JP6124499B2 - High-strength galvannealed steel sheet with excellent plating adhesion and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength galvannealed steel sheet excellent in plating adhesion 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, the applicant, in Patent Document 5, not only improves the plateability by controlling the oxygen potential in the annealing atmosphere, but also reduces the amount of C on the surface of the steel sheet, Improves ductility, suppresses the occurrence of cracks, and further generates Si, Mn oxide in the vicinity of the steel sheet surface layer, even if cracks occur, suppresses the propagation of cracks with this oxide, A technology to ensure bendability is proposed.

  However, in this technique, even if the steel sheet is annealed under the condition of internal oxidation, the oxide generated at the plating / steel sheet interface is not completely lost, and the plating layer / steel sheet interface caused by the generation behavior of the internal oxide is not eliminated. Depending on the properties, the adhesion between the steel sheet and the plating layer is deteriorated, and the problem that the plating peels off during processing is likely to occur.

  Moreover, when a plated steel plate is manufactured using these methods, as described in Patent Document 4, oxide particles containing Si and Mn are dispersed in the plating layer after the alloying treatment.

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 becomes harder and more brittle as the amount of Fe increases. However, when the oxide particles are dispersed in the Zn-Fe alloy phase, the plastic deformability of the alloy phase decreases, When stress is applied to the plating layer, the plating layer is easily cracked or peeled off.

  For example, Patent Document 6 describes a problem that occurs at the interface between a plating layer and a steel sheet in response to the problems of plating peeling and powdering resistance degradation that occur when an alloyed hot-dip galvanized steel sheet is manufactured using a high-strength steel sheet as an original sheet. Paying attention to the shape of the structure consisting of the Si-Mn oxide and the Zn-Fe intermetallic compound to improve 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 Technology is disclosed.

  However, this technique employs a process in which the steel sheet is heated in an oxidizing atmosphere and then held in a reducing atmosphere for a certain period of time in annealing before plating, and the interface state between the plated layer and the steel sheet after alloying treatment is adopted. In order to obtain a predetermined state, the annealing atmosphere must be strictly controlled.

  In Patent Document 7, the penetration depth of the Zn—Fe intermetallic compound in the depth direction on the steel sheet side is controlled to 10 μm or less from the interface between the plating layer and the steel sheet to improve powdering resistance and plating adhesion. Technology is disclosed. However, in recent years, higher workability is required for high-strength galvannealed steel sheets, including those for automobiles. By controlling the maximum penetration depth of Zn-Fe intermetallic compounds, severe processing is required. It is difficult to ensure endurance plating adhesion.

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

  An object of this invention is to provide the high strength alloying hot dip galvanized steel plate which was remarkably excellent in plating adhesiveness, and its manufacturing method in view of the said present condition which concerns on a high strength alloying hot dip galvanized steel plate.

  The present inventors diligently studied a method for improving the plating adhesion of a high-strength galvannealed steel sheet (hereinafter sometimes collectively referred to as “plated steel sheet”). As a result, in the plated steel sheet after the plating treatment, in the vicinity of the interface between the plating layer and the steel sheet, (i) the structure formed on the steel sheet side and the state of oxide formation, and (ii) Zn from the plating layer side to the steel sheet It was found that the presence form of the Zn—Fe alloy phase produced by intrusion into the steel greatly affects the improvement of the plating adhesion.

  And the present inventors discovered that the said subject could be solved if the structure | tissue near the interface of a plating layer and a steel plate was controlled based on the said clear fact.

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

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) One of oxides of (x2) Mn, Si, Al, and Cr, or (x1) having a crystal grain size of 2 μm or less in a region within 10 μm from the interface between the steel plate and the plating layer to the steel plate side or There is a microstructure that includes one or more of two or more and / or a composite oxide composed of two or more of Mn, Si, Al, and Cr,
(Y) A high-strength alloy excellent in plating adhesion, characterized in that a Zn-Fe alloy phase is present around a part of the oxide and / or composite oxide at the grain boundary of the microstructure. Hot-dip galvanized steel sheet.

