JP5264235B2 - High yield ratio type Zn-Al-Mg plated steel sheet having excellent resistance to molten metal embrittlement cracking and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-yield ratio type high-strength hot dip Zn-Al-Mg based steel sheet in which molten metal embrittlement crack can be stably suppressed even under severe welding conditions. <P>SOLUTION: The Zn-Al-Mg based steel sheet of tensile strength &ge;590 MPa and yield ratio &ge;0.7 comprises a substrate steel sheet having a composition containing, by mass%, 0.05 to 0.25% C. &le;1.5% Si, 1 to 2% Mn, &le;0.005% N, 0.0003 to 0.01% B, 0.5 to 2% Cr, 0.05 to 0.2% Ti, 0.01 to 0.2% Nb, if necessary. further containing one or more kinds of &le;0.1% V, &le;1% Mo, and &le;1% Zr, and the balance Fe and inevitable impurities; the mean crystal grain size of a ferrite phase is &le;10 &mu;m; the mean particle diameter of the Ti and Nb precipitate dispersed in the ferrite phase is &le;10 nm; the total of the Ti and Nb existing as the precipitate in a matrix is &ge;0.05 mass%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-yield ratio type high-strength molten Zn—Al—Mg-based plated steel sheet and a method for producing the same, which has the property of stably suppressing molten metal embrittlement cracking even under severe welding conditions.

  In recent years, high-strength steel sheets have been adopted for automobile structural members and the like in order to improve fuel efficiency and collision safety. In particular, for members such as pillars and roof rails, it is desired that the material has high strength and a high yield ratio because of the need to secure a passenger space at the time of collision. On the other hand, from the viewpoint of increasing the rust prevention of the automobile body, the above member needs to be formed of a steel plate having excellent corrosion resistance. Conventionally, hot-dip galvanized steel sheets have been the mainstream, but application of hot-dip Zn-Al-Mg-based steel sheets that are more excellent in corrosion resistance has also been studied.

  Structural members of automobiles are often assembled by welding plated steel sheets. In this case, the plating layer melts together with a part of the steel base during welding. According to the investigations so far, when using a Zn-Al-Mg plated steel sheet, fine intergranular cracks are likely to occur in the weld heat affected zone (HAZ) as compared to a general galvanized steel sheet. I know empirically. This crack is caused by a tensile stress caused by the expansion and contraction of the material during welding, and is a kind of phenomenon called a molten metal embrittlement crack. It is considered that the Zn—Al—Mg-based plating component melted at the time of welding increases the sensitivity of molten metal embrittlement cracking compared to the case of zinc plating.

  Conventionally, it has been known that when a zinc-based plated steel material is assembled as a structural member by welding, a grain boundary crack occurs in the heat-affected zone. As an example, Patent Document 1 can be cited. However, this example is a crack generated when the steel material itself is first assembled as a structural member by welding, and the structural member is immersed in a molten zinc bath (around 450 ° C.) and plated. On the other hand, when the Zn-Al-Mg-based plated steel sheet is subjected to welding such as arc welding or spot welding, the plating layer melts together with the steel substrate during welding, and in the subsequent cooling process, grain boundaries are formed in the heat affected zone (HAZ). Cracking occurs. This cracking mechanism is considered to be different from the cracking mechanism when the welded member is immersed in a zinc-based plating bath.

  In order to improve the molten metal embrittlement cracking resistance of the Zn—Al—Mg plated steel sheet, various studies have been made so far. Patent Documents 2 and 3 describe a technique in which the metal structure of the base steel is a mixed structure of ferrite and bainite, pearlite, or martensite, and the intrusion of the molten metal into the grain boundaries is complicated by making the crystal grain boundaries complicated. Is disclosed. Patent Document 4 uses steel plate materials to which Ti, Nb, V, Mo, Zr, etc. are added, and precipitates of these elements having a pinning action are dispersed to suppress the growth of crystal grains in the austenite region during welding. In addition, there is disclosed a technique for preventing cracks by utilizing the action of these elements segregating at grain boundaries after welding. Patent Document 5 uses a base steel on which a strong and thin film in which Cr, Cu, Ni, etc. are concentrated together with Si is used to suppress the penetration of hot-dipped metal into crystal grain boundaries on the surface of the steel material. A technique is disclosed.

