JP4932376B2 - High-strength hot-dip galvanized steel sheet with excellent plating properties and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet and a method for producing the same, and more specifically, has a good appearance without unplating and excellent plating adhesion and corrosion resistance, and is used in various applications such as building materials and automotive steel sheets. The present invention relates to a plated steel sheet and a method for producing the same.

  A hot-dip galvanized steel sheet is most frequently used as a plated steel sheet having good corrosion resistance. This hot dip galvanized steel sheet is usually degreased, preheated in a non-oxidizing furnace, subjected to reduction annealing in a reducing furnace to clean the surface and secure the material, immersed in a hot dip zinc bath, Manufactured by performing control. As its feature, it is excellent in corrosion resistance, plating adhesion, etc., it is widely used mainly for automobiles, building materials and the like.

  Particularly in recent years, in the automotive field, it has been desired to reduce the thickness of plated steel sheets and to increase the strength of plated steel sheets in order to ensure the function of protecting passengers in the event of a collision and to reduce the weight for the purpose of improving fuel economy. It has been said that.

  In order to increase the strength of the steel sheet without degrading the workability, it is effective to add elements such as Si, Mn, and P. Of these, Si is particularly easy to oxidize than Fe. It is known that when the contained steel plate is plated under normal hot dip galvanizing conditions, Si in the steel is concentrated on the surface during the annealing process, which causes non-plating defects and poor plating adhesion.

  As a technique for suppressing non-plating defects in a steel sheet containing Si, a method is proposed in which the steel surface is oxidized so that the thickness of the oxide film becomes 400 to 10,000 mm, and then annealed and plated in an atmosphere containing hydrogen. (For example, refer to Patent Document 1). However, in this technique, it is practically difficult to adjust the reduction time of the iron oxide film. If the reduction time is too long, it causes Si surface concentration, and if it is too short, the iron oxide film remains on the steel surface. Eventually, there is a problem that the plating defect is not completely eliminated, and that when the iron oxide film on the surface becomes too thick, the peeled oxide adheres to the roll and causes appearance defects.

  For the purpose of improving the above problems, the present inventors have proposed a manufacturing method for preventing the surface concentration of Si by reducing the surface of the steel sheet in a reduction furnace in which the atmosphere is controlled after oxidizing the surface of the steel sheet (for example, , See Patent Document 2).

  Further, a steel sheet having a Si content of 0.2 to 2.0% by mass is made of Al: 2 to 19% by mass, Mg: 1 to 10% by mass, the balance being molten Zn—Al—Mg composed of Zn and inevitable impurities. A Si-containing high-strength hot-dip galvanized steel sheet having a good corrosion resistance with a plating layer formed on the steel sheet surface was proposed (see, for example, Patent Document 3).

  In addition, a method for suppressing non-plating defects by performing pre-plating of Cr, Ni, Fe or the like on the steel sheet surface has been proposed (see, for example, Patent Documents 4 and 5). Furthermore, a method has been proposed in which an internal oxide layer is formed immediately below the surface of a steel sheet in a continuous annealing line, and at the same time, the generated surface oxide is removed by pickling, and then plating is performed in a continuous hot dip galvanizing line (for example, patents). Reference 6).

  In addition, a method of performing plating in a continuous hot dip galvanizing line after forming an oxide on the steel sheet surface layer has been proposed (see, for example, Patent Document 7).

JP 55-122865 A JP 2001-323355 A JP 2001-295018 A JP-A-56-33463 JP-A-57-79160 JP 2002-161315 A JP 2003-147486 A

  However, the above and other manufacturing techniques disclosed so far cannot completely prevent non-plating defects and poor adhesion. In the method of Patent Document 1, it is practically difficult to adjust the reduction time of the iron oxide film. If the reduction time is too long, the Si surface is concentrated, and if it is too short, the iron oxide film remains on the steel surface. Unplating defects cannot be completely prevented.

  For this reason, in Patent Document 3, by increasing the Al concentration in the bath to increase the reducing power of the bath, and further adding Mg to increase the disappearance rate of the iron oxide film, some iron oxide film remains on the surface. Although the plating can be performed even in the state and the range where the plating can be performed is widened, this method cannot prevent the non-plating defect caused by the surface concentration of Si.

Therefore, in Patent Document 2, for the purpose of suppressing non-plating defects generated due to surface concentration of Si, the reducing atmosphere is controlled to make SiO ₂ in an internal oxidation state. Although this method makes it possible to considerably reduce non-plating defects caused by Si concentration, it is still impossible to completely prevent non-plating defects and poor adhesion.

This is because the method described in the above-mentioned patent document cannot completely prevent the exposure of SiO _{2 to} the steel sheet surface even if it can prevent external oxidation of Si. Therefore, in order to prevent non-plating defects and poor adhesion, more strict control of SiO ₂ is required.

  Furthermore, since the pre-plating method as described in Patent Documents 4 and 5 requires plating equipment, it cannot be used when there is no space. There is also a problem that the production cost increases due to the installation of the pre-plating equipment.

  Further, the double annealing as in Patent Document 6 also causes a problem that the production cost increases.

  In addition, when the surface oxide layer is thin, a steel plate like Patent Document 7 diffuses Si in the reducing atmosphere of the continuous hot dipping line, and Si surface concentration occurs, which causes non-plating. Therefore, in order to avoid this, it is necessary to considerably increase the thickness of the oxide layer, which causes a problem that the production cost increases.

  Furthermore, the manufacturing technology of the steel sheet containing Si disclosed so far is focused on ensuring plating properties, and has been made to improve various performances when used as a plated steel plate such as corrosion resistance and formability. It wasn't.

  Therefore, the present invention solves the above problems and proposes a high-strength hot-dip galvanized steel sheet having a good appearance and excellent formability and corrosion resistance, and a method for producing the same.

  As a result of intensive research on the plating treatment of high-strength steel sheets, the present inventors have plated steel to which a certain amount of Si and Mn are added in a continuous hot dip galvanizing facility with optimized heat treatment conditions and plating conditions. By doing so, it was found that a high-strength hot-dip galvanized steel sheet having good appearance and excellent plating adhesion and corrosion resistance can be produced by controlling the type and position of the Si oxide.

  That is, the gist of the present invention is as follows.

(1) In mass%,
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
On the high-strength steel plate containing N: 0.0060% or less, the balance being Fe and inevitable impurities, Al: 35-85 mass%, Si: Al content of 0.5-10 mass% In the hot dip galvanized steel sheet having a galvanized layer consisting of Zn and inevitable impurities in the balance, Si is contained in the grain boundary and the crystal grain on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer A high-strength hot-dip galvanized steel sheet excellent in formability and corrosion resistance, characterized in that an oxide is present at an average content of 0.6 to 10% by mass.

(2) In mass%,
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: On a high-strength steel plate containing 0.0060% or less, the balance being Fe and inevitable impurities, Al: 35 to 85% by mass, Si: Al content of 0.5 to 10% by mass, Mg: 0.5 to 10% by mass of a hot-dip galvanized steel sheet having a galvanized layer composed of Zn and inevitable impurities, and a crystal on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer A high-strength hot-dip galvanized steel sheet with excellent appearance and excellent corrosion resistance, characterized in that an oxide containing Si is present at an average content of 0.6 to 10% by mass in grain boundaries and crystal grains.