  (2) The plating adhesion according to (1), wherein the Zn—Fe alloy phase is formed by a reaction between Zn entering from the plating layer and Fe in the steel sheet during the alloying treatment. High strength alloyed hot-dip galvanized steel sheet with excellent properties.

  (3) The steel sheet further contains, in mass%, B: 0.010% or less, and the high strength alloying and melting excellent in plating adhesion according to (1) or (2) above Galvanized steel sheet.

  (4) The high strength with excellent plating adhesion according to any one of (1) to (3), wherein the steel sheet further contains P: 0.10% or less by mass%. Alloyed hot-dip galvanized steel sheet.

(5) In the method for producing a high-strength galvannealed steel sheet having excellent adhesion according to any one of (1) to (4),
The steel sheet having the component composition according to any one of (1) to (4), comprising hydrogen: 0.1 to 50% by volume, the balance: nitrogen and inevitable impurities, and dew point: an atmosphere of more than −30 ° C. to 20 ° C. In the temperature range of 650 ° C. or higher and 740 ° C. or lower, the region of T1 ° C. or higher and T2 ° C. or lower is set at a heating rate of 4 ° C./second or less, and the residence time in the region of T1 ° C. or higher and T2 ° C. or lower is High strength alloyed hot dip galvanized with excellent adhesion, characterized by heating to 750 to 900 ° C. and annealing, followed by hot dip galvanizing and then alloying A method of manufacturing a steel sheet.

  (6) The method for producing a high-strength galvannealed steel sheet having excellent adhesion as described in (5) above, 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 500 ° C. using a galvanizing bath containing Al: 0.01 to 1%. The manufacturing method of the high intensity | strength galvannealed steel plate excellent in the adhesiveness of description.

  (8) The method for producing a high-strength galvannealed steel sheet having excellent adhesion according to any one of (1) to (7), wherein the alloying treatment is performed at 450 to 550 ° C. .

  According to the present invention, it is possible to provide a high-strength galvannealed steel sheet having improved plating adhesion than ever before.

It is a figure which shows typically the mechanism in which plating adhesiveness improves notably. (A) shows the aspect which gave the zinc plating to the fine-structure steel plate which includes an internal oxide, (b) shows the Zn which penetrate | invaded from the plating layer, and Fe in a steel plate react, and it is in a crystal grain boundary. The aspect of the wedge-shaped Zn-Fe alloy phase produced | generated around the internal oxide which exists is shown, (c) is a figure which shows the aspect of the Zn-Fe plating layer formed by alloying process. It is a figure which shows the correlation of the "microstructure containing an internal oxide" formed in the steel plate surface vicinity, and a plating layer. (A) schematically shows an embodiment of “a fine structure including an internal oxide” formed in the vicinity of the steel sheet surface, and (b) is a “fine structure including an internal oxide” remaining on the steel sheet side. The aspect of is shown typically. It is a figure which shows the fine structure which includes the internal oxide after annealing. 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.

  Hereinafter, the present invention will be described in detail.

The high strength alloyed hot dip galvanized steel sheet (hereinafter sometimes referred to as “the present invention steel sheet”) having excellent plating adhesion of the present invention is mass%, C: 0.05 to 0.50%, and Mn is 0. 0.01 to 3.0%, Si: 3.0% or less, Al: 2.0% or less, Cr: 2.0% or less, or Mn + Si + Al + Cr: 0. 4% or more of alloyed hot dip zinc having a plating layer consisting of Fe: 7-15%, Al: 0.01-1%, the balance Zn and inevitable impurities on the surface of the steel plate consisting of the balance Fe and inevitable impurities In plated steel sheet,
(X) One of oxides of (x2) Mn, Si, Al, and Cr, or (x1) having a crystal grain size of 2 μm or less in a region within 10 μm from the interface between the steel plate and the plating layer to the steel plate side or There is a microstructure that includes one or more of two or more and / or a composite oxide composed of two or more of Mn, Si, Al, and Cr,
(Y) A Zn—Fe alloy phase is present around a part of the oxide and / or the composite oxide at a grain boundary of the microstructure .

