JP2018131669A - BLACK SURFACE-COATED HIGH-STRENGTH MOLTEN Zn-Al-Mg-BASED PLATED STEEL SHEET EXCELLENT IN BENDING PROCESSABILITY AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molten Zn-Al-Mg-based plated steel sheet that has higher tensile strength than the conventional molten Zn-Al-Mg-based plated steel sheet and realizes an improvement in both strength and bending processability at the same time while suppressing an excessive increase in manufacturing cost and to provide a method for manufacturing the plated steel sheet.SOLUTION: There is provided a black surface-coated high-strength molten Zn-Al-Mg-based plated steel sheet excellent in bending processability with a tensile strength of 780 to 1,100 MPa, which is a plated steel sheet having a molten Zn-Al-Mg-based coating layer on the surface of a steel sheet stock having a predetermined chemical composition; comprises as the main phase a single phase composed of either of a bainitic ferrite phase and a ferrite phase each having a dislocation density of 1.8×10/mto 5.7×10/mor a phase including the bainitic ferrite phase and the ferrite phase, has an area ratio of a hard second phase and cementite of 3% or less, has a carbide including Ti with an average particle diameter of 20 nm or less dispersed and precipitated and has a diffusible hydrogen concentration of 0.30 ppm or less; and has a black oxide of Zn distributed in a molten Zn-Al-Mg-based coating layer having a specific composition and has a lightness Lof the coating layer surface of 60 or less. There is also provided a method for manufacturing the black surface-coated high-strength molten Zn-Al-Mg-based plated steel sheet.SELECTED DRAWING: None

Description

  The present invention is an application where high corrosion resistance is required, and is suitable for a member that is mainly subjected to bending work and a material for a steel pipe, and has a high black surface coating with excellent bending workability with a tensile strength of 780 MPa or more. The present invention relates to a high strength Zn—Al—Mg based plated steel sheet.

  In recent years, interest in environmental problems has further increased, and various processed products such as automobile members have been required to be lightened by high strength-thinning. In addition, when various deformation modes such as press working and stretch flange processing are performed, the material steel plate is required to have ductility, high hole expansibility, etc. in addition to strength. Therefore, many inventions have been made in which the metal structure is precisely controlled by combining complicated heat treatment in addition to the addition of expensive alloy elements. Further, in these inventions, a high-strength rust-proof steel sheet is often required from the standpoint of extending the life and post-plating.

  Patent Documents 1 to 3 disclose a high-strength cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof that are excellent in bending workability. However, both are strengthened by transformation strengthening and use of retained austenite to achieve both high strength and workability. It is necessary to add a large amount of expensive alloy elements such as Si and Mn. Therefore, the manufacturing cost becomes high. Further, in transformation strengthening, it is very difficult to ensure stable and good bendability due to a large difference in strength between the hard phase and the soft phase. Patent Document 4 discloses a cold-rolled steel sheet that does not use martensite or retained austenite, is a high-proportional source that uses fine precipitates and dislocation strengthening based on a ferrite structure, and is excellent in bending workability. However, it has been found that the C content is high and the level of bending workability is not always sufficient.

  On the other hand, the present inventors strengthened precipitation using fine precipitates such as Ti based on ferrite or bainite structure without using martensite or retained austenite, and suppressed precipitation of coarse hard second phase and cementite. By doing so, Patent Document 5 discloses a hot-rolled plated steel sheet that has improved hole expandability, which is an indicator of high strength and local ductility. However, although very good bending workability is obtained in Patent Document 5, sufficient strength is not always obtained.

  Furthermore, with this type of plated steel sheet, when high-strength steel is used as the plating base plate, so-called hydrogen embrittlement is likely to occur due to hydrogen inevitably entering the steel in the plating line, which may be a problem depending on the application. Become. In a general hot-dip Zn plating line, a base steel plate that is a plating original plate is subjected to heat treatment in a reducing atmosphere containing hydrogen gas immediately before the plating bath. Hydrogen in this heated atmosphere enters the base steel plate and causes hydrogen embrittlement. In addition, intrusion of hydrogen is conceivable even in a wet process such as electrolytic degreasing performed before plating, which may cause hydrogen embrittlement of the plated steel sheet.

  Hydrogen embrittlement of a hot-dip Zn—Al—Mg-based plated steel sheet is usually considered to be a problem when a high-tensile steel of 980 MPa class or higher is used as a plating original sheet. However, even if a high strength steel having a relatively low strength level such as 780 MPa class or 590 MPa class is used, brittle fracture may occur if extremely severe processing is performed. According to detailed investigations by the inventors, it has been found that this kind of brittle fracture is also an event caused by hydrogen entering the plating line. Therefore, in order to improve the reliability level with respect to the processing of the high-strength steel sheet subjected to hot-dip Zn—Al—Mg plating, it is desired to establish a technique for suppressing hydrogen embrittlement of the steel sheet.

  As a technique for countermeasures against hydrogen embrittlement of steel sheets, Patent Document 6 discloses a technique for suppressing the hydrogen generated by the corrosion reaction in the atmospheric environment from entering the steel sheets by optimizing the chemical composition and metal structure of the steel. Is disclosed. Patent Document 7 discloses a technique for suppressing hydrogen embrittlement caused by hydrogen entering from the environment by reducing microsegregation of Mn at a position deeper than the surface pitting depth. These techniques are measures against hydrogen intrusion when the steel sheet is used in a corrosive environment, and are not effective against hydrogen that has already infiltrated in the hot dipping line.