JP-A-10-96021 JP 2004-315847 A JP 2004-315848 A JP 2006-97129 A JP 2006-89787 A

  Any of the methods of Patent Documents 2 to 5 is effective for improving the resistance to molten metal embrittlement cracking of a Zn—Al—Mg based steel sheet. However, according to further investigations by the present inventors, in actual welding work, there are cases where molten metal embrittlement cracks cannot be stopped depending on the welding conditions even if the methods of these documents are adopted. all right.

  The present invention stabilizes molten metal embrittlement cracking even under severe welding conditions in a high yield ratio type high strength molten Zn-Al-Mg plated steel sheet that combines high strength, high yield ratio, and good corrosion resistance. The purpose is to provide something that can be deterred.

  As a result of research, the inventors have realized that high strength and high yield ratio are simultaneously achieved by increasing the strength of the base steel sheet by making the ferrite average grain size 10 μm or less, and then adding Ti and Nb in the ferrite grains. It has been found that a technique of finely precipitating the carbide and utilizing the pinning effect of the carbide is extremely effective.

  On the other hand, it is known that the molten metal embrittlement crack of the hot dip galvanized steel sheet is suppressed when a large amount of C that enhances the bonding force between Fe atoms is present at the crystal grain boundary. However, since hot-dip Zn-Al-Mg-based plated steel sheets are more sensitive to hot metal embrittlement cracks than hot-dip galvanized steel sheets, this method alone suppresses hot metal embrittlement cracks in hot-dip Zn-Al-Mg plated steel sheets. It is impossible to do. Therefore, as a result of detailed studies, the inventors have adopted a steel in which Ti and B are added in combination to the base steel sheet and the Cr content is increased to a certain level or more, so that even under severe welding conditions, molten Zn -It discovered that the molten metal embrittlement crack of an Al-Mg type plated steel plate could be suppressed stably. The present invention has been completed based on such findings.

  That is, in the present invention, it contains Al: 3 to 22% by mass, Mg: 1 to 10%, Ti: 0.1% by mass or less, B: 0.05% by mass or less, Si: 2% or less, Fe: 1% or less of 2% or less, with the balance being a hot-dip plated steel sheet composed of Zn and inevitable impurities, the base steel sheet is C: 0.05 to 0.25% by mass, Si 1.5% or less, Mn: 1.0 to 2.0%, N: 0.005% or less, B: 0.0003 to 0.01%, Cr: 0.5 to 2.0%, Ti: 0.05-0.2%, Nb: 0.01-0.2%, further if necessary V: 1.0% or less, preferably 0.05-1.0%, Mo: 1.0% or less Preferably it contains 0.05 to 1.0%, Zr: 1.0% or less, preferably 0.05 to 1.0%, and has a composition of the balance Fe and inevitable impurities, In The average grain size of the ferrite phase in the ferrite phase is 10 μm or less, the average grain size of the Ti precipitate and Nb precipitate dispersed in the ferrite phase is 10 nm or less, and exists as a precipitate in the matrix of the base steel plate. Zn—Al— excellent in molten metal embrittlement cracking resistance, in which the total of Ti and Nb is 0.05% by mass or more, the tensile strength of the plated steel sheet is 590 MPa or more, and the yield ratio is 0.7 or more. An Mg-based plated steel sheet is provided.

  Further, in the present invention, as a method for producing a Zn-Al-Mg based plated steel sheet excellent in the resistance to molten metal embrittlement cracking having the above-mentioned high yield ratio and high strength, for a steel slab having the above-mentioned composition, In the process of sequentially performing hot rolling, cold rolling, annealing in a continuous hot dip plating line and hot dip Zn—Al—Mg based plating, a manufacturing method for setting the coiling temperature in hot rolling to 570 to 680 ° C. is provided. The

  According to the present invention, in a high-strength molten Zn-Al-Mg-based plated steel sheet having a tensile strength of 590 MPa or more and a yield ratio of 0.7 or more, molten metal embrittlement cracks can be stably suppressed even under severe welding conditions. Things with properties are now available. Therefore, the plated steel sheet of the present invention is suitable for structural members of automobiles, particularly for applications such as pillars and roof rails.

