JP2013087313A - Galvannealed steel sheet excellent in adhesion strength and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet excellent in adhesion strength.SOLUTION: The galvannealed steel sheet has a plating layer 2 comprising, by mass, 7-15% Fe, 0.01-1% Al and the balance being Zn and unavoidable impurities on a surface of a steel sheet 1 comprising 0.05-0.50% C, 0.01-3.0% Mn, at least one chosen from ≤3.0% Si, ≤2.0% Al and ≤2.0% Cr, provided that Mn+Si+Al+Cr is ≥0.4%, and the balance being Fe and unavoidable impurities. Here, (x) the steel sheet side of the plating layer is a Zn-Fe alloy phase including (x1) at least one chosen from oxides of Mn, Si, Al and Cr and/or (x2) at least one composite oxide comprising at least two chosen from Mn, Si, Al and Cr, and (y) a surface layer of the plating layer 2 is a Zn-Fe alloy phase containing a ζ phase free of the oxide and/or the composite oxide.

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent adhesive strength and a method for producing the same.

  In recent years, particularly in the field of automobile technology, demand for high-strength steel sheets is increasing from the viewpoint of reducing the weight of a vehicle body for the purpose of energy saving by improving fuel consumption. In response to such demand, for example, in Patent Document 1, the steel sheet structure is a structure in which three phases of a ferrite phase, a bainite phase, and an austenite phase are mixed, and the residual austenite is transformed into martensite during molding. A steel sheet using transformation-induced plasticity that exhibits high ductility is disclosed.

  This type of steel sheet contains, for example, 0.05 to 0.4% by mass of C, 0.2 to 3.0% by mass of Si, and 0.1 to 2.5% by mass of Mn. After annealing, the composite structure is formed by controlling the temperature pattern of the cooling process, and the required characteristics can be secured without using expensive alloy elements.

  In order to give a rust prevention function to such a steel sheet, when performing galvanization with a continuous hot dip galvanizing facility, if the Si amount of the steel sheet exceeds 0.3% by mass, the plating wettability is greatly reduced, In the Sendzimer method using a normal Al-containing plating bath, there is a problem that non-plating occurs and the appearance quality deteriorates.

  This is said to be due to the fact that an external oxide film containing Si or Mn is formed on the steel sheet surface during reduction annealing, and the wettability of these oxides to molten Zn is poor.

As means for solving this problem, Patent Document 2 discloses that a steel sheet is heated in advance in an atmosphere having an air ratio of 0.9 to 1.2 to generate Fe oxide, and then a reduction zone containing H _2. Then, after reducing the thickness of the oxide to 500 mm or less, a method of plating in a bath containing Mn and Al has been proposed, but in the actual line, various steel plates containing various additive elements are passed through. It is difficult to accurately control the thickness of the oxide.

  As another non-plating suppressing means, Patent Document 3 discloses a method of improving plating properties by applying specific plating to the lower layer. However, in this method, it is necessary to newly provide a plating facility before the annealing furnace in the hot dipping line, or to perform a plating process in advance in the electroplating line. In either case, a significant increase in manufacturing cost is expected.

  On the other hand, Patent Document 4 discloses a method of manufacturing an alloyed hot-dip galvanized steel sheet without adjusting Fe in the steel sheet by adjusting the oxygen potential of the annealing atmosphere during annealing. In this method, oxidizable elements such as Si and Mn in steel are internally oxidized by controlling the oxygen potential of the atmosphere, and the formation of an external oxide film is suppressed, thereby achieving an improvement in plating performance. Yes.

  By applying this technique, the steel plate is reheated after plating, the Zn plating layer and the steel plate are reacted, and the Zn-Fe alloying reaction is uniformly progressed when an alloy plating layer made of a Zn-Fe alloy is formed. It becomes possible.

  In general, a high-strength steel plate used for a reinforcing member for automobiles is processed by processing mainly consisting of bending. When a high-strength steel plate having a relatively high C content is used as the plating original plate, the plating original plate itself is hard, so that cracks are likely to occur in the steel plate surface layer during bending. This crack becomes a factor that the steel plate breaks in the plate thickness direction when the steel plate is used.

  In order to solve this problem of bendability, Patent Document 5 discloses that not only the plateability is improved by controlling the oxygen potential in the annealing atmosphere, but also the carbon concentration on the surface of the steel sheet is lowered and the ductility of the surface layer is improved. By suppressing the generation of cracks, and by generating oxides of Si and Mn near the surface of the steel sheet, even if cracks occur, this oxide suppresses the propagation of cracks, and bending A technique for ensuring the safety is disclosed.