  Baking treatment is known as a treatment for releasing hydrogen that has entered the steel material to the outside of the steel material. The baking process is a process in which hydrogen that has intruded into hydrogen is heated at a temperature of about 200 ° C. to diffuse the hydrogen that has intruded into the steel and expelled from the surface of the steel. However, the baking treatment generally tends to cause discoloration due to oxidation on the surface of the steel material (the surface of the plating layer in the case of steel material after plating). In a reducing atmosphere using hydrogen, it is difficult to remove hydrogen in the steel. Therefore, to completely prevent discoloration during baking, treatment in a vacuum furnace is required. Since such a process causes an increase in cost, there is a practical aspect as a process for a high-strength part after processing, but it is difficult to adopt it for a plated steel sheet as a processing material. In particular, in the case of a steel plate, uneven surface discoloration is easily noticeable. For this reason, it is generally not easy to realize a steel sheet material having excellent design properties by baking treatment.

  On the other hand, Patent Document 8 discloses a technique of forming a black film resulting from a black oxide of Zn by heating in a steam atmosphere as a post-treatment of a molten Zn—Al—Mg-based steel sheet. However, an example in which high-strength steel is applied to a plating original sheet is not shown.

JP 2009-270126 A JP 2013-117042 A Japanese Patent Laying-Open No. 2015-193897 JP2015-147959A International Publication No. 2015/093596 Japanese Patent Laid-Open No. 7-150241 JP 2012-172247 A Japanese Patent No. 5097305

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

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

Specifically, the present invention is a plated steel plate having a Zn—Al—Mg-based coating layer on the surface of the material steel plate, and the material steel plate is in 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 to 0.1% or less, with the balance being Fe and inevitable impurities, and a dislocation density of 1.8 × 10 ¹⁴ it ^{/ m 2 ~5.7 × 10 14 /} m 2 or more, phases and main phase comprising either single-phase or bainitic ferrite phase and the ferrite phase of bainitic ferrite phase or a ferrite phase, the hard second The area ratio of the two-phase and cementite is 3% or less, and the average particle diameter is 20 nm or less. And a diffusible hydrogen concentration of 0.30 ppm or less, and the Zn-Al-Mg-based coating layer is in mass%, Al: 1.0-22.0%, Mg : 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.10%, B: 0 to 0.05%, Fe: 2.0% or less, remaining Zn and inevitable It consists of impurities, Zn black oxide is distributed in the Zn-Al-Mg-based coating layer, the surface brightness L ^* is 60 or less, the tensile strength is 780 to 1100 MPa, and bending workability is improved. An excellent black surface-coated high-strength molten Zn-Al-Mg-based plated steel sheet is provided.

Furthermore, in the relationship between T and C, the Ti / C equivalent ratio represented by the following formula (1) is controlled to be 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 plate is substituted for the element symbol in the formula (1).

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

  The plated steel sheet may further have an inorganic coating or an organic coating on the surface of the Zn—Al—Mg coating layer.

  The method for producing the hot-dip Zn-Al-Mg-based plated steel sheet includes, for example, hot rolling, pickling, cold rolling, annealing in a continuous hot-dip plating line, hot-melt Zn-Al-Mg Including sequentially performing system plating and baking.

The coiling temperature in the said hot rolling shall be 500 to 650 degreeC. The cold rolling rate in the cold rolling is set to 30% to 60%.
Thereafter, the raw steel plate is heated at an annealing temperature of 550 ° C. to 750 ° C. in a continuous hot dipping line, and then immersed in a hot dipping bath. In this case, the plating composition of the plating bath is, in mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0 .10%, B: 0 to 0.05%, Fe: 2.0% or less, the balance being made of Zn and inevitable impurities.

  In the baking treatment, the plated material steel plate is heated and held at 70 to 250 ° C. in a water vapor atmosphere, and the surface of the plating layer is brought into contact with water vapor so that the diffusible hydrogen concentration in the material steel plate is 0.30 ppm. Reduce to:

  INDUSTRIAL APPLICABILITY The present invention can provide a hot-dip Zn—Al—Mg-based plated steel sheet having a low manufacturing cost, sufficient strength, and excellent bending workability, and a manufacturing method thereof. In particular, the hot-dip Zn—Al—Mg-based plated steel sheet of the present invention can be bent at a tip R of 0.5 mm and 135 ° and has excellent workability.

It is a perspective view explaining the shape of a boss welding test material. It is sectional drawing explaining the procedure which produces a boss | hub weld test material. It is the figure which showed an example of the hot dipping process of the manufacturing method of the hot-dip Zn-Al-Mg type plated steel plate of this invention.

  Hereinafter, the components, metal structure and production 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 forms an carbide containing Ti, finely precipitates in bainitic ferrite or ferrite structure, and is an element 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 added over 0.08%, bending workability decreases due to coarse precipitates and formation of cementite. Moreover, Preferably it is 0.01 to 0.06%, More preferably, it is 0.01 to 0.04%.