  In the present invention, as a base steel plate as a plating base plate, by using a steel having a C content of 0.05 to 0.25% by mass and containing Ti, B and Cr in appropriate amounts, a molten Zn- Prevents molten metal embrittlement cracking during welding of Al-Mg plated steel sheet. Hereinafter, “%” in the steel composition means mass% unless otherwise specified.

[Composition of the base steel sheet]
C is an element indispensable for increasing the strength of the steel sheet. If the content is less than 0.05%, it is difficult to obtain a tensile strength of 590 MPa or more, and addition exceeding 0.25% lowers the weldability and ductility, so the C content is 0.05 to 0.5. The range is 25%.

  Si is also an element effective for increasing the strength of the steel sheet. In addition, it is a useful element for increasing the strength because it is difficult to deteriorate the workability even if the amount added is increased compared to other elements effective for increasing the strength. However, if added excessively, an oxide is formed on the surface of the steel sheet during heating in the hot dipping line and the plating property is hindered, so the upper limit of the addition amount is 1.5%. More preferably, the content is 0.01 to 0.5%.

Mn stabilizes the austenite phase and has an effect of suppressing the formation of pearlite during cooling from the annealing temperature, and contributes to the formation of martensite necessary for increasing the strength. If the content is less than 1%, it is difficult to stably suppress the formation of pearlite, but it cannot be suppressed, which is disadvantageous in obtaining a high strength of 590 MPa or more. On the other hand, if it exceeds 2.0%, Mn segregation in the steel increases, the amount of martensite generated in a band shape increases, and the workability deteriorates. For this reason, Mn content shall be 1.0-2.0%. However, since Mn is a plating line similarly to Si and forms an oxide on the surface of the steel sheet during heating and tends to impair plating properties, it is more preferably 1.8% or less.

Since Ti has a high affinity with nitrogen and fixes N in the steel as TiN, the addition of Ti is extremely effective in securing an amount of B that improves the resistance to molten metal embrittlement cracking. Ti also forms fine carbides during hot rolling and heating in the plating line, contributing to higher strength and an increased yield ratio. In order to obtain these effects sufficiently, the Ti content should be 0.05% or more. However, even if it is added excessively, not only the effect is saturated, but also the workability is lowered, so the upper limit of the Ti addition amount is set to 0.2%.
It was also found that Ti has the effect of promoting the grain boundary segregation of Cr. For this reason, it is effective also in enhancing the improvement effect of the resistance to molten metal embrittlement cracking by Cr described later.

  Nb is effective in increasing the strength and increasing the yield ratio because Nb forms fine carbides during hot rolling and heating in the plating line in the present invention in the same manner as Ti. In order to sufficiently exhibit this action, an Nb content of 0.01% by mass or more is ensured. However, even if it is added excessively, not only the effect is saturated, but also the workability is lowered, so the upper limit of the Nb addition amount is set to 0.2%.

  When N remains as solid solution N in the steel, BN is generated, leading to a decrease in the amount of B that increases the resistance to molten metal embrittlement cracking. Therefore, the content is preferably as low as possible, and the upper limit is made 0.005%. This N is fixed as BN by the Ti described above.

  B is one of the elements forming the gist of the present invention, and is an element effective in suppressing molten metal embrittlement cracking. The effect is considered to be due to the fact that B segregates at the crystal grain boundaries to increase the interatomic bonding force, and by adding it in combination with Cr described later, it is possible to suppress molten metal embrittlement cracking. In the analysis by SIMS (secondary ion mass spectrometry), it is confirmed that when the target steel of the present invention is heated to a high temperature austenite region, B significantly segregates at the austenite grain boundaries. In order to exert such a grain boundary strengthening function by B, as described above, by reducing N or fixing N by Ti, it is possible to suppress the consumption of B accompanying BN formation and secure an effective B amount. is important. In addition, the B content (total amount) needs to be 0.0003% or more, and more preferably 0.001% or more. However, excessive B addition generates boride and causes deterioration of workability, so the B content (total amount) is limited to 0.01% or less.