  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 is deteriorated and the practicality of the steel sheet of the present invention is lowered. And Preferably it is 0.10 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).

  In addition to the above components, the steel sheet of the present invention may contain one or both of B: 0.010% or less and P: 0.10% or less.

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: 0.10% or less P is an element that enhances the strength of steel, but it 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%, on the contrary, the Zn-Fe alloying reaction by Al is suppressed. When the effect becomes remarkable, the Zn-Fe reaction is advanced, so the line speed must be reduced, and the productivity is deteriorated. Therefore, Al in the plating layer is set to 0.01 to 1%.

  Next, organizational features of the present invention will be described.

  When producing alloyed hot dip galvanizing 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 (an easily oxidizable element) 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, by adjusting the oxygen potential and temperature rising pattern in the annealing furnace, before the crystal grains near the steel sheet surface are coarsened, Mn, Si, Al, and Cr (easy oxidizable elements) in the steel sheet are changed. It can be preferentially oxidized at the grain boundaries of the steel sheet.

  The internal oxide produced by the preferential oxidation suppresses the movement of the crystal grain boundary, so that the microstructure is kept fine as it is rolled near the steel sheet surface, and the internal oxide is included in the crystal grain boundary near the steel sheet surface. A fine structure can be formed.

  In the steel sheet of the present invention, Zn entering from the plating layer reacts with Fe in the steel sheet around the internal oxide at the grain boundary of the microstructure obtained by the grain growth inhibiting action of the internal oxide generated by annealing. Thus, the Zn—Fe alloy phase is formed in a wedge shape. Since the Zn—Fe alloy phase has a wedge shape, the adhesion between the steel sheet and the plating layer is significantly improved. This adhesion improving mechanism will be described with reference to the drawings.

  FIG. 1 schematically shows a mechanism for significantly improving plating adhesion. FIG. 1 (a) shows an embodiment in which a fine-structure steel sheet containing an internal oxide is galvanized, and FIG. 1 (b) shows the reaction between Zn invading from the plating layer and Fe in the steel sheet. An embodiment of a wedge-shaped Zn—Fe alloy phase generated around an internal oxide existing at a grain boundary is shown, and FIG. 1C shows an embodiment of a Zn—Fe plating layer formed by alloying treatment.

  As shown in FIG. 1A, a plating layer 2 is formed on a steel plate having a microstructure 1 that contains an internal oxide 4. Although the internal oxide 4 exists at almost all grain boundaries, Zn easily penetrates from the plating layer 2 at the crystal grain boundary where the internal oxide 4 exists, and the internal oxide 4 exists due to alloying treatment. Zn entering from the plating layer 2 and Fe in the steel sheet are bonded to a part of the crystal grain boundary, and a wedge-shaped Zn—Fe alloy is formed around the inner oxide 4 as shown in FIG. A phase (intermetallic compound) 5 is formed.

  As the alloying process proceeds, as shown in FIG. 1 (c), the alloy plating layer 3 takes in a fine structure in the vicinity of the steel sheet surface and grows inside. The wedge-shaped Zn—Fe alloy phase (intermetallic compound) 5 existing inside the microstructure 1 in the vicinity firmly bonds the alloy plating layer 3 and the microstructure 1 in the vicinity of the steel plate surface, and the alloy plating layer 3 and the steel plate. It has been found that the adhesion of can be dramatically improved. This is the knowledge forming the basis of the present invention.

  As described above, the microstructure near the steel sheet surface is taken into the alloy plating layer from the steel sheet surface side by alloying treatment, but the inventors adjusted the annealing atmosphere and heating rate to adjust the internal structure. It has been found that the thickness of the microstructure near the surface of the steel sheet can be controlled by controlling the progress of oxidation. The adjustment of the annealing atmosphere and the heating rate will be described later.

