JP5392116B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to an alloyed hot-dip galvanized steel sheet and a method for producing the same, and specifically, a high-strength hot-dip galvanized steel sheet excellent in powdering resistance, plating adhesion, and surface properties, particularly as an automobile body. The present invention relates to a high-strength hot-dip galvanized steel sheet suitable for press forming, and particularly for applications that require complicated forming, and a method for producing the same.

  In recent years, improvement in fuel efficiency of automobiles is required for protecting the global environment. The demand for high-strength hot-dip galvanized steel sheets is increasing in order to reduce the weight of the vehicle body and ensure the safety of passengers.

  A steel sheet used for a member for an automobile is not sufficient if it has only high strength, and it is necessary to satisfy various performances such as press formability and corrosion resistance. However, an oxide film of Mn or Si is formed on the surface of the high-strength steel sheet in the annealing process of the high-strength steel sheet containing a large amount of Si, Mn and Al for increasing the strength. Wetting is reduced and non-plating is likely to occur. Even when non-plating does not occur, the oxide film of Mn and Si becomes a barrier to the diffusion of iron from the base material during heating for alloying, making alloying extremely difficult, and the oxide film of Mn and Si Is not uniformly formed on the steel sheet, a part where the alloying reaction proceeds slowly and a part where the alloying reaction proceeds earlier are partially formed, resulting in unevenness of the alloying treatment, It is very difficult to secure the properties.

  In the solidification process of molten steel containing a large amount of Mn and Si, local chemical composition fluctuations occur due to solidification segregation, and a heterogeneous structure corresponding to the fluctuations is formed, and the surface of the processed part is visually observed during bending. But noticeable irregularities appear. Since the unevenness promotes non-uniform deformation and induces cracking of the base material, the bendability itself deteriorates. In addition, even when cracks do not occur, the remarkable unevenness present in the machined part deteriorates the collision characteristics required for the parts, and stress concentrates on this unevenness during processing, so that alloyed molten zinc Cracking and peeling of the plating film are likely to occur, resistance to plating peeling at the time of processing (powdering resistance) is deteriorated, and appearance damage and processing deterioration due to plating peeling pieces are generated.

  For this reason, a uniform alloyed molten zinc coating (also referred to as “GA coating” in this specification) having excellent powdering resistance and plating adhesion is formed on a high-strength steel sheet containing a large amount of Si, Mn, and Al. Was very difficult.

  As a method for forming a GA film on a high-strength steel plate containing a large amount of easily oxidizable elements such as Si, Mn, and Al, for example, Patent Document 1 discloses that Fe-based pre-plating is performed in advance on high-Si and Mn steel. An invention for producing a high-strength galvannealed steel sheet by performing galvannealing afterwards has been disclosed. Patent Document 2 discloses an invention in which an easily oxidizable element is concentrated on the surface by annealing and then pickling, and after removing this oxide, hot dip galvanizing is performed. Furthermore, Patent Document 3 discloses an invention that suppresses uneven alloying by performing alloying hot dip galvanization after applying a sulfur compound to the surface of a steel sheet before annealing.

  However, since all of these inventions perform a special treatment before alloying hot dip galvanizing, it is necessary to newly install equipment for performing this special treatment before the hot dip galvanizing line. However, since this causes an increase in manufacturing cost, it cannot be said that it is practically preferable. Even based on these inventions, it is technically difficult to uniformly form an easy oxide on the surface of a steel sheet, and it is not sufficient as a method due to a partial speed difference during GA conversion treatment. .

  On the other hand, as a method for alloying a high-strength steel sheet containing a large amount of easily oxidizable elements, for example, in Patent Document 4, the surface of the steel sheet is actively oxidized in a pre-oxidation furnace during annealing and then melted by annealing. An invention for galvanizing is disclosed. Furthermore, in Patent Document 5, the hot rolling coiling temperature is set to a high temperature, and an easily oxidizable element is actively oxidized in the vicinity of the steel sheet surface, thereby improving the adhesion of hot dip galvanizing. The invention is disclosed. As described above, an invention corresponding to an easily oxidizable element by controlling the coiling temperature during hot rolling, controlling the atmosphere in the annealing furnace, or the like is often taken as one of the methods for promoting GA.

  Although these inventions are effective means in the sense that GA can be secured without the need for special equipment, a stable and uniform internal oxide layer is formed in the vicinity of the steel sheet surface layer, and a very unstable heat. In order to control and form only by the cycle, it is very difficult to exhibit a sufficient effect. In addition, since all of these inventions actively oxidize the surface of the steel sheet, the grain boundary oxidation on the surface of the steel sheet proceeds and the invasion of zinc into the crystal grain boundary becomes prominent. Embrittlement is likely to occur, and not only the base steel plate and the plating interface, but also the phenomenon that the steel plate crystal grains of the surface layer are peeled not only along the grain boundaries where zinc has penetrated into the base steel plate. For this reason, it becomes easy to generate | occur | produce peeling of plating, and it becomes very difficult to ensure favorable powdering property.

  The inventors of the present invention manufactured an alloyed hot-dip galvanized steel sheet having excellent hole expansion resistance while suppressing grain boundary oxidation of the base crystal by finely controlling the atmosphere in the annealing furnace according to Patent Document 6. Disclosed the invention. Patent Document 6 discloses that suppressing grain boundary oxidation in an annealing furnace is important for ensuring workability.

JP-A-5-331537 Japanese Patent Laid-Open No. 7-9055 Japanese Patent Laid-Open No. 11-50220 JP 7-316762 A JP-A-9-316103 JP 7-286253 A JP 2002-146475 A JP 2002-29497A

  However, from the viewpoint of prevention of non-plating and GA processing unevenness, the conventional technology uses internal oxidation of easily oxidizable elements in order to ensure stable surface properties without providing special processing steps. There is only. However, when internal oxidation is promoted, grain boundary oxidation inevitably progresses, so in the hot dip galvanization and the subsequent GA treatment, the penetration of Zn into the grain boundary becomes significant, and the grain boundary at the plating / base metal interface. Brittleness is likely to occur, and it is very difficult to ensure good powdering resistance and plating adhesion during processing.

