JP4837464B2 - High-strength hot-dip galvanized steel sheet with excellent plating adhesion 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 formability, and is used in various applications such as building materials and automobiles. The present invention relates to a plated steel sheet applicable as a steel sheet.

  The hot-dip galvanized steel sheet is most 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.

  In recent years, in particular, in the automobile field, it has been necessary 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 efficiency.

  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 of a steel sheet containing Si, in Patent Document 1 (Japanese Patent Laid-Open No. 55-122865), after oxidation so that the thickness of the oxide film on the surface becomes 400 to 10,000 mm, A method of annealing and plating in an atmosphere containing hydrogen is shown. 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 inventors of the present invention disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2001-323355) by oxidizing the steel sheet surface and then reducing it in a reducing furnace with controlled atmosphere, thereby increasing the surface concentration of Si. A manufacturing method was proposed to prevent crystallization.

  Moreover, in patent document 3 (Unexamined-Japanese-Patent No. 2004-323970), in the inside of the steel plate surface whose Si content is 0.2-3.0 mass%, it is Si oxide, Mn oxide, or the composite of Si and Mn. A high-strength hot-dip galvanized steel sheet with good plating properties characterized by containing one or more oxide particles selected from oxides has been proposed.

  In Patent Document 4 (Japanese Patent Laid-Open No. Sho 56-33463) and Patent Document 5 (Japanese Patent Laid-Open No. 57-79160), non-plating is performed by pre-plating Cr, Ni, Fe, etc. on the steel sheet surface. A method for suppressing defects is shown. Further, in Patent Document 6 (Japanese Patent Laid-Open No. 2002-161315), an internal oxide layer is formed immediately below the surface of a steel sheet in a continuous annealing line, and the formed surface oxide is removed by pickling at the same time, followed by continuous hot dip galvanization. A method of plating in line is shown.

JP 55-122865 A JP 2001-323355 A JP 2004-323970 A JP-A-56-33463 JP-A-57-79160 JP 2002-161315 A

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

For this reason, in Patent Document 2, the reducing atmosphere is controlled and SiO ₂ is brought into an internal oxidation state for the purpose of suppressing non-plating defects generated by the surface concentration of Si. 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 patent cannot completely prevent the exposure of SiO _{2 to} the steel sheet surface even though it can prevent the surface from being concentrated by external oxidation of Si. Therefore, in order to prevent non-plating defects and poor adhesion, more strict control of SiO ₂ is required.

In Patent Document 3, the reducing atmosphere is controlled, and one or more oxide particles selected from Si oxide, Mn oxide, or a composite oxide of Si and Mn are contained inside the steel sheet surface, and plating is performed. Although this method can also significantly reduce the non-plating defects caused by the surface concentration of Si, it is not possible to completely prevent the exposure of SiO _{2 to} the steel plate surface. Defects and poor adhesion cannot be completely prevented.

  Since hot-dip galvanized steel cannot be expected to improve adhesion due to alloying like galvannealed steel, it is likely that the adhesion of the steel / platinum interface is likely to be low, as with steel added with Si. It is difficult to improve. Therefore, also in Patent Document 2 and Patent Document 3, plating adhesion to such an extent that peeling does not occur in the bending test is ensured, but plating adhesion by strict evaluation such as DuPont impact test is not sufficiently ensured.

  Furthermore, the pre-plating method as in Patent Document 4 and Patent Document 5 requires a plating facility, and thus cannot be employed when there is no space. Moreover, the problem that production cost rises by pre-plating equipment installation also arises.