  If the microstructure containing the internal oxide is formed with a certain thickness, alloying at the interface between the steel plate and the plating layer proceeds rapidly, and after the alloying process is completed, Some will remain.

  FIG. 2 shows the interrelationship between the “fine structure containing internal oxides” formed in the vicinity of the steel sheet surface and the plating layer. FIG. 2 (a) schematically shows an embodiment of “a fine structure including an internal oxide” formed in the vicinity of the steel sheet surface, and FIG. 2 (b) illustrates an “internal oxide included” remaining on the steel sheet side. An embodiment of “fine structure” is schematically shown.

  When a plating layer is formed on the surface of the steel plate shown in FIG. 2 (a) and alloying is performed, the alloy plating layer takes in the “microstructure containing the internal oxide” as described above, and on the steel plate side. Although it grows, in the steel sheet of the present invention, as shown in FIG. 2 (b), the "fine structure including the internal oxide" is left on the steel sheet side.

  Since the “wedge-like Zn—Fe alloy phase” present at the grain boundary of the “microstructure containing the internal oxide” plays a role of systematically connecting the alloy plating layer and the steel plate, the steel plate of the present invention is in close contact with the plating. Sexually improves.

  In order to ensure a dramatic improvement in plating adhesion, in the steel sheet of the present invention, (x1) the crystal grain size is 2 μm or less and (x2) internal oxide (on the steel sheet side within 10 μm from the interface between the steel sheet and the plating layer) 1 or 2 or more types of oxides of Mn, Si, Al and Cr, and / or 1 or 2 or more types of composite oxides comprising 2 or more types of Mn, Si, Al and Cr) It is characterized by leaving a fine structure that encloses.

  As described above, the structure of the steel sheet before annealing is usually a rolled structure, and in many cases, the structure is composed of fine crystal grains having a particle size of submicron order. Based on this, in order to form a sufficient amount of “wedge-like Zn—Fe alloy phase” at the crystal grain boundary, the microstructure is defined as a microstructure having a crystal grain size of 2 μm or less. Preferably, the particle size of the fine structure is 1 μm or less.

  When the range of the fine structure exceeds “10 μm from the interface between the steel plate and the plating layer”, the presence of excessive internal oxide structure makes the vicinity of the steel plate surface brittle, and cracks are likely to occur during bending. Therefore, the existence range of the fine structure is defined as “within 10 μm from the interface between the steel plate and the plating layer” on the steel plate side.

  In order to ensure sufficient bendability, it is preferable that the region where the fine structure including the internal oxide is present is “within 5 μm from the interface between the steel plate and the plating layer”.

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

  The method for producing a steel sheet of the present invention comprises a steel sheet having a predetermined component composition comprising hydrogen: 0.1 to 50% by volume, the balance: nitrogen and inevitable impurities, in an atmosphere with a dew point of more than −30 ° C. to 20 ° C. A certain temperature range (T1 or more and T2 or less) at ℃ or higher is heated to 750 to 900 ℃ at 6 ℃ / second or less, then hot dip galvanized, and then alloyed. It is characterized by that.

  The annealing is preferably performed in a total reduction furnace of a continuous hot dip plating facility. The reducing annealing atmosphere before plating is an atmosphere composed of hydrogen: 0.1 to 50% by volume and the balance: nitrogen and inevitable impurities. If the hydrogen content is less than 0.1% by volume, the oxide film present on the surface of the steel sheet cannot be sufficiently reduced, and plating wettability cannot be ensured. Therefore, the amount of hydrogen in the reduction annealing atmosphere is set to 0.1% by volume or more.

If the hydrogen in the reducing annealing atmosphere exceeds 50% by volume, the dew point (corresponding to the water vapor partial pressure P _H2O ) increases too much, and it is necessary to introduce equipment for preventing dew condensation. Since the introduction of new equipment causes an increase in production cost, the amount of hydrogen in the reduction annealing atmosphere is set to 50% by volume or less. Preferably, it is 0.1-40 volume%.

