JP4790525B2 - High-strength galvannealed steel plate with excellent chipping resistance - Google Patents

Description

  The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet with excellent chipping resistance. More specifically, the present invention is applied to automotive steel sheets, architectural steel sheets, etc. that have both chipping resistance and adhesion (non-plating resistance). The present invention relates to a high strength alloyed hot dip galvanized steel sheet.

  An alloyed hot-dip galvanized steel sheet is available as a plated steel sheet having good corrosion resistance. This alloyed hot-dip galvanized steel sheet is usually degreased and preheated in a non-oxidizing furnace or direct-fired furnace, and subjected to reduction annealing in a reducing furnace to clean the surface and secure the material, It is manufactured by dipping and controlling the amount of adhesion, followed by alloying. 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.

  It is effective to add elements such as Si, Mn, and P to increase the strength of the steel sheet without degrading workability. However, the addition of these elements delays alloying, so compared to mild steel. High temperature and long time alloying is required. This alloying for a long time at a high temperature transforms the austenite remaining in the steel sheet into pearlite and lowers the workability. As a result, the effect of the additive element is offset. Moreover, since Si is particularly easier to oxidize than Fe, if a steel sheet containing Si is plated under normal hot dip galvanizing conditions, Si in the steel is concentrated on the surface during the annealing process, causing non-plating defects. It is known.

  In order to solve these problems, a hot dip galvanizing treatment immersed in a hot dip zinc bath is carried out. An effective Al concentration in the bath is 0.07 to 0.105 mass%, and the balance is hot dip zinc having a component composition consisting of Zn and inevitable impurities. In a plating bath, and alloying treatment is performed at a temperature T (° C.) that satisfies 225 + 2500 × [Al%] ≦ T ≦ 295 + 2500 × [Al%] (mass%), and when Si is high Is performed at an effective Al concentration (mass%) in the bath satisfying [Al%] ≦ 0.103−0.008 × [Si%], and is a high-strength alloying melt with excellent workability. It has been proposed to prevent non-plating by a method for producing a galvanized steel sheet (see, for example, Patent Document 1). However, this invention may not improve the chipping of the plating layer.

  Chipping is a phenomenon in which pebbles hit the vehicle body due to stone splashes while the car is running, and the coating or plating is peeled off by the impact. When the plating peels off and the base steel plate is exposed, the corrosion resistance of the steel plate is remarkably increased. Deteriorate. The peeling of the plating due to this chipping occurs remarkably at low temperatures. This is because the coating film hardens and the viscoelasticity decreases due to a decrease in temperature, and the coating film cannot absorb impact energy such as pebbles, so that peeling occurs at the low toughness plating / steel sheet interface. Is considered to be the cause. For this reason, in order to improve the chipping resistance, it is not sufficient to simply increase the plating adhesion, and it is necessary to improve the toughness of the plating / steel interface.

  Various technologies for improving the chipping property of alloyed hot-dip galvanized steel sheets have been proposed. For example, in a continuous hot-dip galvanizing line, the technology for improving the chipping property by improving the bonding force between the steel sheet and the plating interface is proposed. After annealing, controlling the dew point (-10 to -40 ° C) of the atmosphere during heating annealing and the rate of temperature increase, hot dip galvanizing, alloying treatment, unevenness at the interface between the plating and the steel A method for producing an alloyed hot-dip galvanized steel sheet excellent in adhesion and slidability with improved low-temperature chipping resistance by the anchor effect (see, for example, Patent Document 2) has been proposed.

  However, in this method, the dew point of the atmosphere is set to −10 to −40 ° C. so as not to generate an internal oxide layer and to promote Fe diffusion in the grain boundary. That is, it is said that the chipping property can be improved by not generating the internal oxide layer.

  However, the oxidation behavior differs depending on the difference in the components of the steel sheet, and for steel sheets with high Si, Si is particularly easier to oxidize than Fe, so at such dew point, the surface concentration of Si is large and there is no internal oxide layer. In the region, there is a problem that an easily peelable interface is generated, and the chipping property cannot be improved.

  Further, as a technique for improving the chipping property by controlling the thickness of the Γ phase in the plating layer, for example, the average thickness of the Γ phase in the plating layer: 2.0 μm or less, and the center line of the surface of the plating layer Average roughness: 3.0 μm or less, and formula: [T · Γ] × Ra ≦ 1.5 (where [T · Γ] is the average thickness of the Γ phase, and Ra is the center of the plating layer surface) An alloyed hot-dip galvanized steel sheet with excellent chipping resistance (for example, see Patent Document 3), an average crystal grain size of 200 nm to 800 nm, Low-temperature resistance characterized by having a plated film made of an Fe-Zn alloy having an Γ phase having an average thickness of 0.5 μm or more and containing Fe of 10 wt% or more and 15 wt% or less on at least one side Alloyed hot-dip galvanized steel sheet with excellent chipping ( In example, see Patent Document 4) have been proposed.

  However, these technologies are all related to steel sheets equivalent to IF steel (the former is an ultra-low carbon steel containing Ti and Nb and the latter is a P-added IF steel), and a high Si steel (GA-TRIP steel). Even if these techniques are applied as they are, the high Si steel has a different Si surface enrichment behavior from the IF steel, so that an interface easily peels off and the chipping property cannot be improved. Is difficult to alloy because of the large grain boundary segregation on the surface, and there is a problem that the material does not appear when the alloying temperature is raised to increase the surface roughness (roughness).

  To date, no technology has been proposed for improving the chipping properties of high-Si high-strength steel sheets.

  This is because it is known that the IF steel has few impurities at the grain boundary and an Fe—Zn alloy is easily formed from the grain boundary during the alloying treatment. This is known as an outburst organization. Therefore, since the growth of the Γ phase is large, it is considered that various proposals for the above-mentioned chipping countermeasure have been made. However, if the Si concentration is high, there is Si segregation at the grain boundaries, so it is difficult to produce an outburst structure. Conventionally, the growth of the Γ phase is small, so the chipping property is considered to be originally good and more serious. This is thought to be due to the fact that chipping is thought to be improved if non-plating, which is a defect, is suppressed.

  However, the inventors have proceeded with the development of the present invention after confirming that chipping properties are deteriorated even if non-plating is suppressed in a high-Si high-strength steel sheet.

  The presence of oxides in the plating layer, which is also described in the present invention, is known (see, for example, Patent Documents 5 and 6).

  Among these, Patent Document 5 relates to the adhesion of the plating layer, and an oxide of 600 nm or less is present in the plating, and there is no oxide at the steel plate and the plating interface in a portion of 30% or more of the interface length. Proposed. However, as will be described later, when an oxide is present at the interface, it is found that a Γ phase grows from the periphery thereof, so that the chipping property cannot be improved in the present invention.

  Patent Document 6 proposes that when an oxide is present during plating, the unformed portion of the Fe—Zn alloy layer is less than 10% and the formability is improved. In this case, it is stated that the internal oxide moves to the plating layer in the alloying process after the hot dipping. As will be described later, if an oxide is present at the interface in this case, a Γ phase is formed from the surroundings. Therefore, the chipping property cannot be improved by this invention.