  Therefore, at present, it cannot be said that a technique capable of ensuring good surface properties and ensuring good powdering properties without adding a new processing step has yet to be established.

  The present invention is a high-strength alloy having extremely good powdering resistance and excellent surface properties free from non-plating and alloying irregularities while containing a large amount of easily oxidizable elements such as Si, Mn, and Al. An object of the present invention is to provide a galvannealed steel sheet and a method for producing the same.

  As a result of intensive studies on the elements contained in a high-strength steel sheet containing a large amount of easily oxidizable elements such as Mn, Si, and Al, the present inventors have found that Zn after alloying hot-dip galvanizing treatment for this high-strength steel sheet, and Zn In order to suppress penetration of the Fe alloy into the base material and to ensure stable GA processability, it was found that it is effective to control the atmosphere during annealing while containing Bi. Completed the invention.

  As an alloyed hot-dip galvanized steel sheet containing Bi, Patent Document 7 describes that the wettability of plating is improved by containing Sb, Bi, etc. in a P-containing ultra-low carbon steel sheet, and the grain boundary of the base material. An invention has been disclosed in which segregation of P is prevented and alloying unevenness due to local P segregation, called P streak, is prevented. However, since this invention has low Si content, it cannot fully prevent the non-plating and GA nonuniformity resulting from Si oxide in a steel plate with high Si content.

  Further, Patent Document 8 discloses that Bi is contained in a high-tensile hot-dip galvanized steel sheet having a high Si content, Mn content, and Al content, thereby preventing plating wettability and improving plating adhesion. An invention for achieving the above is disclosed. However, as will be described later, the powdering resistance and the surface appearance cannot be made compatible at a high level only by containing Bi and specifying the annealing temperature. Further, the prevention of bending cracking by suppressing the formation of a heterogeneous structure resulting from solidification segregation due to the inclusion of Bi, and the improvement of the adhesion of plating by the improvement thereof are not expected.

  In the present invention, it is impossible to ensure good powdering resistance only by containing Bi, and keeping the annealing atmosphere properly together with Bi, specifically, up to the recrystallization temperature range. This is based on the knowledge that it is possible to achieve both high levels of powdering resistance and surface properties for the first time by keeping the annealing dew point in an appropriate range. The reason why the powdering resistance and the surface properties are compatible at a high level by the synergistic effect of containing Bi and maintaining an appropriate annealing dew point up to the recrystallization temperature is considered as follows.

(A) Si, Mn, Al, etc., which are easily oxidizable elements, are concentrated in the form of an oxide on the steel sheet surface.
(B) At the same time, a Bi concentrated layer is formed on the surface layer, and excessive base material grain boundary oxidation is suppressed.

(C) At the time of hot dip galvanization penetration, the film-like oxide layer of the easily oxidizable element on the surface layer is peeled off from the steel sheet surface layer along with the dissolution of the Bi concentrated layer in the zinc bath.
(D) Since the oxide film serving as a barrier against Fe diffusion during non-plating and alloying treatment is removed from the steel sheet surface, non-plating and alloying unevenness are suppressed.

  The present invention has been completed on the basis of these findings (a) to (d), and provides a hot-dip galvanized steel sheet excellent in formability suitable as a material for a part where corrosion resistance is important, and a method for producing the same. .

The present invention, C: 0.03 to 0.20% (in the present specification, “%” means “% by mass” unless otherwise specified), Mn: 0.03 to 3.0%, Si: 0.1 to 2.5%, S: 0.01% or less, P: 0.1% or less, sol. Al: 1.0% or less, N: 0.01% or less, Bi: 0.0001 to 0.05%, alloying by forming an alloying hot-dip galvanized film on a steel plate having a chemical composition consisting of Fe and impurities. It is a hot-dip galvanized steel sheet, and the penetration depth of zinc and zinc-Fe alloy is 10 μm or less in the depth direction on the steel sheet side from the interface between the plating layer and the base material, and the plating basis weight is 60 g / m ^2. The degree of alloying of the plating layer is 7 to 16%, and the variation in the degree of alloying at an arbitrary distance of 50 mm or more in the plating layer is 1.5% or less with a standard deviation of 10 points or more. It is an alloyed hot-dip galvanized steel sheet.

From another viewpoint, the present invention provides C: 0.03-0.20%, Mn: 0.03-3.0%, Si: 0.1-2.5%, S: 0.01% or less , P: 0.1% or less, sol. Alloying by continuously performing annealing, hot dip galvanizing and alloying treatment on a steel sheet containing Al: 1.0% or less, N: 0.01% or less, Bi: 0.0001 to 0.05% When manufacturing a hot-dip galvanized steel sheet, the coiling temperature after hot rolling of the steel sheet is 650 ° C. or lower, and when heating to the recrystallization temperature in annealing , annealing at least from 650 ° C. to the recrystallization temperature After annealing to the recrystallization temperature at a dew point in the furnace of -25 to 0 ° C. , and after annealing to the recrystallization temperature, a hot dip galvanized layer having a basis weight of 60 g / m ² or less is provided, and immediately in the GA furnace An alloyed hot-dip galvanized steel sheet manufacturing method according to the present invention described above is characterized in that, when an alloyed film is formed by heating, the alloying treatment temperature is 650C or lower .

In these present inventions, the galvannealed steel sheet is excellent in powdering resistance, plating adhesion and surface properties and has high strength.
In these present invention, the steel sheet is replaced with a part of Fe,
(I) containing one or more selected from the group consisting of Ti: 0.25% or less, Nb: 0.25% or less, and V: 0.25% or less,
(Ii) containing one or two selected from the group consisting of Cr: 1% or less and Mo: 1% or less,
(Iii) containing 1 type or 2 types selected from the group consisting of Cu: 1% or less and Ni: 1% or less,
(Iv) Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less selected from the group consisting of 0.01% or less about,
(V) B: containing 0.01% or less,
(Vi) It is preferable to have one or two or more of Sb: 0.05% or less and Sn: 0.05% or less.