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

  Furthermore, the manufacturing technology of the steel sheet containing Si disclosed so far has been focused on ensuring plating properties, and has been made to improve various performances when used as plated steel plates such as formability. There 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 plating adhesion and formability, 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 formability 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) 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: 0.005-0.5%, N: 0.0060% or less, Al: 0.05-10 on the surface of the high-strength steel plate composed of the remainder Fe and inevitable impurities In a hot dip galvanized steel sheet having a galvanized layer containing 0.05% to 3% by mass of Fe and 0.05 to 3% by mass of the balance, the steel sheet having a thickness of 5 μm or less from the interface between the high-strength steel sheet and the plated layer An oxide containing Si is present at an average content of 0.6 to 10% by mass in the grain boundaries and crystal grains on the side, and an Fe—Zn alloy having an average grain size of 0.5 to 3 μm is present on the plating side. A high-strength hot-dip galvanized steel sheet with excellent plating adhesion.

(2) 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: 0.005-0.5%, N: 0.0060% or less, Al: 0.05-10 on the surface of the high-strength steel plate composed of the remainder Fe and inevitable impurities In a hot dip galvanized steel sheet having a galvanized layer containing 0.05% to 3% by mass of Fe and 0.05 to 3% by mass of the balance, the steel sheet having a thickness of 5 μm or less from the interface between the high-strength steel sheet and the plated layer An Fe-Zn alloy having an average grain content of 0.6 to 10% by mass and an average grain size of 0.5 to 3 μm on the plating side is arbitrary. High strength with excellent plating adhesion characterized by being present at a ratio of 1 piece / 500 μm or more in cross section Hot-dip galvanized steel sheet.

(3) The oxide containing Si described in (1) or (2) is at least one selected from SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO _4. A high-strength hot-dip galvanized steel sheet with excellent plating adhesion.

(4) In the high-strength hot-dip galvanized steel sheet according to any one of (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 excellent in plating adhesion, characterized in that at least one kind of Si oxide is present and SiO ₂ is present on the inner surface side of the steel sheet.

(5) 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: 0.0060% or less, when continuously hot-dip galvanizing on high-strength steel plate composed of the remaining Fe and inevitable impurities, As the atmosphere of the reduction zone, ₁ to 60% by volume of H ₂ is contained, and the balance is composed of one or more of N ₂ , H ₂ O, O ₂ , CO ₂ , CO, and unavoidable impurities. Logarithmic log PO ₂ of oxygen partial pressure is −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 in steel sheet (wt%)
The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the plating adhesiveness characterized by performing reduction | restoration in a controlled atmosphere.

(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: Finish rolling a slab containing 0.02% or less, Al: 0.005 to 0.5%, N: 0.0060% or less and comprising the balance Fe and inevitable impurities at a temperature of Ar ₃ or higher. conducted, subjected to a rolling 50% to 85% of the cold, subsequently, when subjected to hot-dip galvanizing, as the atmosphere of the reducing zone, and H ₂ contains 1 to 60 vol%, the remainder _{N 2, H 2 O, O} 2 , CO ₂ , one or more of CO and unavoidable impurities, and the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is −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 in steel sheet (wt%)
In a controlled hot-dip galvanizing facility and annealing in the two-phase coexistence temperature range of 1023K to 1153K ferrite and austenite, and the average cooling rate from the highest temperature to 923K is 0.5 to 10 degrees / By cooling in seconds, subsequently cooling from 923 K to 773 K at an average cooling rate of 3 degrees / second or more, further cooling from 773 K at an average cooling speed of 0.5 degrees / second or more, and performing hot dip galvanizing treatment, In the manufacturing method for forming a hot-dip galvanized layer on the surface of the cold-rolled steel sheet, the time required to reach 623K after plating from 773K is set to 25 seconds or more and 240 seconds or less and excellent in plating adhesion A method for producing high-strength hot-dip galvanized steel sheets.

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

The present invention is described in detail below.
The high-strength hot-dip galvanized steel sheet of the present invention is in mass%, C: 0.05 to 0.25%, Si: 0.3 to 2.5%, Mn: 1.5 to 2.8%, Mn: 1.5 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5%, N: 0.0060% or less, the balance Fe and On a high-strength steel plate made of inevitable impurities, Al: 0.05 to 10% by mass, Fe: 0.05 to 3% by mass, with the remainder having a hot-dip galvanized layer made of Zn and inevitable impurities It is a steel sheet that has been hot dip galvanized.