JP 2003-105516 A Japanese Patent Laid-Open No. 10-130802 JP-A-9-241816 JP-A-10-130804 JP 2004-204280 A JP 2004-315960 A

  High-strength steel sheets are generally manufactured by adding necessary amounts of solid solution strengthening elements such as C, Si, Mn, and P, precipitate forming elements such as Ti and Nb, and combining the structure control in the manufacturing process. However, the influence of steel components on the hot dip galvanizing properties (plating wettability, alloying speed) of steel is large. If elements having a strong affinity for oxygen such as Si and Mn are added, these elements will be added during annealing before hot dip galvanizing. As a result of generating an oxide on the surface of the steel sheet, the wettability with molten zinc is lowered, causing quality defects such as non-plating. On the other hand, the alloy layer of the galvannealed steel sheet is as hard as micro Vickers hardness: M-Hv> 200, and the alloy layer adhesion during processing is an important quality control issue. In particular, when an impact is received in a low temperature range where the viscoelasticity of the coating film is lowered, a so-called chipping phenomenon may occur in which the alloy layer peels off and drops together with the coating film, which is a problem from the viewpoint of appearance and rust prevention. Become.

  Accordingly, an object of the present invention is to propose a high-strength galvannealed steel sheet that does not cause non-plating and has good chipping resistance.

The present invention is a continuous annealing as a pretreatment for plating a high-strength steel sheet containing Si, Mn, etc., and FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO _{3 in the} grain boundaries and crystal grains of 5 μm or less from the steel sheet surface. , Mn ₂ regions Si oxide comprising at least one silicate selected from SiO ₄ is contained more than SiO ₂ is, in the thickness direction of the steel sheet, continuously so as to be positioned on the surface of the steel sheet side annealing, the steel By continuously annealing so that Si of the steel does not concentrate on the surface, non-plating is effectively prevented, and a region of Fe concentration 15 to 90% that can be read by GDS on the plating side of the steel plate and the plating interface is 1.8 to If the Fe concentration gradient at the interface is made small with a thickness of 3.5 μm, even if there is a difference between the hardness of the steel plate and the hardness of the Γ phase in a high-strength steel plate such as GA-TRIP, pebbles will hit. A few mm In a macroscopic area, a gentle gradient of hardness can be formed on the surface of the steel sheet, and even if pebbles hit the high-strength alloyed hot-dip galvanized steel sheet, stress concentration can be eased against the impact from the plated surface. The present invention has been completed by finding that chipping resistance can be improved.

  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%,
N: 0.0060% or less,
In a high-strength galvannealed steel plate having an alloyed hot-dip galvanized layer on the high-strength steel plate composed of the remaining Fe and inevitable impurities, a crystal on the steel plate side of 5 μm or less from the interface between the high-strength steel plate and the plated layer An oxide containing Si is present at an average content of 0.6 to 10% by mass in grain boundaries and crystal grains, and a region having an Fe concentration of 15 to 90% that can be read by GDS from the interface between the steel sheet and the plating layer to the plating side is 1 High in excellent chipping resistance, characterized by being present in a thickness of .8 to 3.5 μm and having an average content of 0.05 to 1.5 mass% of oxide containing Si in the plating layer High strength alloyed hot dip galvanized steel sheet.

  (2) The region having an Fe concentration of 15 to 90% measured by the GDS has a gradual concentration gradient in which the Fe concentration increases toward the steel plate side. Excellent high-strength galvannealed steel sheet.

( 3 ) The oxide according to ( 1 ), wherein the Si-containing oxide is at least one selected from SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ . High-strength galvannealed steel sheet with excellent chipping properties.

( 4 ) The high-strength steel sheet is further in mass%, and Nb, Ti, B, Mo, Cu, Ni, Zr, W, Co, Ca, rare earth elements (including Y), and one or more of V are added in total. % High-strength galvannealed steel sheet excellent in chipping resistance according to any one of the above (1) to ( 3 ).

  According to the present invention, in a high-strength galvannealed steel sheet obtained by plating a high-strength steel sheet containing Si, Mn, etc., it is possible to eliminate non-plating defects and improve chipping resistance. There is an effect.

  The present invention will be described in detail below.

  In recent years, in the automobile field, high-strength alloying and melting with excellent workability has been achieved to reduce the thickness of steel sheets for the purpose of improving the fuel efficiency of automobiles and to ensure the required strength even if they are thin. There is a need for galvanized steel sheets.

  In order to increase the strength of the steel sheet without deteriorating workability, it is effective to add elements such as Si, Mn, and P. However, since Si is particularly easy to oxidize than Fe, it contains Si. It is known that when a steel plate is plated under normal hot dip galvanizing conditions, Si in the steel is concentrated on the surface during the annealing process, causing non-plating defects.

  According to the study by the present inventors, when a non-plating defect occurs in a high-strength hot-dip galvanized steel sheet, it causes an appearance defect. Further, simply improving the non-plating defect in a high-strength hot-dip galvanized steel sheet improves chipping resistance. I knew that I couldn't.

Accordingly, the present inventors have further researched on improving the chipping resistance of the high-strength hot-dip galvanized steel sheet, and as a result, as shown in FIG. When a region of ˜90 mass% is present with a thickness of 1.8 to 3.5 μm and the Fe concentration gradient in this portion is small (the thickness of the concentration portion of 15% to 90 mass% is large), plating is performed from the steel plate side. Since the gradient of the hardness of the interface up to the phase becomes small, the stress concentration at the size hit by the stone can be relaxed, and the concentration gradient of Fe is formed, so that the interface between the plating layer and the steel plate is 1.8. so that the loose areas of concentration gradient of Zn which is inversely proportional to the concentration gradient of Fe in the thickness range of ~3.5μm is formed. And it discovered that chipping resistance could be improved by converting the Si oxide containing a large amount of silicate in the plating layer into an Si concentration and adding an average content of 0.05 to 1.5% by mass. Was completed.

  First, the reason why the conventional high Si steel has poor chipping resistance will be described.

The inventor conducted research on the prevention of the adverse effects of Si oxides, and found that chipping resistance deteriorates when Si oxides remain in the form of SiO ₂ as a layer at the interface.

  As a pretreatment for hot dip galvanization, annealing is performed on the steel sheet. By this annealing, an oxide of Si is generated on the surface of the steel sheet. When hot dip plating is performed, the oxide of Si is layered on the interface between the plating layer and the steel sheet. Remains. It is considered that the Si oxide cannot be removed from the plating interface, and cracks generated from the Si oxide as a starting point develop along the interface due to the impact of the pebbles and the like, thereby deteriorating the chipping resistance.

In particular, when high-Si steel was produced with a low dew point during annealing, the above phenomenon was observed because SiO ₂ was formed in layers at the interface.
Furthermore, since chipping resistance has a greater influence on the interface than galvanized adhesion, it is thought that chipping resistance deteriorates if SiO ₂ remains at the interface between the steel sheet and the plating layer even in a very small amount. It is done.

Since the alloyed hot-dip galvanized steel sheet is alloyed immediately after hot-dip plating, even if SiO ₂ remains on the outermost surface, alloying proceeds from its periphery, and it is considered that there is no problem in plating adhesion. However, when there is a difference in the alloying speed at the interface in this way, the alloying speed around the SiO ₂ remaining is increased, and therefore the hard and brittle Γ phase is formed around the Si oxide remaining at the interface. Is likely to be generated, and this is considered to cause a reduction in chipping resistance. For this reason, it is considered that more strict control of SiO ₂ is required to ensure good chipping resistance. This phenomenon is adjusted such that the dew point is adjusted so as not to generate layered SiO ₂ so that non-plating can be prevented. Even in this case, it is considered that this occurs when Si oxide remains on the surface of the steel sheet.