  According to the present invention, in a high-strength steel sheet containing a large amount of easily oxidizable elements such as Si, Mn, Al, etc., extremely good powdering resistance and excellent high-strength surface properties without non-plating and alloying unevenness It becomes possible to provide an alloyed hot-dip galvanized steel sheet.

Drawing 1 is an explanatory view showing an example of a laboratory plating sample preparation method in an example. It is explanatory drawing which shows the powdering-proof evaluation method. It is a photograph which shows the cross-sectional micro observation (plating / base-material interface condition) in the optical microscope after plating melt | dissolution of Bi containing steel (sample No. 4 in Table 3; example of this invention). It is a photograph which shows the cross-sectional micro observation (plating / base-material interface condition) in the optical microscope after plating melt | dissolution of Bi non-containing steel (sample No. 13 in Table 3; comparative example).

The reason for limiting the chemical composition of the steel sheet as the base material in the galvannealed steel sheet according to the present invention, the hot dip galvanized steel sheet, and the manufacturing conditions thereof as described above will be described.
(A) Alloying hot-dip galvanizing The present invention is a plating layer by limiting the chemical composition and operating conditions of a high-strength hot-dip galvanized steel sheet containing a large amount of easily oxidizable elements described below to a specific range. In addition, it is possible to control the form of the interface of the base material, thereby ensuring both the powdering resistance and the surface properties of the plating.

  The ideal form of the interface between the plating layer and the base material is that the penetration depth of zinc and the zinc-iron alloy from the interface between the plating layer and the steel sheet is 10 μm or less. This zinc intrusion is caused by zinc intrusion when forming hot dip galvanizing after oxidation of crystal grain boundaries of the base material, and alloying progresses in the subsequent GA process. Therefore, in a steel sheet containing a large amount of easily oxidizable elements, it is essential to proceed with oxidation in order to ensure surface properties. Can happen.

  On the other hand, the present inventors have previously disclosed that such grain boundary oxidation affects the deterioration of the hole expandability of the steel material, but in addition to the hole expandability, the plating is performed when sliding is applied. We have found a new fact that it has a great influence on the powdering resistance that induces peeling. Therefore, as a result of paying attention to the shape of peeling of the plating layer and the base material, it was found that the depth of zinc penetrating along the crystal grain boundary has a great influence on the powdering resistance. That is, when the zinc penetration depth exceeds 10 μm, the grain boundary becomes brittle due to zinc penetration into the crystal grain boundary of the base material, and the volume expansion due to iron diffusion of the penetration zinc accompanying the progress of alloying thereafter. Further, embrittlement of the crystal grains of the base material progresses, and as a result, each crystal grain of the base material peels in the vicinity of the interface between the plating layer and the base material during processing, and the powdering resistance is remarkably deteriorated. Therefore, the penetration depth of zinc and zinc-iron alloy from the interface between the plating layer and the steel sheet is 10 μm or less, preferably 6 μm or less.

Further, the basis weight of plating is preferably 60 g / m ² or less. If the basis weight is more than 60 g / m ^2, it is necessary to increase the amount of Fe diffusion after heating in order to ensure a predetermined degree of alloying, and a brittle iron-zinc alloy layer ( The amount of (Γ phase) formation increases. In addition, since it is necessary to raise the temperature for GA formation, a more brittle iron-zinc alloy layer (Γ ₁ layer) is formed, which makes it difficult to ensure good powdering resistance. Therefore, the basis weight of plating is preferably 60 g / m ² or less, more preferably 50 g / m ² or less.

  Similarly, the alloying degree of plating is preferably 7% or more and 16% or less. When the degree of alloying is less than 7%, GA unevenness that does not complete the alloying partially occurs to the surface, and when it exceeds 16%, the brittle iron-zinc layer becomes thick, so that it is resistant to powdering. It will be difficult to secure.

  Furthermore, in the steel sheet containing a large amount of the easily oxidizable element that is the premise of the present invention, the easily oxidizable element is unavoidably present as an oxide on the surface of the steel sheet until it is immersed in the plating bath. Since it is partially unstable, it becomes very difficult to ensure a uniform degree of alloying. The reason why the oxide on the surface is not partially stabilized is that the oxide generated during rolling and winding during hot rolling is not stabilized in the subsequent pickling process, and the oxide film is formed even during the subsequent annealing. Is not stabilized. This reason is understood to be because the composition of the oxide formed on the surface layer varies greatly depending on the atmosphere and temperature. As a result of detailed investigation of the GA film of the high-strength steel sheet containing a large amount of easily oxidizable elements, the inventors of the present invention have made progress in alloying in the portion where the zinc and zinc-iron alloy intrusion amount is large. It becomes extremely fast and the degree of alloying increases locally, and the penetration of such zinc and zinc-iron alloys proceeds along the base crystal grains, so that the grain boundaries become brittle and work. Along with this, it was found that the powdering resistance is remarkably lowered because the base material crystal grains are separated. Therefore, even within the above range of the basis weight and the degree of alloying, the powdering resistance is not stable unless the variation in the degree of alloying is locally suppressed.

  As a preferable variation in the degree of alloying of the GA film in the present invention, the variation in the degree of alloying at 10 points or more at an arbitrary position separated by 50 mm or more is suppressed to 1.5% or less by its standard deviation. When the standard deviation is more than 1.5%, as described above, there is a portion where the zinc penetration depth is locally large, so that stable powdering resistance cannot be ensured. Of course, due to the large variation in the degree of alloying, it is easy to generate powdering defects locally, and this standard deviation is 1.5% or less in order to prevent the occurrence of uneven alloying. Suppress.