It has been found that Cr significantly contributes to the suppression of molten metal embrittlement cracking by segregating at austenite grain boundaries at high temperatures. Moreover, it is an element which improves hardenability like Mn, and contributes to the production | generation of the martensite required for high intensity | strength by suppressing the production | generation of pearlite at the time of cooling from annealing temperature.
As shown in Patent Document 5, conventionally, as a base steel sheet for a Zn—Al—Mg-based plated steel sheet, there is one containing Cr and B. However, when such conventional steel was used, there were cases where molten metal embrittlement cracks occurred in actual welding work. As a result of detailed studies, the inventors have found that the above problem can be solved by increasing the Cr content level compared to the conventional steel and adding Ti. Specifically, the addition amounts of Ti and B are as described above, and it was found that it is extremely effective to set the addition amount of Cr to 0.5% or more. Thus, the mechanism by which the resistance to molten metal embrittlement is remarkably improved by adding Ti and B and increasing the amount of Cr is not clarified at the present time, but a certain amount or more of B and Cr are simultaneously high in temperature. By segregating at the austenite grain boundaries, the synergistic effect of B and Cr lowers the grain boundary energy than in the case of conventional steel and increases the interatomic bonding force at the grain boundaries. It is inferred that the resistance increased significantly. However, if Cr is excessively contained, the workability of the steel material is lowered and the toughness is also adversely affected, so the Cr content is limited to a range of 2.0% or less.

  Since each element of V, Mo, and Zr also has an action of suppressing molten metal embrittlement cracking by segregating at high temperature austenite grain boundaries, one or more of these elements can be added as necessary. The effect becomes more remarkable by the combined addition with B and Cr. In order to sufficiently bring out the above-described action of each element, it is particularly effective to secure a content of V of 0.05% or more, Mo of 0.05% or more, and Zr of 0.05% or more. However, even if these elements are added excessively, the effect of suppressing molten metal embrittlement cracking is saturated, resulting in a decrease in the toughness and workability of the steel material and an increase in manufacturing costs. Nb: 0.3% or less, more preferably 0.05% or less, V: 1.0% or less, more preferably 0.2% or less, Mo: 1.0% or less, more preferably 0.1% or less, Zr: It is carried out in a range of 1.0% or less, more preferably 0.1% or less.

[Structure of base steel sheet]
The metal structure of the base steel sheet applied to the present invention has a ferrite phase as a main phase. The proportion of the ferrite phase in the matrix is desirably 60% by volume or more. The type of the second phase constituting the matrix is not particularly limited. The second phase is composed of, for example, one or more of pearlite, bainite, and martensite. As the amount of ferrite phase increases, the strength decreases, but in the present invention, the tensile strength is 590 MPa or more and the yield ratio is 0.7 or more. The upper limit of the ferrite amount need not be specified. On the other hand, the ratio of the second phase in the matrix is advantageously 5% by volume or more in order to obtain the high strength characteristics as described above. It is more preferable that it is 10 volume% or more.

[Average grain size of ferrite phase]
In order to obtain characteristics of high strength and high yield ratio, it is effective that the crystal grain size of the ferrite phase is small. As a result of various studies, the average crystal grain size is desirably 10 μm or less. When manufacturing by the process of “hot rolling → cold rolling → annealing → dipping bath”, the ferrite crystal grain size can be controlled by the coiling temperature, cold rolling rate and annealing temperature in hot rolling.

[Ti and Nb precipitates]
Ti and Nb are both single or composite and form fine carbides during hot rolling and precipitate, and the strength and yield ratio increase due to the precipitation strengthening action. As a result of various studies, it is extremely effective that the average particle size of the precipitates dispersed in the ferrite phase is 10 nm or less. In the base steel sheet targeted in the present invention, most of the observed precipitates are Ti and Nb precipitates. Ti and Nb precipitates are precipitates from which Ti or Nb is detected. These are mainly carbides and nitrides, and there are also precipitates in which both carbon and nitrogen are fixed. These are collectively called “carbonitrides”. The average particle size of the precipitate is determined by observing the ferrite phase portion with a transmission electron microscope (TEM) for a sample collected from the base steel plate portion of the plated steel plate, and within a certain region containing 30 or more precipitates. It can be determined by measuring the particle size (major axis) of each precipitate and calculating the average value. Whether or not the observed precipitates are Ti and Nb precipitates can be confirmed by analysis means (such as EDX) attached to the TEM.

