JP2011117040A - Hot-dip galvannealed steel sheet and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a hot-dip zinc base plated steel sheet subjected to hot-dip galvanizing or hot dip galvannealing with a steel sheet comprising a relatively large amount of Si as a plating base material at low cost while improving the wettability of the plating. <P>SOLUTION: The hot-dip galvannealed steel sheet is obtained by providing the surface of the base material of a steel sheet having a chemical composition comprising 0.01 to 0.25% C, 0.3 to 2.0% Si, 0.030 to 3.0% Mn, &le;0.050% P, &le;0.010% S, &le;0.0060% N and &le;0.5% sol.Al with a plating layer comprising, by mass, 8 to 15% Fe and 0.15 to 0.50% Al. The independent oxides of Si, Mn or Al, oxides including two or more selected from Si, Mn and Al or their multiple oxides are present in the base material of the steel sheet within a depth of 2 &mu;m from the boundary between the plating layer and the base material of the steel sheet. The maximum grain size of the oxides is &le;0.1 &mu;m. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet and a method for producing the same, and more specifically, an alloying melt produced by subjecting a steel sheet containing a relatively large amount of Si, Mn, and Al to alloy hot-dip galvanizing. The present invention relates to a galvanized steel sheet and a method for producing the same.

  As is well known, automobiles are required to improve fuel efficiency and collision safety. For this reason, plated steel sheets used as automotive steel sheets have high mechanical properties and excellent corrosion resistance, and have low alloying elements in the base material in order to suppress raw material prices, capital investment, and basic units, It is required that the equipment can be manufactured with fewer processes and less energy consumption.

  Combining high mechanical properties, that is, high tensile strength and total elongation, is achieved by optimizing the content of reinforcing elements such as C, Si and Mn in the base material. Of these elements, particularly Si is easily oxidizable. For this reason, in the continuous hot dip galvanizing line (CGL), the Si content is approximately 0.3% or more (“%” in the present specification means “mass%” unless otherwise specified). When a steel sheet containing a relatively large amount of Si is plated under the same conditions as the plating conditions for ordinary steel, the Si oxide formed on the surface layer of the steel sheet, which is the plating base material just before plating, is wettable with the plating. Therefore, non-plating defects are likely to occur. Thus, various inventions have been proposed so far to solve this problem.

  Patent Document 1 discloses an invention in which pre-plating is performed on a steel sheet before the annealing process in CGL, and Patent Document 2 discloses an invention in which the surface of the steel sheet is ground before the annealing process in CGL. Yes. However, in order to implement these inventions, there is a problem that a new equipment investment for providing a pre-plating step and a grinding step is required, and the number of steps increases.

  Patent Document 3 discloses an invention using a steel sheet containing 0.001 to 0.4% of Ni and Cu in addition to Si as an alloy element in steel. However, this invention has a problem that an increase in cost is inevitable due to an increase in the content of alloy elements.

  Patent Document 4 discloses an invention in which a thick oxide film is formed on the surface of a steel sheet by irradiating and oxidizing the surface of the steel sheet with an oxidation band in CGL, and then plated by reduction annealing. Since Si hardly diffuses into the oxide film (substantially iron oxide), it is possible to obtain a steel material surface in which Si (oxide) hardly exists by reducing such a thick oxide film, This improves the plating properties. However, since a thick oxide film is relatively easy to drop off, it adheres to, for example, a roll in the furnace through a CGL annealing furnace, and affects the surface of the subsequent steel sheet to cause surface defects of the product. There is a fear.

Further, by oxidizing Si slightly inside from the surface of the steel plate base metal (this is sometimes referred to as “internal oxidation” of Si, and this term is also used in this specification for the same purpose), There is also known an invention based on a technical idea different from the inventions disclosed in Patent Documents 3 and 4, in which the formation of Si oxide is suppressed to improve the plating property. As an invention utilizing internal oxidation, for example, in Patent Document 5, when hot dip galvanizing is performed on a steel sheet containing 0.7% or more of Si, Mn, and Cr, after hot rolling in the manufacturing process of this steel sheet An invention is disclosed in which the ^coiling temperature of the steel sheet is set as high as 780 × (Si [%] + Mn [%] / 10 + Cr [%] / 10) ^0.148 (° C.) or higher. However, when the coiling temperature in hot rolling is set high, problems due to coarsening of crystal grains of the base material and grain boundary oxidation are likely to occur. For example, hot-rolled steel sheets that have undergone grain boundary oxidation preferentially erode the grain boundaries in the subsequent pickling process to form deep irregularities, and then cold-rolled to create a notched surface on the scab. Is formed. Although this steel sheet surface (plating after plating-base metal interface) works favorably on the wettability of plating, when notching the alloyed hot-dip galvanized steel sheet, notched crystal grains are likely to fall off. Poor powdering resistance. Note that “powdering” usually means dropping due to powdering of the plating layer itself, and the dropping mechanism is different from that of the above plating layer, but it is not distinguished in the evaluation of an actual molded product.