<Si: 0.8% or less>
Si is an element effective for solid solution strengthening. However, if added excessively, an oxide is formed on the surface of the steel sheet during heating in the hot dipping line, impairing the plateability and increasing the manufacturing cost, so the upper limit of the addition amount is set to 0.8%. Moreover, Preferably it is 0.4% or less, More preferably, it is 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. If it is added in excess of 1.8%, segregation tends to occur and bending workability is lowered. In addition, the manufacturing cost increases. Therefore, the upper limit of the addition amount is 1.8%. Moreover, Preferably it is 1.0 to 1.7%, More preferably, it is 1.0 to 1.5%.

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

<S: 0.005% or less>
S forms sulfide with Mn and degrades local ductility including bending workability. For this reason, although S is an element which should be reduced as much as possible, up to 0.005% is acceptable, so the upper limit of the content is limited to 0.005%. Moreover, Preferably it is 0.003% or less, More preferably, it is 0.002% or less. Note that S is an inevitable impurity, and its content does not include zero.

<N: 0.001 to 0.005%>
When N remains as solid solution N in the steel, BN is generated, leading to a decrease in the amount of B effective for resistance to molten metal embrittlement cracking. As a result of the examination, the N content is limited to 0.005% or less, but normally 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 to 0.2%>
Ti combines with C and precipitates as fine Ti carbide, and is an effective element for increasing the strength and suppressing the precipitation of cementite. Further, Ti has a high affinity with N and fixes N in the steel as TiN. Therefore, the addition of Ti is extremely effective in securing an amount of B that increases the resistance to molten metal embrittlement cracking. In order to obtain these effects sufficiently, addition of 0.02% or more is necessary. On the other hand, even if added over 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%, and 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 interatomic bonding force and is effective in suppressing molten metal embrittlement cracking. B is an element that segregates at the grain boundary 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 added over 0.01%, the effects are saturated and the manufacturing cost is increased. Therefore, the addition range is limited to 0.0005% to 0.010%.

<Al: 0.005 to 0.1% or less>
Al is added as a deoxidizer during steelmaking. In order to obtain the effect, addition of 0.005% or more is necessary. On the other hand, even if added over 0.1%, the effect is saturated and, on the contrary, the manufacturing cost is increased.

<One or more of V: 1.0% or less, Nb: 0.1% or less>
Nb and V prevent the coarsening of γ grains during heating and hot rolling, and are effective in making ferrite grains fine. Further, similarly to Ti, a composite carbide containing C is formed, which contributes to an increase in strength. For this reason, it can contain 1 or more types of these elements as needed.

<Ti / C equivalent ratio: 0.4 to 1.5>
The Ti / C equivalent ratio is an important value for improving the bending workability. The Ti / C equivalent ratio is defined by the formula (1).
Ti / C equivalent ratio = (Ti / 48) / (C / 12) (1)
However, the content (mass%) of the element in the material steel plate is substituted for the element symbol in the formula (1).

  When the Ti / C equivalent ratio is less than 0.4, the amount of 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, the effect is saturated and the manufacturing cost is increased. Therefore, it limits to the range of 0.4-1.5.

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

<Metallic structure>
The microstructure of the high-strength molten Zn—Al—Mg-based plated steel sheet according to the present invention has a bainitic ferrite phase having a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² or A single phase of a ferrite phase or a phase including a bainitic ferrite phase and a ferrite phase is used as a main phase, and the area ratio of the hard second phase and cementite is 3% or less, and the average particle diameter is 20 nm or less. It is assumed that the carbide containing contains is dispersed and precipitated. Hereinafter, these will be described.

A bainitic ferrite phase having a dislocation density of 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² or a single phase of a ferrite phase, or a phase including a bainitic ferrite phase and a ferrite phase The reason why the area ratio of the hard second phase and cementite is 3% or less is to make the tensile strength of 780 MPa or more and good bending workability compatible.

That is, by setting the area ratio of the hard second phase and cementite to 3% or less of ferrite and / or bainite structure, good bending workability that does not cause cracking at 135 ° bending of the tip R: 0.5 mm is obtained, By setting the dislocation density to 1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , a tensile strength of 780 MPa or more can be secured. Cementite is liable to cause a smiling crack at the interface with the ferrite phase or bainite phase during bending, and the bending workability is greatly reduced because it becomes the starting point of bending cracking. Since an area ratio of up to 3% is acceptable, the upper limit was made 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 20 nm or less is that the carbide containing Ti precipitates during hot rolling, and the strength increases due to the precipitation strengthening action. In addition, fine precipitation is effective for improving the bending workability. As a result of various studies, it is extremely effective that the average particle size of the carbide dispersed in the bainitic ferrite phase 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 includes carbides such as Nb and V.

<Zn-Al-Mg-based coating layer>
It is necessary to have a Zn—Al—Mg coating layer on the surface of the base steel sheet having the above chemical composition. The coating layer has a structure in which a part of Zn in a plating layer formed by hot-dip Zn—Al—Mg plating is distributed as a black oxide. This black oxide is generated by a baking process described later. In this specification, a coating layer derived from a molten Zn—Al—Mg-based plating layer containing a black oxide generated by baking is referred to as a “Zn—Al—Mg-based coating layer”.