  However, even if the steel plate is annealed under the condition that the oxide is internally oxidized in the steel, the oxide formed at the plating / steel plate interface is not completely lost, and the plating layer / steel plate interface resulting from the formation behavior of the internal oxide Depending on the properties, the adhesion between the steel sheet and the plating layer deteriorates and the problem that the plating peels off during processing is likely to occur.

  In order to improve plating adhesion when producing an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet as an original sheet, for example, in Patent Document 6, in Si—Mn oxide and Zn—Fe formed at the interface between a plating layer and a steel sheet, A technique for improving the adhesion between the plating layer and the steel sheet by controlling the size of the irregularities at the interface between the structure and the steel sheet is disclosed focusing on the form of the structure made of an intermetallic compound.

  However, this technique employs a process of heating the steel sheet before plating in a Fe oxidizing atmosphere and then holding it in a reducing atmosphere for a certain period of time. In order to make the state of the interface between the plating layer and the steel plate a predetermined state, the annealing atmosphere must be strictly adjusted to strictly control the oxidation and reduction of the steel plate.

  In Patent Document 7, from the interface between the plating layer and the steel plate, the depth of penetration of the Zn-Fe intermetallic compound in the depth direction on the steel plate side is controlled to 10 μm or less to improve powdering resistance and plating adhesion. Is disclosed.

  Recently, as disclosed in Patent Documents 8 and 9, there is a movement to use a galvanized steel sheet as a structural material for adhesion, particularly in the body of an automobile. In this case, the galvannealed steel sheet has The above disadvantages become a big problem.

  By comparing the breaking strength of the bonded structure using the unplated base material with the bonded structure using the galvannealed steel sheet, the latter breaking strength is It decreases to about 1/2 of the breaking strength.

  And, in the case of the adhesive structure of the base material, the form of fracture is a cohesive failure of the adhesive, whereas in the case of the adhesive structure of the galvannealed steel sheet, the interface between the plating layer and the steel sheet. Interfacial peeling that peels off at.

  In recent years, when a high-strength galvannealed steel sheet is used as an adhesive structural member, the strength of the steel sheet itself is high, so higher adhesion is required, and the structure of the interface between the plating layer and the steel sheet is controlled. In addition, it is not sufficient, and it is required that the plating layer itself be further improved in plating adhesion by incorporating a device for ensuring adhesion.

In the Zn—Fe alloy plating layer, there are a plurality of phases such as a ζ phase, a δ ₁ phase, a Γ phase, and a Γ ₁ phase in ascending order of Fe amount. In general, the Zn-Fe alloy phase is harder and more brittle as the amount of Fe increases. Patent Document 10 discloses a technique for producing an alloyed hot-dip galvanized steel sheet with high adhesion of the plating layer by making all the structures in the alloy plating layer into the ζ phase.

  However, when an alloyed hot-dip galvanized steel sheet is produced using a technique disclosed in Patent Document 4 using a high-strength steel sheet containing an element such as Si or Mn as an original sheet, the oxide particles described above are Zn-Fe alloys. By being dispersed in the phase, the plastic deformability of the alloy layer is reduced, and when the plating layer is stressed, the plating layer is easily cracked and peeled off.

JP 05-59429 A Japanese Patent Laid-Open No. 04-276057 JP 2003-105514 A Japanese Patent No. 4718782 International Publication WO2011 / 025042 Pamphlet JP 2011-127216 A JP 2011-153367 A JP 2011-007250 A JP 2011-074422 A JP 05-311372 A

  An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet that is excellent in adhesive strength and suitable as a structural material for bonding, and a method for producing the same, in view of the above-described present situation relating to an alloyed hot-dip galvanized steel sheet.

  The present inventors diligently studied a method for securing adhesive strength necessary as a structural material for bonding in an alloyed hot-dip galvanized steel sheet (hereinafter, sometimes referred to as “plated steel sheet”). As a result, it was found that when the ζ phase containing no oxide was formed outside the Zn—Fe alloy phase containing the internal oxide in the plating layer (surface layer of the plating layer), the adhesive strength was dramatically improved. did.

  This invention was made | formed based on the said knowledge, The summary is as follows.