(3) The oxide containing Si as described in the above (1) or (2) is one or more selected from SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ . A high-strength hot-dip galvanized steel sheet excellent in formability and corrosion resistance with a good appearance.

(4) In the hot-dip galvanized steel sheet according to any one of the above items (1) to (3), 1 selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ on the steel sheet surface or surface side A high-strength hot-dip galvanized steel sheet with good appearance and excellent corrosion resistance, characterized by the presence of at least one kind of Si oxide and SiO ₂ on the inner surface of the steel sheet.

(5) In mass%,
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
When N: 0.0060% or less and the balance is continuously hot dip galvanized on a high-strength steel plate made of Fe and inevitable impurities, the atmosphere of the reduction zone contains ₁ to 60% by volume of H _2. , The balance N ₂ , H ₂ O, O ₂ , CO ₂ , CO, and inevitable impurities, and the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is expressed by the following (formula 1) and (2 formula)
−0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0.000038 T ² + 0.107T−90.4 1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum temperature of steel sheet (K)
[Si%]: Si content (mass%) in the steel sheet
After Tsu lines reduced in a controlled atmosphere to satisfy, Al: 35 to 85 wt%, Si: contains 0.5 to 10 mass% of the content of Al, the balance being Zn and unavoidable impurities Hot dip galvanizing bath or Al: 35 to 85% by mass, Si: Al content of 0.5 to 10% by mass, Mg: 0.5 to 10% by mass, the balance being Zn and inevitable impurities method for manufacturing a high strength galvanized steel sheet appearance by immersion in molten zinc plating bath, wherein the row Ukoto the galvanizing treatment was excellent good formability and corrosion resistance consisting of.

(6) In mass%,
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: A slab containing 0.0060% or less and the balance of Fe and inevitable impurities is finish-rolled at a temperature of Ar ₃ point or higher, cold-rolled by 50 to 85%, and subsequently melted. When applying galvanization, the atmosphere of the reduction zone contains ₁ to 60% by volume of H ₂ , and the balance N ₂ , H ₂ O, O ₂ , CO ₂ , one or more of CO, and unavoidable impurities The logarithm log PO ₂ of the oxygen partial pressure in the atmosphere is expressed by the following formulas (1) and (2)
−0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0.000038 T ² + 0.107T−90.4 1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum temperature of steel sheet (K)
[Si%]: Si content (mass%) in the steel sheet
Using a continuous hot-dip galvanizing facility controlled to satisfy the above conditions, annealing in the two-phase coexistence temperature range of ferrite and austenite of 1023K to 1153K, and the average cooling rate from 0.5 to 923K is 0.5 to In a manufacturing method in which 923 K to 773 K are subsequently cooled at an average cooling rate of 3 degrees / second or more at 10 degrees / second, and further cooled at an average cooling speed of 0.5 degrees / second or more from 773 K, a temperature of 823 K to 923 K. A hot dip galvanizing bath containing Al: 35 to 85% by mass and Si: Al content of 0.5 to 10% by mass, the balance being Zn and inevitable impurities, or Al: 35 to 85 Molten zinc containing 0.5% by mass, Si: Al content of 0.5 to 10%, Mg: 0.5 to 10% by mass, the balance being Zn and inevitable impurities By performing the galvanizing treatment was immersed in Kki bath, the hot-dip galvanizing layer formed on the surface of the cold-rolled steel sheet, and the following 240 seconds 25 seconds Time to reach 623K from 773K A method for producing a high-strength hot-dip galvanized steel sheet having a good appearance and excellent corrosion resistance, characterized in that:

  The present invention makes it possible to provide a high-strength hot-dip galvanized steel sheet having good plating properties and adhesion, and excellent formability and corrosion resistance, and a method for producing the same, and contributes greatly to industrial development. is there.

  The present invention is described in detail below.

  The high-strength hot-dip galvanized steel sheet according to the present invention contains, in mass%, Si: 0.3 to 2.5%, Mn: 1.5 to 2.8%, and is composed of the remainder Fe and inevitable impurities. On the steel plate, Al: 35 to 85% by mass, Si: Al content of 0.5 to 10% by mass, the balance being Zn and unavoidable impurities galvanized, or Al: 35 to 35% Hot dip galvanizing having a zinc plating layer containing 85% by mass, 0.5% to 10% by mass of Si: Al content, 0.5% to 10% by mass of Mg, and the balance consisting of Zn and inevitable impurities It is the applied steel plate.

  Si is added in an amount of 0.3 to 2.5% as an element that increases the strength without significantly impairing the workability of the steel sheet, particularly the elongation. The reason why the Si content is 0.3% or more is that it is difficult to ensure the required tensile strength when Si is less than 0.3%, and the Si content is 2.5% or less. The reason for this is that when Si exceeds 2.5%, the effect of increasing the strength is saturated and the ductility is lowered.

  Since Mn lowers the free energy of austenite together with C, 1.5% or more is added for the purpose of stabilizing austenite until the steel strip is immersed in the plating bath. However, if the amount added is excessive, cracks are likely to occur in the slab and spot weldability deteriorates, so the upper limit is 2.8%.

  Furthermore, in order to balance high strength, press workability, good weldability, and the like, it is desirable that C, P, S, Al, and N be in the following ranges.

  C is an essential element for increasing the strength of a steel sheet by strengthening the structure with martensite or retained austenite. The reason why the C content is 0.05% or more is that when C is less than 0.05%, cementite and pearlite are not easily formed in a hot dip galvanizing line where mist or jet water is difficult to rapidly cool from the annealing temperature. This is because it is easy to produce and it is difficult to ensure the required tensile strength. On the other hand, the reason why the C content is 0.25% or less is that when C exceeds 0.25%, it becomes difficult to form a sound weld by spot welding, and at the same time, segregation of C becomes prominent. This is because the property deteriorates.

  P is generally contained in steel as an unavoidable impurity. However, when its amount exceeds 0.03%, the spot weldability is significantly deteriorated, and in a high-strength steel sheet having a tensile strength exceeding 490 MPa as in the present invention. Since the cold rolling property is remarkably deteriorated together with the toughness, the content is set to 0.03% or less. S is also generally contained in steel as an unavoidable impurity. However, if the amount exceeds 0.02%, the presence of MnS stretched in the rolling direction becomes significant, which adversely affects the bendability of the steel sheet. 0.02% or less.

  Al is a deoxidizing element for steel, and it is necessary to add 0.005% or more in order to improve the material by suppressing the grain refinement of the hot rolled material by AlN and the coarsening of crystal grains in a series of heat treatment steps. . However, if it exceeds 0.5%, not only the cost increases, but also the surface properties deteriorate, so the content is made 0.5% or less. N is also generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.006%, the brittleness deteriorates with elongation, so its content is made 0.006% or less.

  In addition, Nb, Ti, B, Mo, Cu, Ni, Sn, Zn, Zr, W, Co, Ca, rare earth elements (including Y), V, Ta, Hf, Pb, Even if Mg, As, Sb, and Bi are contained in a total of 2.0% or less, preferably 1.0% or less, the effect of the present invention is not impaired, and depending on the amount, plating properties and workability are improved. In some cases.