  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.

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 P, S, Al, and N be in the following ranges.

  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 amount of 1% or less, the effects of the present invention are not impaired, and depending on the amount, plating properties and workability may be improved.

  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 hot dip galvanized layer is limited to 0.05 to 10% by mass is that when the Al content exceeds 10% by mass, the Fe-Al alloying reaction proceeds too much, resulting in a decrease in plating adhesion. This is to be seen. The reason for limiting the Al content to 0.05% by mass or more is that when an ordinary hot dipping process is performed with an Al content of less than 0.05% by mass, the Zn—Fe alloying reaction proceeds during the plating process. This is because a brittle alloy layer develops at the iron-iron interface and plating adhesion deteriorates.

  The reason why the Fe content is limited to 0.01 to 3% by mass is that the effect of improving the plating adhesion is insufficient when the content is less than 0.01% by mass. This is because a brittle alloy layer develops and plating adhesion decreases.

In addition to this, Sb, Pb, Bi, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, P, B, Sn, Zr, Hf, Sr, V, Se, REM are included in the plating layer. 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 and sometimes preferred. 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 of 0.6 to 0.6 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 the Fe-Zn alloy having an average particle size of 0.5 to 3 µm on the plating side can be improved by improving the plating adhesion. The reason why the plating adhesion improves when there is an oxide containing Si in the crystal grain boundaries and crystal grains of the high-strength steel sheet is that the surface of the steel sheet is plated by the formation of an oxide containing Si in the steel sheet during the annealing process. It is thought that this is because SiO ₂ that causes a decrease in adhesion is not exposed.

  The reason why the plating adhesion is improved by forming an Fe-Zn alloy having an average particle size of 0.5 to 3 μm on the plating side from the interface between the high-strength steel plate and the plating layer is that the steel plate reacts with the plating bath. This is thought to be due to the improved adhesion.

In general, in a steel sheet having a Si content of less than 0.3%, it is known that the steel sheet and the plating bath react to produce an Fe-Al-Zn-based intermetallic compound, thereby improving adhesion. As a result of various experiments, it has been clarified that, in a steel sheet having a Si content of 0.3% or more, the plating adhesion is improved by the formation of an Fe-Zn intermetallic compound. If SiO ₂ is exposed on the surface of the steel sheet therefore this order to inhibit the reaction of the plating bath and the steel sheet, Fe-Zn alloy without generating believed plating adhesion is reduced at the same time.

  FIG. 1 shows the result of observing a cross-section with an SEM image after embedding and polishing a high-strength hot-dip galvanized steel sheet having good plating adhesion. In FIG. 1, 1 is a plating layer, 2 is a high-strength steel plate, 3 is an internal oxide layer, and 4 is an Fe—Zn-based intermetallic compound. As can be seen from this figure, the Fe—Zn intermetallic compounds present in the plating layer can be clearly distinguished by microscopic observation. When the Fe% of the present intermetallic compound is analyzed to be about 7%, this Fe—Zn intermetallic compound is considered to be a ζ phase.

  Since the ζ phase has a monoclinic crystal structure, it is rectangular or parallelogram as shown in FIG. Therefore, the average particle diameter of the Fe—Zn-based intermetallic compound was determined by measuring the major axis and minor axis of the rectangle or parallelogram, and using the average value.

  The reason why the average particle size of the Fe—Zn intermetallic compound is limited to 0.5 to 3 μm is that if it is less than 0.5 μm, the effect of improving the plating adhesion is not sufficient, and if it exceeds 3 μm, the Zn—Fe alloy This is because the chemical reaction proceeds too much, a brittle alloy layer develops at the interface of the iron-base, and the plating adhesion deteriorates.

  As a result of the inventors investigating a large number of Fe—Zn intermetallic compounds in plating, in a high-strength hot-dip galvanized steel sheet having good plating adhesion, the Fe—Zn intermetallic compound is 1 in any cross section. It was confirmed that they existed at a rate of not less than 500 / μm.