(B) Chemical composition of steel slab or base steel plate
[C: 0.03 to 0.20%]
C is effective for obtaining a high tension, and if the C content is less than 0.03%, the necessary high tension cannot be obtained. If the C content exceeds 0.20%, the toughness and weldability deteriorate. Therefore, in the present invention, the C content is set to 0.03% or more and 0.20% or less.

[Mn: 0.03-3.0%, Si: 0.1-2.5%]
Si is effective in increasing the strength of a steel sheet, strengthening ferrite, and making the structure uniform. Mn is effective for increasing the strength by promoting transformation strengthening. On the other hand, Si and Mn are easier to oxidize than iron, so they are easily concentrated on the surface during annealing to form oxides. In the subsequent hot dip galvanizing process, if these oxidizable elements are present as oxides on the surface, non-plating that repels hot dip plating is likely to occur, and if the oxidizable elements are present on the surface of the steel sheet, hot dip galvanizing is formed. In the GA process where heat treatment is performed immediately thereafter, it becomes a barrier to iron diffusion from the base material, making GA process extremely difficult, and when such easily oxidizable elements are generated unevenly on the surface, partially Variations in the degree of alloying increase, causing unevenness in alloying. Further, when the GA is promoted in order to eliminate such alloying unevenness, a part having a high degree of alloying locally exists, resulting in poor powdering properties. For these reasons, the Mn content is 0.03% to 3.0%, and the Si content is 0.1% to 2.5%. These ranges are ranges in which the surface concentration of Si and Mn and the amount of oxides formed increase, and the GA treatment is difficult under normal operating conditions, so that the effects of the present invention are further exhibited.

[S: 0.01% or less]
S becomes MnS in steel and deteriorates bendability. Therefore, the S content is set to 0.01% or less.

[P: 0.1% or less]
P is an undesirable element that deteriorates toughness. Therefore, the P content is 0.1% or less.

[Sol. Al: 1.0% or less]
Al is originally contained as a deoxidizer for molten steel, but Al is also an element that easily oxidizes, and since it is easy to produce oxides like Si and Mn during annealing, in the GA treatment step, In order to improve surface properties, it is preferable to reduce as much as possible. However, a high strength steel sheet may be actively contained in order to stabilize austenite for ensuring its mechanical properties, and a large amount may be desired.

  In the present invention, similarly to Si and Mn, in order to ensure a stable GA treatment in a high-strength steel sheet containing a large amount of easily oxidizable element Al, and to ensure good powdering properties, sol. Al content shall be 1.0% or less. sol. When the Al content is more than 1.0%, when a large amount of Si and Mn is contained, in the present invention, it is difficult to ensure a stable GA processability, and the GA process temperature is extremely reduced. Since it becomes necessary to raise, it becomes difficult to ensure good powdering resistance. Also, sol. The lower limit of the Al content is not particularly defined, and it is 0.010 to 0.1% which is a normal Al deoxidation level, and the use of a deoxidizer other than Al, or a combination thereof, is 0.010. Even if it is less than%, the effect of the present invention can be sufficiently obtained.

[N: 0.01% or less]
Since N forms a nitride during continuous casting and causes cracks in the slab, it is preferable that the N content is low. Therefore, the N content is 0.01% or less.

[Bi: 0.0001 to 0.05%]
Bi is the most important element in the present invention. The effect of Bi in the present invention has not been fully elucidated, but is thought to be due to the following mechanism. Since the metal Bi has a melting point lower than that of zinc, the metal Bi is easily dissolved during hot dip galvanizing and is said to improve the wettability of the hot dip galvanizing with respect to the steel sheet. In the present invention, by containing Bi having such characteristics, not only the wettability of the plating is improved, but also the concentrated Bi on the surface is concentrated on the surface during the annealing step, so that the concentrated Bi of the surface layer is added to the plating bath. By dissolving in, it becomes possible to peel off easily oxidizable elements present in the surface layer, and the surface state of the steel sheet when immersed in the plating bath can be homogenized. As a result, non-plating can be prevented, and the oxide that becomes a barrier for iron diffusion at the time of GA conversion disappears, and uniform GA conversion becomes possible, so that stable powdering properties can be secured.

  Moreover, as a result of observing the interface state of the plating layer and the base material after forming the GA film on the steel sheet containing Bi, zinc entering the crystal grain boundary after forming the GA film by containing Bi, and zinc-iron There was a tendency for the depth of the alloy to decrease, and as a result, a tendency to improve powdering resistance was observed. Since the surface concentration of Bi occurs, the concentration of oxidizable elements such as Si, Mn, and Al to the grain boundary and the oxidation are suppressed, so the zinc, zinc-iron alloy layer on the subsequent grain boundary This is thought to be due to a decrease in intrusion.

  Furthermore, Bi has the effect | action which concentrates on the solidification interface of molten steel, narrows a dendrite space | interval, and makes solidification segregation small. As a result, there is also an effect of preventing a bending crack at the segregation part. Therefore, as a result of preventing the base material from cracking during processing, plating peeling starting from the base material cracking portion is suppressed, so that an improvement in powdering resistance is expected.

  When the Bi content is less than 0.0001%, the effect of Bi described above is insufficient, and when the Bi content exceeds 0.05%, grain boundary embrittlement due to melting of Bi existing in the crystal grain boundary is caused. As it occurs, zinc penetrates along the Bi melted from the grain boundary during immersion in the zinc bath, so that the powdering resistance decreases. Therefore, the Bi content is set to 0.0001% or more and 0.05% or less. The Bi content is preferably 0.0003% or more and 0.01%, more preferably 0.0003% or more and 0.0050% or less.

Next, arbitrary elements will be described.
[One or more selected from the group consisting of Ti: 0.25% or less, Nb: 0.25% or less, and V: 0.25% or less]
Ti, Nb, and V are optional elements that are contained as necessary because they slow down recrystallization and refine crystal grains. However, this effect is saturated and disadvantageous in cost when the Ti content exceeds 0.25%, the Nb content exceeds 0.25%, and the V content exceeds 0.25%. Therefore, the Ti content is 0.25% or less, the Nb content is 0.25% or less, and the V content is 0.25% or less. For example, in order to more stably secure a tensile strength of 980 MPa or more, the content of any element of Ti, Nb, and V is preferably 0.003% or more.

