JP5070862B2 - Plated steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a plated steel sheet and a method for producing the same. Specifically, the present invention relates to a plated steel sheet used for automobiles, home appliances, mechanical structures, architectural uses, and the like, and a manufacturing method thereof.

  In recent years, there has been a demand for reduction of total carbon dioxide emissions on a global scale. Taking an automobile that consumes a large amount of fossil fuel as an example, the weight of the vehicle body is being reduced for the purpose of reducing the amount of exhaust gas and improving fuel consumption.

  As part of this, the use of high-tensile steel sheets that can ensure strength even when thinned is increasing for automobile cross members and side members. The high-tensile steel sheet is usually decarburized sufficiently at the steel making stage, for example, an ultra-low carbon Ti-added steel to which Ti is added after being made as an ultra-low carbon steel having a carbon content of 0.01% or less, and carbon 0.05 to Based on medium and low carbon aluminum killed steels in the range of 0.2%, high strength steels with increased strength by adding P, Si, Mn, Cr, Al, etc. are used as raw materials. Has been. In particular, high-strength steel to which Si or Cr is added is excellent in both strength and ductility, and a high-tensile steel sheet having a high content of these elements is also promising from the viewpoint of improving corrosion resistance.

  In addition to the use of automobile bodies, in addition to the use of high-strength steel sheets, in addition to the use of high-strength steel sheets, plated steel sheets with hot dip galvanizing etc. are used to improve corrosion resistance and appearance. Has been. Therefore, when using a high-tensile steel plate for the body of an automobile, a high-tensile steel plate that is the base material of the plated steel plate will be used. The plating wettability to the surface and the interfacial adhesion between the steel plate surface and the plating layer are particularly important.

  Various methods have been proposed for improving the plating wettability on the surface of the steel sheet and the interfacial adhesion between the steel sheet surface and the plating layer (hereinafter, sometimes referred to as “interfacial adhesion” including plating wettability). Yes.

  In Patent Document 1, the base steel plate is heated in a weak oxidizing atmosphere to form an Fe-based oxide film on the base steel plate surface, and then the base steel plate is heated in a reducing atmosphere, It has been disclosed that active and porous reduced iron is formed on the surface and then hot-dip plated.

  In Patent Document 2 and Patent Document 3, the surface of the base steel plate is activated by heating the base steel plate in a reducing atmosphere after pre-electroplating such as Cu, Ni and Fe on the base steel plate. It is disclosed that the hot-dip plating is performed after the porous state.

  Patent Document 4 proposes that Si in steel is fixed as an oxide in grain boundaries and / or crystal grains in the hot rolling process in the previous stage in order to prevent a decrease in interfacial adhesion. ing.

Japanese Patent Publication No.53-44141 JP-A-56-33463 JP-A-57-79160 JP 9-310148 A

  The method disclosed in Patent Document 1, that is, heating the base steel plate in a weak oxidizing atmosphere to form an Fe-based oxide film on the base steel plate surface, and then heating the base steel plate in a reducing atmosphere. Thus, the method in which active and porous reduced iron is formed on the surface of the base steel sheet and then hot-dip plated is effective for ordinary steel sheets.

  However, since Si is easily oxidizable as compared with Fe, when applied to a high-Si steel sheet having a high Si content, it is easy to concentrate on the steel sheet surface in the annealing process, hindering interfacial adhesion, and after press forming, etc. In the machining process, it may cause the peeling of the plating film, and in extreme cases, non-plating defects may occur. The current situation is that it cannot be obtained.

  In addition, the method disclosed in Patent Document 2 and Patent Document 3, that is, by performing pre-electroplating such as Cu, Ni, Fe, etc. on the base material steel plate, and then heating the base material steel plate in a reducing atmosphere. In the method of hot dip plating after making the surface of the base steel plate active and porous, it is necessary to install a new electroplating device in addition to the hot dip plating equipment or to pass through a separate electroplating line Therefore, the cost increase is inevitable.

  Furthermore, in the method proposed in Patent Document 4, it is necessary to form a sufficient internal oxide layer in the previous hot rolling process. However, in that case, an increase in the coiling temperature in the hot rolling process and a subsequent decrease in the cooling rate are unavoidable, and there is a risk of surface defects due to an increase in scale thickness and a collapse due to an increase in the coiling temperature. It can be drastically reduced.

  The foregoing has been described with respect to a plated steel plate using a high Si steel plate as a base material. However, a similar problem occurs with a plated steel plate using a high Cr steel plate as Cr is more easily oxidized than Fe. In addition, the present situation is that a satisfactory product has not been obtained for a plated steel plate using a high Cr steel plate as a base material.

  Then, this invention solves these problems, and it aims at providing the plated steel plate which can make the base material the steel plate containing many oxidizable elements, such as Si and Cr, and its manufacturing method.

  As a result of studies on various steel sheets, the present inventors have obtained the knowledge shown in the following (a) to (n).

  (a) It seems that the cause of plating failure when performing hot dipping on steel sheets with a high Si content is a decrease in plating wettability due to the concentration of Si on the base steel sheet surface during annealing. I have already said that.

  In this regard, the present inventors have reexamined the conventionally proposed measures for preventing plating defects mainly from the viewpoint of concentration of Si on the surface of the base steel plate.

  As a result, after forming an Fe-based oxide film on the surface of the base steel plate, the surface is made active and porous by reduction, or after pre-electroplating of Cu, Ni, Fe, etc., in a reducing atmosphere In the case of the heating method, as long as the existing plating equipment premised on in-line processing is used, Si in the steel easily diffuses and concentrates on the surface in the reduced iron layer or pre-electroplated layer during annealing, It has been found that it is difficult to ensure sufficient plating wettability.

  In addition, it has been proposed to suppress Si from concentrating on the steel sheet surface during annealing by fixing Si in the steel as an oxide in the grains and at the grain boundaries in the previous hot rolling process. However, since the internal oxide layer is thinned in the cold rolling process, the effect of suppressing the surface concentration of Si during annealing is reduced. Therefore, in order to exert the effect of suppressing the surface concentration of Si during annealing, it is necessary to form a sufficient internal oxide layer in the previous hot rolling process. However, in that case, an increase in the coiling temperature in the hot rolling process and a subsequent decrease in the cooling rate are unavoidable, and there is a risk of surface defects due to an increase in scale thickness and a collapse due to an increase in the coiling temperature. It can be drastically reduced.