  The mechanical properties of the high-strength galvannealed steel sheet of the present invention are not particularly specified, but the tensile strength F is 490 MPa or more, and the relationship between the tensile strength F (MPa) and the elongation L (%) is L ≧ It is desirable to have performance that satisfies 51-0.035 × F.

  When the elongation L is lower than [51-0.035 × F], it breaks during severe processing such as deep drawing, so when workability is required, the performance satisfying L ≧ 51−0.035 × F It is desirable to have

  Next, the plating layer will be described.

  The reason why the Al content in the plating layer of the present invention is limited to 35 to 85% by mass is that when the Al content is less than 35% by mass, the bare corrosion resistance decreases, and when it exceeds 85% by mass, the corrosion resistance of the cut end face is reduced. This is because of a decrease. Further, when the Al content is high, it is necessary to keep the temperature of the plating bath high, which causes problems such as an increase in manufacturing cost.

  The reason why the Si content is limited to 0.5 to 10% by mass of the Al content is that when 0.5% by mass or more of the Al content is added and Si is added, This is because it is possible to suppress the formation of an excessively thick Fe—Al-based alloy layer at the interface with the plating layer and improve the adhesion between the steel sheet surface and the plating layer. Moreover, since the effect which suppresses formation of a Fe-Al alloying layer will be saturated when it contains exceeding 10 mass% of Al content, and there exists a possibility of causing the workability of a plating layer to fall, content of Si is The upper limit is 10% by mass of the Al content. When emphasizing the workability of the plating layer, the upper limit is preferably 5% by mass of the Al content.

The reason why the content of Mg is limited to 0.5 to 10% by mass is that if it is less than 0.5% by mass, the effect of improving the corrosion resistance is insufficient, and if it exceeds 10% by mass, the plated layer becomes brittle. This is because the adhesiveness decreases. Since the corrosion resistance increases as the added amount of Mg increases, the Mg content is desirably 2 to 10% by mass in order to significantly improve the corrosion resistance of the plating layer.
And in order to form the hot dip galvanized layer of the said composition, what is necessary is just to use the hot dip galvanizing bath of the same composition as the composition of a plated layer.

In the plating layer, Fe, Sb, Pb, Bi, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, P, B, Sn, Zr, Hf, Sr, V, Sc, REM Even if contained alone or in combination within 0.5% by mass, the effects of the present invention are not impaired, and depending on the amount, the appearance may be further improved. There are no particular restrictions on the amount of hot dip galvanized coating, but it is preferably 10 g / m ² or more from the viewpoint of corrosion resistance and 350 g / m ² or less from the viewpoint of workability.

The high-strength hot-dip galvanized steel sheet according to the present invention has an average content ratio of 0.6 to 5 of an oxide containing Si in crystal grain boundaries and crystal grains on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plating layer. The presence of 10% by mass makes it possible to eliminate non-plating defects. The reason why it is possible to eliminate non-plating defects when there is an oxide containing Si in the crystal grain boundaries and crystal grains of the high-strength steel sheet is that the oxide containing Si is generated in the steel sheet during the annealing process. This is thought to be because SiO ₂ that causes non-plating defects is not exposed on the steel sheet surface.

  The crystal grain boundary and the oxide containing Si existing in the crystal grain can be clearly distinguished by microscopic observation. A cross-sectional observation result is shown in FIG. 1 as an example of a crystal grain boundary on the steel plate side of 5 μm or less from the interface between the high-strength steel plate and the plating layer and an oxide containing Si in the crystal grains. FIG. 1 is a photomicrograph of the result of embedding and polishing by inclining the cross section of the high-strength hot-dip galvanized steel sheet without plating, after removing the plating layer, by 10 degrees. In FIG. 1, 1 is an internal oxide layer, 2 is an oxide containing Si existing in crystal grains, and 3 is an oxide containing Si existing in crystal grain boundaries. FIG. 2 is an enlarged photomicrograph of FIG. As can be seen from these figures, the crystal grain boundaries of the high-strength steel sheet and the oxide containing Si present in the crystal grains can be clearly distinguished by microscopic observation.

Furthermore, when these crystal grain boundaries and oxides in the crystal grains are analyzed by EDX, peaks of Si, Mn, Fe, and O are observed. Therefore, the observed oxides are SiO ₂ , FeSiO ₃ , Fe ₂ SiO _4. , MnSiO ₃ , Mn ₂ SiO ₄ .

  In the present invention, a steel layer containing an oxide containing Si is a layer in which the oxide is observed in a microscope. Moreover, the average content of oxides containing Si indicates the content of oxides contained in the steel layer, and the thickness of the steel layers containing oxides containing Si refers to the oxides from the steel sheet surface. Indicates the width up to the observed part.

  The content of the oxide containing Si can be measured by any method as long as the mass% of the oxide can be measured, but the layer containing the oxide containing Si is dissolved with an acid, and the oxide containing Si is obtained. After separation, the method of measuring the mass is reliable. Moreover, although the measuring method of the thickness of the steel layer containing the oxide containing Si is not prescribed | regulated in particular, the method of measuring with a microscope observation from a cross section is certain.

  In the present invention, the reason why the average content of the oxide containing Si is limited to 0.6 to 10% by mass is that if the amount is less than 0.6% by mass, the suppression of the external oxide film is insufficient and the effect of preventing non-plating defects is achieved. This is because the effect of preventing non-plating defects is saturated when the content exceeds 10% by mass.

  Moreover, the reason which limited the thickness of the steel layer containing the oxide containing Si to 5 micrometers or less is because the effect which improves plating adhesiveness will be saturated when it exceeds 5 micrometers.

  Next, the reasons for limiting the manufacturing conditions will be described. In the present invention, in order to actively produce a steel layer containing an oxide containing Si, a method of internally oxidizing the oxide containing Si in the annealing process of the continuous hot dip plating line is effective.

  Here, the internal oxidation of the oxide containing Si is a phenomenon in which oxygen diffused in the steel plate reacts with Si in the vicinity of the surface layer of the alloy to precipitate the oxide. The internal oxidation phenomenon occurs when the inward diffusion rate of oxygen is much faster than the outward diffusion rate of Si, that is, when the oxygen potential in the atmosphere is relatively high or the concentration of Si is low. At this time, since Si hardly oxidizes and is oxidized in-situ, it is possible to prevent the concentration of Si oxide on the steel sheet surface, which is the cause of the decrease in plating adhesion.

However, even a steel plate prepared by an internal oxidation method has a difference in the subsequent plating properties depending on the type of Si oxide and its positional relationship. Therefore, the oxide of Si is FeSiO ₃ on the steel plate surface or surface side. One or more Si oxides selected from Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO ₄ are present, and SiO ₂ is present on the inner surface side of the steel sheet. This is because even if SiO ₂ is in the internal oxidation state, if it is present on the surface of the steel sheet, the plating property is lowered.

Since FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ are stable in a region where the oxygen potential is larger than that of SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn _{2 is} formed on the steel sheet surface or surface side. In order to make one or more Si oxides selected from SiO ₄ and SiO ₂ exist on the inner surface side of the steel sheet, it is necessary to make the oxygen potential larger than when SiO ₂ alone oxidizes internally. is there.