(1) By mass%, C: 0.05 to 0.50%, Mn 0.01 to 3.0%, Si: 3.0% or less, Al: 2.0% or less, Cr : 2.0% or less of one or more, Mn + Si + Al + Cr: 0.4% or more, Fe: 7 to 15%, Al: 0. In an alloyed hot-dip galvanized steel sheet having a plating layer consisting of 01 to 1%, the balance Zn and inevitable impurities,
(X) The steel plate side of the plating layer is one or more of (x1) Mn, Si, Al, and Cr oxides and / or (x2) Mn, Si, Al, and Cr. A Zn-Fe alloy phase containing one or more of complex oxides composed of two or more,
(Y) An alloyed hot-dip galvanized steel sheet having excellent plating adhesive strength, wherein the surface layer of the plating layer is a Zn—Fe alloy phase containing a ζ phase that does not contain the oxide and / or the composite oxide. .

  (2) The adhesive strength according to (1), wherein the Zn—Fe alloy phase is formed by a reaction between Zn that has entered from the plating layer and Fe in the steel sheet during the alloying treatment. Excellent galvannealed steel sheet.

  (3) The alloyed hot-dip galvanized steel sheet having excellent adhesive strength as described in (1) or (2) above, wherein the steel sheet further contains B: 0.010% or less by mass%. .

  (4) The steel sheet further contains P: 0.10% or less by mass%, and alloying and melting with excellent adhesive strength according to any one of the above (1) to (3) Galvanized steel sheet.

(5) In the method for producing an galvannealed steel sheet excellent in adhesive strength according to any one of (1) to (4),
The steel sheet having the composition described in any one of (1) to (4) above is composed of hydrogen: 0.1 to 50% by volume, balance: nitrogen and inevitable impurities, and log (P _H2O / P _H2 ) is 0 or less In the atmosphere, a certain temperature range of 600 ° C. or higher (T1 to T2 range) is annealed by heating to a maximum of 750 to 900 ° C. at a rate of 6 ° C./second or less, and then performing hot dip galvanization Next, a method for producing an alloyed hot-dip galvanized steel sheet having excellent adhesive strength, wherein alloying treatment is performed at 420 to 500 ° C.

  (6) The method for producing an galvannealed steel sheet having excellent adhesive strength according to (5), wherein the annealing is performed in a total reduction furnace of a continuous hot dip plating facility.

  (7) Any of the above (1) to (6), wherein the hot dip galvanizing is performed at a bath temperature of 430 to 465 ° C. using a galvanizing bath containing Al: 0.01 to 1%. The manufacturing method of the galvannealed steel plate excellent in the adhesive strength as described in 2.

  ADVANTAGE OF THE INVENTION According to this invention, the galvannealed steel plate which adhesive strength improved dramatically can be provided.

It is a figure which shows the microstructure of the interface vicinity of the steel plate after an alloying process, and an alloy plating layer. It is a figure which shows the surface layer of the plating layer containing the (zeta) phase which does not include an internal oxide.

  Hereinafter, the present invention will be described in detail.

The alloyed hot-dip galvanized steel sheet (hereinafter sometimes referred to as “the steel sheet of the present invention”) having excellent adhesive strength according to the present invention is mass%, C: 0.05 to 0.50%, and Mn 0.01 to Contains 3.0%, Si: 3.0% or less, Al: 2.0% or less, Cr: 2.0% or less, one or more, Mn + Si + Al + Cr: 0.4% or more In an alloyed hot-dip galvanized steel sheet having a plating layer consisting of Fe: 7 to 15%, Al: 0.01 to 1%, the balance Zn and inevitable impurities on the surface of the steel sheet consisting of the remainder Fe and inevitable impurities ,
(X) The steel plate side of the plating layer is one or more of (x1) Mn, Si, Al, and Cr oxides and / or (x2) Mn, Si, Al, and Cr. A Zn-Fe alloy phase containing one or more of complex oxides composed of two or more,
(Y) The surface layer of the plating layer is a Zn—Fe alloy phase containing a ζ phase that does not include the oxide and / or the composite oxide.

  The thickness (mm) of the steel plate to which galvanization is applied is not particularly limited. Usually, the thickness of the steel sheet to which galvanization is applied is 0.4 to 3.2 mm, but 1.0 to 3.2 mm is preferable in consideration of the load and productivity of the rolling mill.

  First, the reason which limits the component composition of this invention steel plate is demonstrated. % Related to the component composition means mass%.

C: 0.05-0.5%
C is an element that ensures the strength of steel. If it is less than 0.05%, the effect of improving the strength cannot be expected, and if it exceeds 0.5%, the weldability deteriorates and the practicality of the steel sheet of the present invention decreases, so C is 0.05 to 0.5%. And Preferably it is 0.1 to 0.4%.