  Therefore, a completely different approach is required to prevent defective plating on the steel surface when a high Si steel plate or the like is used as a base material.

  (b) The present inventors pay attention to ultrafine graining in the surface layer of a hot-rolled steel sheet that serves as a base material of a plated steel sheet as a measure for preventing poor plating on the steel surface when a high Si steel sheet or the like is used as a base material. As a result of various examinations and experiments, the following new findings were obtained.

  Regarding the Si content when using a high-Si hot-rolled steel sheet or a high-Si / high-Cr hot-rolled steel sheet as a base material, we examined it from the viewpoint of mechanical properties such as strength and workability and from the viewpoint of interfacial adhesion. As a result, it was found that 0.2 to 1.8% by mass was necessary.

  And regarding the content of Cr when using a high Cr hot rolled steel sheet or a high Si / high Cr hot rolled steel sheet as a base material, from the viewpoint of mechanical properties such as strength and workability, and from the viewpoint of interfacial adhesion As a result of investigation, it was found that 0.5 to 1.2% by mass is necessary.

The average crystal grain size of ferrite in the surface layer of the hot-rolled steel sheet that is the base material of the plated steel sheet is 3 μm or less, and

When the rate of increase in the average grain size of ferrite on the steel sheet surface layer at 800 ° C. is thermally stable at 0.05 μm / second or less, when a hot-rolled steel sheet containing a large amount of Si or Cr is used as a base material Even so, it has been found that a plated steel sheet having excellent plating wettability and interfacial adhesion can be obtained. In addition, the ferrite of the surface layer of a hot-rolled steel plate is a ferrite close | similar to the equiaxed crystal which exists in the 1st layer on the steel plate surface.

  (c) In the surface layer of the hot-rolled steel sheet that is the base material of the plated steel sheet, it is excellent when the base material is a hot-rolled steel sheet that satisfies the above-mentioned chemical composition of the hot-rolled steel sheet and the ultrafine grain condition of ferrite of the surface layer. The reason why high plating wettability and interfacial adhesion are obtained has not been fully elucidated. However, the hot-rolled steel sheet that satisfies the above-mentioned chemical composition of the hot-rolled steel sheet and the ultrafine grain condition of the ferrite of the layer is used as a base material, and after hot-dip plating, the plating surface layer is plated / plated to the cross section. When EPMA line analysis was performed toward the inside of the base material through the interface, it was found that concentration on the surface of Si and Cr due to preferential oxidation during annealing in the pre-plating process was suppressed. It is presumed that the suppression of the concentration of Si and Cr on the surface of the hot-rolled steel sheet led to the improvement of the plating property.

  That is, in the hot dipping process, in the process from temperature rise to reduction (annealing in the pre-plating process), oxidizable elements such as Si and Cr are oxidized on the steel sheet surface and generally concentrated on the steel sheet surface. This is a cause of reducing plating wettability and interfacial adhesion. However, if the ferrite grain size of the surface layer of the hot-rolled steel sheet that is the base material is fine and does not grow during the annealing in the pre-plating process as in the present invention, oxygen is passed from the surface through the ferrite grain boundaries that exist in large quantities. It diffuses inside, and most of the oxidation of Si and Cr occurs inside the steel sheet, not the steel sheet surface, that is, because oxidizable elements such as Si are oxidized inside the steel sheet, Therefore, it is estimated that plating wettability and interface adhesion are improved.

  In order to support this reasoning, after subjecting the hot-rolled steel sheet that satisfies the above-mentioned chemical composition and superfine grain condition of the surface ferrite to hot metal plating, the average crystal grain size of ferrite on the surface of the base material The maximum intensity of the EPMA line analysis of Si and Cr within 1 μm from the surface of the base material was measured.

  As a result, the condition for being an excellent plated steel sheet is that the average crystal grain size of ferrite on the surface of the base material is 4 μm or less, and the maximum intensity of EPMA line analysis of Si and Cr within 1 μm from the base material surface is It was found that the average strength of the Si and Cr in the base material was 10 times or less as compared with the average intensity of EPMA line analysis. When this comparative value was 10 times or less, it was confirmed that hot-plating could be performed without occurrence of non-plating and that the plating adhesion of the obtained hot-dip plated steel sheet was sufficiently ensured.

  In addition, about the measurement of EPMA (electron probe microanalyser) line, EPMA line segmentation was implemented using JEOL JXA-8100. As measurement conditions, an acceleration voltage of 15.0 KV and an irradiation current of 1.02e-07A were employed.

(d) In order to obtain a hot-rolled steel sheet having an average crystal grain size of ferrite of the surface layer of 3 μm or less and an increase rate of the average crystal grain size of ferrite of the steel sheet surface layer at 800 ° C. of 0.05 μm / second or less When the slab having the chemical composition is hot-rolled, the final rolling pass is completed at a temperature of Ar ₃ points or higher and 780 ° C. or higher, and then cooled to 720 ° C. or lower within 0.4 seconds, and then 600 to 720 It was also found that holding for 2 seconds or more in the temperature range of ° C is effective. In this method, not only the surface layer of the hot-rolled steel sheet, but also the inside of the hot-rolled steel sheet, a fine and thermally stable ferrite structure can be obtained, and it was also found that the mechanical properties of the entire hot-rolled steel sheet are improved. .

According to this hot rolling method, multi-pass hot rolling is started from the austenite temperature range, and the final rolling pass is finished at a high temperature of Ar ₃ points or higher and 780 ° C. or higher. Strain accumulates in the crystal grains. Then, the cooling to a temperature of 720 ° C. or less is completed within 0.4 seconds immediately after the end of hot rolling, but since the release of this strain is suppressed until reaching 720 ° C., the strain is within the austenite grains. Remain in the accumulated state. Only when the temperature reaches 720 ° C. or lower, transformation from austenite to ferrite is activated, and a large number of ferrite crystal grains are generated with the accumulated strain as a nucleus, thereby forming a fine ferrite structure. Furthermore, it hold | maintains for 2 second or more in a 600-720 degreeC temperature range after that. As a result, fine and nearly equiaxed ferrite can be sufficiently generated, and a desired ferrite structure in which the dislocation density in the ferrite is small and grain growth is difficult even at high temperatures can be obtained.