  In Patent Document 6, after hot rolling or after cold rolling, the steel sheet is heated and cooled with an annealing facility, and when annealing and plating are further performed in CGL, the temperature during heating and cooling in the annealing facility or CGL is disclosed. And the invention for controlling the atmosphere and the like are disclosed, and as a preferred embodiment, it is disclosed that the winding temperature is 600 ° C. or higher and 850 ° C. or lower, and the cooling rate after winding is 3 ° C./min or lower. Yes. However, this method requires the addition of another heat treatment step before CGL, which is cumbersome in process and disadvantageous in cost.

  In Patent Document 7, by controlling the temperature and atmosphere (water vapor partial pressure and hydrogen partial pressure) in a reduction furnace of a continuous hot dip galvanizing facility, a steel sheet containing C, Si, Mn is used as a base, Manufacturing a hot-dip galvanized steel sheet in which an internal oxide such as Si having an average diameter of 1 μm or less is formed inside a steel sheet having a mean diameter of 2 μm or less from the interface.

JP 2001-279409 A Japanese Patent No. 2555821 Japanese Patent Laid-Open No. 2004-2111157 JP 2006-176806 A Japanese Patent Laid-Open No. 10-017936 JP 2000-290730 A JP 2004-323970 A

  However, in the invention disclosed in Patent Document 7, since the internal oxide is coarse, irregularities are formed on the surface of the plated steel sheet, causing problems unfavorable for product characteristics (for example, poor visibility of the member). .

  In the present invention, referring to the mechanism of internal oxidation of Si, the process itself is not a method of adding a step that is not in the normal manufacturing process of an alloyed hot dip galvanized steel sheet as in the invention disclosed by Patent Document 6, Although it is a normal one, it has been found that, for example, it is possible to improve the plating property (wetting property and alloying treatment property) under production conditions different from the invention disclosed in Patent Document 7. Furthermore, it discovered that the steel plate manufactured by such a method has the characteristic which is not in the invention disclosed by patent document 7 mentioned above. The present invention is based on these findings.

  In the present invention, C: 0.01 to 0.25%, Si: 0.3 to 2.0%, Mn: 0.030 to 3.0%, P: 0.050% or less, S: 0.010 % Or less, N: 0.0060% or less, and sol. A plating layer containing Fe: 8.0 to 15% and Al: 0.15 to 0.50% on the surface of a steel plate base material having a chemical composition comprising Al: 0.5% or less, the balance Fe and impurities An alloyed hot-dip galvanized steel sheet comprising: a single oxide of Si, Mn, or Al, or two or more of these in a steel sheet base material having a depth of 2 μm or less from the interface between the plating layer and the steel sheet base material. The alloyed hot-dip galvanized steel sheet is characterized in that there is an oxide containing or a composite oxide containing two or more of these and Fe, and the maximum particle size of the oxide is 0.10 μm or less.

  Moreover, this invention is C: 0.01-0.25%, Si: 0.3-2.0%, Mn: 0.030-3.0%, P: 0.050% or less, S: 0 .010% or less, N: 0.0060% or less, and sol. A plating layer containing Fe: 8.0 to 15% and Al: 0.15 to 0.50% on the surface of a steel plate base material having a chemical composition consisting of Al: less than 0.01%, the balance Fe and impurities. An alloyed hot-dip galvanized steel sheet comprising: a single oxide of Si, Mn, or Al, or two or more of these in a steel sheet base material having a depth of 2 μm or less from the interface between the plated layer and the steel sheet base material. The alloyed hot-dip galvanized steel sheet is characterized in that there is an oxide containing or a composite oxide containing two or more of these and Fe, and the maximum particle size of the oxide is 0.10 μm or less.