[Cr: 1% or less selected from the group consisting of 1% or less and Mo: 1% or less]
Both Cr and Mo have the function of promoting transformation strengthening by stabilizing austenite in the same manner as Mn, and are effective in increasing the strength of the steel sheet, so are optional elements contained as necessary. . However, since Cr and Mo are also easily oxidizable elements, a large amount is not preferable. If the Cr content exceeds 1% and the Mo content exceeds 1%, the workability deteriorates and it becomes difficult to ensure stable powdering resistance and surface properties. Therefore, the Cr content is 1% or less, and the Mo content is 1% or less.

[One or two selected from the group consisting of Cu: 1% or less and Ni: 1% or less]
Cu and Ni are corrosion-inhibiting effects, are concentrated on the surface, suppress the penetration of hydrogen, and suppress delayed fracture, so are optional elements to be contained as necessary. However, in any case, when the content exceeds 1%, this effect is saturated and disadvantageous in cost. Therefore, both the Cu content and the Ni content are 1% or less.

[One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less]
Ca, Mg, REM, and Zr are all optional elements that are contained as needed in order to contribute to inclusion control, particularly to finely disperse inclusions and further improve bendability. However, if the content is excessive, the surface properties are deteriorated, so each content is set to 0.01% or less. In order to obtain the above effect more reliably, the content of any element is preferably 0.001% or more.

[B: 0.01% or less]
B suppresses nucleation from the grain boundary, enhances hardenability and contributes to high strength, and is therefore an optional element contained as necessary. If the B content exceeds 0.01%, the effect is saturated, so the B content is 0.01% or less. In order to obtain this effect more reliably, the B content is preferably 0.0005% or more.

[One or two of Sb: 0.05% or less and Sn: 0.05% or less]
Since Sb and Sn can be expected to dissolve in the molten zinc bath, they are optional elements that are expected to have the same effect as Bi. However, as with Bi, there is a concern about grain boundary embrittlement due to segregation in the base steel sheet, so the content of each is 0.05% or less. When Sb and Sn are contained in combination, it is preferable that the total content of Sb, Sn and Bi does not exceed 0.05%.

The balance other than the above is Fe and impurities.
The present invention aims to improve the surface properties by stabilizing the powdering resistance and GA treatment properties, so it is limited to the alloyed hot-dip galvanized steel sheet, but in the sense of ensuring the surface properties, a pure zinc bath The present invention is sufficient even for other hot dip galvanized steel sheets such as hot dip galvanized steel sheets that form films with galvanized steel sheets or zinc-aluminum alloyed galvanized steel sheets that form films with a bath containing 55% Al. Applies to Moreover, the plating layer may be given to both surfaces of the steel plate, and may be given to one side.

(C) Manufacturing conditions
In the present invention, the operating conditions of the hot dip galvanizing line for securing a suitable GA film structure will be described.

A high-strength alloyed hot-dip galvanized steel sheet is produced by successively performing annealing, hot-dip galvanizing and alloying treatment on the steel sheet having the above-described chemical composition.
The hot dip galvanizing line is immersed in a plating bath after annealing and immediately alloyed in a GA furnace to produce an alloyed hot dip galvanized steel sheet, but ensures the above-described suitable GA coating in the present invention. What is important in the above is the control of the atmosphere in the annealing furnace and the temperature of GA conversion. Specifically, when heating to the recrystallization temperature in annealing, the dew point in the annealing furnace at least from 650 ° C. to the recrystallization temperature is from −25 to 0 ° C. and annealing is performed to the recrystallization temperature.

  In the annealing atmosphere, when the steel sheet is heated and annealed, the annealing dew point at the time of heating from at least 650 ° C. to the recrystallization temperature is set to −25 ° C. or more and 0 ° C. or less. By making the dew point in the annealing furnace within this temperature range, in steel sheets containing a large amount of easily oxidizable elements and containing Bi, it is possible to prevent intrusion into the zinc base metal grain boundary and to achieve a stable GA process. It can be secured.

  The reason why zinc can be prevented from entering and a stable GA treatment can be ensured by controlling the atmosphere in the annealing furnace as described above is as follows. As a result of the easily oxidized element oxides that have been concentrated on the surface layer being softened and discharged to the surface, these oxides are formed into a film, which improves the cleanliness of the underlying layer and suppresses subsequent grain boundary oxidation. Thus, the progress of grain boundary oxidation, which is the starting point of zinc penetration, is suppressed. At that time, in the Bi-containing steel, as described above, when the film-like oxide formed on these surface layers is immersed in a hot dip galvanizing bath, the film-like oxide is dissolved together with the dissolution of the Bi concentrated layer. As a result of the separation of the oxide, the steel sheet surface is in a highly clean state, and subsequent alloying proceeds uniformly.

  On the other hand, below this temperature range, oxidation of easily oxidizable elements occurs, but softening of the oxide occurs, it does not exist as a film-like oxide on the steel sheet surface, and oxidation to the grain boundaries proceeds. In addition, the amount of zinc intrusion increases, and even after dissolution of the steel sheet surface of the Bi-concentrated layer occurs, oxides remain from the grain boundaries, resulting in a decrease in GA treatment and uneven alloying. It becomes easy to do, and the variation in the alloying degree in a steel plate becomes large.

  In addition, since the formation of a film-like oxide occurs above this temperature, the effect of the present invention can be maintained. However, excessive surface oxidation promotes grain boundary oxidation, which is not preferable, and an increase in dew point is caused by oxidation. As a result of accelerating the formation of the object, the oxide is pseudo-attached to a roll such as a hearth roll in the annealing furnace, resulting in a decrease in surface properties due to indentation defects such as a roll flaw. Therefore, the dew point in the temperature range up to the recrystallization temperature at which mechanical properties can be ensured should be increased.