Mn: 0.01 to 3.0%
Mn is an element that ensures the strength of steel. Mn is an element that forms an internal oxide that suppresses coarsening of crystal grains in the vicinity of the surface of the steel sheet during annealing. If it is less than 0.01%, the effect of addition cannot be expected, and if it exceeds 3.0%, the weldability deteriorates and the practicality of the steel sheet of the present invention decreases, so Mn is 0.01 to 3.0%. . Preferably it is 0.07 to 3.0%.

Si: 3.0% or less Si is an element that ensures the strength of steel. Si is an element that forms an internal oxide that suppresses coarsening of crystal grains in the vicinity of the surface of the steel sheet during annealing. If it exceeds 3.0%, a coarse internal oxide is generated and the plating layer is easily peeled off, so Si is made 3.0% or less. Preferably it is 2.0% or less. The lower limit includes 0%, but when added, 0.01% or more is preferable.

Al: 2.0% or less Al is an element that deoxidizes steel. In addition, Al is an element that forms an internal oxide that suppresses the coarsening of crystal grains in the vicinity of the surface of the steel sheet during annealing. If it exceeds 2.0%, coarse inclusions and internal oxides are produced, workability is lowered, and the plating layer is easily peeled off, so Al is made 2.0% or less. From the viewpoint of ensuring high workability, it is preferably 1.5% or less. The lower limit includes 0%, but when added, 0.01% or more is preferable.

Cr: 2.0% or less Cr is an element that ensures the strength of the steel without impairing the workability of the steel sheet, particularly the elongation. Cr is an element that forms an internal oxide that suppresses coarsening of crystal grains in the vicinity of the surface of the steel sheet during annealing. If it exceeds 2.0%, the grain boundary becomes brittle due to grain boundary segregation, and the alloying speed becomes slow. Therefore, Cr is made 2.0% or less. Preferably it is 1.5% or less. The lower limit includes 0%, but when added, 0.01% or more is preferable in terms of securing strength.

Mn + Si + Al + Cr: 0.4% or more As described above, Mn, Si, Al, and Cr are all elements that form an internal oxide that suppresses coarsening of crystal grains in the vicinity of the surface of a steel sheet during annealing. is there. If Mn + Si + Al + Cr is less than 0.4%, the amount of internal oxide produced is not sufficient, and the crystal grains near the surface of the steel sheet are coarsened, and a desired microstructure cannot be obtained. Therefore, Mn + Si + Al + Cr is 0.4% or more. Preferably it is 0.9% or more. Although an upper limit is decided by the upper limit of each element, 6.0% or less is preferable at the point which suppresses the excessive production | generation of an internal oxide.

  Here, the internal oxide is an oxide of Mn, Si, Al, and Cr and a composite oxide composed of two or more of Mn, Si, Al, and Cr.

  Specifically, Si oxide, Mn oxide, Si—Mn oxide, Al oxide, Al—Si composite oxide, Al—Mn composite oxide, Al—Si—Mn composite oxide, Cr oxide, Cr-Si composite oxide, Cr-Mn composite oxide, Cr-Si-Mn composite oxide, Cr-Al composite oxide, Cr-Al-Si composite oxide, Cr-Al-Mn composite oxide, Cr- Al-Mn-Si composite oxide.

  The size of the internal oxide is preferably not more than 1 μm in average diameter so that the elongation does not decrease, and in order to exhibit the effect of suppressing the movement of the grain boundary of the steel sheet, it should be 10 nm or more. preferable. The number of oxides is not particularly limited, but at the time of cross-sectional observation, it is preferable that at least one oxide is present in the length of 100 μm in the plate width direction of the cross section at a depth d (μm).

  The steel sheet of the present invention may contain one or both of B: 0.010% or less and P: 0.100% or less in addition to the above components.

B: 0.010% or less B is an element that reinforces grain boundaries and improves secondary workability, but is also an element that deteriorates plating properties. Therefore, the upper limit is 0.010%, preferably 0.0075%. Although a minimum is not specifically limited, 0.0001% or more is preferable at the point which ensures the said improvement effect.

P: 0.1% or less P is an element that increases the strength of steel, but is also an element that segregates at the center of the plate thickness of the steel sheet and embrittles the weld. Therefore, the upper limit is made 0.10%. Preferably it is 0.08% or less. Although a minimum is not specifically limited, 0.001% or more is preferable at the point which ensures the strength improvement effect.

  The steel sheet of the present invention inevitably contains S and N as elements other than those described above, but S is preferably 0.02% or less, and N is preferably 0.01% or less. In addition, the steel sheet of the present invention is one or more of Ti, Nb, Mo, W, Co, Cu, Ni, Sn, V, and REM as necessary, as long as the properties of the steel sheet of the present invention are not impaired. It may contain.