  This coating layer has a structure in which a part of the Zn phase existing in the molten Zn—Al—Mg-based plating layer is oxidized, but its chemical composition is determined by the composition ratio of metal elements. The composition of the original molten Zn—Al—Mg-based plating layer is substantially maintained. As the original molten Zn—Al—Mg-based plating layer itself, those conventionally used for highly corrosion-resistant Zn-based plated steel sheets can be applied. That is, the chemical composition of the Zn—Al—Mg-based coating layer is such that the composition ratio of metal elements is mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 -2.0%, Ti: 0-0.10%, B: 0-0.05%, Fe: 2.0% or less, the remainder Zn and inevitable impurities. It was confirmed that even if a part of Zn was changed to black oxide, an excellent rust prevention effect was obtained in the above composition range.

  In order to sufficiently obtain this rust prevention effect over a long period of time, the average thickness of the Zn—Al—Mg-based coating layer is preferably 3 μm or more. Moreover, the average thickness of the Zn—Al—Mg-based coating layer may be in the range of 100 μm or less. Forming an excessively thick film is uneconomical and leads to a decrease in workability of the coating layer itself. Here, the average thickness of the coating layer can be obtained by observing a cross section parallel to the plate thickness direction.

The black oxide of Zn in the Zn—Al—Mg-based coating layer is generated when the surface of the molten Zn—Al—Mg-based plating layer comes into contact with water vapor, which is an atmospheric gas, during the baking process described later. Accordingly, the black oxide of Zn is distributed in a relatively large amount in the upper layer portion of the Zn—Al—Mg-based coating layer, and exhibits an effect of giving a black-like surface appearance. As a result of various studies, when a Zn black oxide having a surface lightness L ^{* of} the Zn-Al-Mg-based coating layer of 60 or less is formed, it is excellent in design that the discoloration unevenness associated with the baking treatment is less noticeable. It was found to have a black appearance. When the brightness L ^* is adjusted to be 40 or less, a deeper black appearance is exhibited. The black appearance due to the black oxide of Zn can be realized within the baking treatment condition range for reducing the diffusible hydrogen concentration in the steel to 0.30 ppm or less.

<Diffusion hydrogen concentration in base steel plate>
The hydrogen concentration in the base steel sheet that causes hydrogen embrittlement can be evaluated by measuring the diffusible hydrogen concentration. The diffusible hydrogen concentration can be determined by measuring the amount of hydrogen released when heated from room temperature to 300 ° C. at a heating rate of 5 ° C./min with an atmospheric pressure ionization mass spectrometer. As a measurement sample, a sample composed only of a base steel plate from which a Zn—Al—Mg coating layer is removed with abrasive paper can be used.

  Usually, in the case of a hot-dip Zn—Al—Mg-based plated steel sheet manufactured by a continuous hot dipping line using high-strength steel having the above composition range as a plating original sheet, the diffusible hydrogen concentration in the base steel sheet before baking is 0. It becomes 35 ppm or more. According to the study by the inventors, when the diffusible hydrogen concentration in the base steel sheet is reduced to 0.30 ppm or less by baking treatment, a molten Zn—Al—Mg system using a high-tensile steel of 980 MPa class or higher as the base steel sheet Not only the phenomenon of hydrogen embrittlement, which is likely to be a problem with plated steel sheets, but also extremely severe processing with hot-strength steels with a relatively low strength level of 780 MPa class or 590 MPa class as base steel sheets It has been found that the phenomenon of hydrogen embrittlement, which can be manifested when subjected to, is remarkably suppressed. Therefore, in the present invention, the diffusible hydrogen concentration in the base steel sheet is regulated to 0.30 ppm or less. More preferably, it is 0.20 ppm or less.

・ Manufacturing method The high-strength hot-dip Zn-Al-Mg-based plated steel sheet with excellent workability is, for example, hot-rolled, pickled, cold-rolled, continuously hot-plated on steel materials (such as continuous cast slabs) with adjusted components. It can be manufactured by a process of sequentially performing annealing in a line, hot-dip Zn—Al—Mg-based plating and baking treatment. Hereinafter, the manufacturing conditions in that case will be exemplified.

  A steel slab satisfying the above component 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 up at the following winding temperature. Thereafter, a hot-rolled steel strip is obtained at the following winding temperature. Further, the steel strip is pickled, cold-rolled under the following conditions, and subjected to a plating process in a continuous hot dipping line.

<The coiling temperature in hot rolling is 500 ° C. to 650 ° C.>
When the coiling temperature is less than 500 ° C., the amount of precipitation of carbide containing Ti becomes insufficient and the strength is lowered. On the other hand, when the coiling temperature exceeds 650 ° C., coarsening of the carbide containing Ti occurs, resulting in a decrease in strength and bending workability.

<Cold rolling rate: 30-60%>
After hot rolling, a continuous pickling line is passed through to remove surface scale and cold rolling. At that time, when the rolling rate of cold rolling is less than 30%, the dislocation density after annealing in the continuous hot dipping 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, when it exceeds 60%, the dislocation density exceeds 5.7 × 10 ¹⁴ / m ² , and the ductility decreases greatly, and the bending workability may deteriorate. Therefore, the cold rolling rate is preferably in the range of 30% to 50%.