  The amount of Ti and Nb precipitates in the matrix is important as well as the particle size. According to the study by the inventors, in order to stably obtain mechanical properties having a tensile strength of 590 MPa or more and a yield ratio of 0.7 or more, Ti and Nb existing as precipitates in the matrix of the base steel plate are used. It is extremely effective that the total is 0.05% by mass or more. The amount of Ti and Nb in the precipitate can be determined by analyzing Ti and Nb in the residue obtained by dissolving the matrix as described later.

[Mechanical properties of plated steel sheet]
The plated steel sheet of the present invention has the characteristics that the tensile strength is 590 MPa or more and the yield ratio is 0.7 or more. The tensile strength can be obtained by conducting a tensile test in a direction perpendicular to the rolling direction. The yield ratio YR is represented by the ratio of the yield strength YS (the maximum strength at the yield point in the stress strain curve) and the tensile strength TS, YR = YS / TS. A plated steel sheet having a tensile strength of 590 MPa or more and a yield ratio of 0.7 or more is suitable for automobile structural members, particularly pillars and roof rails.

[Taking temperature in hot rolling]
Although the base steel plate applied to this invention can be manufactured using a general steel plate production line, the coiling temperature in hot rolling greatly affects the amount of precipitates and the particle size. Even if a predetermined amount of carbonitride-forming elements such as Ti and Nb is contained in the raw steel plate, if the coiling temperature is low, carbonitrides are not formed or the amount produced is reduced. In order to increase the amount of Ti or Nb present as carbonitride, it is extremely effective to set the coiling temperature to 570 ° C. or higher. On the other hand, the ferrite crystal grain size and the carbonitride grain size become smaller as the coiling temperature is lower. As a result of various studies, it is desirable that the coiling temperature be 680 ° C. or lower.

[Annealing and cooling]
The final annealing performed before plating is desirably in the range of 750 to 950 ° C. If the annealing temperature is too low, the cementite is difficult to dissolve completely. In addition, austenite is not generated, and the amount of martensite in the steel sheet in a state of becoming a molten Zn—Al—Mg plated steel sheet may be insufficient for obtaining a tensile strength of 590 MPa or more. When the annealing temperature is too high, oxides of Si and Mn are easily generated on the surface of the steel sheet, and the plating property is deteriorated.
In the cooling process from the heating temperature during annealing, it is desirable to cool at a cooling rate that does not cause pearlite transformation. Specifically, the average cooling rate may be 3 ° C./sec or more, preferably 5 ° C./sec or more, from the heating and holding temperature to a temperature range of at least Ac ₁ or less.

[Hot Zn-Al-Mg plating]
In the present invention, a known hot-dip Zn—Al—Mg-based plating method can be applied.
Al in a plating layer has the effect | action which improves the corrosion resistance of a plated steel plate. Moreover, it has the effect | action which suppresses generation | occurrence | production of Mg oxide type dross by containing Al in a plating bath. In order to obtain these effects sufficiently, the Al content of the hot dipping needs to be 3% by mass or more, and more preferably 4% by mass or more. On the other hand, if the Al content exceeds 22% by mass, the growth of the Fe—Al alloy layer becomes remarkable at the interface between the plating layer and the base steel sheet, and the plating adhesion deteriorates. In order to ensure excellent plating adhesion, the Al content is preferably 15% by mass or less, and more preferably 10% by mass or less.

  Mg in the plating layer exhibits a function of generating a uniform corrosion product on the surface of the plating layer and remarkably increasing the corrosion resistance of the plated steel sheet. In order to fully exert its action, the Mg content of the hot-dip plating needs to be 1% by mass or more, and it is desirable to ensure 2% by mass or more. On the other hand, when the Mg content exceeds 10% by mass, an adverse effect of easily generating Mg oxide-based dross increases. In order to obtain a higher quality plating layer, the Mg content is preferably 5% by mass or less, and more preferably 4% by mass or less.