  The reason for limiting the component composition of the plating layer of the steel sheet of the present invention will be described. % Related to the component composition means mass%.

Fe: 7 to 15%
If it is less than 7%, it becomes unalloyed and not only the surface appearance is bad, but also the flaking resistance during pressing is inferior. On the other hand, if it exceeds 15%, it becomes an overalloy and the powdering resistance at the time of pressing becomes inferior, so Fe in the plating layer is made 7 to 15%.

Al: 0.01 to 1%
If it is less than 0.01%, the alloying reaction of Zn-Fe proceeds excessively in the plating layer during the production of the steel sheet, and if it exceeds 1%, conversely, the effect of suppressing the Zn-Fe alloying reaction by Al Becomes noticeable, the line speed must be reduced to advance the Zn-Fe reaction, and the productivity is deteriorated. Therefore, Al in the plating layer is set to 0.01 to 1%.

  Next, the structural features of the steel sheet of the present invention will be described.

When manufacturing an alloyed hot-dip galvanized steel sheet using a steel sheet containing an easily oxidizable element such as Mn, Si, Al, and Cr as the plating base plate, the hydrogen partial pressure (P _H2 ) in the atmosphere during annealing before plating And adjusting the water vapor partial pressure (P _H2O ) to control the high-temperature oxidation behavior of the original plating plate, and to generate internal oxides such as Mn, Si, Al, and Cr inside the steel plate, so that the plating wettability There is a technique for improving the Zn-Fe alloying reaction between the original plate and the plating layer.

  In this method, the internal oxide generated in the vicinity of the steel sheet surface during annealing is dispersed throughout the plating layer after the alloying treatment.

  This is thought to be because, when alloying treatment, Fe diffuses into the plating layer from the steel plate side, and at the same time, the internal oxide diffuses into the plating layer. When the oxide is dispersed, the plastic deformability of the alloy plating layer is reduced, and cracking and peeling of the plating layer are likely to occur when stress is applied to the plating layer.

  In order to provide an alloyed hot-dip galvanized steel sheet that is excellent as a structural material for bonding, the present inventors diligently studied the form of the plating layer directly responsible for bonding with other members.

  As a result, in the plated steel sheet after the plating treatment, the form of the plating layer is such that the steel plate side is (x1) one or more of oxides of Mn, Si, Al, and Cr, and / or (x2) Zn—Fe alloy phase including one or more complex oxides of two or more of Mn, Si, Al, and Cr, and the surface layer includes Zn— containing a ζ phase that does not include the oxide. It has been found that when the Fe alloy phase is used, the adhesive strength with other members is dramatically improved.

  The reason why the adhesive strength is dramatically improved is considered as follows.

  Among the Zn-Fe alloy phases, the ζ phase is relatively soft and does not contain the oxide, so it has a certain degree of deformability, and when stress is applied to the surface layer of the plating layer, It can be deformed to some extent. Therefore, when it adheres to another member with an adhesive, the adhesion with the other member becomes dense.

  According to the Fe-Zn phase diagram, the steel sheet of the present invention in which the surface layer of the plating layer includes the ζ phase is obtained by performing the alloying process at a relatively low temperature of 500 ° C. or less, which is the peritectic temperature of the ζ phase. When the alloying temperature exceeds 500 ° C., the ζ phase becomes unstable and separates into a δ1 phase and a Zn phase. When the alloying process is further advanced, the entire plated layer becomes a δ1 phase due to diffusion of Fe from the steel sheet.

  When an alloyed hot-dip galvanized steel sheet is manufactured at a temperature of 500 ° C. or less when a high-strength steel sheet containing Si or the like is used as a base sheet by using a general Sendzimer method, if the alloying process is performed at 500 ° C. or less, Fe Enormous amount of time is required to secure a certain amount of Therefore, it is usually not easy to produce an alloyed hot-dip galvanized steel sheet by alloying at 500 ° C. or lower.

  However, the present inventors dramatically improved the Zn-Fe alloying reaction during the alloying treatment by adjusting the oxygen potential and the temperature rising rate of the atmosphere during annealing before plating. It has also been found that an alloyed hot-dip galvanized steel sheet can be manufactured at an alloying temperature of 500 ° C. or lower even in a steel sheet production line.

  Here, the manufacturing method of this invention steel plate is demonstrated.

  When producing alloyed hot-dip galvanized steel sheets in the total reduction furnace type (RTF) line, while adjusting the oxygen potential in the annealing furnace to reduce the oxide film present on the steel sheet surface, Mn, Si, Al, and Cr (easily oxidizable elements) can be oxidized.