  From another viewpoint, the present invention provides C: 0.01 to 0.25%, Si: 0.3 to 2.0%, Mn: 0.030 to 3.0%, P: 0.050% or less. , S: 0.010% or less, N: 0.0060% or less, and sol. A hot slab is obtained by hot rolling a steel slab having a chemical composition having Al: 0.5% or less and winding the obtained hot rolled steel sheet at a coiling temperature of 650 ° C. or less. A pickling process to wash, a cold rolling process to cold-roll the hot-rolled steel sheet pickled in this pickling process at a reduction ratio of 50% or more, and a continuous hot-dip zinc to the cold-rolled steel sheet that has undergone this cold rolling process When reducing the surface of a steel sheet in a reduction annealing furnace on a plating line, annealing is performed in a nitrogen-hydrogen atmosphere at a hydrogen concentration of 1 to 30% by volume and a dew point of −30 ° C. to 10 ° C. in a temperature range of 700 ° C. or higher. And a hot dip galvanizing step, followed by a hot dip galvanizing step in which an alloying treatment is performed. The alloyed hot-dip galvanized steel sheet according to the present invention described above can be manufactured by the manufacturing method according to the present invention.

  In these present inventions, the chemical composition is (a) Ti: 0.50% or less, preferably 0.0040 to 0.50%, Nb: 0.50% or less, preferably 0.0040 to 0.50%. And B: 0.0050% or less, preferably one or more selected from the group consisting of 0.0001 to 0.0050%, (b) Cu: 1.0% or less, Ni: 1. 1 type or 2 or more types selected from the group consisting of 0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, or (c) Bi: 0.05% or less, and Ca: It is preferable to satisfy at least one of having one or two of 0.01% or less.

  According to the present invention, an alloyed hot-dip galvanized steel sheet that is subjected to alloying hot-dip galvanizing using a steel sheet containing a relatively large amount of Si as a plating base is manufactured at a low cost while improving the wettability of plating. be able to.

FIG. It is a metallographic photograph which shows the SEM image of the 11 plated steel plate. FIG. 2 is a photograph showing a cross-section STEM. FIG. 3 is a graph showing the EDS measurement results. FIG. 6 is a metallographic photograph showing an SEM image of a plated steel sheet of No. 6; FIG. It is a metallographic photograph which shows the SEM image of the plated steel plate of 1. FIG. FIG. It is a graph which shows the EDX point analysis result of 1 plated steel plate.

Hereinafter, the best mode for carrying out the present invention will be described.
In the present invention, a hot rolling process, a pickling process for removing the scale of the surface of the steel sheet formed in the hot rolling process, a cold rolling process for cold rolling the steel sheet that has undergone the pickling process, An alloyed hot-dip galvanized steel sheet is produced through a continuous hot-dip galvanizing step in which reduction annealing and hot-dip plating are performed in this order on the cold-rolled steel sheet in a continuous hot-dip plating facility.

The reason for limiting the chemical composition of this steel sheet will be described.
[C: 0.01 to 0.25%]
The galvannealed steel sheet according to the present invention improves the balance between strength and ductility by containing a large amount of C. The C content may be appropriately changed depending on the target strength, but is set to 0.01% or more in order to improve ductility. However, if the C content exceeds 0.25%, the local ductility deteriorates remarkably, so the content is made 0.25% or less.

[Si: 0.3-2.0%]
Since Si is useful for increasing the strength of a steel sheet by solid solution strengthening at low cost, it is contained in an amount of 0.3% or more for the purpose of improving the strength. However, if the Si content exceeds 2.0%, scale wrinkles are likely to occur, so the content is made 2.0% or less.

[Mn: 0.030 to 3.0%]
Mn increases the strength of the steel sheet by solid solution strengthening. If the Mn content exceeds 3.0%, the yield strength increases and the elongation deteriorates, and wrinkles and cracks are likely to occur during processing. However, if the Mn content is less than 0.030%, it is difficult to ensure the strength, so the content is made 0.030% or more.

[P: 0.050% or less]
P is unavoidably contained as an impurity, but positively increases the strength of the steel sheet. However, if P is excessively contained, the alloying processability is lowered during the production of the galvannealed steel sheet, so the P content is 0.050% or less.

[S: 0.010% or less]
Since S content is contained in steel as an impurity, it is preferable that the S content is low. When the S content exceeds 0.010%, precipitation of MnS becomes conspicuous, which not only inhibits the ductility of the steel sheet but also consumes Mn as an austenite stable element. Therefore, the S content is set to 0.010% or less.