The weight per unit area of plating is preferably 60 g / m ² or less for the reason described above.
Next, GA formation is performed after forming a hot-dip galvanized film under such annealing conditions. The GA formation temperature when the basis weight and the degree of alloying of the present invention can be ensured is 650 ° C. or less. Is desirable. This is because when the temperature exceeds 650 ° C., the hard Γ ₁ phase is formed thick, and the powdering resistance deteriorates. The lower limit temperature of GA formation is not particularly defined as long as the degree of alloying of the present invention can be ensured. For example, in the case of residual austenitic steel, the GA temperature is high when alloying hot dip galvanizing. Since the austenite is decomposed, the ductility is lowered, and the GA conversion temperature is preferably as low as possible to ensure the mechanical properties of the high-strength steel sheet.

  In addition, in a high-tensile steel sheet containing a large amount of easily oxidizable elements, it is difficult for GA to occur originally, and it is inevitably necessary to raise the GA temperature. However, in the present invention, by including Bi and optimizing the annealing dew point. As a result of stably ensuring the cleanliness of the steel sheet surface when immersed in the zinc bath, the temperature of GA conversion can be lowered, and there is also an advantage that good powdering resistance can be ensured more stably.

  In the present invention, the preferable production conditions other than those described above are not particularly specified, but from the viewpoint of suppressing the invasion of zinc into the remarkable grain boundary which is the gist of the present invention, grains during hot rolling are used. Since field oxidation is also a concern, the coiling temperature after hot rolling is preferably 650 ° C. or lower. When the coiling temperature is higher than 650 ° C., after winding on the coil, grain boundary oxidation to the base crystal grain boundary proceeds in the hot rolling stage, and after pickling or subsequent cold rolling This is because the grain boundary oxidation portion remains and becomes the entry point of zinc.

  In this way, according to the present invention, it has extremely excellent powdering resistance, which is difficult to manufacture with the conventional technology, excellent surface properties without non-plating and GA unevenness, and plating adhesion. It becomes possible to provide a high-strength, high-tensile alloyed hot-dip galvanized steel sheet.

The effects of containing Bi in the present invention will be described together with the production method thereof with reference to examples.
Steel having the chemical composition shown in Table 1 was melted in an experimental furnace to produce a slab having a thickness of 40 mm.

  Next, this slab was hot-rolled under the conditions shown in Table 2, and then the hot-rolled steel sheet was pickled, further cold-rolled at a rolling rate of 50%, and a sheet thickness of 1.2 mm. A rolled steel sheet was used. The coiling temperature of the hot rolling was set by charging the furnace at that temperature for 30 minutes or more and performing a furnace cooling simulation to 20 ° C./hour to 300 ° C. or less.

  A test material (width: 220 mm x length: 120 mm) for use in a hot dipping simulator is collected from the cold-rolled steel sheet obtained and subjected to annealing, plating and alloying treatment corresponding to a heat pattern in a continuous hot dipping galvanizing facility. It was. Specifically, after annealing under the conditions shown in Table 3, it is cooled to 460 ° C., immersed in a molten zinc bath for plating, and then immersed in a salt bath at 550 ° C. for a predetermined time for alloying treatment. Went.

At that time, in order to confirm the influence of the annealing dew point, as shown in the graph of FIG. 1, which is an explanatory diagram showing an example of a method for preparing a lab plating sample in this example, dew point = −60 ° C. N ₂ − in advance. The inside of the furnace is purged with 10% by volume H ₂ gas, and when the firing temperature starts, the gas is switched to a predetermined high dew point gas. When the plate temperature reaches 650 ° C., the dew point in the furnace is purged with the predetermined high dew point gas. I did it. Then, when the plate temperature reaches 860 ° C. above the recrystallization temperature, the gas is switched to the initial low dew point gas, and the dew point in the furnace reaches −60 ° C. until the plate temperature immersed in the plating bath reaches 460 ° C. After confirming this, it was immersed in a hot dip galvanizing bath (bath Al concentration = 0.10%), and after adjusting the amount of zinc per unit area so as to be 40 to 90 g / m ² , it was cooled as it was.

  About the hot dip galvanized plate obtained in this way, in order to confirm the surface properties, after confirming the presence or absence of non-plating visually, it is immersed in a salt bath at 500 ° C. for 30 seconds, and alloyed. Then, the powdering properties of the coating, the plating / matrix interface, the basis weight in the plate, and the degree of alloying variation were measured.

An evaluation method of the galvannealed steel sheet in this example will be described.
[Plating appearance evaluation method]
The appearance of the plating was evaluated by visual evaluation at the stage of the hot dip galvanized steel sheet. Specifically, the case where no unplating is evaluated is evaluated as “◯”, and the case where non-plating is 0.5 mm or less at the maximum diameter is evaluated as “Δ”, the maximum diameter is 0.5 mm or more. In the case where the non-plating of 1 was present even at one location in the plate, the evaluation was “x”, and the score was Δ or more. The non-plated area in the hot-dip galvanized steel sheet with a maximum diameter of 0.5 mm or more was rejected if it was less than 0.5 mm, and after GA, substantially non-plating could not be visually recognized. Judging from the sacrificial corrosion prevention distance of zinc, it is because the rust preventive property does not decrease significantly.

[Powdering evaluation method]
In the present invention, as a more rigorous method for evaluating the powdering property, the plating film is peeled off by bending and bending back, which occurs because the alloyed hot-dip galvanized film cannot follow the deformation of the base steel sheet due to the conventional compression deformation. Peeling of the plating film by cylindrical deep drawing in consideration of actual processing was evaluated as powdering property instead of powdering. In other words, in actual processing, the plating film undergoes sliding due to contact with the mold as well as compressive deformation, so the conventional powdering property of only compressive deformation is insufficient, and plating peeling due to sliding is also not possible. There is a need to consider. In particular, in the high-strength steel that is the target steel type in the present invention, since the strength of the material is high, the surface pressure received during processing is also high, and thus the sliding resistance is also high, so the occurrence of plating peeling due to sliding is more problematic. Become.