In addition, the oxide containing Si existing in the crystal grain boundary and the crystal grain can be clearly distinguished by microscopic observation. A cross-sectional observation result is shown in FIG. 2 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. 2 shows a result of observing an SEM image by performing embedded polishing with a cross section of a high-strength hot-dip galvanized steel sheet with good plating adhesion inclined at 10 degrees. In FIG. 2, 1 is a plating layer, 2 is a high-strength steel plate, 3 is an internal oxide layer, 4 is an oxide containing Si existing in crystal grains, and 5 is an oxide containing Si existing in crystal grain boundaries. . As can be seen from FIG. 2, the crystal grain boundary of the high-strength steel plate and the oxide containing Si existing in the crystal grain 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 weight 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 plating adhesion is improved. This is because the effect of improving plating adhesion 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 occurs, 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 : K2 = 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: K3 = 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 is defined or all the above values are set. You need to do one of the following:

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. It is made of impurities, and the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is −0.000034T ² + 0.105T−0.2 [Si%] ² +2.1 [Si%] − 98.8 ≦ log PO ₂ ≦ −0 .000038T ² + 0.107T-90.4 (1 set)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum steel sheet temperature (K)
[Si%]: Si content in steel sheet (wt%)
Reduction is performed in a controlled atmosphere.
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 why logPO ₂ is limited to −0.000038T2 + 0.107T-90.4 or less is to reduce iron oxide in the reduction zone. When logPO ₂ exceeds −0.000038T2 + 0.107T-90.4, the iron oxide region is entered, so an iron oxide film is generated on the steel sheet surface, and non-plating defects occur.

LogPO ₂ The -0.000034T2 + 0.105T-0.2 [Si%] 2 + 2.1 [Si%] - 98.8 reason for limiting above is, LogPO ₂ is -0.000034T2 + 0.105T-0.2 [Si %] 2 + 2.1 [Si%] − less than 98.8, the Si oxide SiO ₂ is exposed on the surface and causes non-plating defects.

By setting logPO ₂ to −0.000034T2 + 0.105T−0.2 [Si%] 2 + 2.1 [Si%] − 98.8 or more, FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , One or more Si oxides selected from Mn ₂ SiO ₄ are present, and an oxidized state in which SiO ₂ is present on the inner surface side 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. For this reason, the plating adhesion is reduced. 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 -0.000038T2 + 0.107T-90.4, -0.000038T2 + 0.107T-90.4 in the following areas In order to be reduced, the iron oxide film formed first is reduced, and one or more kinds of Si selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ are formed on the target steel sheet surface or surface side. In order to obtain a steel sheet in which oxide is 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, logPO ₂ passes through the region where -0.000038T2 + 0.107T-90.4 is exceeded before the plate temperature exceeds 923K in the temperature rising process, and an iron oxide film is formed on the steel plate surface. Therefore, while reducing this, the external oxidation of Si is suppressed, and one or more Si oxides selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ are formed on the steel sheet surface or surface side. For this purpose, 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 suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

  The hot rolling finishing temperature must 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 cooled to 627 K through galvanizing at an average cooling rate of 0.5 degrees / second or more from 773 K, and until 623 K is reached from 773 K after plating. Is held for 25 seconds or more and 240 seconds or less.

  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, increase the C concentration of austenite, thereby lowering its free energy of formation, The purpose is to set the temperature at which site transformation starts to be equal to or lower than 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 20 degrees / second in a dry atmosphere.

  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 20 degrees / second in a dry atmosphere.

  The reason why the time from 773K to 623K after plating 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 with excellent workability is obtained. . If the time from 773K to reach 623K after plating 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. This is because if it exceeds 240 seconds, the bainite transformation proceeds too much, the amount of austenite decreases, and a sufficient amount of retained austenite cannot be generated.

  While cooling from 773K to 623K, it passes through a hot dip galvanizing bath, but there is no problem if the above average cooling rate and the time from 773K to 623K are within the range of the present invention. Although the bath temperature of the hot dip galvanizing bath varies depending on the bath composition, 673 to 753 K is appropriate in the bath composition range of the present invention.