[N: 0.0060% or less]
N is an impurity that deteriorates stretch flangeability. Therefore, in the present invention, the N content is set to 0.0060% or less.

[Sol. Al: 0.5% or less]
Al is an element that is contained for deoxidizing steel, and is also an element that effectively acts to improve the yield of carbonitride-forming elements such as Ti. On the other hand, in order to sufficiently internally oxidize Si in steel for improving plating properties, it is preferable that Al, which is an easily oxidizable element, be as small as possible in the same manner as Si. From this viewpoint, Al is made 0.5% or less.

  If Si is contained in an amount of 0.5% or more, the need to contain a large amount of Al for the purpose of improving mechanical properties is reduced, so that the Al content should be further reduced to 0.01% or less from the viewpoint of plating properties. Is preferred.

Next, arbitrary elements will be described.
[Ti: 0.0040 to 0.50%, Nb: 0.0040 to 0.50%, and B: one or more selected from the group consisting of 0.0050% or less]
By containing Ti, Nb or B alone or in combination of two or more, the strength, hole expansibility and elongation are improved. However, even if Ti and Nb are contained in excess of 0.50% and B is contained in excess of 0.0050%, the effect of improving the characteristics is saturated and the cost is increased. Therefore, when these elements are contained, Ti and Nb are 0.50% or less, and B is 0.0050% or less. Furthermore, in order to surely obtain the effect of improving the strength, hole expansibility and elongation, Ti and Nb are contained in an amount of 0.0040% or more, and in B, 0.0001 or more is preferably contained.

[One or more selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less]
Cu, Ni, Cr, and Mo are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain one or more of Cu, Ni, Cr and Mo. In order to acquire the said effect more reliably, it is preferable to contain any element 0.01% or more. However, if Cr, Mo, Cu and Ni are contained in excess of 1%, the above effect is saturated and not only economically wasteful, but the hot-rolled steel sheet is hard and cold-rolled. It becomes difficult. For this reason, 1 type (s) or 2 or more types of Cr, Mo, Cu, and Ni are contained in said amount.

[Bi: 0.005% or less and Ca: 0.01% or less]
In a steel sheet containing a large amount of Mn, the mechanical properties of the steel sheet may deteriorate due to Mn segregation, but this can be prevented by containing Bi or Ca alone or in combination. However, when the Bi content exceeds 0.05%, hot workability is lowered and hot rolling becomes difficult. If the Ca content exceeds 0.01%, the surface properties deteriorate. For this reason, Bi content shall be 0.05% or less, and Ca content shall be 0.01% or less. In order to reliably obtain such an effect, the Bi content is preferably 0.0001% or more, and the Ca content is preferably 0.0001% or more.

The balance other than the above is Fe and impurities.
Next, the reason for limiting the chemical composition of the plating layer of the galvannealed steel sheet according to the present invention will be described.

[Fe: 8 to 15%]
When the Fe content in the galvanizing is 8% or more and 15% or less, the weldability and post-coating corrosion resistance of the galvannealed steel sheet are improved. However, if the content is too small, the press mold and the plating adhere to each other at the time of press forming, causing cracks in the steel sheet and peeling (flaking) of the plated layer, so the lower limit is made 8%. Moreover, since the plating layer at the time of press molding will peel off in powder form and cause a press wrinkle if it contains excessively, an upper limit shall be 15%.

[Al: 0.15 to 0.50%]
When the Al content in the plating layer is less than 0.15%, Fe and Zn are excessively alloyed. On the other hand, when the Al content exceeds 0.50%, the reaction rate during the alloying treatment is inferior. Then, Al content in a plating layer shall be 0.15% or more and 0.50% or less.

[Oxide in the base metal near the plating / base metal interface]
Within 2 μm from the interface between the plating layer and the steel plate base material, a single oxide of Si, Mn or Al, a single oxide of Si, Mn or Al, an oxide containing two or more of these, or these and Fe There are composite oxides containing two or more.

  In the conventional alloyed hot-dip galvanized steel sheet, non-plating defects are likely to occur because the Si oxide formed on the surface layer of the steel sheet inhibits the wettability with the plating. In the present invention, Si, Mn, and Al are converted into oxides within 2 μm from the base metal plating / base metal interface, thereby inhibiting the generation of oxides on the steel sheet surface layer and ensuring plating properties.