When Ti and B are contained in the hot dipping bath, the generation and growth of the Zn ₁₁ Mg ₂ phase which gives speckled appearance defects in the hot-dip Zn—Al—Mg based steel sheet are suppressed. Even if Ti and B are each contained alone, the effect of suppressing the Zn ₁₁ Mg ₂ phase is produced, but it is desirable to contain Ti and B in combination in order to greatly relax the degree of freedom of the production conditions. In order to sufficiently obtain these effects, it is effective that the Ti content of the hot dipping is 0.0005 mass% or more and the B content is 0.0001 mass% or more. However, if the Ti content is excessively large, Ti—Al-based precipitates are generated in the plating layer, and irregularities called “bumps” are generated in the plating layer, thereby impairing the appearance. For this reason, when adding Ti to a plating bath, it is necessary to make it the content range of 0.1 mass% or less, and it is desirable to set it as 0.01 mass% or less. On the other hand, when the B content is excessively large, Al—B or Ti—B based precipitates are generated and coarsened in the plating layer, and irregularities called “bumps” are also generated to impair the appearance. For this reason, when adding B to a plating bath, it is necessary to make it the content range of 0.05 mass% or less, and it is desirable to set it as 0.005 mass% or less.

  When Si is contained in the hot dipping bath, the growth of the Fe—Al alloy layer is suppressed, and the workability of the hot dipped Zn—Al—Mg plated steel sheet is improved. Moreover, Si in the plating layer is effective in preventing the black change of the plating layer and maintaining the gloss of the surface. In order to sufficiently bring out such an action of Si, it is effective to set the Si content of the hot dipping to 0.005 mass% or more. However, since the amount of dross in the hot dipping bath increases when Si is added excessively, the content range is 2.0% by mass or less when Si is contained in the plating bath.

  In general, it is unavoidable to mix Fe in the hot dipping bath because the base steel plate is immersed and passed. In the Zn—Al—Mg based plating, Fe is allowed to be contained up to about 2% by mass. The plating bath may contain, as other elements, for example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr, and Mn, but their total content The amount is desirably 1% by mass or less.

It is desirable to adjust the plating adhesion amount within a range of ^{20 to} 300 g / m ² per one side of the steel plate. The amount of plating adhesion can be controlled using a gas wiping nozzle in accordance with the production of a general galvanized steel sheet. The wiping gas or the atmosphere gas during solidification of the plating layer can be air (atmosphere). That is, an air cooling method can be adopted. If the plating bath temperature exceeds 550 ° C., the evaporation of zinc from the bath becomes remarkable, so that plating defects are likely to occur and the amount of oxidized dross on the bath surface increases, which is not preferable.

The steel shown in Table 1 is melted, and the slab is heated to 1250 ° C. and then extracted, hot rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 480 to 700 ° C., and hot rolled to a thickness of 2.4 mm. A steel strip was obtained. The hot-rolled steel strip was pickled and then subjected to cold rolling to obtain a cold-rolled steel strip having a thickness of 1.4 mm. These cold-rolled steel strips were annealed at 850 ° C. in a hydrogen-nitrogen mixed gas in a continuous hot dipping line and cooled at an average cooling rate of 5 ° C./sec to about 420 ° C. below the Ac ₁ point. Dipping in a molten Zn-Al-Mg-based plating bath having the following bath composition with the surface not exposed to the air, pulling it up and reducing the amount of plating to about 90 g / m ² per side by the gas wiping method (air) An adjusted molten Zn—Al—Mg-based plated steel sheet was obtained. The plating bath temperature was about 410 ° C. The cooling when the plating layer is solidified is air cooling. The main production conditions are shown in Table 2. In addition, all S content of each base steel plate is 0.03 mass% or less.

[Plating bath composition]
The “%” below is mass%.
Al: 6%, Mg: 3%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: balance

  A sample was cut out from the obtained plated steel sheet, and the ferrite content of the base steel sheet, the ferrite average crystal grain size (referred to as “ferrite grain size”), and the total amount of Ti and Nb present as precipitates in the matrix ( The average particle size of the precipitates of Ti and Nb dispersed in the ferrite phase (referred to as “precipitate Ti + Nb amount”) (referred to as “precipitate average particle size”) was determined. In addition, the mechanical properties of the plated steel sheet were examined. Furthermore, in order to evaluate the maximum weld crack length caused by molten metal embrittlement, welding tests by spot welding and arc welding were conducted.