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 make a 1.6 mm cold rolled steel strip, it is passed through the conditions shown in Table 2 using a continuous hot dip galvanizing facility of the in-line annealing method, A plated 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%.

Annealing is adjusted so that the maximum reached temperature becomes the value shown in Table 2, and the soaking time in the soaking temperature (the range from the maximum attainable temperature minus 20 degrees to the maximum attainable temperature) is set to 60 seconds. From the maximum temperature reached 923K at an average cooling rate of 1 degree / second, subsequently from 923K to 773K at an average cooling rate of 4 degrees / second, and further from 773K at an average cooling rate of 1.7 degrees / second or more at 723K. Until the plating bath is kept at 723 K, and the time from 773 K to the plating bath is secured for 30 seconds, then hot-dip plating is performed for 3 seconds in a Zn-Al plating bath with a bath temperature of 723 K, and the amount of plating deposited by N ₂ wiping And cooled to 623K over 20 seconds. The composition in the plating layer of the obtained plated steel sheet was the value shown in Table 2 and Table 3 (continued in Table 2).

The PO ₂ in the reduction furnace is the measured value of the hydrogen concentration, water vapor concentration, CO ₂ concentration, CO concentration, ambient temperature in the furnace and the equilibrium reaction H ₂ O · H ₂ + 1 / 2O _2.
CO ₂ · CO + 1/2 O 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 gravimetric 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 weight 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 by observing an oxide containing Si in a CMA image from the cross-section of the embedded polished steel sheet and evaluated according to the following criteria. .

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 The oxide in which Si and O are observed at the same position is not observed SiO ₂ position ○: The oxide in which Si and O are observed at the same position is observed at the same position as Fe or Mn and Si and O What is observed inside the steel plate from the oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate ×: Oxide where Si and O are observed at the same position is steel plate The Fe—Zn-based intermetallic compound present in the plated layer was evaluated by burying a cross section in the rolling vertical direction of the plated steel sheet by 2 cm and observing the cross section with an SEM image after polishing. The grain size of the Fe—Zn-based intermetallic compound was determined by measuring the major axis and minor axis of the observed crystals and averaging them. For the average particle size, 4 to 10 particles having a large particle size were selected from the observed crystals, and the average value was calculated. In the inventions observed this time, four or more crystals were observed.

  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 of less than 1% 3: Unplated area ratio of 1% to less than 5% 2: Unplated area ratio of 5% to less than 10% 1: Unplated area ratio of 10% or more After affixing the tape, it is bent 180 degrees, bent back, and the tape is peeled off. The width of the plating attached to the tape is taken as the peel width, and when the peel width becomes 3 mm or less, the peel width is 3 mm. When it became super, it was set as x.

  The plating adhesion was evaluated as “◯” when the adhesive tape was applied 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 die having a 1/2 inch roundness at the tip and dropping a 3 kg weight from a height of 1 m.

The evaluation results are as shown in Tables 2 and 3 (continued in Table 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 product caused non-plating and poor plating 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 plating adhesion. 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, cold rolling to obtain a 1.6 mm cold rolled steel strip, followed by plating under the conditions shown in Table 3 using an in-line annealing method of continuous hot dip galvanizing equipment, A plated 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 values shown in Tables 4 and 5 (continued in Table 4).

The annealing was adjusted so that the maximum temperature reached the value shown in Table 3, and the soaking time in the soaking temperature (the range from the maximum temperature to -20 degrees to the maximum temperature) was set to 60 seconds. From the maximum temperature reached 923K at an average cooling rate of 1 degree / second, subsequently from 923K to 773K at an average cooling rate of 4 degrees / second, and further from 773K at an average cooling rate of 1.7 degrees / second or more at 723K. Until the plating bath is held at 723 K, and the time from 773 K to the plating bath is secured for 30 seconds, then hot-dip plating is performed in the Zn-Al plating bath for 3 seconds, and the plating adhesion amount is adjusted by N ₂ wiping. Cooled to 623K over 20 seconds. The composition in the plating layer of the obtained plated steel sheet was the value shown in Table 4 and Table 5 (continued in Table 4).