[Maximum particle size of oxide: 0.10 μm or less]
The maximum particle size of the above oxide is 0.10 μm or less. The reason for this is that, when the oxide size is large, there is a difference in reactivity between the region where the oxide is present and the region where the oxide is not present in the alloying process in which Fe of the base material is diffused during plating during the production of the plated steel sheet. This is because unevenness of the plating thickness occurs. Unevenness in the plating thickness is not preferable because air bubbles are mixed in during coating and become a starting point of generation of foreign matter, or the friction coefficient is increased during press molding, resulting in deterioration of workability. For this reason, it is preferable that the size of the oxide is small. Specifically, the maximum particle size of the oxide is 0.10 μm or less, preferably 0.05 μm or less.

  FIG. 5 shows No. 2 in Table 2 described later. It is a metallographic photograph which shows the SEM image of the plated steel plate of 1. FIG. In this photograph, a test piece was taken from a steel plate, the cross section was mechanically polished, C-deposited, and photographed with SEM (acceleration voltage 10 kV).

  As shown in FIG. 5, a plurality of white spots are observed within 2 μm from the plating / base metal interface or the base crystal grain boundary. When this white spot is subjected to elemental analysis by EDX, one or more of Si, Mn, and Al are detected, and these are recognized as single or complex oxides. These oxides are very fine and have a maximum value of 0.10 μm or less.

The galvannealed steel sheet according to the present invention is configured as described above. Next, the manufacturing method of the galvannealed steel plate concerning this invention is demonstrated.
The manufacturing method of the galvannealed steel sheet according to the present invention is not necessarily limited, but the presence of the characteristic internal oxide and the maximum diameter thereof include hot rolling, pickling, cold rolling, and plating ( Since any process of annealing) has an influence, the optimum manufacturing conditions will be described below.

  The alloyed hot-dip galvanized steel sheet according to the present invention is slab-created, hot-rolled, pickled, cold-rolled, degreased in a continuous hot-dip galvanizing facility, annealed, galvanized and It is manufactured by a manufacturing process called alloying.

Slab creation may be performed according to a conventional method.
In hot rolling, the winding temperature is preferably set to 650 ° C. or less. When the coiling temperature is too high, although internal oxidation of Si occurs, the grain size may grow and a large oxide may be formed. In addition, the oxidation at the grain boundary proceeds and the above-mentioned powdering problem may occur.

  In pickling, not only when the coiling temperature in hot rolling is high, but also for relatively large oxides that are formed even if they are low, pickling conditions that are stronger than usual are used to remove them. Is preferred. Specifically, it is preferable that the temperature of the pickling solution is 85 ° C. or higher and the hydrochloric acid concentration is 12% or higher. The hydrochloric acid concentration is more preferably 20% or more. You may add the inhibitor which suppresses per pickling.

  In cold rolling, the rolling reduction is preferably 50% or more, and more preferably 65% or more. Of course, the rolling reduction needs to be set so as to obtain a predetermined mechanical characteristic. However, if the cold rolling reduction is set higher, large oxides that have not been removed until pickling are mechanically destroyed. In addition to being easy, a large number of transitions are introduced into the steel sheet, increasing the number of oxide generation starting points during annealing and making the oxide finer.

Next, each stage in the continuous hot dip galvanizing line (CGL) will be described.
On the entry side of the CGL, alkali cleaning for removing dirt on the steel plate or pickling as necessary may be performed, and the surface of the steel plate may be ground prior to cleaning.

  Furthermore, in a CGL equipped with equipment such as a non-oxidizing furnace (NOF), it is preferable that the air-fuel ratio is set to about 1 or less and heated to about 500 to 600 ° C. In the pre-oxidation, it is advantageous for the wettability of the plating to sufficiently oxidize the surface of the steel sheet. For that purpose, the higher the air-fuel ratio is advantageous, but due to the fall of the oxide film in the reduction furnace. Problems are likely to occur. In recent CGLs, there are often no non-oxidizing furnaces (pre-oxidation before annealing is also performed in a reducing atmosphere), and this CGL is not particularly limited.

  As long as the reduction annealing is performed in a reducing atmosphere for the steel, the lower the reducibility (typically the lower the hydrogen concentration and the higher the dew point), the easier the internal oxidation of Si occurs. However, this direction also tends to cause decarburization of the steel sheet surface (plating interface after plating). Suitable conditions are as follows: When a nitrogen-hydrogen mixed atmosphere is used in the range of 700 ° C. or higher (the maximum temperature varies depending on the steel components and required mechanical properties), the hydrogen concentration is 1% by volume to 30% by volume and the dew point is It is preferable to be more than −30 ° C. and less than ± 10 ° C.