<Hot plating>
What is necessary is just to manufacture a fusion | melting Zn-Al-Mg type plated steel plate by the conventional general method. Continuous hot dipping lines in mass production sites can be used. Specifically, the heat treatment that also serves as the surface reduction treatment performed immediately before the hot dipping is performed by heating to 550 to 750 ° C. in a mixed gas of hydrogen and nitrogen. If the heating temperature is less than 550 ° C., the surface of the steel sheet is not sufficiently reduced and the plateability is lowered. 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 , resulting in a decrease in strength. That is, the present invention is characterized in that annealing is performed at a temperature lower than recrystallization annealing to maintain a high dislocation density, and the metal structure of the base material is based on the structure at the end of hot rolling. .

  The proportion of hydrogen gas in the mixed gas is preferably 25 to 35% by volume. The time during which the material temperature is in the above temperature range is desirably adjusted within a range of 20 to 200 seconds, for example. When the base steel sheet is heated in a mixed gas of hydrogen and nitrogen in this way, hydrogen enters the steel. The concentration of hydrogen in the steel can be greatly reduced by a baking process described later. The plate | board thickness of a base-material steel plate is 0.8-4.5 mm, for example. After this heat treatment, it is immersed in a hot dipping bath without exposure to the atmosphere.

  FIG. 3 shows an outline of an example of the hot dipping method of the present invention. The material steel plate 10 is introduced into the hot dipping bath 50 accommodated in the plating tank 40 through the snout 30 after the oxide film on the surface is removed in the reduction furnace 20. The material steel plate 10 is pulled up from the hot dipping bath after circling the sink roll 60 immersed in the hot dipping bath 50.

  The snout 30 exhibits an effect of preventing contact between the material steel plate 10 delivered from the reduction furnace 20 and the atmosphere, and is provided between the reduction furnace 20 and the hot dipping bath 50 in a cylindrical shape surrounding the material steel plate 10. The lower end of the snout 30 is slightly immersed in the plating bath 50. The snout 30 is filled with reducing gas.

  The composition of the hot dipping bath is Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.10% by mass%, B: 0 to 0.05%, Fe: 2.0% or less, the balance being Zn and inevitable impurities. The plating layer composition of the obtained plated steel sheet substantially reflects the plating bath composition. The steel sheet pulled up from the plating bath is cooled by a conventional method after adjusting the plating adhesion amount by a gas wiping method or the like. The plating adhesion amount is preferably 3 to 100 μm in terms of the average thickness of the plating layer per side.

<Baking treatment>
The baking treatment is a heat treatment for reducing the hydrogen concentration in the steel by releasing hydrogen that has entered the steel to the outside. Here, the baking process is performed in a steam atmosphere. Specifically, the molten Zn—Al—Mg plated steel sheet is heated and held at 70 to 250 ° C. in a water vapor atmosphere in a space shielded from the atmosphere. The content of impurity gas components (gas components other than water vapor) in the water vapor atmosphere is preferably 5% by volume or less.

When the molten Zn—Al—Mg-based plating layer is brought into contact with water vapor at the above temperature, Zn in the plating layer is preferentially oxidized to form a black Zn oxide. By utilizing this black change, the color unevenness that becomes a problem when baking a steel sheet material becomes very inconspicuous, and a black-like surface appearance having a lightness L ^* of 60 or less with high designability is obtained. The partial pressure of water vapor may be adjusted so that the relative humidity (the partial pressure of water vapor actually present in the atmosphere with respect to the saturated water vapor pressure at that temperature) is 70 to 100%. When the relative humidity is less than 70%, the formation rate of the black oxide of Zn is slow, and uneven coloring tends to occur in the time when the release of hydrogen in the steel is sufficiently achieved.

The temperature of the baking process greatly affects the release of diffusible hydrogen present in the steel. As a result of various studies, when the temperature at which the molten Zn—Al—Mg based steel sheet is maintained in the water vapor atmosphere is less than 70 ° C., the diffusible hydrogen concentration in the base steel sheet is stably reduced to 0.30 ppm or less. It becomes difficult to reduce. In addition, it is difficult to obtain a black appearance having a lightness L ^* of 60 or less. Therefore, the baking temperature is 70 ° C. or higher. More preferably, the temperature is 100 ° C. or higher. On the other hand, when the baking temperature exceeds 250 ° C., the formation speed of the black oxide increases, and it becomes difficult to control the black tone with high uniformity. Therefore, the baking temperature is 250 ° C. or lower. More preferably, the temperature is 210 ° C. or lower.

The time for the baking treatment, that is, the time for maintaining the molten Zn—Al—Mg-based plated steel sheet at a predetermined temperature set in the range of 70 to 250 ° C. results in the diffusible hydrogen concentration in the base steel sheet being 0.00. A time that can be reduced to a target level such as 30 ppm or less or 0.20 ppm or less is set. An appropriate processing time may be determined by conducting preliminary experiments in advance in accordance with the hot dip plating conditions, the atmospheric gas conditions of the baking process, and the baking process temperature. Usually, the processing time in which a favorable result is obtained in the range of 1 to 50 hours can be set. More preferably, it is in the range of 2 to 36 hours. In addition, when it is desired to obtain a deep black appearance having a lightness L ^* of 40 or less, blackening becomes insufficient in a very short time, so it is desirable to ensure a relatively careful processing time.