The PO ₂ in the reduction furnace is the measured value of the hydrogen concentration, water vapor concentration, CO ₂ concentration, CO concentration, ambient temperature in the furnace and the equilibrium reaction H ₂ O · H ₂ + 1 / 2O _2.
CO ₂ · CO + 1/2 O 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 gravimetric 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 weight 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 by observing an oxide containing Si from a cross-section of the embedded and polished plated steel sheet using a CMA image and the following criteria.

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 plate surface ×: Fe or Mn Oxides where Si and O are observed at the same position are not observed SiO ₂ position ○: O and 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 the oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate ×: Oxide where Si and O are observed at the same position is steel plate The Fe—Zn-based intermetallic compound present in the plated layer was evaluated by burying a cross section in the rolling vertical direction of the plated steel sheet by 2 cm and observing the cross section with an SEM image after polishing. The grain size of the Fe—Zn-based intermetallic compound was determined by measuring the major axis and minor axis of the observed crystals and averaging them. For the average particle size, 4 to 10 particles having a large particle size were selected from the observed crystals, and the average value was calculated. In the inventions observed this time, four or more crystals were observed. In Comparative Example No. 11, no Fe—Zn intermetallic compound was observed, but a thick Fe—Al intermetallic compound was observed.

  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 of 1% or more and less than 5% 2: Unplated area ratio of 5% or more and less than 10% 1: Unplated area ratio of 10% or more Adhesive tape was applied to the subsequent hot-dip plated steel sheet, and then peeled off. The case where the plating did not peel off was marked with ◯, and the case where the plating peeled off was marked with x. The DuPont test was conducted by using a shooting die having a 1/2 inch roundness at the tip and dropping a 3 kg weight from a height of 1 m.

  The evaluation results are as shown in Tables 4 and 5 (continued in Table 4). No. 1 is rejected because the Al-concentration in the plating layer is outside the range of the present invention, the Zn-Fe alloying reaction has progressed too much, a brittle alloy layer has developed at the iron-iron interface, and the plating adhesion has deteriorated. It became. No. 11 was rejected because the Al concentration in the plating layer was outside the range of the present invention, so that the Fe-Al alloying reaction proceeded too much 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 obtain a 1.6 mm cold rolled steel strip, plating is performed under the conditions shown in Table 4 using an in-line annealing method of continuous hot dip galvanizing equipment, A plated 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.

  The annealing was adjusted so that the maximum temperature reached the value shown in Table 4, 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, ambient temperature in the furnace and the equilibrium reaction H ₂ O · H ₂ + 1 / 2O _2.
CO ₂ · CO + 1/2 O 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 dipping, hot dipping was performed for 3 seconds in a Zn—Al plating bath, and the amount of plating adhered 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 Al 0.4 to 0.5% and Fe 0.4 to 0.6%.
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 weight 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 by observing an oxide containing Si from a cross-section of the embedded and polished plated steel sheet using a CMA image and the following criteria.

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 plate surface ×: Fe or Mn Oxides where Si and O are observed at the same position are not observed SiO ₂ position ○: O and 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 the oxide Δ: Oxide where Si and O are observed at the same position is observed inside the steel plate ×: Oxide where Si and O are observed at the same position is steel plate The Fe—Zn-based intermetallic compound present in the plated layer was evaluated by burying a cross section in the rolling vertical direction of the plated steel sheet by 2 cm and observing the cross section with an SEM image after polishing. The average particle diameter of the Fe—Zn intermetallic compound of the obtained plated steel sheet was 0.5 to 3 μm. In addition, in the inventions observed this time, four or more crystals were observed.