  The structure of the steel sheet before annealing is usually a structure as it is rolled, and in many cases, it is composed of fine crystal grains having a particle size of the order of submicrons. When this microstructure is heated in an annealing furnace and reaches a certain temperature or higher, recovery and recrystallization occur, and the crystal grains gradually become coarser.

  However, if the oxidation of the easily oxidizable element in the steel sheet can proceed before the crystal grains near the surface of the steel sheet become coarse in the heating process of the steel sheet, the internal oxidation proceeds preferentially at the grain boundaries. Since the internal oxide generated by the internal oxidation suppresses the movement of the crystal grain boundary, the structure in the vicinity of the steel sheet surface can be kept fine.

  At this time, by adjusting the atmosphere and heating rate of the heating process, the progress of internal oxidation can be controlled, and the thickness of the microstructure near the steel sheet surface can be controlled. When the steel plate surface is composed of a fine structure, Zn easily penetrates into the steel plate through the grain boundaries on the steel plate surface.

  Therefore, if the thickness of the microstructure is greater than the thickness corresponding to the amount of Fe consumed during the alloying process, the reaction between Fe and Zn corresponding to the thickness of the microstructure proceeds explosively during the alloying process. However, in a normal hot-dip plated steel sheet production line, even if the alloying temperature is 500 ° C. or lower, the amount of Fe in the plating layer can be sufficiently secured.

  At this time, there is no internal oxide generated in the vicinity of the steel sheet surface during annealing in the ζ phase generated on the surface side of the plating layer. The reason for this is not clear, but the ζ phase is not generated during the alloying process, and during the immersion in the plating bath, Fe eluted from the steel plate surface into the plating bath reacts with Zn in the bath to cause Zn on the steel plate surface. It is thought that it was precipitated as a -Fe alloy phase.

  Here, FIG. 1 shows a microstructure near the interface between the steel sheet and the plated layer after the alloying treatment. Between the steel plate 1 and the plating layer 2, a fine structure including the internal oxide 3 is formed. FIG. 2 shows a surface layer of a plating layer containing a ζ phase that does not include an internal oxide. In the plating layer 2 shown in FIG. 2, black spots are internal oxides, but black spots (internal oxides) do not exist on the surface layer of the plating layer 2. That is, the surface layer of the plating layer is a ζ phase.

  When Fe elutes in the plating bath, some amount of internal oxide generated during annealing also elutes in the plating bath, but when the Zn-Fe alloy layer precipitates in the bath, the internal oxide It is assumed that it is not incorporated into the strata.

The reducing annealing atmosphere before plating is composed of nitrogen containing 0.1 to 50% by volume of hydrogen. When hydrogen is less than 0.1% by volume, the oxide film present on the surface of the steel sheet cannot be sufficiently reduced before annealing, and the plating wettability cannot be improved. If hydrogen exceeds 50% by volume, the dew point corresponding to the required water vapor partial pressure (P _H2O ) becomes too high, leading to an increase in production costs such as the introduction of equipment for preventing condensation in the apparatus. Therefore, hydrogen in the nitrogen atmosphere is 0.1 to 50% by volume.

The log (P _H2O / P _H2 ) of the annealing atmosphere is 0 or less. If log (P _H2O / P _H2 ) is increased, alloying is promoted, but if it exceeds 0, the oxide film formed on the surface of the steel sheet before annealing cannot be sufficiently reduced, and plating wettability cannot be secured. The upper limit of (P _H2O / P _H2 ) was 0. More preferably, it is -0.1 or less.

  In addition, the dew point of the annealing reduction atmosphere shall be more than -30 degreeC-20 degreeC. If it is −30 ° C. or lower, it becomes difficult to secure an oxygen potential necessary for internal oxidation of easily oxidizable elements such as Si and Mn in the steel, so the dew point is set to be over −30 ° C. Preferably, it is −25 ° C. or higher. On the other hand, if the temperature exceeds 20 ° C., dew condensation on the piping through which the reducing gas flows becomes remarkable, and stable atmosphere control becomes difficult. Therefore, the dew point is set to 20 ° C. or less. Preferably, it is 15 degrees C or less.

  The lower limit of the maximum temperature reached for annealing is 750 ° C. If the maximum temperature reached is less than 750 ° C., the oxide film generated on the surface of the steel plate before annealing cannot be sufficiently reduced, and the plating wettability cannot be ensured. The upper limit of the maximum temperature reached for annealing is 900 ° C. When the annealing temperature exceeds 900 ° C., the press formability deteriorates and the amount of heat necessary for heating increases, leading to an increase in manufacturing cost. Preferably it is 760-880 degreeC.