Since the oxygen potential in steel decreases from the steel sheet surface toward the inside, one or more Si oxides selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ and Mn ₂ SiO _{4 are present} on the steel sheet surface or surface side. When the surface of the steel sheet is controlled to the generated oxygen potential, one or more Si oxides selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ are generated on the steel sheet surface or surface side, and the oxygen potential is increased. SiO ₂ is generated on the inner surface side of the reduced steel plate.

By setting the kind of Si oxide and the positional relationship as described above, it is possible to prevent non-plating defects due to SiO ₂ in the subsequent immersion process in the hot dip galvanizing bath.

Since the oxidation state of Si is determined by the oxygen potential in the atmosphere, it is necessary to directly control the PO ₂ in the atmosphere in order to produce the oxide defined in the present invention under the desired conditions.

When the gas in the atmosphere is H ₂ , H ₂ O, O ₂ , and the balance N ₂ , it is considered that the following equilibrium reaction occurs, and PH ₂ O / PH ₂ is the 1/2 power of PO ₂ and the equilibrium constant 1 / K. Proportional to ₁ .
H ₂ O = H ₂ + 1 / 2O ₂ : K ₁ = P (H ₂ ) · P (O ₂ ) ^1/2 / P (H ₂ O)
However, since the equilibrium constant K ₁ is a variable depending on temperature, when the temperature changes, PH ₂ O / PH ₂ and PO ₂ change separately. That is, even in a region where the ratio of moisture pressure and hydrogen partial pressure, which corresponds to the oxygen potential of the internal oxidation region of Si at a certain temperature range, corresponds to the oxygen potential of the region where iron is oxidized at another temperature range, This is to cope with the oxygen potential of the external oxidation region.

Therefore, even if PH ₂ O / PH ₂ is controlled, the oxide defined in the present invention cannot be generated.

Further, when the gas in the atmosphere is H ₂ , CO ₂ , CO, O ₂ , and the balance N ₂ , it is considered that the following equilibrium reaction takes place, and PCO ₂ / PCO is the _second power of PO ₂ and the equilibrium constant 1 / proportional to K _2.
CO ₂ = CO + 1 / 2O ₂ : K ₂ = P (CO) · P (O ₂ ) ^1/2 / P (CO ₂ )
It is also contemplated that simultaneously for the following equilibrium reaction occurs, H ₂ O is generated in the atmosphere.
CO ₂ + H ₂ = CO + H ₂ O: K ₃ = P (CO) · P (H ₂ O) / P (CO ₂ ) · P (H ₂ )
Therefore, since PO ₂ is not determined unless the temperature is determined as PH ₂ O, PH ₂ , PCO ₂ , and PCO, in order to produce the oxide defined in the present invention, PO ₂ must be defined or the above value. It is necessary to do either one of the above.

Specifically, the external oxidation of Si is suppressed while reducing iron in the reduction zone, and at least one selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ on the steel sheet surface or surface side. For the purpose of generating Si oxide, ₁ to 60% by volume of H ₂ is contained as the reducing zone atmosphere, and the balance is one or more of N ₂ , H ₂ O, O ₂ , CO ₂ , CO, and unavoidable. Logarithmic log PO ₂ of oxygen partial pressure in the atmosphere, which is made of impurities, is expressed by the following formulas (1) and (2)
−0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0.000038 T ² + 0.107T−90.4 1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum steel sheet temperature (K)
[Si%]: Si content in the steel sheet (mass%)
Reduction is performed in an atmosphere controlled to satisfy the above conditions.

  Here, in this invention, all logarithms are shown by a common logarithm.

The reason for limiting H ₂ to ₁ to 60% by volume is that if it is less than 1%, the oxide film formed on the steel sheet surface cannot be sufficiently reduced, and plating wettability cannot be ensured. This is because there is no improvement and costs increase.

The reason for limiting the LogPO ₂ below -0.000038T ² + 0.107T-90.4 is to reduce oxides of iron in the reduction zone. If logPO ₂ exceeds −0.000038T ² + 0.107T-90.4, it enters the oxidized region of iron, so that an iron oxide film is generated on the surface of the steel sheet and non-plating defects occur.

The logPO ^₂ -0.000034T ₂ + 0.105T-0.2 [Si%] ² +2.1 [Si%] - reason for limiting 98.8 above is, LogPO ₂ is -0.000034T ² + 0.105T- If it is less than 0.2 [Si%] ² +2.1 [Si%] − 98.8, the Si oxide SiO ₂ is exposed on the surface and non-plating defects are generated.

By setting logPO ₂ to −0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 or more, FeSiO ₃ , Fe ₂ SiO ₄ , One or more Si oxides selected from MnSiO ₃ and Mn ₂ SiO ₄ are present, and an oxidized state in which SiO ₂ is present on the inner surface of the steel sheet can be obtained.

Further, in an atmosphere where logPO ₂ is smaller, the plating adhesion is remarkably lowered because it enters the external oxidation region of Si.

In the present invention, the maximum reached sheet temperature T of the steel sheet that defines the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is set to 923K or more and 1173K or less.

  The reason for limiting T to 923 K or more is that if T is less than 923 K, the oxygen potential of Si being externally oxidized is small, and if the oxygen potential is within an industrially operable range, it becomes an iron oxidation region and generates FeO on the steel sheet surface. Therefore, the corrosion resistance after coating is deteriorated. On the other hand, the reason for limiting T to 1173K or less is that annealing at a temperature exceeding 1173K requires a lot of energy and is uneconomical. For the purpose of obtaining the mechanical properties of the steel sheet, it is sufficient that the maximum reached plate temperature is 1153 K or less, as will be described later.

  Further, the higher the atmospheric temperature in the furnace, the easier it is to raise the plate temperature of the steel sheet. However, if the atmospheric temperature is too high, the life of the refractory in the furnace is shortened and the cost is increased.

In the present invention, PO ₂ is operated by introducing one or more of H ₂ O, O ₂ , CO ₂ and CO. In the above-described equilibrium reaction formula, when the temperature is determined, the equilibrium constant is determined, and the oxygen partial pressure, that is, the oxygen potential is determined based on the equilibrium constant. At atmospheric temperatures from 773 K to 1273 K, the gas reaction reaches an equilibrium state in a short time, so that PO ₂ is determined to determine the atmospheric temperature of PH ₂ , PH ₂ O, PCO ₂ and PCO in the furnace.

O ₂ and CO do not need to be consciously introduced. However, when H ₂ O and CO ₂ are introduced into a furnace containing 1% by volume or more of H ₂ at the main annealing temperature, a part of them and H ₂ O ₂ and CO are produced by the equilibrium reaction. The introduction method of H ₂ O and CO ₂ is not particularly limited as long as a necessary amount can be introduced. For example, a method in which a gas in which CO and H ₂ are mixed is burned and the generated H ₂ O and CO ₂ are introduced. , CH ₄ , C ₂ H ₆ , C ₃ H _{8 and} other hydrocarbon gases, and a mixture of hydrocarbons such as LNG, and the generated H ₂ O and CO ₂ are introduced, gasoline, light oil, heavy oil Such as burning a mixture of liquid hydrocarbons and introducing the generated H ₂ O, CO ₂ , burning alcohols such as CH ₃ OH, C ₂ H ₅ OH and mixtures thereof, various organic solvents, A method of introducing the generated H ₂ O and CO ₂ is raised.