  Perform deep drawing with the conditions shown in Fig. 2 (cylinder drawing condition blank diameter: 90 mm, drawing height: 30 mm, lubricating oil: general rust preventive oil (Nox-Rust550HN; Parker Kosan)), and affix tape to the side wall Then, the film was peeled off, and the powdering resistance was evaluated from the weight difference before and after the peeling. Evaluation criteria were defined as follows.

X: Powdering peeling amount> 100 mg / P (failed)
Δ: Powdering peeling amount = 50 to 100 mg / P (accepted)
○: Powdering peeling amount <50 mg / P (pass)
Of the alloyed hot-dip galvanized steel sheets for automobiles, poor powdering properties may occur on mild steel bases for proven interior use, because this evaluation exceeds 100 mg / P, so the pass is 100 mg. / P or less, more preferably less than 50 mg / P.

[Plating / base metal interface state]
In order to exhibit good powdering properties and plating adhesion in the present invention, the interface state between the plating layer and the base material is controlled. Specifically, the penetration depth of the alloyed hot-dip galvanized film that penetrates into the base material is 10 μm or less, and this evaluation method will be described.

  A sample of the galvannealed steel sheet prepared as described above (effective area 100 mm × 150 mm) is divided into 6 parts having a size of 50 mm × 50 mm, and arbitrary parts (six places) of the small pieces are collected, From the cross-sectional observation, the depth of the alloyed hot-dip galvanized film penetrating into the base steel sheet was measured by micro observation. At that time, the measurement magnification at the time of micro observation is 500 times, the length per visual field is 200 μm, and the maximum penetration depth is a representative value in one visual field, and an average value of five places evaluated.

  In this case, the micro observation method is a method in which only the plating film is dissolved with a hydrochloric acid solution to which an inhibitor is added, and then the surface state is measured with an optical microscope without etching, or from a mapping image of Zn by EPMA. Any method of directly observing the penetration depth into the base material may be used, but the observation magnification was determined to be 1000 times or less.

  The reason why the observation magnification is set to 1000 times or less is that zinc or a zinc alloy penetrates into the base material mainly along the crystal grain boundary of the base material. If there is zinc penetration that can be clarified even at such a relatively low magnification, the base crystal grain boundary becomes brittle and cracks originate from that during processing. This is because, since it becomes a starting point for lowering the powdering resistance, it is easy to predict whether the powdering resistance is good or bad.

  FIG. 3 is a photograph showing cross-sectional micro observation (plating / base metal interface state) with an optical microscope after plating dissolution of Bi-containing steel (sample No. 4 in Table 3; example of the present invention). It is a photograph which shows the cross-sectional micro observation (plating / base-material interface condition) in the optical microscope after plating melt | dissolution of Bi non-containing steel (sample No. 13 in Table 3; comparative example).

[Adhesion amount, alloying degree variation]
In the present invention, the possibility of expressing such an intrusion state of zinc more macroscopically appears as variations in the degree of alloying. That is, the progress or delay of local alloying appears as a discontinuity at the plating / matrix interface.

  Therefore, after preparing three lab samples with the same plating conditions in this example and cutting them into an effective area of 100 mm × 150 mm, they were further cut into small pieces of 50 mm × 50 mm (6 sheets / P, a total of 18 sheets) ), Dissolve only the plating film with 10% hydrochloric acid solution with inhibitor added, measure the Zn content, Fe content and Al content by atomic absorption method, and calculate the basis weight per one side and the degree of alloying. did. At that time, the basis weight and the degree of alloying in each sample were expressed by the average value of the 18 sheets as the representative basis weight and the degree of alloying, and the standard deviation of the variation in the degree of alloying at that time was obtained. The plating conditions and the evaluation results are listed in Table 3.

  As shown in Table 3, in Bi-containing steel, No. As shown in 1-6, when the annealing dew point until recrystallization is increased, the amount of zinc alloy plating intruding into the base steel sheet becomes large even under the same alloying conditions, but in order to achieve both plating appearance and powdering resistance. It can be seen that the dew point is compatible between −25 ° C. and 0 ° C., and the plating penetration depth at that time is 8 μm or less. Further, as the plating penetration depth for ensuring the powdering resistance, No. As shown in FIG. 8, it must be 10 μm or less. In order to obtain more preferable powdering resistance, no. 7 indicates that 6 μm or less is necessary. No. As shown in FIG. 16, if high dew point annealing is continued even after the completion of recrystallization, the powdering appearance and the powdering resistance also deteriorate, so that it is also important to increase the dew point up to the recrystallization temperature. .

On the other hand, no. As shown in 8 and 9, the GA temperature is found to be 650 ° C. or lower. No. As shown in 9, 10, when the representative weight is 60 g / m ² or more and the alloying degree is 16% or more, it is difficult to ensure the powdering resistance.

  Further, when a steel sheet not containing Bi is plated under the same conditions, for example, No. 4, 5 and No. Comparison with 13 and 14 shows that the penetration of plating into the base steel sheet is larger in the steel sheet not containing Bi, and it is difficult to ensure the powdering resistance. In particular, it can be seen that annealing at a high dew point significantly increases the amount of plating penetration, and it is difficult to ensure stable powdering properties.

  In the standard deviation of the alloying degree, for example, No. As shown in FIGS. 1 and 2, the standard deviation is high under the condition that non-plating occurs. As shown in FIGS. 8 and 9, when the standard deviation exceeds 1.5%, it can be seen that the penetration amount of the plating increases even under the condition that no plating occurs, and the powdering resistance is deteriorated.