  Thus, after carrying out reduction annealing, alloying hot dip galvanization is performed. For example, the plating bath temperature is 440 ° C. or more and 470 ° C. or less, the Al concentration in the bath is 0.10% or more and 0.50% or less, and the lower limit of the intrusion material temperature is the same temperature as the bath temperature from the viewpoint of bath temperature stability. Is exemplified. What is necessary is just to determine suitably the conditions of the alloying process in manufacture of a galvannealed steel plate so that a predetermined alloying degree and the structure | tissue of plating may be obtained.

The present invention will be specifically described with reference to examples.
Several slabs having the chemical composition shown in Table 1 (unit: mass%, balance other than those shown in Table 1: Fe and impurities) were prepared.

  These were hot rolled to a thickness of 2.8 mm on a hot rolling line, and then wound on a coil. Subsequently, the hot-rolled steel strip was passed through a pickling line to remove the surface scale. Two types of pickling conditions were performed: pickling at a liquid temperature of 85 ° C. and a hydrochloric acid concentration of 13% was set to “1”, and pickling at a liquid temperature of 85 ° C. and a hydrochloric acid concentration of 20% was set to “2”.

Further, the pickled steel strip is 0.8 mm thick (cold rolling rate 70%), 1.4 mm thick (cold rolling rate 50%), 2.0 mm (cold rolling rate 30%) in the cold rolling line. Cold rolled.
Furthermore, reduction annealing, galvanization, and alloying treatment were performed by a continuous hot dip galvanizing facility to prepare an alloyed hot dip galvanized steel sheet.

  The actual water vapor concentration in the atmosphere during reduction (nitrogen-20% hydrogen) is converted into a dew point. In the production of the present invention, the range is from -30 ° C to + 5 ° C, and in the production of the comparative example, from -50 It was set to -20 ° C. The dew point range partially overlaps due to the difference in the combination of conditions between the components and the coiling temperature (Example: Dew point actual ranges of the present invention and comparative examples are described respectively).

The prepared galvanized plate was held in a 500 ° C. salt bath for 20 seconds to 120 seconds, and alloyed so that Fe% during plating was 8 to 15%.
The mechanical properties of the alloyed hot-dip galvanized steel sheet were evaluated according to JIS Z 2201, and the product (MPa x%) of the values of tensile strength (MPa) and elongation (%) was determined to be 17500 or more. Less than was judged as x.

  The plating wettability of the alloyed hot-dip galvanized steel sheet was evaluated by non-plating defects (parts where the base material was exposed without plating). When there was no non-plating defect, it was marked as ◯, when there was a non-plating defect and the maximum size was 5 mm or less, it was marked as Δ, and when there was a non-plating defect and the maximum size exceeded 5 mm, it was marked as x.

  The unevenness of the plating thickness was evaluated by the roughness Ra of the plating surface. Ra was measured using a surface roughness / contour shape measuring instrument SURFCOM1900DX manufactured by Tokyo Seimitsu. The case where Ra was 1.5 μm or less was rated as ◯, the case where Ra was more than 1.5 μm and 2.0 μm or less was rated as Δ, and the case where Ra was more than 2.5 μm was rated as x.

This alloyed hot-dip galvanized steel sheet was subjected to SEM-EDX measurement and FIB-STEM measurement.
At the time of SEM-EDX measurement, a test piece is taken from the steel plate, the cross section is mechanically polished, C is deposited, SEM observation-EDX (acceleration voltage 10 kV) is measured, and the presence or absence of oxide and the maximum value of the oxide diameter are evaluated. did. Since measurement was performed at an accelerating voltage of 10 kV without slicing, the measurement range was an area having a diameter of about 1 μm, and the detected Fe is not an oxide but a possibility of being a surrounding base material Fe.

The FIB-STEM observation was performed by FE-STEM / EDS (acceleration voltage 300 kV) measurement after slicing to a thickness of 100 nm or less with FIB.
Table 2 summarizes the presence / absence of oxides in the base metal and the maximum diameter of the oxides of the galvannealed steel sheets prepared.

  As shown in Table 2, those having Si, Mn, and Al oxide within 2 μm from the plating / matrix interface or matrix grain boundary showed good plating properties, while those not having poor plating properties. It was good.