CO only burned, but also conceivable to introduce CO ₂ generated, the annealing temperature, if CO ₂ was introduced into the furnace atmosphere, part is reduced by H _2, CO and H ₂ O is Therefore, there is essentially no difference from the case where H ₂ O and CO ₂ are introduced.

In addition to the method of introducing H ₂ O and CO ₂ generated by combustion, a gas containing a mixture of CO and H ₂ , a hydrocarbon gas such as CH ₄ , C ₂ H ₆ , C ₃ H ₈ , LNG and other hydrocarbon mixtures, gasoline, light oil, heavy oil, liquid hydrocarbon mixtures, CH ₃ OH, C ₂ H ₅ OH and other alcohols and mixtures, various organic solvents, etc. A method of introducing H ₂ O and CO ₂ by introducing the gas into the furnace and burning it in a furnace can also be used.

Such method utilizes N ₂ raising the N ₂ and dew point saturated with water vapor compared to the method of supplying steam, simple and the control is excellent. In addition, since there is no fear of condensation in the pipe, it is possible to save the trouble of heat insulation of the pipe.

In the present invention, the reduction time at the PO ₂ and temperature specified in the claims is not particularly specified, but is preferably 10 seconds or more and 3 minutes or less. Increasing the PO ₂ in the reducing furnace, the Atsushi Nobori process, after the LogPO ₂ has passed the region exceeding ^{-0.000038T 2 + 0.107T-90.4, -0.000038T} 2 + 0.107T-90.4 1) selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ on the target steel plate surface or surface side because the iron oxide film formed first is reduced because it is reduced in the following regions. In order to obtain a steel sheet in which more than seeds of Si oxide are present and SiO ₂ is present on the inner surface side of the steel sheet, it is desirable to hold for 10 seconds or more. However, holding for more than 3 minutes is not preferable because it not only wastes energy but also reduces productivity in a continuous line.

If the PO ₂ and temperature in the reducing atmosphere are within the range of the present invention, a normal non-oxidizing furnace type hot dipping method or a hot dipping method using an all radiant tube type annealing furnace can be used. Regardless of which method is used, the log PO ₂ passes through the region where −0.000038T ² + 0.107T-90.4 exceeds the plate temperature until the plate temperature exceeds 923K in the temperature rising process, and the iron oxide film is formed on the steel plate surface. Therefore, the external oxidation of Si is suppressed while reducing this, and one or more Si oxides selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ on the surface or the surface side of the steel sheet are reduced. For the purpose of generating, the PO ₂ and temperature of the reducing zone atmosphere may be appropriately controlled within the scope of the present invention.

  Next, the reasons for limiting other manufacturing conditions will be described. The purpose is to provide a metal structure containing 3 to 20% martensite and retained austenite, and to achieve both high strength and good press workability. When the volume ratio of martensite and retained austenite is less than 3%, the strength is not high. On the other hand, if the volume ratio of martensite and retained austenite exceeds 20%, the workability of the steel sheet is deteriorated although it is high in strength, and the object of the present invention is not achieved.

  The slab to be used for hot rolling is not particularly limited as long as it is manufactured with a continuously cast slab or a thin slab caster. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

  The hot rolling finishing temperature needs to be Ar3 or higher from the viewpoint of ensuring the press formability of the steel sheet. The cooling conditions and coiling temperature after hot rolling are not particularly limited, but the coiling temperature is to avoid large material variations at both ends of the coil and to avoid pickling deterioration due to increased scale thickness. Is 1023K or less, and when bainite or martensite is partially formed, ear cracks are likely to occur during cold rolling. Cold rolling may be performed under ordinary conditions, and the rolling rate is set to 50% or more for the purpose of finely dispersing martensite and retained austenite so that the ferrite is easily work-hardened to maximize workability. On the other hand, it is not realistic to perform cold rolling at a rolling rate exceeding 85% because a large cold rolling load is required.

  When annealing is performed in a continuous hot dip galvanizing facility using an in-line annealing method, the annealing temperature is in the range of 1023 K or more and 1153 K or less of ferrite and austenite. If the annealing temperature is less than 1023 K, recrystallization is insufficient and the press workability necessary for the steel sheet cannot be achieved. Annealing at a temperature exceeding 1153 K is not preferable because the production cost is increased and the deterioration of the equipment is accelerated. Further, in the process of immersing and cooling in the plating bath, ferrite having a sufficient volume ratio does not grow even if it is slowly cooled to 923K, so that it is difficult to achieve both high strength and good press workability.

  The steel strip is cooled in the process of being subsequently immersed in the plating bath after annealing. In this case, the cooling rate is 0.5 to 10 degrees / second on average from its maximum temperature to 923 K, and subsequently from 923 K to 773 K. Is cooled at an average cooling rate of 3 degrees / second or more, further from 773 K to 627 K is cooled at an average cooling rate of 0.5 degrees / second or more, and the time to reach 623 K to 623 K is 25 seconds or more 240 Hold for less than a second.

  The average of 0.5 to 10 degrees / second up to 923K is to increase the volume fraction of ferrite in order to improve workability, and at the same time, to increase the C concentration of austenite, thereby lowering the free energy of formation, and martensite The purpose is to make the temperature at which transformation starts below the plating bath temperature. In order to make the average cooling rate up to 923K less than 0.5 degree / second, it is necessary to lengthen the line length of the continuous hot dip galvanizing equipment, resulting in high cost, so the average cooling rate up to 923K is 0.5 degree / Second or more.

  In order to make the average cooling rate up to 923 K less than 0.5 degrees / second, it is conceivable to lower the maximum temperature and anneal at a temperature at which the volume fraction of austenite is small, but in that case, in actual operation If the appropriate temperature range is narrower than the allowable temperature range and the annealing temperature is low even a little, austenite is not formed and the purpose is not achieved.

  On the other hand, when the average cooling rate up to 923 K is made to exceed 10 degrees / second, not only the increase in the volume fraction of ferrite is not sufficient, but also the increase in the C concentration in austenite is small, so that the high strength and workability are good. It becomes difficult to achieve both.

  The reason why the average cooling rate from 923 K to 773 K is set to 3 degrees / second or more is to avoid the transformation of austenite to pearlite during the cooling, and is specified by the present invention when the cooling rate is less than 3 degrees / second. Even if it is annealed at a temperature to cool to 923K, the formation of pearlite is inevitable. The upper limit of the average cooling rate is not particularly defined, but it is difficult to cool the steel strip so that the average cooling rate exceeds 50 degrees / second.

  The reason why the average cooling rate from 773 K is 0.5 degrees / second or more is to avoid the transformation of austenite to pearlite during the cooling, and if the cooling rate is less than 0.5 degrees / second, the present invention. Even if it is annealed at a temperature specified in (1) and cooled to 773K, the formation of pearlite is inevitable. The upper limit of the average cooling rate is not particularly defined, but it is difficult to cool the steel strip so that the average cooling rate exceeds 50 degrees / second.