The final rolling pass is completed at a temperature of Ar ₃ points or higher and 780 ° C. or higher, and then the cooling rate during water cooling when cooling to 720 ° C. or lower within 0.4 seconds is 400 ° C./second or higher. It is preferable to do this.

Here, as described above, the cooling condition immediately after the end of the hot rolling needs to complete the cooling to a temperature of 720 ° C. or less within 0.4 seconds. Conventionally, even the earliest one has passed 0.2 seconds or more from the end of rolling to the start of cooling. The cooling rate was at most about 250 ° C./second. Taking a low carbon steel with an Ar ₃ point of 800 ° C. as an example, even if the hot rolling of the low carbon steel is terminated at the Ar ₃ point, while the cooling is performed from 800 ° C. to 720 ° C., Since 0.52 seconds or more had elapsed, it was difficult to complete the cooling to a temperature of 720 ° C. or less within 0.4 seconds.

  (e) The heat obtained in this manner was such that the average crystal grain size of ferrite on the surface layer was 3 μm or less and the rate of increase in the average crystal grain size of ferrite on the steel sheet surface layer at 800 ° C. was 0.05 μm / second or less. After removing the black skin by pickling, the rolled steel sheet is washed with water, and then subjected to heating at 700 ° C. or higher in a reducing atmosphere. A plated steel sheet can be obtained.

  (f) In addition, the most common sheet passing of hot-rolled steel sheets to hot-dip plating lines is a mixed sheet of cold-rolled steel sheets and hot-rolled steel sheets. Is required. When a cold-rolled steel sheet is used as a base material, it is exposed to a reducing atmosphere at 700 ° C. or higher in order to improve the plating wettability on the surface of the cold-rolled steel sheet. In the case of a rolled steel sheet, if it stays in a high temperature region of 700 ° C. or higher, there is a problem of surface concentration of Si on the surface of the steel sheet. However, even though the above hot-rolled steel sheet having a fine grain surface is a high-Si steel sheet or a high-Cr steel sheet, even if it remains in a high temperature region of 700 ° C. or higher in the hot dipping process, the surface concentration of Si or Cr is increased. Is less likely to occur. Therefore, since the hot-rolled steel sheet having the fine grain surface can be mixed with the cold-rolled steel sheet, the operability can be greatly improved.

(h) As described in (d) above, when hot-rolling a slab having a predetermined chemical composition, the final rolling pass is completed at a temperature of Ar ₃ points or higher and 780 ° C. or higher, and then 0.4 seconds. After being cooled to 720 ° C. or less, and kept at a temperature range of 600 to 720 ° C. for 2 seconds or more, not only the surface layer of the hot-rolled steel plate but also the fine and thermally stable ferrite not only in the hot-rolled steel plate Since the structure is obtained, the mechanical properties of the entire hot-rolled steel sheet are also improved.

  As a result of various studies on the ferrite structure inside the hot-rolled steel sheet obtained as described above, the present inventors have excellent mechanical properties of the hot-rolled steel sheet itself because fine-grained ferrite particles are formed. In addition, even after the hot-rolled steel sheet is weld-plated as a base material, the fine-grained ferrite particles are not coarsened, although the fine-grained ferrite particles are exposed to high temperatures in the plating process. As such, it was found that the mechanical properties were maintained.

(i) In order to obtain a plated steel sheet having such mechanical properties and thermal stability, it is necessary to keep the average crystal grain size of ferrite within a certain range, and a temperature in the vicinity of 700 ° C. just below A ₁ point. It is preferable to provide an upper limit for the rate of increase X (μm / min) of the average crystal grain size D (μm) of ferrite and the product D · X (μm ² / min) of the average crystal grain size D (μm). I found out. In order to obtain better thermal stability, it is preferable to keep the ferrite crystal grain size distribution within a certain range or to leave no strain due to rolling in the ferrite crystal grains. I found it.

  In order to evaluate the mechanical properties of the hot-rolled steel sheet as a whole, the average crystal grain diameter D (μm) of ferrite at a depth of 1/4 of the sheet thickness from the surface of the base metal and its increase rate X at 700 ° C. (μm / min) can be used as an index.

(j) About keeping the average crystal grain size of ferrite within a certain range, the strength increases as the crystal grain size of ferrite decreases, but if the crystal grain size becomes too small, the driving force of grain growth by grain boundary energy Therefore, it was found that grain growth at high temperature is promoted. Specifically, when the average crystal grain size is less than 1.2 μm, it becomes difficult to suppress grain growth at high temperatures, and conversely, the average crystal grain size is 2.7 + 5000 / (5 + 350 · C + 40 · It has been found that if the value exceeds either Mn) ² μm or 7 μm, improvement in mechanical properties due to miniaturization cannot be sufficiently expected. Therefore, in order to achieve both mechanical properties and thermal stability, 1.2 μm is adopted as the lower limit of the average grain size of ferrite, and 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² μm as the upper limit. And the smaller value of 7 μm needs to be adopted.

(k) A For the upper limit of the product D · X of the average crystal grain size D and the increase rate X of the average grain size D of ferrite at a temperature in the vicinity of 700 ° C. just below _one point, the grain growth of ferrite grains at high temperatures The speed increases with increasing temperature. In general, a temperature range where problem grain growth of ferrite occurs by welding or hot-dip plating process is the temperature range from just under ₁ point A (730 ° C. vicinity) to the vicinity of A ₃ points, grain growth rate of the ferrite in the temperature range It changes a lot. However, since it was found that the temperature characteristic of the grain growth rate of the steel sheet having the average crystal grain size of ferrite in the range of (a) is determined by the ferrite grain growth rate at a temperature in the vicinity of 700 ° C., 700 ° C. The upper limit is the grain growth rate of ferrite at a nearby temperature, that is, the product D · X (μm ² / min) of the average crystal grain size increase rate X (μm / min) and the average crystal grain size D (μm). It was found that no problem occurs even when heated to a higher temperature in the welding or hot dipping process. As a result of the experiment, it was also found that the product D · X needs to be set to 0.1 μm ² / min or less. Incidentally, the product D · X is preferably from 0.07 .mu.m ² / min, more preferably 0.05 .mu.m ² / min or less.