  The dew point of the annealing reduction atmosphere is more than −30 ° C. to 20 ° C. 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 steel, so the dew point is 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.

It is preferable to adjust the log (P _H2O / P _H2 ) of the reduction annealing atmosphere to 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 plate before annealing cannot be reduced sufficiently and plating wettability cannot be ensured. Therefore, the upper limit of log (P _H2O / P _H2 ) is set to zero. More preferably, it is -0.1 or less.

In the reduction annealing atmosphere, when the steel sheet reaches 600 ° C., a certain temperature region of 600 ° C. or higher (T1 ° C. or higher and T2 ° C. or lower region) is heated at a heating rate of 6 ° C./second or lower, and a maximum of 750 to 900 Heat to ℃ and anneal. When the heating rate in the region of T1 ° C. or more and T2 ° C. or less exceeds 6 ° C./second, the heating rate is too fast, and the crystal grains inside the steel sheet become coarse before the internal oxidation sufficiently proceeds. 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.
In the claims, based on the examples, the region of T1 ° C. or more and T2 ° C. or less is set within the temperature region of 650 ° C. or more and 740 ° C. or less, and the heating rate of the region of T1 ° C. or more and T2 ° C. or less is 4 ° C. / The time for heating at a heating rate of 4 ° C./second or less, that is, the residence time between T1 ° C. and T2 ° C. was set to 10 seconds or longer.

  If the annealing temperature is less than 750 ° C., the oxide film formed on the surface of the steel plate before annealing cannot be sufficiently reduced, and plating wettability may not be ensured. Therefore, the annealing temperature is set to 750 ° C. or higher. If the annealing temperature exceeds 900 ° C., the press formability deteriorates and the amount of heat required for heating increases, resulting in an increase in manufacturing cost. Therefore, the annealing temperature is set to 900 ° C. or less. Preferably, it is 760-880 degreeC.

  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%.

  When Al is less than 0.01%, a Zn-Fe alloy layer grows rapidly in the plating bath, and depending on the steel type, not only a desired plating layer of only immersion time can be formed, but also the plating bath The amount of bottom dross generated inside increases, surface defects due to dross occur, and the appearance of the steel sheet deteriorates.

  On the other hand, if the Al content 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 increased. Deteriorate.

  When the bath temperature of the galvanizing bath is less than 430 ° C., the melting point of zinc is about 420 ° C., so that the bath temperature control becomes unstable and there is a concern that the bath partially solidifies. When the bath temperature exceeds 500 ° C., the service life of facilities such as sink rolls and zinc pots is shortened. Therefore, the bath temperature of the galvanizing bath is preferably 420 to 500 ° C. More preferably, it is 440-480 degreeC.

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

  The alloying treatment is performed at 450 to 550 ° C. When the alloying temperature is lower than 450 ° C., the progress of alloying is slowed and there is a possibility that the Zn layer remains on the plating surface layer. When the alloying speed exceeds 550 ° C., alloying proceeds too much, and a brittle Γ phase can be thickened at the plated steel sheet interface, so that the plating adhesion during processing is reduced.

  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 component composition shown in Table 1 as a plating original sheet and using a vertical hot-dip plating simulator. Table 2 shows the reduction annealing conditions before plating. The maximum temperature reached 800 ° C., and the holding temperature at 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 0.13% 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 550 ° 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.

  In addition, the observation of the structure of the interface between the plating layer and the steel plate was performed by processing a steel plate cut into 10 mm × 10 mm using a cross section polisher, and then using FE-SEM at a magnification of 10000 times and 20 fields for each sample. The above was observed. The obtained image data was subjected to image analysis, and the crystal grain size in the direction parallel to the initial interface of the steel plate was measured in the structure on the steel plate side of the plating / steel plate interface. A fine grain having a crystal grain size of 2 μm or less was used.