  The baking process is performed in a furnace cut off from the atmosphere. It is desirable to use a highly airtight container for the furnace body. When accommodating the molten Zn—Al—Mg-based plated steel sheet in the furnace, consideration is given so that the surface of the plating layer comes into contact with the atmospheric gas. Exclude air in the furnace by nitrogen replacement or evacuation, and then introduce water vapor to make the atmosphere in the furnace a water vapor atmosphere, raise the temperature to a predetermined temperature, and perform the baking process by holding at that temperature . The furnace atmosphere is managed so that a predetermined gas composition is maintained even during the baking process.

<Formation of inorganic film>
An inorganic coating can be formed on the surface of the Zn—Al—Mg coating layer modified by the baking treatment. As the inorganic coating, various known coatings conventionally applied to hot-dip Zn—Al—Mg plated steel sheets can be applied. Among them, one or more compounds selected from the group consisting of valve metal oxides, valve metal oxyacid salts, valve metal hydroxides, valve metal phosphates and valve metal fluorides ( Those containing a “valve metal compound”) are also suitable objects. Examples of the valve metal include Ti, Zr, Hf, V, Nb, Ta, W, Si, and Al. It is desirable to apply the valve metal compound containing at least one of these valve metals. The inorganic film can be formed by a known method. For example, a method of applying an inorganic coating material containing a valve metal compound or the like on the surface of the Zn—Al—Mg coating layer by a roll coating method, a spin coating method, a spray method or the like can be employed.

<Formation of organic film>
An organic coating can also be formed on the surface of the Zn—Al—Mg coating layer modified by the baking process. Various known organic resin coatings conventionally applied to hot-dip Zn—Al—Mg-based plated steel sheets can also be applied. For example, urethane resins, epoxy resins, olefin resins, styrene resins, polyester resins, acrylic resins, fluorine resins, combinations of these resins, copolymers or modified products of these resins, etc. The film to contain is mentioned. The organic film can also be formed by a known method. For example, a method of applying the organic paint containing the resin component to the surface of the Zn—Al—Mg coating layer by a roll coating method, a spin coating method, a spray method, or the like can be employed.

  Each steel having the composition shown in Table 1 is melted and the slab is heated to 1250 ° C., then hot-rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 520 to 680 ° C., and hot rolled to a thickness of 2.6 mm. A steel strip was obtained. The coiling temperature of each hot-rolled steel strip is shown in Tables 2 and 3, respectively.

After pickling the hot-rolled steel strip and subjecting it to cold rolling at a cold rolling rate of 30% and 50%, in a continuous hot dipping line, hydrogen is 30% by volume-nitrogen is 70% by volume in a mixed gas of 500- Annealed at 790 ° C., cooled to about 420 ° C. at an average cooling rate of 5 ° C./sec to obtain a raw steel plate (plating original plate), and then the molten Zn having the following plating bath composition with the steel plate surface not exposed to the atmosphere -Dipped in an Al-Mg-based plating bath and then pulled up to obtain a molten Zn-Al-Mg-based plated steel sheet in which the amount of plating adhesion was adjusted to about 90 g / m ² per side by a gas wiping method. The plating bath temperature was about 410 ° C. The cold rolling rate and annealing temperature of each steel are also shown in Tables 2 and 3.

  Each molten Zn—Al—Mg-based plated steel sheet was baked. The molten Zn—Al—Mg-based plated steel sheet was placed in a heating furnace and placed so that the surface of the plating layer was in contact with the atmospheric gas. Then, the inside of the furnace is sealed, evacuated with a vacuum pump, steam is introduced from the gas introduction pipe, and the furnace temperature is controlled to a predetermined baking temperature while controlling the furnace pressure so that the relative humidity becomes 100%. The temperature was raised to 1, and kept at that temperature for a predetermined time, then the temperature was lowered, and the inside of the furnace was opened to the atmosphere. The atmosphere gas during the baking treatment was 100% by volume of water vapor and 100% of relative humidity (common to each example).

A sample was taken from the steel sheet after baking, and a tensile test, a bending test, a tolerant hot metal embrittlement test, a diffusible hydrogen concentration in the base steel sheet, and a lightness L ^* of the Zn-Al-Mg coating layer surface were measured. Various characteristics such as the above were examined and shown in Table 3. For comparison, Table 2 shows the results of various characteristic investigations before baking. In addition, since the change was not recognized in the microstructure of the raw material even if it baked, description of the microstructure in Table 3 was abbreviate | omitted.

[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 diameter of Ti-containing carbide]
The thin film produced from the collected molten Zn—Al—Mg based plated steel sheet sample is 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. 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 from the collected hot-dip Zn-Al-Mg plated steel plate sample to 1/4 of the plate thickness, chemical polishing is applied to remove the processing strain, and the dislocation density is calculated by X-ray analysis. Determined by For X-ray diffraction, the Kα1 line of the 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: Calculated.
ρ = 14.4 × η ² / b ²
Here, ρ is the dislocation density and b is the Burgers vector (0.25 nm). The dislocation density was calculated using the Modified Williamson-Hall / Warren-Aberbach method. The calculated dislocation density is also shown in Table 2.