  No. in Table 2 11 and Table 3 show the SEM images and EDX point analysis results of the plated steel sheet, and FIG. 2 and FIG. 3 show the FIB-processed cross-sectional STEM and EDS measurement results.

  1 to 3 and Table 3, No. In 11 plated steel sheets, although internal oxides whose main components are Si, Mn, and Al exist in the vicinity of the base metal side interface of the plating / base metal interface, the grain size was large. This is thought to be due to the high winding temperature.

  In Table 2, No. The SEM image and EDX point analysis result of the plated steel sheet 6 are shown in FIG.

  From FIG. In the plated steel sheet No. 6, although the internal oxide mainly composed of Si and a main component was present in the vicinity of the base metal side interface of the plating / base metal interface, the particle size was large. This is thought to be due to the small cold rolling rate.

In Table 2, No. The SEM image and EDX point analysis result of the plated steel sheet 1 are shown in FIGS.
From FIG. 5 and FIG. It can be seen that the plated steel sheet 1 has fine internal oxides containing Si, Al and Mn as main components in the vicinity of the base metal side interface of the plating / base metal interface.

Claims (6)

  In mass%, C: 0.01 to 0.25%, Si: 0.3 to 2.0%, Mn: 0.030 to 3.0%, P: 0.050% or less, S: 0.010 % Or less, N: 0.0060% or less, and sol. On the surface of a steel plate base material having a chemical composition consisting of Al: 0.5% or less, balance Fe and impurities, in mass%, Fe: 8.0-15% and Al: 0.15-0.50% An alloyed hot-dip galvanized steel sheet comprising a plating layer containing, in a steel sheet base material having a depth of 2 μm or less from the interface between the plating layer and the steel sheet base material, a single oxide of Si, Mn, or Al, There is an oxide containing two or more of the above, or a composite oxide containing two or more of these and Fe, and the maximum particle size of the oxide is 0.10 μm or less. Plated steel sheet.   In mass%, C: 0.01 to 0.25%, Si: 0.3 to 2.0%, Mn: 0.030 to 3.0%, P: 0.050% or less, S: 0.010 % Or less, N: 0.0060% or less, and sol. On the surface of a steel plate base material having a chemical composition consisting of Al: less than 0.01%, balance Fe and impurities, Fe: 8.0-15% and Al: 0.15-0.50% in mass% An alloyed hot-dip galvanized steel sheet comprising a plating layer containing, in a steel sheet base material having a depth of 2 μm or less from the interface between the plating layer and the steel sheet base material, a single oxide of Si, Mn, or Al, There is an oxide containing two or more of the above, or a composite oxide containing two or more of these and Fe, and the maximum particle size of the oxide is 0.10 μm or less. Plated steel sheet.   2. The chemical composition according to claim 1, wherein the chemical composition has one or more selected from the group consisting of Ti: 0.50% or less, Nb: 0.50% or less, and B: 0.0050% or less, in mass%. The galvannealed steel sheet according to claim 2.   The chemical composition is one or two selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less in terms of mass%. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3, having at least seeds.   The alloy according to any one of claims 1 to 4, wherein the chemical composition has one or two of Bi: 0.005% or less and Ca: 0.01% or less in mass%. Hot-dip galvanized steel sheet. In mass%, C: 0.01 to 0.25%, Si: 0.3 to 2.0%, Mn: 0.030 to 3.0%, P: 0.050% or less, S: 0.010 % Or less, N: 0.0060% or less, and sol. A hot rolling step of hot rolling a steel slab having a chemical composition having Al: 0.5% or less and winding the obtained hot rolled steel sheet at a winding temperature of 650 ° C. or less;
A pickling process for pickling the hot-rolled steel sheet; a cold-rolling process for cold-rolling the hot-rolled steel sheet pickled in the pickling process at a reduction rate of 50% or more;
When the cold-rolled steel sheet that has undergone the cold rolling process is reduced on the steel sheet surface in a reduction annealing furnace in a continuous hot-dip galvanizing line, the hydrogen concentration is 1 to 30% by volume and the dew point is −30 ° C. in the temperature range of 700 ° C. or higher. A method for producing an alloyed hot-dip galvanized steel sheet, comprising: a hot-dip galvanizing step in which hot-dip galvanizing is performed after annealing in a nitrogen-hydrogen atmosphere at 10 ° C, followed by alloying treatment.