  When the steel sheet reaches 600 ° C. in a reduced annealing atmosphere, a certain temperature region of 600 ° C. or higher (T1 or more and T2 region) is heated to 750 to 900 ° C. at a heating rate of 6 ° C./second or less. I do. When the heating rate exceeds 6 ° C./second, the heating rate is too fast, and the crystal grains inside the steel sheet become coarse before the internal oxidation proceeds sufficiently, and the structure form required by the present invention cannot be obtained. Therefore, the heating rate is set to 6 ° C./second or less. Preferably it is 4 degrees C / sec or less. The lower limit is not particularly defined, but is preferably 0.2 ° C./second or more from the viewpoint of productivity.

  The component composition and dew point of the reduction annealing atmosphere, and the heating rate and annealing temperature of the steel sheet are important in forming a desired “microstructure containing internal oxides” in the vicinity of the steel sheet surface.

  The steel sheet of the present invention is characterized in that after the alloying treatment, a “microstructure containing an internal oxide” remains on the steel sheet side adjacent to the plating layer. Therefore, it is necessary to form a “fine structure including an internal oxide” having a predetermined thickness in the vicinity of the surface of the steel plate. The thickness of the “microstructure containing the internal oxide” is adjusted by adjusting the component composition and dew point of the reducing annealing atmosphere, the heating rate and the annealing temperature of the steel sheet, and including the “internal oxide” of the desired thickness. A “fine structure” is formed.

  The hot dip galvanizing is performed at a bath temperature of 430 to 500 ° C. using a galvanizing bath containing Al: 0.01 to 1%. If the Al content is less than 0.01%, the Zn-Fe alloy phase grows rapidly in the plating bath, and depending on the steel type, a plating layer with excellent powdering resistance can be obtained by controlling only the immersion time. At the same time, the amount of bottom dross generated in the plating bath is remarkably increased, and surface defects due to dross cause poor appearance of the plated steel sheet.

  If Al exceeds 1%, the effect of suppressing the Zn-Fe alloying reaction by Al becomes remarkable, so that the line speed must be reduced to advance the Zn-Fe reaction, and the productivity is deteriorated. .

  If the bath temperature of the galvanizing bath is less than 430 ° C., the melting point of zinc is about 420 ° C. Therefore, if temperature control becomes unstable, there is a concern that a part of the plating bath solidifies. When it exceeds 500 ° C., the life of equipment such as a sink roll and a zinc pot is shortened. Therefore, the bath temperature of the galvanizing bath is preferably 430 to 500. More preferably, it is 440-480 degreeC.

  The plating adhesion amount is not particularly limited, but is preferably 1 μm or more in terms of one-side adhesion amount from the viewpoint of corrosion resistance. From the viewpoint of workability, weldability, and economy, the amount of adhesion on one side is desirably 20 μm or less.

  The alloying temperature is 420 to 500 ° C. When the temperature is lower than 420 ° C., the progress of the alloying reaction is delayed, and the possibility that Zn remains on the surface of the plating layer increases. If it exceeds 500 ° C., the ζ phase becomes unstable during the alloying treatment, and the desired adhesive strength cannot be obtained.

  The plated steel sheet of the present invention is subjected to upper layer plating for the purpose of improving paintability and weldability, and various chemical conversion treatments such as phosphate treatment, weldability improvement treatment, lubricity improvement treatment, etc. This does not depart from the present invention.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
An alloyed hot-dip galvanized steel sheet was manufactured using a cold-rolled steel sheet having the composition shown in Table 1 as a plating original sheet and a vertical hot-dip plating simulator. Table 2 shows the conditions for reduction annealing before plating. The retention temperature after reaching the maximum temperature was 100 seconds.

  After annealing, the steel sheet was continuously cooled to 450 ° C. in nitrogen gas and immersed in a molten zinc bath containing 0.12% Al for 3 seconds. The temperature of the hot dip galvanizing bath was 450 ° C., the same as the temperature at which the steel sheet entered the bath.

  After plating, the basis weight of zinc was adjusted to 5 to 15 μm with a gas wiper, and an alloying treatment was performed. The alloying temperature was 450 to 500 ° C., and the amount of Fe in the plating layer was 9 to less than 11%. After the alloying treatment, the steel sheet was cooled to room temperature with nitrogen gas. The component composition of the plating layer was measured by dissolving the plating layer with an acid and then chemically analyzing it using ICP.