  (l) From the results of (j) and (k) above, the average crystal grain diameter D (μm) of ferrite at a depth position of ¼ of the plate thickness from the surface of the base material is expressed by the following equations (1) and (2 ) And an increase rate X (μm / min) at 700 ° C. of the average crystal grain size of ferrite at a depth of 1/4 of the plate thickness from the base material surface and the average crystal grain size D (μm). Is preferably a plated steel sheet satisfying the following expression (3).

1.2 ≦ D ≦ 7 (1) Formula D ≦ 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² (2) Formula D · X ≤ 0.1 · · · · · · · · · ······································································· The average crystal grain size (μm) of ferrite at a depth position of ¼ of C, C and Mn, respectively, the content (mass%) of each element in the base material, and X from the surface of the base material The increase rate (micrometer / min) in 700 degreeC of the average crystal grain diameter D (micrometer) of the ferrite in the depth position of 1/4 of base material board thickness is shown.

(m) For keeping the ferrite crystal grain size distribution within a certain range or not leaving any strain due to rolling in the ferrite crystal grains, the ferrite crystal grain size distribution and the ferrite crystal grain Strain is closely related to grain growth at high temperatures. Grain growth at high temperatures is caused by the grain boundary energy and intra-granular distortion as driving forces. Therefore, when relatively large ferrite crystal grains are mixed in a fine ferrite structure, the large ferrite crystal grains are easily integrated with the surrounding fine ferrite crystal grains using the grain boundary as a driving force. Further, if there is strain in the ferrite crystal grains, adjacent ferrite crystal grains are easily integrated with each other using the strain in the grains as a driving force. In this way, grain growth proceeds rapidly. For this reason, in order to prevent rapid progress of grain growth, in addition to the refinement of ferrite crystal grains, the crystal grain size distribution of ferrite is 80% or more in the range of 1/3 to 3 times the average grain size. It is preferable to keep the grains of the particles. Further, the intragranular dislocation density showing the strain in the ferrite crystal grains is preferably 10 ⁹ / cm ² or less, and more preferably 10 ⁸ / cm ² or less.

  (n) From the result of the above (m), the ferrite grain size in which the crystal grain size d (μm) satisfies the following formula (4) at a depth position of ¼ of the base material plate thickness from the base material surface. It is preferable to use a plated steel sheet satisfying that the area ratio of the ferrite in the position is 80% or more.

D / 3 ≦ d ≦ 3D (4) where D is the ferrite at a depth of 1/4 of the base metal plate thickness from the base metal surface. The average crystal grain size (μm) is shown.

  The present invention has been completed on the basis of such knowledge, and the gist thereof is a plated steel sheet shown in the following (1) and a method for producing a plated steel sheet shown in the following (2) and (3). . Hereinafter, the present invention (1) to the present invention (3), respectively. The present invention (1) to the present invention (3) may be collectively referred to as the present invention.

(1) A base material of a steel plate made of carbon steel or low alloy steel containing Si: 0.2 to 1.8% and Cr: 0.01 to 1.2% and containing ferrite as a main phase in mass%. And the average crystal grain size of ferrite on the surface of the base material obtained by observing from the surface using a cutting method using a linear test line in the rolling direction and its vertical direction is 4 μm or less, and the base material A plated steel sheet characterized in that the maximum intensity of EPMA line analysis of Si and Cr within 1 μm from the surface is 10 times or less than the average intensity of EPMA line analysis of Si and Cr in the base material.

(2) By mass%, Si: 0.2-1.8% and Cr: 0.01-1.2%, consisting of carbon steel or low alloy steel with ferrite as the main phase , the average grain size of the ferrite of the first layer is 3μm or less, and an increase rate at 800 ° C. of the average crystal grain size of the ferrite of the first layer of the steel sheet surface is 0.05 .mu.m / sec of the following hot-rolled steel sheet plated steel sheet A method for producing a plated steel sheet used as a base material, which is pickled, heated in a reducing atmosphere at a temperature of 700 ° C. or higher and Ae ₃ points + 50 ° C. or lower, and then in a hot dipping line. A method for producing a plated steel sheet, characterized by performing hot dipping.

(3) The method for producing a plated steel sheet according to (2) above, wherein the final rolling pass is finished at a temperature of Ar ₃ points or higher and 780 ° C. or higher, and then cooled to 720 ° C. or lower within 0.4 seconds. The manufacturing method of the plated steel plate characterized by using the hot-rolled steel plate obtained by hold | maintaining for 2 second or more in the temperature range of 600-720 degreeC as a base material of a plated steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, the plated steel plate which can make the base material the steel plate containing many easily oxidizable elements, such as Si and Cr, and its manufacturing method can be provided.

  Below, the plated steel plate concerning this invention and its manufacturing method are demonstrated. Note that “%” in concentration and the like means weight% unless otherwise specified.

(1) Alloy component of base steel sheet Si:
If the Si content in the base steel sheet is less than 0.2%, the Si amount concentrated on the steel sheet surface during annealing is small, so that the conventional continuous annealing conditions are sufficient even without special pretreatment. Plating adhesion can be obtained, but mechanical properties such as strength and workability are not sufficient. And if content of Si in a base-material steel plate exceeds 1.8%, the fall of plating interface adhesiveness will be recognized by normal plating operation conditions. Therefore, the Si content in the base steel sheet needs to be 0.2 to 1.8%. A preferable range is 0.2 to 1.5%, and a more preferable range is 0.2 to 1.2%.