[Cementite area ratio]
The area ratio of the hard second phase and cementite was determined by polishing a sample cut from the collected molten Zn-Al-Mg-based plated steel sheet sample in the rolling direction, etching it with a Picral reagent, and observing it with an SEM. And calculated by image analysis. Table 2 shows the measured area ratio of the hard second phase and cementite.

(Tensile properties)
Using a JIS No. 5 test piece taken so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the material steel plate, the tensile strength TS and total elongation T.E1 were determined in accordance with JISZ2241.

[Bending workability]
A 20 × 50 mm sample was taken from the hot-dip Zn—Al—Mg-based plated steel sheet in the direction perpendicular to the rolling direction and subjected to a 135 ° bending test. That is, with a pressing force of 20 kN using a V-shaped punch and a die having a tip R of 1.0 mm, 0.5 mm, and a tip angle of 45 ° so that the rolling direction is the axis of bending at the center of the sample in the longitudinal direction. Bending was performed, and the presence / absence of cracks on the outer surface at the tip of the bent portion was evaluated by ○ ×.

[Evaluation of molten metal embrittlement cracking]
The molten metal embrittlement characteristics were evaluated by conducting a welding test according to the following procedure.
A sample of 100 mm × 75 mm was cut out from the molten Zn—Al—Mg-based plated steel sheet, and this was used as a test piece for evaluating the maximum crack depth due to molten metal embrittlement. In the welding test, “boss welding” was performed to create a boss weld material having the appearance shown in FIG. That is, a boss (projection) 1 made of a steel bar (SS400 material defined in JIS) having a diameter of 20 mm and a length of 25 mm is vertically set at the center of the plate surface of the test piece 3, and this boss 1 is arc welded to the test piece 3. It joined with. The welding wire is YGW12. The welding bead 6 goes around the boss from the welding start point around the boss, and after passing the welding start point, welding is further advanced to pass the welding start point and the weld bead overlap portion 8 is formed. At that point, welding was finished. The welding conditions were 190A, 23V, welding speed 0.3 m / min, shielding gas: Ar-20 vol. % CO ₂ , shielding gas flow rate: 20 L / min.

  In welding, as shown in FIG. 2, a test piece 3 previously joined to a restraint plate 4 was used. First, a constrained plate 4 (SS400 material stipulated in JIS) 120 mm × 95 mm × 4 mm thick is prepared, and the test piece 3 is placed at the center of the plate surface. It is welded to the restraint plate 4. The boss weld material is manufactured by fixing the joined body (the test piece 3 and the restraint plate 4) on the horizontal test bench 5 with the clamp 2, and performing boss welding in this state.

  After the boss welding, the boss 1 / test piece 3 / restraint plate 4 joined body is cut at a cut surface 9 passing through the central axis of the boss 1 and passing through the overlapping portion 8 of the beads. Observation was performed, the maximum depth of cracks observed in the test piece 3 was measured, and this was taken as the maximum base material crack depth. This crack corresponds to a molten metal embrittlement crack. A maximum base material crack depth of 0.1 mm or less was evaluated as acceptable, and a material exceeding 0.1 mm was evaluated as unacceptable.

(Measurement of diffusible hydrogen concentration)
By removing the Zn—Al—Mg coating layer on the surface layer of the steel plate sample with abrasive paper, a sample consisting only of the base steel plate was produced. The measurement conditions for the diffusible hydrogen concentration are shown below.
Sample heating unit: Infrared gold image furnace (RHL-E410P manufactured by ULVAC-RIKO)
Analyzer: APS-MS / atmospheric pressure ionization mass spectrometer (FLEX-MS400 manufactured by Japan API Corporation)
・ Analytical sample: Analyzed three pieces cut into 10 mm × 3 mm dimensions.
・ Raising rate: 5 ° C / min
Measurement atmosphere: Ar (1000 mL / min)

[Measurement of lightness L ^* value]
Using a spectroscopic color difference meter (manufactured by Tokyo Denshoku Co., Ltd .; TC-1800), the lightness L ^* value was measured by a spectral reflection measurement method based on JIS K5600. The measurement conditions are shown below.
・ Optical conditions: d / 8 ° method (double beam optical system)
-Field of view: 2 degree field of view-Measuring method: Reflected light measurement-Standard light: C
-Color system: CIELAB
・ Measurement wavelength: 380 to 780 nm
・ Measurement wavelength interval: 5 nm
-Spectroscope: diffraction grating 1200 / mm
・ Lighting: Halogen lamp (voltage 12V, power 50W, rated life 2000 hours)
・ Measurement area: 7.25mmφ
-Detection element: Photomultiplier tube (R928; Hamamatsu Photonics Co., Ltd.)
-Reflectance: 0-150%
・ Measurement temperature: 23 ℃
・ Standard plate: White

※ ＢＦ：ベイニティックフェライト F：フェライト P：パーライト

* BF: Bainitic ferrite F: Ferrite P: Pearlite

In Table 2, no. Nos. 1 to 15 have a dislocation density of (1.8 × 10 ¹⁴ / m ^{2 to} 5.7 × 10 ¹⁴ / m ² , a tensile strength of 780 to 1100 MPa, and 135 ° bending with a tip bending R of 1.0 mm is possible. On the other hand, in Table 3, Nos. 1 to 15 which are the scope of the present invention in Table 3 indicate that if baking treatment is performed under appropriate conditions, Since the diffusible hydrogen concentration and the lightness of the surface are low and good values, it has good bending workability capable of 135 ° bending with a tip R of 0.5 mm.