  Therefore, the present inventors have further researched on improving the chipping resistance of the high-strength hot-dip galvanized steel sheet, and as a result, an area having an Fe concentration of 15 to 90% that can be read by GDS is 1 on the steel sheet side of the plated layer. By adding a Si oxide containing a large amount of silicate in the plating layer to a Si concentration, the average content is 0.05 to 1.5% by mass. The present invention was completed by finding that the chipping property can be improved.

The process until the steel plate surface reaches the above-mentioned shape will be described below in order.
It is considered that the steel sheet surface is formed through the following processes (1) to (6).
(1) Annealing so that there is a difference in the type of oxide.
(2) Microscopic irregularities are generated on the steel sheet surface.
(3) A ζ phase is generated in the plating bath.
(4) Apparently the oxide moves into the plating layer (5) Alloying treatment is performed.
(6) A region having an Fe concentration of 15 to 90% is formed with a thickness of 1.8 to 3.5 μm.

First, (1) Si oxide is 50% by mass of one or more selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ as Si oxide formed on the surface of the steel sheet. or contains an oxide of SiO ₂ to the balance _{SiO 2, FeSiO 3, Fe 2} SiO 4, MnSiO 3, Mn 2 SiO 4 is selectively generated so that the surface of the steel sheet side. Although details of this generation method will be described later, the inventors have found that if the oxygen potential during annealing is unprecedented and precisely controlled, an oxide distribution that has not been conventionally known can be obtained.

  In addition, since these oxides are not layered, they can solve the problem of chipping failure when they are manufactured at a conventional low dew point. However, if these oxides are present at the steel plate and plating interface during the alloying process, the Γ phase grows. It becomes easy to do and chipping property deteriorates.

Next, (2) among the oxides of Si formed immediately below the surface of the steel sheet, if FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ are the main components, these oxides are converted into SiO ₂ . Since the volume expansion is larger than that, it is considered that the microscopic unevenness of the interface becomes larger. This is because the generation of internal oxide is caused by the diffusion of oxygen from the atmosphere and the diffusion of Si and Mn from the inside of the steel sheet, so the larger the number of oxygen molecules in the oxide, the larger the oxide, and the volume expansion at that time It is thought to cause.

Thus, the oxide formed on the steel sheet surface portion as an oxide of Si contains 50% by mass or more of one or more selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ , It has been found that the microscopic unevenness of the interface is further increased when the oxide is composed of the remaining SiO ₂ . On the other hand, if it is 50% or less, the effect of forming microscopic unevenness is reduced. In the end, these oxides are present during plating, but an average content of 0.05 to 1 is calculated by converting Si oxide containing a large amount of silicate in the plating layer into Si concentration. It has been found that when the content is 5% by mass, the above-described effect is produced.

In addition, there is finally an oxide mainly composed of SiO ₂ inside the steel plate side of the plating interface, and its concentration is 0.6 to 10% by mass in terms of the average content of oxide containing Si. The present inventors have found that the above-mentioned oxide is a condition for being present in the plating layer.

  (3) Next, a ζ phase is generated in the plating bath.

  As will be described later, when the ζ phase is generated on the surface of the steel sheet, the oxide present at the interface between the plating layer and the steel sheet can be moved into the plating layer, so that generation of a hard and brittle Γ phase from the periphery of the oxide can be suppressed.

  It is known in IF steel that the ζ phase is formed in the plating bath.

However, since high Si steel has larger grain boundary segregation than IF steel, it is difficult for the Fe—Zn alloy to grow from the grain boundary. However, the inventor, after appropriately adjusting the oxygen potential at the time of annealing described above, promotes heat transfer from the plating bath to the steel plate in the plating bath before the alloying treatment, so that the ζ phase is sufficiently grown in the plating bath. I found out.
FIG. 2 shows the result of observation of the ζ phase generated when the steel plate is immersed in the plating bath after the oxygen potential during annealing is properly adjusted.
Thus, the inventor has found that the ζ phase has already been formed before the alloying treatment.

  First, the growth process of the ζ phase will be described. The ζ phase 1a is generated by the reaction of Fe and Zn in the steel sheet at the site where zinc in the plating bath is in contact with the steel sheet. For this reason, since the steel plate surface is replaced by the ζ phase, the interface between the steel plate and the plating layer moves to the steel plate side. Therefore, in order for the ζ phase to grow, it is necessary to diffuse Zn into the interface, and this Zn is supplied from the molten plating bath. Therefore, the growth of the ζ phase becomes Zn diffusion-limited. This diffusion increases as the temperature increases. For this purpose, it is important to promote heat transfer from the plating bath to the steel plate in the plating bath. For this reason, when the steel sheet enters the plating bath, the inventor, for example, in Japanese Patent Application Laid-Open No. 2003-293107, arranges one metal pump at each end in the steel sheet width direction in the snout for guiding the steel sheet to the molten metal bath, One is a discharge type in the snout, the other is a suction type in the snout, and the metal pump of the discharge type in the snout has a variable discharge port height. Heat transfer if a transverse flow is generated around the steel plate using a metal pump in the apparatus that has been proposed as a molten metal plating apparatus with excellent appearance, characterized by being arranged one by one parallel to the bath surface Found to promote. However, since the steel sheet is moving, it is necessary to increase the gradient of the boundary layer flow at the interface in order to promote heat transfer, so this speed needs to be approximately equal to the plate passing speed of the steel sheet. Specifically, it has been found that heat transfer is promoted by adjusting the range from 0.3 m / s to 2.5 m / s. If it is 0.3 m / s or less, the heat transfer is not sufficiently promoted and the thickness of the ζ phase is not sufficiently grown. If it is 2.5 m / s or more, the growth of the ζ phase is not further promoted. In order to strictly control dross and flow patterns, it is necessary to select the range of 0.5 m / s to 1.5 m / s shown in Japanese Patent Application Laid-Open No. 2003-293107. For this purpose, this range is not necessary.

  (3 ') Effect of oxygen potential during annealing on ζ phase generation.

  The inventor has also found that the ζ phase does not grow sufficiently from a steel sheet annealed under an inappropriate oxygen potential.

  Conversely, from a steel sheet annealed under an appropriate oxygen potential, even if there is no flow, if the temperature of the plating bath and the immersion time in the plating bath are properly selected, the ζ phase is generated, although not as much as in the state of flow. , It was found to develop.

Annealing under an appropriate oxygen potential is such that FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ are mainly formed among the above-mentioned Si oxides formed immediately below the steel sheet surface. Annealing conditions. When such oxides are formed, microscopic irregularities are formed on the surface, so that the effective interfacial area increases in the plating bath, and surface deformation occurs when microscopic irregularities are formed. Since there are relatively many dislocations on the surface of the steel plate before entering the bath, the diffusion of Zn is promoted, and in the state where such an oxide is generated, the case where annealing is performed with an inappropriate low oxygen potential. Similarly, it is conceivable that Zn is easily diffused because no component concentrated layer is formed on the surface.

  The inventors investigated the conditions under which the ζ phase grows in the absence of flow. Assuming that annealing is performed under an appropriate oxygen potential, the following conditions result in 0.08% Al in the bath and the ζ phase. Was confirmed to be generated.