  The reason why the time from 773K to 623K is maintained for 25 seconds or more and 240 seconds or less is that concentration of C in austenite is promoted and high-strength hot-dip galvanizing excellent in workability is obtained. If the time from 773 K to 623 K is less than 25 seconds, the concentration of C in the austenite becomes insufficient, and the C concentration in the austenite does not reach a level that allows austenite to remain at room temperature. If it exceeds 240 seconds, the bainite transformation proceeds excessively, the amount of austenite decreases, and a sufficient amount of retained austenite cannot be generated.

  While cooling from 923K to 773K, it passes through the hot dipping bath, but there is no problem if the average cooling rate is within the range of the present invention. Although the bath temperature of the hot dipping bath varies depending on the bath composition, 823 to 923K is appropriate in the range of the bath composition of the present invention.

  In addition, since the solidification end temperature of hot dipping differs depending on the bath composition, even if the cooling rate from the hot dipping bath temperature to the solidification end temperature is determined according to the solidification conditions of the plating, the above average cooling rate is within the scope of the present invention. If the time from 773K to 623K is maintained for 25 seconds or more and 240 seconds or less, there is no problem.

  Hereinafter, the present invention will be described specifically by way of examples.

[Example 1]
A slab having the composition shown in Table 1 was heated to 1423K to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 1183 to 1203K, and wound at 853 to 953K. After pickling and cold rolling to obtain a 1.6 mm cold rolled steel strip, using a continuous hot dip galvanizing facility with an in-line annealing method, the plate is passed under the conditions shown in Table 2-1. A hot dip galvanized steel sheet was produced. The continuous hot-dip galvanizing equipment used a method of reducing and annealing in a reduction zone after heating in a non-oxidizing furnace. Reduction zone is CO and H ₂ The mixed gas by burning generated H ₂ O, fitted with a device for introducing the CO _2, introducing H ₂ O and CO ₂ and H ₂ in N ₂ gas containing 10 vol%.

For annealing, the maximum temperature reached was adjusted to the value shown in Table 2-1, and the soaking time in the soaking temperature (the range from the maximum temperature to -20 degrees to the maximum temperature) was 60 seconds. Thereafter, the temperature from the highest temperature reached to 923K was cooled at an average cooling rate of 1 degree / second, hot-dip plating was performed for 3 seconds in a Zn-Al-Si plating bath having a bath temperature of 873K, and the amount of plating adhered was adjusted by N ₂ wiping. Then, it cooled to room temperature. The average cooling rate from 923 K to 773 K was 10 degrees / second, and the average cooling rate from 773 K to 623 K was 2.5 degrees / second. The composition in the plating layer of the obtained plated steel sheet was Al: 55% and Si: 1.6%.

The PO ₂ in the reduction furnace is the measured value of the hydrogen concentration, water vapor concentration, CO ₂ concentration, CO concentration, atmospheric temperature in the furnace and the equilibrium reaction H ₂ O = H ₂ + 1 / 2O _2.
CO ₂ = CO + 1 / 2O ₂
The equilibrium constants K ₁ and K ₂ were used.

  Tensile strength (TS) and elongation (El) were determined by cutting out a JIS No. 5 test piece from each steel plate and conducting a tensile test at room temperature.

  The adhesion amount of the plating was measured by a mass method after dissolving the plating with hydrochloric acid containing an inhibitor.

  The oxide containing Si present in the crystal grain boundaries and in the crystal grains of the steel sheet was evaluated by observing the embedded and polished plated steel sheet with a SEM image from the cross section. The state of the internal oxide layer was observed with an SEM image, where an oxide containing Si was observed in the crystal grain boundary and in the crystal grains, and x was not observed. Similarly, the thickness of the internal oxide layer was observed with an SEM image, and the thickness of the portion where the oxide was observed in the crystal grain boundary and in the crystal grain from the interface between the steel sheet and the plating layer was measured. The composition of the internal oxide layer was analyzed using EDX attached to the SEM, and the case where the Si and O peaks were observed was evaluated as ◯, and the case where the Si and O peaks were not observed as ×.

  Measurement of the content ratio of oxides containing Si in the steel sheet uses the steel sheet after the plating is dissolved with hydrochloric acid containing an inhibitor, and the oxide-containing layer is oxidized by dissolving the layer containing the oxide containing Si with an acid. After separating the object, its mass was measured and determined.

  The presence or absence of FeO was measured by XRD from the surface of the steel sheet. The case where the diffraction peak of FeO was not observed was evaluated as ◯, and the case where the diffraction peak was observed was evaluated as ×.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ , and SiO ₂ were evaluated on the basis of the following criteria by observing an oxide containing Si from a cross-section of the embedded and polished plated steel sheet using a CMA image.
Position of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ ○: Fe or Mn and Si, O observed at the same position Oxide observed on steel sheet surface ×: Fe or Mn Oxides where Si and O are observed at the same position are not observed SiO ₂ position O: Oxides where Si and O are observed at the same position, Fe or Mn and Si and O are observed at the same position What is observed inside the steel plate from oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate x: Oxide where Si and O are observed at the same position is steel plate What is not observed inside

The plating appearance was determined by visually observing the entire length of the passed coil and rating the unplated area ratio as shown below. A score of 3 or more was accepted.
4: Unplated area ratio less than 1% 3: Unplated area ratio from 1% to less than 5% 2: Unplated area ratio from 5% to less than 10% 1: Unplated area ratio of 10% or more

  For adhesion, a cellophane tape was affixed to the hot-dip plated steel sheet after the DuPont impact test, and then peeled off. The DuPont test was conducted by using a shooting type having a 1/2 inch roundness at the tip and dropping a 1 kg weight from a height of 1 m.

The evaluation results are as shown in Table 2-2. Nos. 3, 6, 9, 12, 17, ₂₀ , 23, 26, 29, 33, 35, 38, 41, 45, and 48 indicate that the log PO ₂ in the furnace is outside the scope of the present invention, so that the surface of the steel plate is oxidized with Si. Thickening of the material caused non-plating and poor adhesion, resulting in failure. Nos. ₂ , 5, 8, 11, 18, 21, 24, 27, 30, 32, 36, 39, 42, 44, 47 indicate that the log PO ₂ in the furnace is outside the scope of the present invention, so The oxide could not be reduced, resulting in non-plating and poor adhesion, resulting in failure. The steel plate produced by the method of the present invention other than these was a high-strength hot-dip galvanized steel plate excellent in plating properties and adhesion.

[Example 2]
A slab having the composition shown in Table 1 was heated to 1423K to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 1183 to 1203K, and wound at 853 to 953K. After pickling and cold rolling to obtain a 1.6 mm cold rolled steel strip, plating is performed under conditions as shown in Table 3-1, using a continuous hot dip galvanizing facility of an in-line annealing method, A hot dip galvanized steel sheet was produced. The continuous hot-dip galvanizing equipment used a method of reducing and annealing in a reduction zone after heating in a non-oxidizing furnace. Reduction zone is fitted with a device for introducing of H ₂ O, CO ₂ generated by burning gas of a mixture of CO and H _2, is introduced of H ₂ O and CO ₂ and H ₂ in N ₂ gas containing 10 vol%, The logarithm log PO ₂ of the oxygen potential in the furnace was adjusted to the value shown in Table 3-1.