DESCRIPTION OF SYMBOLS 1 Boss 2 Clamp 3 Test piece 4 Restraint plate 5 Test bench 6 Weld bead 7 Weld bead 8 of welded part around the test piece 8 Weld bead overlap part 9 Cut surface 10 Material steel plate 20 Reduction furnace 30 Snout 40 Plating tank 50 Hot dipping bath 60 Syncroll

Claims (8)

A plated steel sheet having a Zn—Al—Mg coating layer on the surface of the material steel sheet,
The material steel plate is in mass%, C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.00. 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 consists of Fe and inevitable impurities, the Ti / C equivalent ratio represented by the following formula (1) is 0.4 to 1.5, and the dislocation density is 1.8 × 10 ¹⁴ / m ² to 5.7 × 10 ¹⁴ / m ² , a single phase of either a bainitic ferrite phase or a ferrite phase or a phase including a bainitic ferrite phase and a ferrite phase, and a hard second phase and cementite A carbide containing Ti having an area ratio of 3% or less and an average particle diameter of 20 nm or less is dispersed and precipitated. And diffusible hydrogen concentration is equal to or less than 0.30 ppm,
The Zn—Al—Mg-based coating layer is mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.10%, B: 0 to 0.05%, Fe: 2.0% or less, remaining Zn and unavoidable impurities, and a black oxide of Zn is distributed in the Zn-Al-Mg coating layer. And the surface brightness L ^* is 60 or less,
A black surface-coated high-strength molten Zn—Al—Mg-based plated steel sheet having 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 plate is substituted for the element symbol in the formula (1).   The bending material having a tensile strength of 780 to 1100 MPa according to claim 1, wherein the raw steel plate further has a composition containing at least one of Nb: 0.1% or less and V: 0.1% or less by mass%. Black surface coated high strength molten Zn-Al-Mg plated steel sheet with excellent workability.   The black surface-coated high-strength molten Zn-Al having excellent bending workability with a tensile strength of 780 to 1100 MPa according to claim 1 or 2, further comprising an inorganic coating on the surface of the Zn-Al-Mg-based coating layer. -Mg based plated steel sheet.   The black surface-coated high-strength molten Zn-Al- having excellent bending workability with a tensile strength of 780 to 1100 MPa according to claim 1 or 2, further comprising an organic coating on the surface of the Zn-Al-Mg-based coating layer. Mg-based plated steel sheet. The steel sheet is subjected to hot rolling, pickling, cold rolling, annealing in a continuous hot dipping line, hot-dip Zn-Al-Mg plating and baking treatment in order, with a tensile strength of 780-1100 MPa and excellent bending workability. A method for producing a black surface-coated high-strength molten Zn-Al-Mg-based steel sheet,
The material steel plate is, by mass, C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.00. 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 Fe and inevitable impurities, and the Ti / C equivalent ratio represented by the following formula (1) is 0.4 to 1.5,
The coiling temperature in the hot rolling is 500 ° C. to 650 ° C., the cold rolling rate in the cold rolling is 30% to 60%,
The raw steel sheet is heated at an annealing temperature of 550 ° C. to 750 ° C. in a continuous hot dipping line, and then immersed in a hot dipping bath.
The plating composition of the hot dipping bath is Al: 1.0-22.0%, Mg: 1.3-10.0%, Si: 0-2.0%, Ti: 0-0. 10%, B: 0 to 0.05%, Fe: 2.0% or less, the balance consists of Zn and inevitable impurities,
In the baking treatment, the plated material steel plate is heated and held at 70 to 250 ° C. in a water vapor atmosphere, and the surface of the plating layer is brought into contact with water vapor, so that the diffusible hydrogen concentration in the material steel plate is 0.30 ppm. Including reducing to:
A method for producing a black surface coated high-strength Zn-Al-Mg plated steel sheet having 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 plate is substituted for the element symbol in the formula (1).   The material steel plate further contains at least 1% of Nb: 0.1% or less and V: 0.1% or less in terms of mass%, and the tensile strength according to claim 5 is 780 to 1100 MPa, and the bending workability is excellent. A method for producing a black surface-coated high-strength molten Zn-Al-Mg-based plated steel sheet.   In the baking step, the diffusible hydrogen concentration in the base steel sheet is reduced to 0.20 ppm or less, and the black surface-coated high-strength molten Zn having excellent bending workability at a tensile strength of 780 to 1100 MPa according to claim 5 or 6. -Manufacturing method of Al-Mg type plated steel sheet.   The diffusible hydrogen concentration in the base steel sheet of the plated steel sheet to be subjected to the baking treatment step is 0.35 ppm or more, and the black surface-coated high strength excellent in bending workability at a tensile strength of 780 to 1100 MPa according to claim 6. A method for producing a molten Zn-Al-Mg-based steel sheet.

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