Similarly, the ζ phase is generated even when the Al concentration in the bath is in the range of 0.07% to 0.1%. However, the formation density and thickness of the ζ phase are optimal when the Al content in the bath is 0.08%. It was.
Bath temperature Immersion time 440 ° C. 2.5 s or more 450 ° C. 2 s or more 460 ° C. 1.5 s or more In addition, when the flow rate is as small as 0.3 m / s or 0.5 m / s, the chipping property of the steel sheet is rated 2. Although it was inferior to the case where there was fluidity (the chipping property was 1), it was satisfactory as the range of use.

  (4) When the ζ phase is generated and grown in the plating bath, the oxide apparently moves into the plating layer. This is because the Fe—Zn interface moves to the Fe side due to the diffusion of Zn. As a result, when the ζ phase is generated on the surface of the steel sheet, the oxide present at the interface between the plating layer and the steel sheet can be moved into the plating layer. Furthermore, since a ζ phase is generated at the Fe-Zn interface at this time, the ζ phase continues to grow around the granular oxide, so that a hard and brittle Γ phase is generated from the periphery of the oxide in contact with the interface. Can be suppressed.

When the oxide moves in the plating layer, the Si oxide containing a large amount of silicate is converted to the Si concentration and the average content is 0.05 to 1.5 mass%.
(5) Alloying treatment is performed.

  The alloying temperature in the present invention may be any number as long as the intended plating layer is obtained. When the alloying temperature is high, the alloying proceeds too much, and the brittle Γ phase can be thickened at the plated steel plate interface, so that the plating adhesion at the time of processing is lowered and the chipping property is deteriorated. Therefore, the lower alloying temperature is preferable. . Moreover, by controlling the alloying treatment time (3 to 60 seconds), the diffusion of Fe into the plating layer is controlled, and a region having an Fe concentration of 15 to 90% that can be read by GDS between the steel sheet and the plating layer. , And a thickness of 1.8 to 3.5 μm.

  If it is less than 3 seconds, the ξ phase may remain at the interface after alloying. On the other hand, if it exceeds 60 seconds, the Fe concentration in the plating layer may exceed 12%.

In order to move the oxide from the interface and to make the Fe concentration region of 15 to 90% by mass exist in a thickness of 1.8 to 3.5 μm, in the plating bath, It is preferable to develop the phase thickness to about 2 μm to 4 μm. Further, in the region of the Fe concentration from 15 to 90 wt%, it will be appreciated that for a Fe-Zn alloy plating layer, region Zn concentration is inversely proportional to the value of F e concentration is formed.

  Although depending on the alloying temperature, when the alloying temperature is relatively high, a Γ phase is formed in a layer on the lower side of the oxide. When the alloying temperature is relatively low, the Γ phase may not be generated at the interface.

  (6) A 4 mm diameter portion is measured macroscopically by GDS to form a region having an Fe concentration of 15 to 90% with a thickness of 1.8 to 3.5 μm.

  First, if the microscopic irregularities on the surface of the steel sheet are large, the thickness of a region having an Fe concentration of 15 to 90% increases when measured macroscopically by GDS.

In addition, if the thickness of the ζ phase is sufficiently grown in the plating bath, the initial Fe concentration at the interface before the alloying treatment becomes larger than when the ζ phase is not grown. It is considered that the Fe concentration increases, and the thickness of the region having the Fe concentration of 15 to 90% increases.
Furthermore, it is preferable that the orthorhombic ζ phase is sufficiently developed compared to the case where a Γ phase or Si oxide having an fcc crystal structure is present at the interface without the ζ phase growing sufficiently. It can also be estimated that the diffusion of Fe further proceeds during the conversion.

  Thus, when the Fe concentration gradient is small (the thickness of the concentration portion of 15% to 90% is large), the gradient of the hardness of the interface from the steel plate side to the plating phase becomes small macroscopically. The reason why the stress concentration can be relaxed is estimated as follows.

  The Fe—Zn alloy plating layer is composed of a Γ phase, a δ1 phase, and a ζ phase in which Fe in the steel sheet is diffused during Zn plating. The hardness of the Γ phase in the plating layer is Hv = 326 (Fe concentration: 21.7-27.7%), the δ1 phase is Hv = 284-300 (Fe: concentration 7.3-11.3%), ζ The phase is Hv = 200 (Fe concentration: 5.8 to 6.2%). Moreover, it is said that the phylite phase of a steel plate is Hv = 150-200, and hard phases, such as a retained austenite phase, are Hv = 300-500.

  In the present invention, for the purpose of improving chipping resistance, an Fe concentration of 15 to 90%, which can be read by GDS (glow discharge spectroscopic analysis), is 1.8 to 3.5 μm thick on the plating side of the steel plate and the plating interface. It is characterized by the existence.

  A region where the Fe concentration read by GDS is less than 15% corresponds to a plating-based region, and a region where the Fe concentration exceeds 90% corresponds to a steel plate-based region. The region where the Fe concentration is 15 to 90% corresponds to the region where the plating and the steel plate exist. In this region, a hard phase such as a ferrite phase and a retained austenite phase is present on the steel plate side, and a layered Γ phase or δ1 phase is present on the plating side. Are mixed. Therefore, in these regions, as the Fe concentration increases, the proportion of the ferrite phase increases, so that the macro hardness decreases as it proceeds from the plating surface to the steel plate side. As a result, the impact energy of the surface is gradually converted into elastic energy, so that stress concentration at the interface can be relaxed.

  As described above, in a high-strength steel sheet such as GA-TRIP having a high Si, stress can be relieved in a macroscopic region of millimeter units when a region having an Fe concentration of 15 to 90% is selected widely. As described above, it can be controlled so that Si oxide does not exist at the interface, but industrially, Si oxide should remain at the interface and a Γ phase of about 1.5 μm, for example, is extremely part from the edge. Even if there are several (for example, several in a 4 mm diameter region), the region having an Fe concentration of 15 to 90% is larger than the Γ phase, so that chipping resistance can be ensured.

  As described above, it was found that in the high-Si steel, if the conditions are appropriately selected, the ζ phase is generated in the plating bath and affects the subsequent phase change in the plating layer. Conventionally, ζ phase is generated before alloying even in IF steel. However, alloying is said to proceed mainly by outburst from grain boundaries during the alloying process. Therefore, although there is no measurement example, in the case of IF steel, when the Fe concentration increases during the alloying process, it is estimated that the Γ phase grows in proportion to the Fe concentration. Therefore, in the case of IF steel, it is estimated that the chipping property deteriorates when the Fe concentration region increases in the range of 15 to 90%.

As described above, the phenomenon is complicated, but as a phenomenon that appears conspicuously on the surface of the steel sheet after the plating alloying treatment, an Fe concentration range of 15 to 90% can be understood by analyzing a macro range such as GDS. Is present in a thickness of 1.8 to 3.5 μm, and an Si content containing a large amount of silicate in the plating layer is converted to a Si concentration and contains an average content of 0.05 to 1.5 mass%. That is, there is an oxide mainly composed of SiO ₂ inside the steel plate, and the concentration thereof is 0.6 to 10% by mass in terms of the average content of the oxide containing Si.

  In particular, the presence of a region with an Fe concentration of 15 to 90% with a thickness of 1.8 to 3.5 μm means that microscopic unevenness of the interface exists microscopically, and the thickness of the ζ phase grows sufficiently. And the fact that the Γ phase is developed in layers. Further, in a macro view, a small concentration gradient of Fe at the interface in a region of about 4 mm where pebbles hit is effective for reducing stress concentration at the interface.

  In order to explain these, FIGS. 3A to 3D schematically show the interface state.