  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 of 1% or more and less than 5% 2: Unplated area ratio of 5% or more and less than 10% 1: Unplated area ratio of 10% or more Adhesive tape was applied to the subsequent hot-dip plated steel sheet, and then peeled off. The case where the plating did not peel off was marked with ◯, and the case where the plating peeled off was marked with x. The DuPont test was conducted by using a shooting die having a 1/2 inch roundness at the tip and dropping a 3 kg weight from a height of 1 m.

  The evaluation results are as shown in Tables 6 and 7 (continued in Table 6). 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 a figure which shows the result of having observed the cross section by the SEM image after embedding | polishing and polishing the high intensity | strength hot-dip galvanized steel plate with favorable plating adhesiveness, and etching. It is a figure which shows the result of having carried out embedding grinding | polishing by inclining the cross section of a high intensity | strength hot-dip galvanized steel plate with favorable plating adhesiveness to 10 degree | times, and observing the cross section with a SEM image.

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%,
N: 0.0060% or less, Al: 0.05 to 10% by mass, Fe: 0.05 to 3% by mass on the surface of the high-strength steel plate composed of Fe and unavoidable impurities, and the balance In a hot-dip galvanized steel sheet having a galvanized layer consisting of Zn and unavoidable impurities, the average of the grain boundaries on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer and the oxide containing Si in the crystal grains A high-strength hot-dip galvanized steel sheet excellent in plating adhesion, characterized in that it is present at a content of 0.6 to 10% by mass and an Fe—Zn alloy having an average particle size of 0.5 to 3 μm is present on the plating side . % 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: 0.0060% or less, Al: 0.05 to 10% by mass, Fe: 0.05 to 3% by mass on the surface of the high-strength steel plate composed of Fe and unavoidable impurities, and the balance In a hot-dip galvanized steel sheet having a galvanized layer consisting of Zn and unavoidable impurities, the average of the grain boundaries on the steel sheet side of 5 μm or less from the interface between the high-strength steel sheet and the plated layer and the oxide containing Si in the crystal grains Fe-Zn alloy having a content of 0.6 to 10% by mass and having an average particle size of 0.5 to 3 μm is present on the plating side at a ratio of 1 piece / 500 μm or more in an arbitrary cross section. High-strength hot-dip galvanized steel sheet with excellent plating adhesion. The oxide containing Si in claim 1 or 2 is one or more selected from SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ and has excellent plating adhesion High strength hot dip galvanized steel sheet. The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to ₃ , wherein at least one kind of Si oxide selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO _{4 is} formed on the steel sheet surface or surface side. A high-strength hot-dip galvanized steel sheet with excellent plating adhesion, 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%,
N: 0.0060% or less, and when continuously hot dip galvanizing to a high-strength steel plate composed of the remainder Fe and inevitable impurities, as an atmosphere of the reduction zone, H2 is contained ₁ to 60% by volume, The balance consists of one or more of 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 −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 formula)
923 ≦ T ≦ 1173 (2 formulas)
T: Maximum temperature of steel sheet (K)
[Si%]: Si content in steel sheet (wt%)
The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the plating adhesiveness characterized by performing reduction | restoration in a controlled atmosphere. % 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: The slab containing 0.0060% or less, the balance of Fe and inevitable impurities is finish-rolled at a temperature of Ar ₃ or higher, 50% to 85% cold-rolled, 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 logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is −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 in steel sheet (wt%)
In a controlled hot-dip galvanizing facility and annealing in the two-phase coexistence temperature range of 1023K to 1153K ferrite and austenite, and the average cooling rate from the highest temperature to 923K is 0.5 to 10 degrees / By cooling in seconds, subsequently cooling from 923 K to 773 K at an average cooling rate of 3 degrees / second or more, further cooling from 773 K at an average cooling speed of 0.5 degrees / second or more, and performing hot dip galvanizing treatment, In the manufacturing method for forming a hot-dip galvanized layer on the surface of the cold-rolled steel sheet, the time required to reach 623K after plating from 773K is set to 25 seconds or more and 240 seconds or less and excellent in plating adhesion A method for producing high-strength hot-dip galvanized steel sheets.