  Furthermore, no. 11 and 16, when the hot rolling coiling temperature is high, the penetration amount of the plating becomes large even under the same plating conditions. This is because the formation of oxide in the winding stage during hot rolling becomes remarkable, and this oxide is dissolved in the subsequent pickling, and in the subsequent cold rolling stage, it is in a scab shape in advance. In addition, since the formation of grain boundary oxides during annealing is added, the result is that the plating remarkably penetrates the base steel sheet when entering the zinc bath, and it can be seen that good powdering resistance cannot be ensured.

Next, the influence of the chemical composition of the base steel sheet in the present invention will be described using examples.
Steel having the chemical composition shown in Table 4 was melted in an experimental furnace to produce a slab having a thickness of 40 mm. Furthermore, hot rolling was performed under the conditions shown in Table 4, then pickling was performed, and further cold rolling was performed to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm.

The coiling temperature of the hot rolling was charged in a furnace at that temperature for 30 minutes or more, and the furnace cooling simulation was performed at 20 ° C./hour to 300 ° C. or less.
A test material (width: 220 mm × length: 120 mmL) for use in a hot dipping simulator was sampled from the obtained cold-rolled steel sheet and subjected to annealing, plating and alloying treatment corresponding to a heat pattern in a continuous hot dipping galvanizing facility. . As shown in FIG. 1, the annealing and plating conditions at that time are set to 860 ° C., the holding time is set to 60 seconds, and then cooled to 460 ° C. and immersed in a hot dip galvanizing bath. A hot dip galvanized steel sheet was prepared with a target of 50 g / m ² and the surface appearance was observed. The atmosphere during annealing is switched to a high dew point gas (dew point = −10 ° C.) when the plate temperature is 650 ° C., and is changed to a low dew point gas (−60 ° C.) at the recrystallization temperature (= 860 ° C.). Switched.

Next, after the prepared hot-dip galvanized steel sheet was immersed in a salt bath at 500 to 630 ° C. for 30 seconds and subjected to alloying treatment, it was immediately quenched to prepare an alloyed hot-dip galvanized steel sheet.
In the same manner as in Example 1, the sample was subjected to powdering resistance evaluation, plating and base material interface observation, basis weight measurement, and alloying degree variation. The acceptance / rejection determination criteria are the same as those in the first embodiment.

  Table 5 shows the alloying conditions and the investigation results.

Since the most important components in the base steel sheet in the present invention are Si and Al which are oxidizable elements and Bi which has an interface control effect, the relationship between these elements will be described.
No. As shown in 1 to 6, regardless of the presence or absence of Bi, if the Si content in the steel is less than 0.10%, when high dew point annealing is performed as in the present invention, the alloying processability is too advanced. Thus, it is difficult to secure good powdering resistance. No. As shown in FIG. If the Al content is more than 1.0%, it is difficult to ensure a good plating appearance even with high dew point annealing, and it is impossible to ensure powdering resistance. On the other hand, the effect of the presence or absence of Bi is, for example, No. 30, 31, or no. 18, 19 or even No. As shown in 20, 21, it can be seen that by containing Bi, the penetration of the plating into the base steel sheet is suppressed, and the powdering resistance can be secured.

  From these results, it is found that the sol. If the Al content is 1.0% or less, annealing is performed at a high dew point up to the recrystallization temperature range, and the subsequent dew point is lowered to obtain a stable surface property and excellent powdering resistance. It is possible to ensure the sex.

Claims (8)

In mass%, C: 0.03 to 0.20%, Mn: 0.03 to 3.0%, Si: 0.1 to 2.5%, S: 0.01% or less, P: 0.1 % Or less, sol. Al: 1.0% or less, N: 0.01% or less, Bi: 0.0001 to 0.05%, alloying by forming an alloying hot-dip galvanized film on a steel plate having a chemical composition consisting of Fe and impurities. It is a hot-dip galvanized steel sheet, and the penetration depth of zinc and zinc-Fe alloy is 10 μm or less in the depth direction on the steel sheet side from the interface between the plating layer and the base material, and the plating basis weight is 60 g / m ^2. The degree of alloying of the plating layer is 7 to 16% by mass, and the variation in the degree of alloying at an arbitrary distance of 50 mm or more in the plating layer is 1.5% or less with a standard deviation of 10 points or more. galvannealed steel sheet, characterized in that it.   The steel sheet is replaced with a part of the Fe, and is selected from the group consisting of Ti: 0.25% or less, Nb: 0.25% or less, and V: 0.25% or less in mass% or The alloyed hot-dip galvanized steel sheet according to claim 1 containing two or more kinds.   The said steel plate replaces a part of said Fe, and contains 1 type or 2 types chosen from the group which consists of Cr: 1% or less and Mo: 1% or less by the mass%. The alloyed hot-dip galvanized steel sheet described in 1.   The steel sheet contains one or two selected from the group consisting of Cu: 1% or less and Ni: 1% or less in mass% instead of a part of the Fe. The alloyed hot-dip galvanized steel sheet described in any one of items 1 to 4.   The steel sheet is composed of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less in mass% instead of part of the Fe. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4, comprising one or more selected from the group.   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 5, wherein the steel sheet contains B: 0.01% by mass or less instead of a part of the Fe.   The steel sheet contains one or two of Sb: 0.05% or less and Sn: 0.05% or less in mass%, instead of a part of the Fe. The alloyed hot-dip galvanized steel sheet described in any one of the items. An alloyed hot-dip galvanized steel sheet is produced by continuously performing annealing, hot-dip galvanizing and alloying treatment on a steel sheet containing the chemical composition according to any one of claims 1 to 7. In this case, the coiling temperature after hot rolling of the steel sheet is 650 ° C. or less, and when the steel sheet is heated to the recrystallization temperature in the annealing, the dew point in the annealing furnace from at least 650 ° C. to the recrystallization temperature is − After annealing to a recrystallization temperature at 25 to 0 ° C. and annealing to the recrystallization temperature, a hot dip galvanized layer having a basis weight of 60 g / m ² or less is provided and immediately heated in a GA furnace to form an alloyed film The method for producing an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 7 , wherein the alloying treatment temperature is set to 650 ° C or lower when forming the steel sheet.