  As shown in the figure, there were fine irregularities at the interface, and the ζ phase was also present along the interface. Therefore, when the Fe concentration at the interface was measured by GDS, the region including these irregularities was It is detected as a region of 15% to 90% in Fe concentration.

  When the oxygen potential in the atmosphere during annealing was low (FIG. 3 (a)), silica 3 was present at the interface, and Γ phase 5 grew greatly from that time to deteriorate the chipping property. Further, even if the oxygen potential is slightly increased (FIG. 3B), silica 3 and silicate 4 are often present at the interface, and Γ phase 5 grows greatly from there and deteriorates the chipping property. . On the other hand, when the oxygen potential is appropriately adjusted and a flow is provided in the snout (FIG. 3 (d)), the old ζ phase 1b generated in the bath is observed, but silica 3 and silicate 4 are present at the interface. And the chipping resistance was improved because the Γ phase 5 did not grow significantly. However, even if the oxygen potential is adjusted appropriately, if no flow is given into the snout (FIG. 3 (c)), there remains a case where the silica 3 and the silicate 4 remain at the interface and the Γ phase 5 grows greatly. There was little cost to improve chipping resistance.

  As described above, in high-strength galvannealed steel sheets with high Si, the presence of oxides should be adjusted by appropriately adjusting the oxygen potential in the annealing atmosphere, and the width of the bath should be adjusted within the plating bath snout. The flow in the direction greatly affects the formation of the ζ phase and the formation of the Fe concentrated layer at the plating interface in the subsequent alloying treatment. For example, if the Si concentration of the oxide existing in the steel plate near the plating interface is outside the case of 0.6% to 10% and exceeds 10%, the oxygen potential with more silica than silicate is appropriate. The value is lower than the value, and fine irregularities before plating by silicate are reduced. On the other hand, when it is less than 0.6%, the silica is not sufficiently formed and the silicate is too much. In this case, iron oxide is likely to be formed on the surface and inhibits formation of ζ phase.

  If the Si present in the plating is out of the range of 0.05% to 1.5% and exceeds 1.5%, the oxygen potential with more silica than silicate is lower than the appropriate value. The fine irregularities before plating by silicate are reduced, and the formation of the ζ phase is delayed.

  On the other hand, when it is less than 0.05%, the silica is not sufficiently formed and the silicate becomes too much. In this case, iron oxide is likely to be formed on the surface and inhibits formation of ζ phase.

  Even if the Si concentration range is satisfied, if the flow rate is less than 0.3 m / s out of 0.3 m / s to 2.5 m / s, heat transfer in the plating bath is not sufficient, The growth of ζ phase is delayed. On the other hand, when it exceeds 2.5 m / s, the heat transfer becomes excessive and a coarse ζ phase is easily formed, and unevenness appears in the plating.

  In addition, oxide conditions with many silicates, that is, the Si concentration in the steel sheet in the vicinity of the plating interface is in the vicinity when the Si concentration is 0.6% or more, and the Si existing in the plating is in the vicinity when it is 0.05% or more. Under such conditions, the ζ phase is sufficiently formed even if the flow velocity is as small as 1.2 m / s.

  However, under conditions where the Si concentration of the oxide present in the steel plate near the plating interface is near at 10% or less, and the Si concentration in the plating is near at 1.5% or less, the flow rate is The ζ phase is more likely to be formed in the vicinity of 2.5 m / s and larger.

  From these results, as a result of the alloying treatment, the chipping resistance is improved when the Fe concentration of 15 to 90% is 1.8 to 3.5 μm. When the thickness is less than 1.8 μm, the chipping resistance score is 3 or less, which is not good. On the contrary, if the condition is 3.5 μm or more, the Fe concentration during plating exceeds 12% and is not appropriate.

  Next, the reason why the steel components for obtaining a high-strength steel plate excellent in workability is limited will be described.

  The reasons for limiting the numerical values of C, Si, Mn, P, S, Al, and N, which are components of the steel sheet, will be described.

C: 0.05 to 0.25%
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: 0.3 to 2.5%,
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. Desirably, by setting the mass% to 4% or more of the C content, the progress of pearlite and bainite transformation is significantly retarded by reheating for the alloying treatment performed immediately after plating, and even after cooling to room temperature, the volume ratio A metal structure in which 3 to 20% of martensite and retained austenite are mixed in the ferrite can be obtained.

  The Si concentration is preferably in the range of 0.6% to 1.6% in consideration of the elongation, strength, and chipping resistance of the steel sheet.

Mn: 1.5 to 2.8%,
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. In addition, by adding a mass% of 12 times or more of the C content, the progress of pearlite and bainite transformation is significantly delayed by reheating for alloying performed immediately after plating, and in volume ratio after cooling to room temperature. A metal structure in which 3 to 20% of martensite and retained austenite are mixed in the ferrite can be obtained. 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%.

P: 0.03% or less,
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: 0.02% or less,
S is 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 and adversely affects the bendability of the steel sheet. 0.02% or less.

Al: 0.005 to 0.5%,
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: 0.0060% or less,
N is generally contained in steel as an inevitable impurity, but if its amount exceeds 0.006%, the brittleness deteriorates with elongation, so its content is made 0.006% or less.

  Further, the steel containing these as a main component contains 1% or less in total of at least one of Nb, Ti, B, Mo, Cu, Ni, Zr, W, Co, Ca, rare earth elements (including Y), and V. However, the effects of the present invention are not impaired, and depending on the amount, the corrosion resistance and workability may be improved. In addition, even if one or more of Sn, Zn, Ta, Hf, Pb, Mg, As, Sb, and Bi are contained in a total of 1% or less, the effects of the present invention are not impaired. It does not matter.

  Next, the alloyed hot-dip galvanized layer will be described.

In the present invention, the above-described high-strength steel plate is continuously annealed so that Si in the steel does not concentrate on the surface, that is, Si is added to the grain boundaries and crystal grains within 5 μm from the steel plate surface before plating. As the oxide to be contained, SiO ₂ containing one or more silicates selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ is contained in the thickness direction of the steel sheet in a larger amount of silicate than SiO _2. Continuous annealing is performed so that the region is located on the steel sheet surface side, and then an alloying hot-dip galvanizing treatment is performed to form an alloyed hot-dip galvanized layer.

  The alloyed hot-dip galvanized layer is a plated layer mainly composed of an Fe—Zn alloy formed by diffusion of Fe in steel during Zn plating by an alloying reaction.

  The Fe content is not particularly limited, but if the Fe content in the plating is less than 7% by mass, a soft Zn—Fe alloy is formed on the plating surface to deteriorate the press formability. Since a brittle alloy layer develops too much at the interface and plating adhesion deteriorates, 7 to 15% by mass is appropriate.

  In general, when continuously hot dip galvanizing is performed, Al is added to the plating bath for the purpose of controlling the alloying reaction in the plating bath. Is included. In addition, during the alloying process, the elements added to the steel diffuse simultaneously with the diffusion of Fe, so these elements are also included in the plating.

The steel sheet of the present invention is one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and rare earth elements during hot dip galvanizing bath or galvanizing. Alternatively, even if two or more kinds are contained or mixed, the effects of the present invention are not impaired, and depending on the amount, there are cases where the corrosion resistance and workability are improved, and so on. There are no particular restrictions on the amount of galvannealed coating, but it is preferably 20 g / m ² or more from the viewpoint of corrosion resistance and 150 g / m ² or less from the viewpoint of economy.