For annealing, the maximum temperature reached was adjusted to the value shown in Table 3-1, and the soaking time in the soaking temperature (the range from the maximum temperature to -20 degrees to the maximum temperature) was 60 seconds. Thereafter, from the highest temperature reached to 923 K at an average cooling rate of 1 degree / second, subsequently from 923 K to 773 K at an average cooling rate of 10 degrees / second, and further from 773 K at an average cooling rate of 2.5 degrees / second. After cooling to 623K, it was cooled to room temperature. In hot dip plating, hot dip plating was performed for 3 seconds in a Zn-Mg-Al plating bath or a Zn-Mg-Al-Si plating bath in a temperature range of 823 K to 923 K, and the amount of plating adhered was adjusted by N ₂ wiping. The composition in the plating layer of the obtained plated steel sheet was the value shown in Table 3-1.

The PO ₂ in the reduction furnace is the measured value of the hydrogen concentration, water vapor concentration, CO ₂ concentration, CO concentration, atmospheric temperature in the furnace and the equilibrium reaction H ₂ O = H ₂ + 1 / 2O _2.
CO ₂ = CO + 1 / 2O ₂
The equilibrium constants K ₁ and K ₂ were used.

  Tensile strength (TS) and elongation (El) were determined by cutting out a JIS No. 5 test piece from each steel plate and conducting a tensile test at room temperature.

  The adhesion amount of the plating was measured by a mass method after dissolving the plating with hydrochloric acid containing an inhibitor. The composition of the plating layer was measured by chemical analysis after dissolving the plating with hydrochloric acid containing an inhibitor.

  The oxide containing Si present in the crystal grain boundaries and in the crystal grains of the steel sheet was evaluated by observing the embedded and polished plated steel sheet with a SEM image from the cross section. The state of the internal oxide layer was observed with an SEM image, where an oxide containing Si was observed in the crystal grain boundary and in the crystal grains, and x was not observed. Similarly, the thickness of the internal oxide layer was observed with an SEM image, and the thickness of the portion where the oxide was observed in the crystal grain boundary and in the crystal grain from the interface between the steel sheet and the plating layer was measured. The composition of the internal oxide layer was analyzed using EDX attached to the SEM, and the case where the Si and O peaks were observed was evaluated as ◯, and the case where the Si and O peaks were not observed as ×.

  Measurement of the content ratio of oxides containing Si in the steel sheet uses the steel sheet after the plating is dissolved with hydrochloric acid containing an inhibitor, and the oxide-containing layer is oxidized by dissolving the layer containing the oxide containing Si with an acid. After separating the object, its mass was measured and determined.

The presence or absence of FeO was measured by XRD from the surface of the steel sheet. The case where the diffraction peak of FeO was not observed was evaluated as ◯, and the case where the diffraction peak was observed was evaluated as ×.
The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ , and SiO ₂ were evaluated on the basis of the following criteria by observing an oxide containing Si from a cross-section of the embedded and polished plated steel sheet using a CMA image.
Position of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ ○: Fe or Mn and Si, O observed at the same position Oxide observed on steel sheet surface ×: Fe or Mn Oxides where Si and O are observed at the same position are not observed SiO ₂ position O: Oxides where Si and O are observed at the same position, Fe or Mn and Si and O are observed at the same position What is observed inside the steel plate from oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate x: Oxide where Si and O are observed at the same position is steel plate What is not observed inside

The plating appearance was determined by visually observing the entire length of the passed coil and rating the unplated area ratio as shown below. A score of 3 or more was accepted.
4: Unplated area ratio less than 1% 3: Unplated area ratio from 1% to less than 5% 2: Unplated area ratio from 5% to less than 10% 1: Unplated area ratio of 10% or more

  For adhesion, a cellophane tape was affixed to the hot-dip plated steel sheet after the DuPont impact test, and then peeled off. The DuPont test was conducted by using a shooting type having a 1/2 inch roundness at the tip and dropping a 1 kg weight from a height of 1 m.

  The evaluation results are as shown in Table 3-2. Nos. 1 and 15 were rejected because the Si-concentration in the plating layer was outside the range of the present invention, so that the Fe-Al alloying reaction occurred and the plating adhesion decreased. The steel plate produced by the method of the present invention other than these was a high-strength hot-dip galvanized steel plate excellent in plating properties and adhesion.

[Example 3]
A slab having the composition shown in Table 1 was heated to 1423K to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 1183 to 1203K, and wound at 853 to 953K. After pickling and cold rolling to make a 1.6 mm cold rolled steel strip, plating is performed under conditions as shown in Table 4-1 using a continuous hot dip galvanizing facility of an in-line annealing method, A hot dip galvanized steel sheet was produced. The continuous hot-dip galvanizing equipment used a method of reducing and annealing in a reduction zone after heating in a non-oxidizing furnace. Reduction zone is fitted with a device for introducing of H ₂ O, CO ₂ generated by burning gas of a mixture of CO and H _2, is introduced of H ₂ O and CO ₂ and H ₂ in N ₂ gas containing 10 vol%, The logarithm log PO ₂ of the oxygen potential in the furnace was adjusted to the value shown in Table 4-1.

  For annealing, the maximum temperature reached was adjusted to the value shown in Table 4-1, and the soaking time in the soaking temperature (the range from the maximum temperature to -20 degrees to the maximum temperature) was 60 seconds. .

The PO ₂ in the reduction furnace is the measured value of the hydrogen concentration, water vapor concentration, CO ₂ concentration, CO concentration, atmospheric temperature in the furnace and the equilibrium reaction H ₂ O = H ₂ + 1 / 2O _2.
CO ₂ = CO + 1 / 2O ₂
The equilibrium constants K ₁ and K ₂ were used.

  Tensile strength (TS) and elongation (El) were determined by cutting out a JIS No. 5 test piece from each steel plate and conducting a tensile test at room temperature.

For hot dip plating, hot dip plating was performed for 3 seconds in a Zn-Mg-Al-Si plating bath having a bath temperature of 873 K, and the plating adhesion was adjusted to 100 g / m ² on one side by N ₂ wiping. The composition in the plating layer of the obtained plated steel sheet was Mg 3%, Al 55%, and Si 1.6%.

  The oxide containing Si present in the crystal grain boundaries and in the crystal grains of the steel sheet was evaluated by observing the embedded and polished plated steel sheet with a SEM image from the cross section. The state of the internal oxide layer was observed with an SEM image, where an oxide containing Si was observed in the crystal grain boundary and in the crystal grains, and x was not observed. Similarly, the thickness of the internal oxide layer was observed with an SEM image, and the thickness of the portion where the oxide was observed in the crystal grain boundary and in the crystal grain from the interface between the steel sheet and the plating layer was measured. The composition of the internal oxide layer was analyzed using EDX attached to the SEM, and the case where the Si and O peaks were observed was evaluated as ◯, and the case where the Si and O peaks were not observed as ×.