  The cross-sectional structure of the plating layer was observed using a SEM after polishing the cross section of the hot dip galvanized steel sheet, and the distribution of oxide particles in the plating layer was confirmed. The phase configuration of the plating layer was performed by a constant current electrolysis method, and the presence or absence of the ζ phase was confirmed.

  Table 2 shows the results of conducting a tensile shear test on the manufactured plated steel sheet and examining the adhesion at the plating / steel sheet interface, together with the annealing conditions and the observation results of the sectional structure of the plated layer.

  Evaluation of joining strength was performed as follows in a tensile shear test.

  The alloyed hot-dip galvanized steel sheet with a thickness of 0.8 mm produced by the above method is cut into 100 mm × 25 mm, and two of them are prepared, and in a state where they are shifted from each other by 12.5 mm in the plate length direction, Adhesive was applied to the overlapping portions and joined.

  A commercially available epoxy adhesive was used as the adhesive, and it was applied to a 25 mm × 12.5 mm adhesive surface with a thickness of about 100 μm. The prepared test piece was left in a freezer for 5 hours, and then pulled at a rate of 50 mm / min in an atmosphere of 0 ° C. to perform a tensile shear test. The maximum load until failure was measured, and the adhesive strength was evaluated by the tensile shear strength obtained by dividing the maximum load by the shear area (adhesion area). The evaluation results are shown in Table 2. The evaluation criteria are as follows.

◎: 180kgf / mm ² or more ○ +: 160kgf / mm ² or more and less than ^{180kgf / mm 2 ○: 140kgf /} mm 2 or more and less than ^{160kgf / mm 2 △: 120kgf /} mm 2 or more, 140kgf / mm ² less than ×: 120kgf / Mm ² less

  As described above, according to the present invention, an alloyed hot-dip galvanized steel sheet having dramatically improved adhesive strength can be provided. Therefore, the present invention has high applicability in the galvanized steel sheet manufacturing industry.

1 Steel plate 2 Plating layer 3 Internal oxide

Claims (7)

In mass%, C: 0.05 to 0.50%, Mn 0.01 to 3.0%, Si: 3.0% or less, Al: 2.0% or less, Cr: 2. One or two or more of 0% or less, Mn + Si + Al + Cr: 0.4% or more, Fe: 7 to 15%, Al: 0.01 to 1 on the surface of the steel plate made of the remaining Fe and inevitable impurities %, In the alloyed hot-dip galvanized steel sheet having a plating layer consisting of the balance Zn and inevitable impurities,
(X) The steel plate side of the plating layer is one or more of (x1) Mn, Si, Al, and Cr oxides and / or (x2) Mn, Si, Al, and Cr. A Zn-Fe alloy phase containing one or more of complex oxides composed of two or more,
(Y) An alloyed hot-dip galvanized steel sheet having excellent plating adhesive strength, wherein the surface layer of the plating layer is a Zn—Fe alloy phase containing a ζ phase that does not contain the oxide and / or the composite oxide. .   2. The alloy having excellent adhesive strength according to claim 1, wherein the Zn—Fe alloy phase is formed by a reaction between Zn infiltrated from a plating layer and Fe in a steel plate during alloying treatment. Hot-dip galvanized steel sheet.   The alloyed hot-dip galvanized steel sheet with excellent adhesive strength according to claim 1 or 2, wherein the steel sheet further contains B: 0.010% or less in terms of mass%.   The alloyed hot-dip galvanized steel sheet with excellent adhesive strength according to any one of claims 1 to 3, wherein the steel sheet further contains P: 0.10% or less by mass%. In the manufacturing method of the galvannealed steel plate excellent in the adhesive strength of any one of Claims 1-4,
The steel sheet having the component composition according to any one of claims 1 to 4, comprising hydrogen: 0.1 to 50% by volume, balance: nitrogen and inevitable impurities, and log (P _H2O / P _H2 ) is 0 or less. In an atmosphere, a certain temperature region of 600 ° C. or higher (T1 or more and T2 region) is annealed by heating to 750 to 900 ° C. at a maximum of 6 ° C./second, followed by hot dip galvanization, A method for producing an alloyed hot-dip galvanized steel sheet having excellent adhesive strength, characterized by performing an alloying treatment at 420 to 500 ° C.   6. The method for producing an alloyed hot-dip galvanized steel sheet having excellent adhesive strength according to claim 5, wherein the annealing is performed in a total reduction furnace of a continuous hot-dip plating facility.   The adhesion according to any one of claims 1 to 6, wherein the hot dip galvanizing is performed at a bath temperature of 430 to 500 ° C using a galvanizing bath containing Al: 0.01 to 1%. A method for producing a galvannealed steel sheet having excellent strength.