The composition of the oxide is measured by EDX. Since these crystal grain boundaries, oxides 4a and 4b in the crystal grains, and oxide 5 in the plating layer are analyzed by EDX, peaks of Si, Mn, Fe, and O are observed. _{SiO 2, FeSiO 3, Fe 2} SiO 4, considered MnSiO _3, an 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 the silicate is excessive when the content is less than 0.6% by mass, and Fe oxide is generated as an external oxide film. This is because non-plating defects are likely to occur, and when it exceeds 10% by mass, the silicate is reduced and the silica is increased, and the feature of the present invention that the silicate is present on the surface side is lost.

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

  Also, the reason why the oxide containing Si in the plating layer is limited to an average content of 0.05 to 1.5% by mass is that the silicate is excessive when the content is less than 0.05% by mass, and Fe oxide is used as an external oxide film. This is because non-plating defects are likely to occur, and when the amount exceeds 1.5% by mass, the silicate is reduced and the silica is increased, and the feature of the present invention that the silicate is present on the surface side is lost. Because.

  The content ratio of the oxide containing Si in the plating layer may be any method as long as the mass% of the oxide can be measured, but only the plating layer is dissolved with an acid, and the oxide containing Si is separated. After that, the method of measuring the mass is reliable.

  Next, the continuous hot dip galvanizing process will be described.

  FIG. 4 is a diagram showing an outline of the continuous hot dip galvanizing process.

  As shown in FIG. 4, the continuous hot dip galvanizing process is performed by passing the steel sheet through the heating zone, soaking zone, and cooling zone of the annealing furnace through the in-furnace roll from the direction of travel of the steel sheet, and entering the molten zinc through the snout. The steel sheet is immersed in a hot dip galvanizing tank, pulled up through a sink roll, the amount of plating is adjusted with a gas wiping nozzle, and alloyed in an alloying furnace to produce a high-strength galvannealed steel sheet.

  In the soaking zone of the annealing furnace, the combustion gas obtained by burning the fuel gas from the fuel gas pipe and the air from the air pipe in the combustion device is supplied through the combustion gas pipe. Moreover, reducing gas is supplied to the soaking zone and cooling zone of the annealing furnace from the reducing gas flow direction through the reducing gas pipe.

  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 adjusted 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 chipping resistance is lowered as a result.

Since FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ are stable in a region where the oxygen potential is higher 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 obtain a state in which one or more Si oxides selected from SiO ₄ are present and SiO ₂ is present on the inner surface side of the steel sheet, the oxygen potential needs to be larger than the oxygen potential at which SiO ₂ is independently oxidized.

Since the oxygen potential in the 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 , Mn ₂ SiO _{4 are present} on the steel sheet surface or surface side. When the steel sheet surface is controlled to the oxygen potential to be generated, 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 reduced. 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, the above-described microscopic unevenness of the interface increases. In addition, non-plating defects due to SiO ₂ can be prevented 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 manage 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 , the following equilibrium reaction is considered to occur, and PH ₂ O / PH ₂ is the 1/2 power of PO ₂ and the equilibrium constant 1 / K 1. Is proportional to
H ₂ O = H ₂ + 1 / 2O ₂ : K1 = P (H ₂ ) · P (O ₂ ) ^1/2 / P (H ₂ O)
However, since the equilibrium constant K1 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.

In addition, 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 K2.
CO ₂ = CO + 1 / 2O ₂ : K2 = P (CO) · P (O ₂ ) ^1/2 / P (CO ₂ )
Moreover, since the following equilibrium reaction occurs simultaneously, it is thought that H2O generate | occur | produces in 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 generate 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. It consists of impurities, and the logarithmic log PO2 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)
923 ≦ T ≦ 1173 (2)
T: Maximum steel sheet temperature (K)
[Si%]: Si content in the steel sheet (mass%)
Reduction is performed in a controlled atmosphere.

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

The reason for limiting H2 to ₁ to 60% by volume is that if it is less than 1%, the oxide film formed on the surface of the steel sheet 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 iron oxidation region, so an iron oxide film is formed on the steel plate surface, resulting in bluing.

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 less than 0.2 [Si%] ² +2.1 [Si%] − 98.8, the Si oxide SiO ₂ is exposed on the surface and the plating adhesion is lowered.

LogPO ₂ The -0.000034T ² + 0.105T-0.2 [Si%] ² +2.1 [Si%] - 98.8 or more and FeSiO ₃ on the surface of the steel sheet or surface side by, _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 attainable plate temperature T of the steel plate that defines the logarithmic log PO ₂ of the oxygen partial pressure in the atmosphere is 923K or more and 1173K or less.

The reason for limiting the T above 923K may, T is small the oxygen potential Si to external oxidation is less than 923K, to become an oxidation zone of the iron in the oxygen potential range that can be industrially operated, SiO ₂ is produced on the surface of the steel sheet This is because the corrosion resistance does not deteriorate after painting. 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 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 773K to 1273K, 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, but 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 H ₂ and CO ₂ O ₂ and CO are produced by the equilibrium reaction. H ₂ O, CO ₂ may if the amount introduced necessary, but not limited the manner of its introduction in particular, for example, by burning gas of a mixture of CO and _{H 2,} generated _H 2 O, a method of introducing a CO ₂ Ya , CH ₄ , C ₂ H ₆ , C ₃ H _8, etc., hydrocarbon gas such as LNG, or a mixture of hydrocarbons such as LNG, and a method of introducing the generated H ₂ O, CO ₂ , 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 since _generated, H 2 O, there is essentially no difference between the case of introducing CO _2.

In addition to the method of introducing H ₂ O and CO ₂ generated by combustion, a gas in which CO and H ₂ are mixed, 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 and other liquid hydrocarbon mixtures, alcohols such as CH ₃ OH and C ₂ H ₅ OH and mixtures thereof, various organic solvents, etc. A method of introducing H ₂ O and CO ₂ by introducing into the furnace and burning it in the 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 in terms of PO ₂ and temperature is not particularly defined, but is desirably 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 Si oxide of a seed or more exists and SiO ₂ exists 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.

  As a method of forming the iron oxide film, for example, a method of forming an iron oxide film by controlling the combustion air ratio in the oxidation zone to 0.9 to 1.2, or a dew point of the oxidation zone is controlled to 273 K or more and the iron oxide film is formed. Any method of forming can be used.

  The reason for adjusting the combustion air ratio in the range of 0.9 to 1.2 is that a combustion air ratio of 0.9 or more is necessary to generate an iron oxide film sufficient to suppress external oxidation of Si. If the ratio is less than 0.9, a sufficient iron oxide film cannot be formed. Further, when the combustion air ratio exceeds 1.2, the iron oxide film formed in the oxidation zone is too thick, and the peeled oxide adheres to the roll and generates appearance defects.

  The reason for controlling the dew point of the oxidation band to 273K or more is that a dew point of 273K or more is necessary to generate an iron oxide film sufficient to suppress external oxidation of Si, and the dew point is less than 273K. This is because an iron oxide film cannot be formed. The upper limit of the dew point is not specified, but it is preferably 373K or less in consideration of the influence on the deterioration of the equipment.

  The thickness of the oxide film affects not only the combustion air ratio and the dew point, but also the line speed, the reaching plate temperature, etc., so these are controlled appropriately, and the plate is passed under the condition that the thickness of the oxide film is 200 to 2000 mm. It is desirable to do.

However, in order to finish the reduction of the generated iron oxide film, the reduction time at PO ₂ and temperature specified in the claims is preferably set to 20 seconds or more.