[Amount of ferrite]
The steel structure of the plated steel sheet was observed in the metal structure of a cross section (C cross section) perpendicular to the rolling direction, and the ferrite content was determined by image analysis. In all of the inventive examples, the amount of the ferrite phase in the matrix was in the range of 60 to 95% by volume. In the examples of the present invention, it has been separately confirmed that the phases other than the ferrite phase constituting the matrix are composed of one or more of pearlite, bainite, and martensite.

[Ferrite particle size]
A metal structure of a cross section perpendicular to the rolling direction (C cross section) was observed for the steel base portion of the plated steel sheet, and the average particle size was determined for the ferrite phase portion by a cutting method in accordance with JIS G0551: 2005.

[Deposited Ti + Nb amount]
The sample cut out from the plated steel sheet was determined by electrolytic extraction residue analysis. After electrolysis at a constant potential using a 10% acetylacetone-1% methylammonium chloride-methanol (10% AA system) solution, the solution was filtered through a filter having a pore size of 0.05 μm. Although the individual precipitate particles are nanoparticles, since they are aggregated, it may be considered that almost the entire amount is recovered as a residue by the filter. The obtained residue was thermally decomposed with a mixed acid, the amount of precipitation of each element was calculated using an inductively coupled plasma emission spectrometer (ICP), and the amount of precipitated Ti + Nb was calculated based on the analysis value.

[Average particle size of precipitates]
It calculated | required by the above-mentioned method using TEM.

(Mechanical properties)
Using a JIS No. 5 test piece taken so that the longitudinal direction of the test piece is perpendicular to the rolling direction, the tensile strength TS, the yield strength YS, and the total elongation (breaking elongation) T.EL are determined in accordance with JIS Z2241. Asked. And the yield ratio YR was calculated | required by YR = YS / TS from the yield strength and the tensile strength.

[Arc welding]
A 100 mm × 75 mm sample was cut out from the plated steel sheet, and this was used as a test piece for evaluating the maximum weld crack length caused by molten metal embrittlement by arc welding.
The welding test was performed by “boss welding” for producing a boss welded member having an appearance as shown in FIG. 1 and examining the cross section of the welded portion to examine the occurrence of cracks. That is, a boss (projection) 1 made of a steel bar having a diameter of 20 mm and a length of 25 mm was set up vertically at the center of the plate surface of the test piece 3, and the boss 1 was joined to the test piece 3 by arc welding. As the welding wire, YGW12 was used. After the welding start point, the circumference of the boss was made one round, and after passing the welding start point, the bead was further piled up and the welding was completed when the welding proceeded a little. That is, the weld bead 6 overlaps between the welding start point and the welding end point. The welding conditions were a welding current: 217 A, a welding voltage of 25 V, a welding speed of 0.2 m / min, a shielding gas: CO ₂ , and a shielding gas flow rate: 20 L / min. A member after welding composed of the boss 1, the test piece 3, and the weld bead 6 is called a “boss welded member”.

  In the arc welding, the test piece 3 was restrained as shown in FIG. 2 for the purpose of experimentally facilitating weld cracking. That is, the test piece 3 was placed at the center of the plate surface of the restraint plate 4 (SS400 steel material defined in JIS) having a size of 120 mm × 95 mm × 4 mm, and the entire circumference of the test piece 3 was welded to the restraint plate 4 in advance. The integrated test piece 3 / restraint plate 4 assembly was fixed on a horizontal test bench 5 by two clamps 2, and the boss welding was performed in this state. In this specification, the boss welding performed in such a restrained state is referred to as “restraint boss welding”. According to this method, since the test piece 3 is integrated with the restraint plate 4 by all-around welding, the expansion / contraction caused by heat input during boss welding is restrained. Stress makes it easier for weld cracks to occur during boss welding, enabling clear evaluation of weld cracks.