  FIG. 3 shows the microstructure containing the internal oxide after annealing, and FIG. 4 shows the microstructure near the interface between the steel sheet after alloying and the alloy plating layer. From FIG. 3, it can be seen that a fine structure including an internal oxide is formed in the vicinity of the surface of the steel sheet. Moreover, it turns out from FIG. 4 that the fine structure which includes an internal oxide remains from the interface of a steel plate and an alloy plating layer to the steel plate side.

  For those in which crystal grains having a crystal grain size of 2 μm or less were not observed, the average grain size of the microstructure was not measured. Further, from the above image data, it was confirmed whether or not the fine structure of the Zn—Fe alloy layer as shown in FIG.

  These steel sheets were investigated for powdering resistance. The results are shown in Table 2 together with the reduction annealing conditions and the observation results of the interface structure.

  The method for evaluating the powdering resistance was as follows.

Powdering resistance Alloyed hot-dip galvanized steel sheet manufactured by the above method is cut into a width of 40 mm and a length of 250 mm, and formed with a half-round bead die of r = 5 mm with a punch shoulder radius of 5 mm and a die shoulder radius of 5 mm. Processed to a height of 65 mm. During processing, the separated plating layer was measured and evaluated according to the following criteria.

Evaluation criteria Plating peeling amount: Less than 3 g / m ² : ◎
3 g / m ² or more and less than 6 g / m ² : ○
6 g / m ² or more and less than 10 g / m ² :
10 g / m ² or more: ×

  As described above, according to the present invention, it is possible to provide a high-strength galvannealed steel sheet having dramatically improved plating adhesion. Therefore, the present invention has high applicability in the galvanized steel sheet manufacturing industry.

DESCRIPTION OF SYMBOLS 1 Fine structure 2 Plating layer 3 Alloy plating layer 4 Internal oxide 5 Zn-Fe alloy phase 6 Oxide film

Claims (8)

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) One of oxides of (x2) Mn, Si, Al, and Cr, or (x1) having a crystal grain size of 2 μm or less in a region within 10 μm from the interface between the steel plate and the plating layer to the steel plate side or There is a microstructure that includes one or more of two or more and / or a composite oxide composed of two or more of Mn, Si, Al, and Cr,
(Y) A high-strength alloy excellent in plating adhesion, characterized in that a Zn-Fe alloy phase is present around a part of the oxide and / or composite oxide at the grain boundary of the microstructure. Hot-dip galvanized steel sheet.   2. The plating adhesion 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. High strength galvannealed steel sheet.   The high-strength galvannealed steel sheet with excellent plating adhesion according to claim 1 or 2, wherein the steel sheet further contains, in mass%, B: 0.010% or less.   The high-strength alloyed molten zinc with excellent plating adhesion according to any one of claims 1 to 3, wherein the steel sheet further contains, in mass%, P: 0.10% or less. Plated steel sheet. In the manufacturing method of the high intensity | strength galvannealed steel plate excellent in the adhesiveness 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 dew point: in an atmosphere of more than -30 ° C to 20 ° C In the region of T1 ° C. to T2 ° C. within the temperature region of 650 ° C. to 740 ° C., the residence time in the region of T1 ° C. to T2 ° C. is 10 seconds or more at a heating rate of 4 ° C./second or less. High strength alloyed hot dip galvanized steel sheet with excellent adhesion, characterized by heating to 750 to 900 ° C. and annealing, followed by hot dip galvanization and then alloying Manufacturing method.   6. The method for producing a high-strength galvannealed steel sheet with excellent adhesion 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 galvanization is performed at a bath temperature of 430 to 500 ° C using a zinc plating bath containing Al: 0.01 to 1%. Of high strength alloyed hot-dip galvanized steel sheet with excellent properties.   The said alloying process is performed at 450-550 degreeC, The manufacturing method of the high intensity | strength galvannealed steel plate excellent in the adhesiveness of any one of Claims 1-7 characterized by the above-mentioned.