The manufacturing method described above, a continuous hot dipping facility, the CO ₂ containing 1 to 100% by volume, balance _{N 2,} H 2 _O, a device for introducing a gas consisting of O 2, CO and unavoidable impurities reduction furnace be arranged and the CO or hydrocarbon is combusted in a reducing furnace, the CO ₂ containing 1 to 100% by volume, balance _{N 2,} H 2 _O, a gas consisting of O 2, CO and unavoidable impurities This can be achieved by providing a generating device. Examples of specific manufacturing equipment are shown in FIGS. In the example shown in FIG. 4, the steel sheet 6 is passed through the heating zone 7 of the annealing furnace, the soaking zone 8 of the annealing furnace, and the cooling zone 9 of the annealing furnace from the steel plate traveling direction 11, and annealed, and the molten zinc 13 is passed through the snout 14. It is an apparatus that is immersed in the hot dip galvanized layer 12 and pulled up through a sink roll 15, the amount of adhesion is adjusted by a gas wiping nozzle 16, and alloying treatment is performed by an alloying furnace 17. In the soaking zone 8 of the annealing furnace, the combustion gas obtained by burning the fuel gas from the fuel gas pipe 24 and the air from the air pipe 26 in the combustion device 21 is distributed through the combustion gas pipe 22. Further, the reducing gas is supplied from the reducing gas flow direction (indicated by the arrow 20) to the soaking zone 8 of the annealing furnace and the cooling zone 9 of the annealing furnace through the reducing gas pipe 19.

  The example shown in FIG. 5 has the same basic structure as FIG. 4, but includes a combustion device 28 installed in the soaking zone 8 of the combustion furnace, and fuel from the fuel gas pipe 24. And the air from the air pipe 26 are burned by the combustion device 28.

Thus, contain CO ₂ 1 to 100% by volume, and be disposed balance _{N 2,} H 2 _O, a device for introducing a gas consisting of O 2, CO and unavoidable impurities in the reduction furnace, the reduction An apparatus for combusting CO or hydrocarbons in a furnace and generating 1 to 100% by volume of CO ₂ and generating a gas composed of the balance N ₂ , H ₂ O, O ₂ , CO and inevitable impurities is provided. As a result, the reduction furnace can be controlled in an atmosphere in which a target oxide layer can be obtained.

  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 with sufficient volume fraction does not grow even if it is slowly cooled to 923K, and austenite is transformed into martensite during the cooling from 923K to the plating bath. Martensite is tempered by reheating for alloying treatment, and cementite is precipitated, 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 from 773 K to 693 K to 733 K at an average cooling speed of 0.5 degrees / second or more, and maintained from 773 K to the plating bath for 25 seconds or more and 240 seconds or less. To do.

  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 volume fraction of ferrite is not increased sufficiently, but also the increase in C concentration in austenite is small, so the steel strip is immersed in the plating bath. A part of it is transformed into martensite before, and then martensite is tempered by heating for alloying treatment and precipitates as cementite. Therefore, it is difficult to achieve both high strength and good workability.

  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. Moreover, the reason why the cooling end temperature is set to 693 K to 733 K is that concentration of C in austenite is promoted and high strength alloyed hot dip galvanizing excellent in workability is obtained.

  The reason why the temperature from 773K to the plating bath is maintained for 25 seconds or more and 240 seconds or less is that if less than 25 seconds, the concentration of C in the austenite becomes insufficient, and the C concentration in the austenite allows austenite to remain at room temperature. This is because when the time exceeds 240 seconds, the bainite transformation proceeds excessively, the amount of austenite decreases, and a sufficient amount of retained austenite cannot be generated.

  Further, while holding from 773K to the plating bath, once cooling to 673K to 723K and holding, concentration of C in the austenite is promoted and high strength alloyed hot dip galvanizing excellent in workability is obtained. However, if the plate is continuously immersed in the plating bath at 703K or less, the plating bath is cooled and solidified, and thus it is necessary to perform hot dip galvanizing after reheating to a temperature of 703 to 743K.

Next, the hot dip galvanizing process will be described.
In the thickness direction of the steel _sheet, the region _{FeSiO 3, Fe 2 SiO 4,} MnSiO 3, Mn 2 1 or more silicates selected from SiO ₄ is contained more than SiO ₂ is, SiO ₂ is contained more than the silicate A steel plate that has been annealed so that it is located on the surface side of the region is immersed in a hot dip galvanizing bath through a snout to perform hot dip galvanizing. After adjusting the amount, alloying treatment is performed in an alloying furnace.

In the production of the high strength alloyed hot dip galvanized steel sheet of the present invention, the hot dip galvanizing bath used preferably has an Al concentration adjusted to 0.07 to 0.105 mass% as an effective Al concentration in the bath. Here, the effective Al concentration in the plating bath is a value obtained by subtracting the Fe concentration in the bath from the Al concentration in the bath. The reason for limiting the effective Al concentration to 0.07 to 0.105 mass% is that when the effective Al concentration is lower than 0.07%, the formation of an Fe—Al—Zn phase that becomes an alloying barrier at the initial stage of plating is performed. This is because only the galvannealed steel sheet with poor plating film adhesion at the time of processing can be obtained because it is insufficient and the brittle Γ phase can be thickened at the interface of the plated steel sheet during the plating process. On the other hand, when the effective Al concentration is higher than 0.105%, alloying for a long time at high temperature is required, and austenite remaining in the steel is transformed into pearlite, so that high strength and workability are good. It becomes difficult to achieve both.
The alloying temperature in the present invention may be any number as long as the intended plating layer is obtained.

  When the alloying temperature is high, the alloying proceeds too much, and the brittle Γ phase can be thickened at the plated steel plate interface, so that the plating adhesion at the time of processing is lowered and the chipping property is deteriorated. Therefore, the lower alloying temperature is preferable. . Further, by controlling the alloying treatment time (3 to 60 seconds), the diffusion of Fe into the plating layer is controlled, and an Fe concentration region of 15 to 90% that can be read by GDS between the steel plate and the plating layer; A region having a Zn concentration of 5 to 87% is made to exist with a thickness of 1.8 to 3.5 μm. By forming a layer with a concentration gradient of Fe and Zn in this way, the hardness gradient gradually changes in the plating layer, and the stress concentration at the size that the stone hits against the impact from the plating surface It can be mitigated and chipping resistance can be improved.

Further, by alloying, an oxide of Si containing one or more silicates selected from FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO _{4 is} diffused from the steel plate surface portion into the plating layer. Since these Si oxides are present in a granular form in the plating layer, they do not hinder the diffusion of Fe during the alloying process.

  Further, when the steel sheet is immersed in the plating bath through the snout, the surface of the hot dip galvanizing bath in the portion in contact with the steel sheet is subjected to a hot dip galvanizing bath at a flow rate of 0.3 to 2.5 m / s in the width direction of the steel plate in the snout. It is preferable to make it flow. As described above, this method is the flow of the plating bath inside the snout by the metal pump.

  In the present invention, the alloying furnace heating method is not particularly limited. If the temperature of the present invention can be secured, radiation heating by a normal gas furnace or high frequency induction heating may be used. In addition, the method of cooling from the highest plate temperature after alloying heating is not questioned. If the heat is shut off by air seal etc. after alloying, it is sufficient to leave open, and the gas that cools more rapidly There is no problem with cooling.