Cr:
If the Cr content in the base steel sheet is less than 0.5%, the amount of Cr concentrated on the steel sheet surface during annealing is small, so that the conventional continuous annealing conditions are sufficient even without special pretreatment. Plating adhesion can be obtained, but mechanical properties such as strength and workability are not sufficient. And if content of Cr in a base material steel plate exceeds 1.2, the fall of plating interface adhesiveness will be recognized by normal plating operation conditions. Therefore, the Cr content in the base steel sheet needs to be 0.2 to 1.2%. A preferred range is 0.5-1.2%.

  The carbon steel or low alloy steel used in the present invention preferably contains C: 0.01 to 0.25%, and Mn, Al, Ti, Nb, V, Cu, P, Mo, Ni , Ca, REM, or B may be included. About Al concentration in steel, it is known as a retained austenite formation element, and even if it contains to 2.0%, there is no problem in particular.

C:
C is an element useful for promoting the refinement of ferrite crystal grains because it can lower the transformation temperature from austenite to ferrite and lower the finishing temperature of hot rolling. Moreover, it is an element for ensuring strength. For this reason, it is preferable to make it contain 0.01% or more. Moreover, in order to further promote the refinement of ferrite crystal grains, the content is preferably 0.03% or more. However, if it is contained excessively, ferrite transformation after hot rolling is delayed, and the volume fraction of ferrite is lowered. Therefore, the content is preferably 0.25% or less.

Mn:
Mn can be contained for securing the strength. Moreover, since the transformation temperature from austenite to ferrite can be lowered and the finishing temperature in hot rolling can be lowered, it is preferably contained in order to promote refinement of ferrite crystal grains. However, if it is contained excessively, ferrite transformation after hot rolling is delayed and the volume fraction of ferrite is lowered, so the content is preferably made 3% or less. More preferably, it is 2.7% or less. The lower limit may be an impurity level, but when added for the purpose of improving the strength, it is preferable to contain 0.5% or more. Moreover, when producing | generating a retained austenite in a ferrite structure, it is preferable to contain 0.5% or more, and it is more preferable to contain 0.8% or more.

Al:
Al may be added to improve ductility. However, if excessively contained, the austenite at high temperature becomes unstable, and it is necessary to excessively increase the finishing temperature in hot rolling, and it becomes difficult to achieve stable continuous casting. The following is preferable. The lower limit may be an impurity level.

Ti:
Ti precipitates as carbide or nitride to increase the strength, and this precipitate suppresses the coarsening of austenite and ferrite to promote the refinement of crystal grains during hot rolling, and during the heat treatment In order to suppress grain growth, it may be added. However, if excessively contained, a large amount of coarse Ti carbide or nitride is generated at the time of heating before hot rolling to inhibit ductility and workability, so the content is preferably 0.3% or less. . In order to facilitate the formation of ferrite, the total amount of Ti + Nb is preferably 0.1% or less, more preferably 0.03% or less, and even more preferably 0.01% or less. The lower limit may be an impurity level.

Nb:
Nb precipitates as carbide or nitride to increase the strength, and this precipitate suppresses the coarsening of austenite and ferrite to promote the refinement of crystal grains during hot rolling, and during the heat treatment In order to suppress grain growth, it may be added. However, if excessively contained, a large amount of coarse NbC is generated at the time of heating before hot rolling to inhibit ductility and workability, so the content is preferably 0.1% or less. In order to facilitate the formation of ferrite, the total amount of Ti + Nb is preferably 0.1% or less, more preferably 0.03% or less, and still more preferably 0.01%. The lower limit may be an impurity level.

V:
V precipitates as a carbide to increase the strength, and this precipitate may be added to suppress the coarsening of ferrite and promote the refinement of crystal grains. However, for the same reason as Ti and Nb, the ductility and workability are inhibited, so the content is preferably 1% or less. More preferably, it is 0.5% or less, More preferably, it is 0.3% or less. The lower limit may be an impurity level.

Cu:
Since Cu has an action of precipitating at a low temperature to increase the strength, Cu may be added for the purpose of these actions. However, since there is a risk of causing grain boundary cracking of the slab, the content is preferably 3% or less. More preferably, it is 2% or less. In addition, when adding, it is preferable to make it content 0.1% or more. The lower limit may be an impurity level.

P:
P may be added to increase the strength. However, if excessively contained, embrittlement due to grain boundary segregation occurs. Therefore, when added, the content is preferably 0.5% or less. More preferably, it is 0.2% or less, More preferably, it is 0.1% or less. The lower limit may be an impurity level.

Mo:
Mo may be added to increase the strength by precipitating MoC, and because this precipitate suppresses the coarsening of ferrite and promotes the refinement of crystal grains. However, for the same reason as Ti and Nb, the ductility and workability are inhibited, so the content is preferably 1% or less. More preferably, it is 0.5% or less, More preferably, it is 0.3% or less. The lower limit may be an impurity level.

Ni:
Ni may be added for the purpose of increasing the stability of austenite at high temperatures. Further, when Cu is contained, it may be added in order to prevent grain boundary embrittlement of the slab. However, since the production | generation of a ferrite will be suppressed when it contains excessively, it is preferable to make content 1% or less. The lower limit may be an impurity level.

Ca, REM, B:
Ca, rare earth elements (REM), and B may be added alone or in combination to refine oxides and nitrides precipitated during solidification and maintain the soundness of the slab. However, since it is expensive, the total content is preferably 0.005% or less. The lower limit may be an impurity level. Here, the rare earth element (REM) means 17 elements including 15 elements of lanthanoid and Y and Sc.

  In addition, S, N, Sn etc. are mentioned as an "impurity" mixed in steel. About S and N, if possible, it is desirable to regulate the content thereof as follows.

S:
Since S is an impurity element that forms sulfide inclusions and degrades workability, the content is preferably suppressed to 0.05% or less. And when securing the further outstanding workability, it is preferable to set it as 0.008% or less. More preferably, it is 0.003% or less.

N:
N is an impurity element that lowers workability, and its content is preferably suppressed to 0.01% or less. More preferably, it is 0.006% or less.

(2) Slab heating and hot rolling conditions At the final hot rolling stage of the slab, Ar ends at a temperature of ₃ points or higher and 780 ° C or higher, and then at a cooling rate of 400 ° C / second or higher, within 720 ° C within 0.4 seconds. And is kept at a temperature of 600 to 720 ° C. for 2 seconds or longer.