  After welding the restraint boss, the joined surface 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 overlap portion 8 of the weld bead. About the metal structure of the test piece 3 (namely, the base steel plate which is a plating steel plate base material) part of the welding bead vicinity was observed with the microscope. About the crack observed in the part of the test piece 3 in the cross section by the microscopic observation, the length of the crack from the surface on the boss weld side of the test piece 3 to the tip of the crack is measured, and the measured value for the longest crack is obtained. “Maximum crack length”. Considering the strength and fatigue characteristics of the welded part, those having a maximum crack length of 0.2 mm or less were accepted and those other than that were rejected. Such a crack in the base steel sheet occurs along the prior austenite grain boundary of the weld heat affected zone, and this is judged to be a “molten metal embrittlement crack”.

[Spot welding]
Moreover, the test of the molten metal embrittlement crack by spot welding was done. Similarly, a sample of 100 mm × 75 mm is cut out from the obtained plated steel sheet, two samples are overlapped, and this is used as a test piece to evaluate the maximum weld crack length caused by molten metal embrittlement by spot welding. Went.
Spot welding was performed using a DR-type electrode having a tip diameter of 6 mm and supplying a welding current of 10 kA that generates dust when a pressure of 3.2 kN was applied. And the length of a crack was measured by observing the metal structure of the cross section of a welding part similarly to the test by arc welding. Here, too, those having a maximum crack length of 0.2 mm or less were accepted and those other than that were rejected.
These results are shown in Table 2.

Each of the inventive examples exhibited performance with a tensile strength TS of 590 MPa or more and a yield ratio YR of 0.7 or more.
On the other hand, in Comparative Example No. 11, although the Ti amount and Nb amount of the base steel sheet were appropriate, the amount of precipitates was very small because the amount of C was small, and sufficient strength was not obtained. Nos. 12 to 14 had good mechanical properties, but the Cr content of the base steel sheet was small, and therefore, both arc welding and spot welding were inferior in resistance to molten metal embrittlement cracking. In No. 15, the base steel sheet did not contain Nb, so the amount of precipitated Ti + Nb was insufficient and the yield ratio was inferior. In No. 16, since the coiling temperature was too low, the amount of precipitated Ti + Nb was small and the yield ratio was low. In No. 17, since the coiling temperature was too high, precipitates were formed, but the precipitates grew and the average particle size thereof was too large, and the ferrite crystal particle size was too large. For this reason, both the tensile strength and the yield ratio were low.

The figure which showed typically the shape of the boss | hub welding member. Sectional drawing which showed typically the restraint method of the test piece at the time of performing restraint boss welding.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Boss 2 Clamp 3 Test piece 4 Restraint plate 5 Test bench 6 Weld bead 7 Weld bead of the test piece circumference weld part 8 Overlap part of weld bead

Claims (3)

  Al: 3 to 22% by mass, Mg: 1 to 10%, Ti: 0.1% by mass or less, B: 0.05% by mass or less, Si: 2% or less, Fe: 2% or less In the plated steel sheet containing one or more of the following, the balance being hot-plated with Zn and inevitable impurities, the base steel sheet is C: 0.05-0.25% by mass%, Si: 1.5% Mn: 1.0 to 2.0%, N: 0.005% or less, B: 0.0003 to 0.01%, Cr: 0.5 to 2.0%, Ti: 0.05 to 0 0.2%, Nb: 0.01-0.2%, balance Fe and inevitable impurities composition, the average grain size of ferrite phase in the base steel sheet is 10 μm or less, dispersed in the ferrite phase The Ti and Nb precipitates have an average particle diameter of 10 nm or less, and Ti and N present as precipitates in the matrix of the base steel sheet Zn-Al-Mg-based plating having excellent resistance to molten metal embrittlement cracking, in which the total strength is 0.05 mass% or more, the tensile strength of the plated steel sheet is 590 MPa or more, and the yield ratio is 0.7 or more. steel sheet.   2. The molten metal according to claim 1, wherein the base steel sheet has a composition containing at least one of V: 1.0% or less, Mo: 1.0% or less, and Zr: 1.0% or less in terms of mass%. Zn—Al—Mg plated steel sheet with excellent embrittlement cracking properties. Hot rolling, cold rolling, in sequentially performing step annealing and hot-dip Zn-Al-Mg plated in a continuous hot-dip plating line, according to claim 1 or 2, the winding temperature at the hot rolling and 570 to 680 ° C. The manufacturing method of the Zn-Al-Mg type plated steel plate excellent in the molten metal embrittlement cracking resistance described in 1.

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