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

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 2 using an in-line annealing method of continuous hot dip galvanizing equipment, and alloying 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. The combustion air ratio of the non-oxidizing furnace was adjusted to 1.0 and used as an oxidation zone. Reduction zone is CO and _H 2 which was gas mixture is burned to generate H2 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. After cooling to 723 K and holding at 723 K until 30 seconds were secured from 773 K to the plating bath, hot dip galvanization was performed and alloying was performed at 450 ° C. In addition, when a metal pump (push, pull pump) was installed in the snout and the steel plate was immersed in the plating bath, the metal bath was used to cause the plating bath to flow in the steel plate width direction.

The oxygen potential PO ₂ in the reduction furnace is determined by measuring 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
Using the equilibrium constants K1 and K2.

  Elongation (El) was calculated | required by cutting out a JIS5 test piece from each steel plate, and performing the tensile test at normal temperature.

  The adhesion amount of the plating was measured by a gravimetric method after dissolving the plating with hydrochloric acid containing an inhibitor. The Fe% during plating was obtained by dissolving the plating with hydrochloric acid containing an inhibitor and measuring by ICP.

  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 by observing an oxide containing Si from a cross-section of the embedded polished 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 SiO2 position O: Oxides where Si and O are observed at the same position are observed inside the steel plate x: Si and O are the same position The oxides observed in (1) are not observed inside the steel plate. The oxides containing Si present in the plating layer were evaluated by observing the embedded and polished plated steel plate from the cross section with an SEM image. The state of the oxide was observed with an SEM image, where the oxide containing Si was observed in the plating layer was marked with ◯, and the oxide was not observed with x.
GDS Measurement Method Analymat 2504 manufactured by RSV was used to calculate the measurement depth and concentration from the integrated values of measurement measurement time and emission intensity.
The measurement depth was calibrated by measuring the depth after measurement.
Evaluation of low temperature chipping resistance:
For coating, the steel sheet was subjected to chemical conversion treatment, electrodeposition coating, intermediate coating, and top coating. The chemical conversion coating film had a zinc phosphate adhesion amount of ^{2 to 3} g / m ² , the electrodeposition coating had a film thickness of 25 μm, the intermediate coating had a film thickness of 35 μm, and the top coating had a base of 15 μm + clear 35 μm, for a total of 50 μm.
Low temperature chipping resistance was investigated using a gravel test. Table 3 shows the test conditions.
Stone type: Crushed stone No. 7 (JIS A 5001)
Stone amount: 50 g
Shot condition: 2.0 kgf / cm ²
Test piece temperature: -20 ° C
Test plate angle: 90 degree evaluation is performed by tape peeling after gravel test, and after completely removing the destroyed coating film and the coating film where the coating film is lifted with the tip of the cutter knife, evaluated. Evaluation is performed by selecting 50 × 50 mm from the sample so that the peeled area is maximized, and the peeled area ratio (mm ² / cm ² ) obtained by dividing the total peeled area by the area to be evaluated (50 × 50 mm). Was determined by scoring as shown below, and 3 or less was accepted. The peeled area was immersed in a saturated copper sulfate aqueous solution at room temperature for 1 minute, and was the area of a circle whose diameter was the major axis of the copper-plated part (plating peeled part).

The following chipping scores were used.
Peeling area ratio (mm ² / cm ² ) Score ≦ 0.3 1
≦ 0.6 2
≦ 0.9 3
≦ 1.5 4
1.5 <5
As can be seen from Tables 2 and 3, the thickness of Fe in the region of Fe concentration of 15 to 90% as measured by GDS (GDS thickness) is obtained for steel sheets that have an appropriate PO ₂ during annealing and are flow-treated in the snout. ) Is within the scope of the invention (Zn is a layer within the scope of the invention with the same thickness and a gentle concentration gradient in inverse proportion to the Fe concentration), and the chipping score was as good as 1. Even if the PO ₂ at the time of annealing was appropriate, the thickness of Fe measured by GDS was out of the scope of the invention and the chipping score was 4 if no flow treatment was performed in the snout.

  In addition to the components in the same range as in Table 1, 1 or more of Nb, Ti, B, Mo, Cu, Ni, Zr, W, Co, Ca, rare earth elements (including Y), and V in total 1 As a result of carrying out the present invention under the conditions according to Tables 2 and 3 using steel containing not more than%, it was confirmed that the chipping property was good.

It is the figure which described typically the Fe density | concentration and Zn density | concentration at the side of the steel plate of the plating layer read by GDS. It is a schematic diagram which shows the zeta phase which copied the microscope picture observed about the zeta phase produced | generated when a steel plate was immersed in a plating bath. It is a figure which shows the state of the interface of a plating layer and a steel plate by the influence of the oxygen potential in the atmosphere at the time of continuous annealing, and the flow of the plating bath in a snout. (A) when the oxygen potential is low, (b) when adjusted to the conventional oxygen potential, (c) when the oxygen potential is adjusted appropriately and no flow in the snout is performed, (d) is when oxygen is It is a figure which shows the state of the interface at the time of adjusting a potential appropriately and performing the flow in a snout. It is a figure which shows the outline | summary of a continuous hot dip galvanization process. It is a figure which shows the other outline | summary of a continuous hot-dip galvanization process.

Explanation of symbols

1a ζ phase 1b old ζ phase generated in bath 2 Zn layer 3 silica 4 silicate 5 Γ phase 6 high strength steel plate 7 heating zone of annealing furnace 8 soaking zone of annealing furnace 9 cooling zone of annealing furnace 10 in-roll roll 11 steel plate Traveling direction 12 Hot dip galvanizing tank 13 Hot dip 14 Snout 15 Sink roll 16 Gas wiping nozzle 17 Alloying furnace 18 Gas flow control valve 19 Reducing gas pipe 20 Reducing gas flow direction 21 Combustion device 22 Combustion gas pipe 23 Combustion gas flow Direction 24 Fuel gas pipe 25 Fuel gas flow direction 26 Air pipe 27 Air flow direction 28 Combustion device installed in the furnace

Claims (4)

% 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 high-strength galvannealed steel sheet having an alloyed hot-dip galvanized layer on a high-strength steel sheet containing 0.0060% or less and the balance Fe and inevitable impurities. The grain boundary on the steel sheet side of 5 μm or less from the interface with the oxide and the oxide containing Si in the crystal grains exist at an average content of 0.6 to 10% by mass, and the GDS from the interface between the steel sheet and the plating layer to the plating side A region having an Fe concentration of 15 to 90% which can be read is present at a thickness of 1.8 to 3.5 μm, and an oxide containing Si is present in the plating layer at an average content of 0.05 to 1.5% by mass. A high-strength galvannealed steel sheet with excellent chipping resistance.   2. The high strength excellent in chipping resistance according to claim 1, wherein the region of Fe concentration of 15 to 90% measured by GDS has a gradual concentration gradient in which the Fe concentration increases toward the steel plate side. Alloyed hot-dip galvanized steel sheet. _{2. The} chipping resistance according to claim 1, wherein the oxide containing Si is at least one selected from SiO ₂ , FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , and Mn ₂ SiO ₄ . High strength alloyed hot dip galvanized steel sheet. The high-strength steel sheet further contains 1% or less in total of one or more of Nb, Ti, B, Mo, Cu, Ni, Zr, W, Co, Ca, rare earth elements (including Y), and V in mass%. The high-strength galvannealed steel sheet having excellent chipping resistance according to any one of claims 1 to 3.

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