ferrite phase or a ferrite phase. Carbides containing Ti having a bainitic ferrite phase and a ferrite phase as the main phase, the hard second phase and cementite having an area ratio of 3% or less, and an average particle diameter of 20 nm or less are dispersed and precipitated. Full growth T. The production method, wherein El is 7% to 15%.
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 formula (1). The tensile strength according to claim 4, wherein the material steel sheet further contains one or more of Nb: 0.1% or less and V: 0.1% or less in mass% . A method for manufacturing a high-strength molten Zn-Al-Mg-based galvanized steel sheet having excellent bending workability.

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