(2-1) About rolling Rolling is performed in the austenite temperature range from a temperature exceeding 1000 ° C. using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages.

Use slabs obtained by continuous casting or casting / bundling, steel plates obtained by strip casting, etc., or once subjected to hot or cold processing as necessary. Reheat to higher temperature and roll. When the rolling start temperature is 1000 ° C. or less, the rolling load becomes excessive and it becomes difficult to obtain a sufficient rolling rate, and rolling at a sufficient rolling rate may be terminated at a temperature of ₃ or more points at Ar. This makes it difficult to obtain desired mechanical properties and thermal stability. Rolling is preferably started at a temperature of 1025 ° C. or higher, more preferably 1050 ° C. or higher. The upper limit is set to 1350 ° C. or lower, preferably 1250 ° C. or lower in order to suppress coarsening of austenite grains and to suppress equipment costs and heating fuel costs. In the case of a steel type in which it is not necessary to sufficiently dissolve precipitates such as TiC and NbC in austenite, it is preferable to reheat to a relatively low temperature (1050 to 1150 ° C.) within this range. This is because the initial austenite crystal grains are refined and the final ferrite crystal grains are easily refined.

The rolling finishing temperature is set to a temperature range of Ar ₃ points or more and 780 ° C. or more in order to transform from austenite to ferrite after rolling. When the finishing temperature is lower than Ar ₃ point, ferrite is generated during rolling. If the temperature is lower than 780 ° C., the rolling load increases and it becomes difficult to apply sufficient reduction, and ferrite transformation may occur in the surface layer portion during rolling. Preferably, the rolling is finished at a temperature of Ar ₃ points or higher and 800 ° C. or higher.

The temperature to terminate the rolling, the better low if the temperature range of more than Ar ₃ point and 780 ° C.. This is because the effect of accumulating processing strain introduced into austenite by rolling increases, and the refinement of crystal grains is promoted. The Ar ₃ point of the steel type used in the present invention is approximately 780 to 900 ° C.

  The total reduction amount is 90% or more, preferably 92%, more preferably 94% or more in terms of sheet thickness reduction rate in order to promote the refinement of ferrite. The sheet thickness reduction rate in the temperature range from the rolling end temperature to [rolling end temperature + 100 ° C.] is preferably 40% or more. More preferably, the sheet thickness reduction rate in the temperature range from the rolling end temperature to [rolling end temperature + 80 ° C.] is 60% or more. The rolling is continuous multi-pass rolling.

  The amount of reduction per pass is preferably 15 to 60%. A larger rolling reduction per pass is preferable from the viewpoint of accumulating strain into austenite and refining the crystal grain size of ferrite produced by transformation, but it requires an increase in rolling load. Not only increases in size but also makes it difficult to control the shape of the plate. In the method of the present invention, fine ferrite crystal grains can be obtained even by rolling in a plurality of passes with a reduction amount per pass of 40% or less. Therefore, in particular, when it is desired to easily control the plate shape, it is preferable that the rolling reduction rate of the final two passes is 40% / pass or less.

(2-2) Cooling after rolling After rolling is completed, the processing strain introduced into austenite is transformed to austenite to ferrite as a driving force without releasing the working strain, and a fine ferrite grain structure is generated. Therefore, it cools to the temperature of 720 degrees C or less within 0.4 second after completion | finish of rolling. Preferably, it is cooled to a temperature of 720 ° C. or less within 0.2 seconds from the end of rolling. It is desirable to use water cooling for cooling, and the cooling rate is preferably 400 ° C./second or more as an average cooling rate during the period of forced cooling excluding the air cooling period.

  Here, the reason for prescribing the time until cooling to a temperature of 720 ° C. or lower was introduced by processing before fine ferrite was formed when cooling was stopped or slowed at a temperature exceeding 720 ° C. This is because the strain is released or the existence form of the strain is changed, so that it becomes ineffective for nucleation of ferrite and the ferrite crystal grains are remarkably coarsened.

  When the temperature reaches 720 ° C. or lower, it enters a transformation temperature range in which ferrite transformation is activated. The ferrite transformation temperature range where the above ferrite structure is obtained is a temperature range between this temperature and 600 ° C. Therefore, after reaching 720 ° C. or lower, the cooling is temporarily stopped, or the speed thereof is slowed down and held at this temperature range for 2 seconds or more, so that the formation of the above thermally stable ferrite crystal grain structure is ensured. Can be. If the holding time in this temperature range is short, the formation of the thermally stable ferrite crystal grain structure may be hindered. More preferably, it is good to make it stay for 3 seconds or more in the temperature range of 620-700 degreeC.

  In the case of a multi-phase structure steel having a fine ferrite crystal grain structure as a main phase and martensite of 5% or more in volume ratio dispersed therein, after cooling and holding as described above, the temperature reaches 350 ° C. or less. It is preferable to cool. It is more preferable to cool to a temperature of 250 ° C. or lower at a cooling rate of 40 ° C./s or higher. In addition, when cooling to a temperature of 350 ° C. or less is performed at a cooling rate of 20 ° C./s or less, bainite is likely to be generated, and martensite formation may be hindered.

  On the other hand, in the case of a dual phase structure steel mainly composed of a fine ferrite crystal grain structure and 3 to 30% of retained austenite is dispersed in volume ratio, after cooling as described above, the cooling rate is set to 350 ° C. at a cooling rate of 20 ° C./s or more. It is preferable to cool to ˜500 ° C. and then gradually cool at a cooling rate of 60 ° C./h or less. The cooling rate from 400 to 500 ° C. is more preferably 50 ° C./s or more.

(2-3) About cooling equipment In this invention, the equipment which performs said cooling is not limited. Industrially, it is preferable to use a water spray device having a high water density. For example, a water spray header can be arrange | positioned between rolling plate conveyance rollers, and it can cool by injecting high-pressure water with sufficient water quantity density from the upper and lower sides of a plate.

(3) Average crystal grain size and grain growth rate of hot-rolled surface layer part In the plating process, in order to reduce Fe oxide on the surface of the steel sheet, the steel sheet is heated and annealed in a reducing atmosphere before plating and then plated. In order to suppress the surface concentration of the easily oxidizable element at this time, the average crystal grain size of ferrite on the surface layer of the hot-rolled steel sheet used in the plating process is 3 μm or less, and the rate of increase of the crystal grain size at 800 ° C. is 0.00. It is necessary to set it to 05 μm / second or less. The average crystal grain size of ferrite on the surface layer of the hot-rolled steel sheet is preferably 2.5 μm or less. And the increase rate in 800 degreeC of the average crystal grain diameter of the ferrite in the surface layer of a hot-rolled steel plate becomes like this. Preferably it is 0.03 micrometer / second or less, More preferably, it is 0.015 micrometer / second or less.

  Here, the ferrite in the surface layer portion of the hot-rolled steel sheet is a ferrite close to an equiaxed crystal existing in the first layer on the surface of the steel sheet. If this range is exceeded, interfacial adhesion decreases. The average crystal grain size of ferrite on the surface layer of the base metal of the plated steel sheet after the plating process is 4 μm or less, and the maximum strength of the EPMA line analysis of Si and Cr within 1 μm from the surface of the base material indicates that the Si and Cr in the base material It is necessary to be 10 times or less of the average intensity of EPMA line analysis. If a hot rolled steel sheet with an average grain size of ferrite of the above surface layer of 3 μm or less and an increase rate of the crystal grain size at 800 ° C. of 0.05 μm / second or less is used, under normal hot dipping conditions, The ferrite grain size of the steel sheet surface is in this range.

(4) Hot dipping temperature The lower limit of the reduction heat treatment temperature of hot dipping may be a temperature at which the surface of the hot-rolled steel sheet can be reduced, that is, about 600 ° C. Since it is necessary, the recrystallization temperature of the cold-rolled full hard material, that is, 700 ° C. or higher is preferable. In order to control the second phase of the steel sheet, it may be heated to a ferrite / austenite two-phase coexistence temperature of Ac ₁ point (approximately 720 ° C.) or higher. Heat having an average crystal grain size of the above-mentioned ferrite of the surface layer of 3 μm or less and an increase rate of the crystal grain size at 800 ° C. of 0.05 (preferably 0.03, more preferably 0.015) μm / sec or less. The ferrite grain size of the rolled steel sheet is sufficiently stable even at such temperatures. The upper limit of the temperature is preferably Ae ₃ points + 50 ° C., more preferably Ae ₃ points + 30 ° C. When this temperature is exceeded, the steel sheet structure is once transformed into an austenite single phase, and the structure becomes coarse.

(5) Plating type The fine-grain hot-rolled steel sheet having the above-described structure and its thermal stability is coated on the steel sheet surface with Zn, Zn-Al alloy, Al-Si alloy alloy, etc. using a hot dipping line. It is possible.

  As a composition of the plating bath of Zn and Zn—Al alloy, a Zn— (0.1-60)% Al bath, a plating bath to which Si and / or Mg are added in combination, and the like can be used. Moreover, as a composition of the plating bath of the Al—Si alloy, an Al— (7-13)% Si bath or the like can be used.

The plating bath may contain 0.1% or less of Fe, V, Mn, Ti, Nb, Ca, Cr, Ni, W, Cu, Pb, Sn, Cd, Sb, Si, and Mg. There is no particular problem. After the plating, the composition of the film on the surface of the cooled steel sheet is generally a composition having a slightly higher Fe concentration than the plating bath composition because element mutual diffusion occurs between the steel material and the molten metal during immersion and cooling. The amount of plating adhesion is not particularly limited, but is preferably 30 to 200 g / m ² per side.
(6) Manufacturing method of hot dip galvanized steel sheet In order to manufacture the hot dip galvanized steel sheet according to the present invention, basically, it may be performed according to the manufacturing method of hot dip galvanized steel sheet. An example of a method for manufacturing a hot-dip galvanized steel sheet will be described below with reference to the continuous hot-dip galvanizing equipment shown in FIG.

In FIG. 1, the continuous hot dip galvanizing equipment 100 is composed of an entrance side portion 10, a processing portion 20, and an exit side portion 30. The entrance side portion 10 includes a payoff reel 11 for rewinding a coiled steel plate, a shearing device 12, a welding device 13, and the like. The processing unit 20 includes a continuous heating furnace 21, a galvanizing bath 22, an air wiper 23, an air cooling zone 24, and the like. The continuous heating furnace 21 includes a non-oxidation furnace 21a and a reduction annealing furnace 21b, and N ₂ gas and H ₂ gas are adjusted to a predetermined concentration in the reduction annealing furnace 21b to constitute an atmosphere in the furnace. Yes. A skin pass mill 31, a tension leveler 32, a chromate treatment device 33, a shearing device 34, an electrostatic oiling machine 35, and a carousel reel 36 for finally winding a steel plate are arranged on the outlet side 30. ing.

  The base material unwound from the payoff reel 11 is passed through an alkali cleaning device in order to remove rolling oil and Fe powder according to a normal method, and then cooled to near the plating bath temperature after annealing in a continuous heating furnace 21, for example. Then, it is immersed in the galvanizing bath 22 and pulled up to adjust the zinc adhesion amount by the air wiper 23. When the base material is a cold-rolled base material and requires recrystallization annealing, it is heated in a reducing atmosphere of at least 700 ° C. and then cooled to near the plating bath temperature and then immersed in the galvanizing bath 22.

  If the plating bath temperature is excessively high, the alloy layer develops excessively during immersion in the Al alloy and zinc alloy plating bath 22. Conversely, if it is too low, it will be difficult to adjust the amount of plating. For this reason, the temperature of the zinc alloy plating bath 22 is preferably set 30 to 60 ° C. higher than its melting point.

The base material immersed in the hot dipping bath 22 is pulled up from the plating bath 22 and the amount of plating adhered is adjusted by a normal gas drawing method using an air wiper 23. However, in order to suppress surface unevenness such as ripples, it is non-oxidizing. In some cases, a wiping process may be used with this gas. The gas species may be any of N ₂ , Ar, He, etc., and there is no problem if the purity is 97% or more.

  Rust preventive oil is applied to the surface of the product after plating by an electrostatic oil coater 35, but chromic acid treatment or the like may be performed by a chromate treatment device 33 as necessary. Further, instead of the chromate treatment apparatus, a phosphate treatment apparatus, a resin film coating apparatus, or the like may be installed to perform post-treatment such as phosphate treatment or resin film coating. After these treatments, the steel plate is wound by a carousel reel 36 for a predetermined length and sent to the next process or shipped.

  A 50 mm-thick slab having the chemical composition shown in Table 1 is hot-rolled at a total reduction rate of 96% in six consecutive passes under the rolling conditions shown in Table 2, and then cooled under the cooling conditions shown in the same table. Thus, a steel plate having a thickness of 2.3 mm was obtained. About these steel plates, the surface was corroded with nital after polishing, the structure was observed using SEM, and the ferrite particle size was measured. For thermal stability, immerse in a salt bath at 800 ° C., hold for 1 minute after heating, then rapidly cool to room temperature, and calculate the average crystal grain size by the same method as described above to obtain the rate of grain size increase. It was. The results are shown in Table 2.

  Test No. 2 shown in Table 2 is particularly fine because the final hot rolling pass is under a large pressure of 50%. Test Nos. 7 to 8 are low-temperature rolling and the rolling rate of the final pass is 65%, which is outside the present invention. Therefore, the grain growth rate at 800 ° C. is excessive. After rolling, pickling treatment was performed to remove the scale. This specimen was cut into a size of 80 × 200 mm, and plated using a vertical hot-dip Zn plating apparatus under the following conditions.

First, a steel plate having a plate thickness of 2.3 mm was degreased and washed with a NaOH solution at 75 ° C. and annealed at 750, 800, or 820 ° C. for 60 seconds in an atmosphere of N ₂ + 10% H ₂ and a dew point of −40 ° C. After annealing, the steel sheet was cooled to near the bath temperature, immersed in various plating baths for 3 seconds, and then the amount of coating on one side of the plating was adjusted to 50 g / m ² by a wiping method. The cooling rate was adjusted by changing the air volume and mist volume.

  The property of the plating film of the obtained plated steel sheet was investigated by the following method. The results are summarized in Table 3.

  The average crystal grain size of the surface layer of the base material is obtained by removing the film by the above-described method, and then clarifying the grain boundary by etching with nital, and the grain boundary crossing the length of 50 μm by the SEM observation from the surface is defined as the rolling direction. In the vertical direction, N = 3 was counted, and the crystal grain size was measured from the average value.

The plating wettability was evaluated by converting the number of pinholes per 1 m ² . The evaluation criteria were as follows.
○: No pinhole
Δ: 1-20 pieces / m ² ,
X: 21 pieces / m ² or more or hardly wet.

For interfacial adhesion, a 2T bending test method by peeling the gum tape outside the bent part at a test temperature of 23 ° C. was adopted. The evaluation criteria are as follows.
○: No peeling of the plating layer,
Δ: Partial peeling of the plating layer,
X: Plating layer is completely peeled off.

  The analysis method of the easily oxidizable element concentrated at the interface between the plating layer and the steel sheet was prepared by preparing a sample for observing a cross section of the plated steel sheet, performing mirror polishing carbon deposition, and performing EPMA line analysis using JXA-8100 manufactured by JEOL. . As the measurement conditions, an acceleration voltage of 15.0 KV and an irradiation current of 1.02e-07A were adopted, and the maximum strength and average strength of the base material concentrations of Si and Cr within 1 μm from the base material surface (5 μm from the base material surface). The above was compared, and the magnification was calculated.

  ADVANTAGE OF THE INVENTION According to this invention, the plated steel plate which can make the base material the steel plate containing many easily oxidizable elements, such as Si and Cr, and its manufacturing method can be provided.

It is an example of a continuous hot dip galvanizing facility.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Entering side part 11 Payoff reel 12 Shearing device 13 Welding device 20 Processing part 21 Continuous heating furnace 21a Non-oxidizing furnace 21b Reduction annealing furnace 22 Zinc plating bath 23 Air wiper 24 Air cooling zone 30 Outlet part 31 Skin pass mill 32 Tension leveler 33 Chromate Processing equipment 34 Shearing equipment 35 Electrostatic oiling machine 36 Carousel reel 100 Continuous hot dip galvanizing equipment

Claims (3)

Plating containing, as a base material, a steel plate made of carbon steel or low alloy steel containing Si: 0.2 to 1.8% and Cr: 0.01 to 1.2% and containing ferrite as a main phase. It is a steel plate, and the average crystal grain size of ferrite on the base material surface obtained by observing from the surface and using a cutting method with a linear test line in the rolling direction and its vertical direction is 4 μm or less, and 1 μm from the base material surface. The maximum strength of EPMA line analysis of Si and Cr within 10 mm or less is 10 times or less than the average intensity of EPMA line analysis of Si and Cr in the base material. By mass% Si: 0.2 to 1.8% and Cr: it contains 0.01 to 1.2%, made of ferrite from carbon steel or low alloy steel as a main phase, a first layer of the steel sheet surface A hot-rolled steel sheet having an average crystal grain size of 3 μm or less and an average crystal grain size of ferrite of the first layer on the steel sheet surface at 800 ° C. of 0.05 μm / second or less is used as a base material for the plated steel sheet. A method for producing a plated steel sheet to be used, wherein after pickling this base material, heating at a temperature of 700 ° C. or higher and Ae ₃ points + 50 ° C. or lower in a reducing atmosphere, followed by hot dipping in a hot dipping line The manufacturing method of the plated steel plate characterized by performing. After A method of manufacturing a plated steel sheet according to claim 2, the final rolling pass end with Ar ₃ point or higher and 780 ° C. or higher, and then cooled to within 0.4 seconds 720 ° C. or less, 600 A method for producing a plated steel sheet, comprising using a hot-rolled steel sheet obtained by holding for 2 seconds or more in a temperature range of ˜720 ° C. as a base material of the plated steel sheet.

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