  Measurement of the content ratio of oxides containing Si in the steel sheet uses the steel sheet after the plating is dissolved with hydrochloric acid containing an inhibitor, and the oxide-containing layer is oxidized by dissolving the layer containing the oxide containing Si with an acid. After separating the object, its mass was measured and determined.

  The presence or absence of FeO was measured by XRD from the surface of the steel sheet. The case where the diffraction peak of FeO was not observed was evaluated as ◯, and the case where the diffraction peak was observed was evaluated as ×.

The positions of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ , and SiO ₂ were evaluated on the basis of the following criteria by observing an oxide containing Si from a cross-section of the embedded and polished plated steel sheet using a CMA image.
Position of (Fe, Mn) SiO ₃ , (Fe, Mn) ₂ SiO ₄ ○: Fe or Mn and Si, O observed at the same position Oxide observed on steel sheet surface ×: Fe or Mn Oxides where Si and O are observed at the same position are not observed SiO ₂ position O: Oxides where Si and O are observed at the same position, Fe or Mn and Si and O are observed at the same position What is observed inside the steel plate from oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate x: Oxide where Si and O are observed at the same position is steel plate What is not observed inside

The plating appearance was determined by visually observing the entire length of the passed coil and rating the unplated area ratio as shown below. A score of 3 or more was accepted.
4: Unplated area ratio less than 1% 3: Unplated area ratio from 1% to less than 5% 2: Unplated area ratio from 5% to less than 10% 1: Unplated area ratio of 10% or more

  For adhesion, a cellophane tape was affixed to the hot-dip plated steel sheet after the DuPont impact test, and then peeled off. The DuPont test was conducted by using a shooting type having a 1/2 inch roundness at the tip and dropping a 1 kg weight from a height of 1 m.

  The evaluation results are as shown in Table 4-2. The method of the present invention makes it possible to produce a high-strength hot-dip galvanized steel sheet having excellent plating properties and adhesion.

It is the microscope picture which carried out embedding grinding | polishing by inclining the cross section after the plating removal of the high intensity | strength hot-dip galvanized steel plate which did not generate | occur | produce the plating at 10 degree | times, and observed with the SEM image. 2 is an enlarged photomicrograph of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal oxide layer 2 Oxide containing Si which exists in a crystal grain 3 Oxide containing Si which exists in a crystal grain boundary

Claims (6)

% By mass
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
On the high-strength steel plate containing N: 0.0060% or less, the balance being Fe and inevitable impurities, Al: 35-85 mass%, Si: Al content of 0.5-10 mass% In the hot dip galvanized steel sheet having a galvanized layer consisting of Zn and inevitable impurities in the balance, Si is contained in the grain boundary and the crystal grain on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer A high-strength hot-dip galvanized steel sheet excellent in formability and corrosion resistance, characterized in that an oxide is present at an average content of 0.6 to 10% by mass. % By mass
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: On a high-strength steel plate containing 0.0060% or less, the balance being Fe and inevitable impurities, Al: 35 to 85% by mass, Si: Al content of 0.5 to 10% by mass, Mg: 0.5 to 10% by mass of a hot-dip galvanized steel sheet having a galvanized layer composed of Zn and inevitable impurities, and a crystal on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer A high-strength hot-dip galvanized steel sheet with excellent appearance and excellent corrosion resistance, characterized in that an oxide containing Si is present at an average content of 0.6 to 10% by mass in grain boundaries and crystal grains. Characterized in that said oxide containing Si as set forth in claim 1 or claim 2 is _{SiO 2, FeSiO 3, Fe 2} SiO 4, MnSiO 3, Mn 2 SiO 4, 1 or more selected from A high-strength hot-dip galvanized steel sheet with excellent appearance and excellent formability and corrosion resistance. _4. The hot-dip galvanized steel sheet according to claim 1, wherein at least one Si oxide selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO _{4 is} formed on the surface or the surface of the steel sheet. A high-strength hot-dip galvanized steel sheet with good appearance and excellent corrosion resistance, characterized by the presence of an object and the presence of SiO ₂ on the inner surface side of the steel sheet. % By mass
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
When N: 0.0060% or less and the balance is continuously hot dip galvanized on a high-strength steel plate made of Fe and inevitable impurities, the atmosphere of the reduction zone contains ₁ to 60% by volume of H _2. , The balance N ₂ , H ₂ O, O ₂ , CO ₂ , CO, and inevitable impurities, and the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is expressed by the following (formula 1) and (2 formula)
−0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0.000038 T ² + 0.107T−90.4 1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum temperature of steel sheet (K)
[Si%]: Si content (mass%) in the steel sheet
After Tsu lines reduced in a controlled atmosphere to satisfy, Al: 35 to 85 wt%, Si: contains 0.5 to 10 mass% of the content of Al, the balance being Zn and unavoidable impurities Hot dip galvanizing bath or Al: 35 to 85% by mass, Si: Al content of 0.5 to 10% by mass, Mg: 0.5 to 10% by mass, the balance being Zn and inevitable impurities method for manufacturing a high strength galvanized steel sheet appearance by immersion in molten zinc plating bath, wherein the row Ukoto the galvanizing treatment was excellent good formability and corrosion resistance consisting of. % By mass
C: 0.05 to 0.25%
Si: 0.3 to 2.5%,
Mn: 1.5 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: A slab containing 0.0060% or less and the balance of Fe and inevitable impurities is finish-rolled at a temperature of Ar ₃ point or higher, cold-rolled by 50 to 85%, and subsequently melted. When applying galvanization, the atmosphere of the reduction zone contains ₁ to 60% by volume of H ₂ , and the balance N ₂ , H ₂ O, O ₂ , CO ₂ , one or more of CO, and unavoidable impurities The logarithm log PO ₂ of the oxygen partial pressure in the atmosphere is expressed by the following formulas (1) and (2)
−0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0.000038 T ² + 0.107T−90.4 1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum temperature of steel sheet (K)
[Si%]: Si content (mass%) in the steel sheet
Using a continuous hot-dip galvanizing facility controlled to satisfy the above conditions, annealing in the two-phase coexistence temperature range of ferrite and austenite of 1023K to 1153K, and the average cooling rate from 0.5 to 923K is 0.5 to In a manufacturing method in which 923 K to 773 K are subsequently cooled at an average cooling rate of 3 degrees / second or more at 10 degrees / second, and further cooled at an average cooling speed of 0.5 degrees / second or more from 773 K, a temperature of 823 K to 923 K. A hot dip galvanizing bath containing Al: 35 to 85% by mass and Si: Al content of 0.5 to 10% by mass, the balance being Zn and inevitable impurities, or Al: 35 to 85 Molten zinc containing 0.5% by mass, Si: Al content of 0.5 to 10%, Mg: 0.5 to 10% by mass, the balance being Zn and inevitable impurities By performing the galvanizing treatment was immersed in Kki bath, the hot-dip galvanizing layer formed on the surface of the cold-rolled steel sheet, and the following 240 seconds 25 seconds Time to reach 623K from 773K A method for producing a high-strength hot-dip galvanized steel sheet having a good appearance and excellent corrosion resistance, characterized in that: