JP2018165398A - Methods for manufacturing galvanized steel sheet and galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide methods for manufacturing galvanized steel sheets and galvannealed steel sheets, capable of making, during heating for annealing, rates of temperature rise uniform at an end and the center of an annealing material in sheet width direction.SOLUTION: A steel material containing Si 0.8-2.7 mass% is hot-rolled and wound up at 600°C or more, and then acid-cleaned, to obtain an annealing material. First, an end of the annealing material in width direction is oxidized, and then the full width of it is oxidized, to form an oxide film. The oxide film is reduced for galvanization. Optionally the plating layer may be galvannealed.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for producing a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet.

  As a high-strength steel sheet having corrosion resistance and excellent formability, a plated steel sheet obtained by subjecting a cold rolled material of Si-added steel to hot dip galvanizing or galvannealing is used. For example, patent document 1 is disclosing that this plated steel plate is obtained with the following manufacturing method.

First, a steel material containing 0.5 to 2.5 mass% of Si is hot-rolled and wound at a temperature of 600 ° C. or higher, and then pickled and cold-rolled. Then, the obtained cold-rolled material (ie, annealed material) is oxidized at an air-fuel ratio of 0.9 to 1.4 and reduced at an annealing temperature in the range of Ac ₃ point to Ac ₃ point + 100 ° C. Apply reduction annealing. Thereafter, the annealed material is immersed in a hot dip galvanizing bath to form a plating layer, and the plating layer is alloyed as necessary. Thereby, the said plated steel plate is obtained.

JP 2015-34334 A

  In the manufacturing method disclosed in Patent Document 1, the annealing temperature, which is the target temperature, is such that the temperature rising rate at the end in the plate width direction of the annealing material is smaller than the temperature rising rate at the center in the plate width direction of the annealing material. In some cases, the time required to reach the end becomes longer at the end in the plate width direction than at the center in the plate width direction. This phenomenon is not limited to the manufacturing method disclosed in Patent Document 1, and a hot rolled material obtained by hot rolling a steel material containing 0.8 mass% or more of Si and winding it at a temperature of 600 ° C. or higher is used. Occurs when oxidation-reduction annealing is performed. When this phenomenon occurs, there arises a problem of uneven annealing temperature in the plate width direction, which is not preferable in the production of a plated steel plate.

  The present invention has been made in view of the above circumstances, and a manufacturing method and alloying of a hot-dip galvanized steel sheet in which the rate of temperature increase at the end part in the sheet width direction and the center part in the sheet width direction of the annealing material are approximately the same during annealing heating. It aims at providing the manufacturing method of a hot-dip galvanized steel plate.

  In one aspect of the present invention, a hot-rolled material is obtained by hot rolling a steel material containing 0.8 to 2.7% by mass of Si and winding at 600 ° C. or higher, and pickling the hot-rolled material. To obtain an annealed material, oxidize the width end of the annealed material, then oxidize the entire width of the annealed material to form an oxide film, reduce the oxide film, and then hot dip galvanize It is the manufacturing method of the hot dip galvanized steel plate which obtains the hot dip galvanized steel plate by performing.

  Another aspect of the present invention is a method for producing an alloyed hot-dip galvanized steel sheet, in which an alloyed hot-dip galvanized steel sheet is obtained by alloying the plating layer of the hot-dip galvanized steel sheet obtained by the above production method. .

  According to the present invention, in the production of hot dip galvanized steel sheet and the production of alloyed hot dip galvanized steel sheet, the rate of temperature rise at the end in the width direction of the annealing material and the central portion in the width direction of the annealing material should be approximately the same during annealing heating. Can do. As a result, the non-plating at the time of hot-dip galvanized layer formation can be suppressed.

It is a cross-sectional schematic diagram showing the distribution state of solute Si in the plate | board thickness direction of the hot-rolled material of Si addition steel. (A) represents the case where the winding temperature immediately after hot rolling is low, and (b) represents the case where the winding temperature immediately after hot rolling is high. It is a cross-sectional schematic diagram of the oxidation process, the reduction | restoration process, and the hot dipping process of the annealing raw material of Si addition steel wound up at high temperature immediately after hot rolling. It is a schematic diagram of the continuous hot dip galvanizing line used in the embodiment of the present invention. It is a graph of the test piece temperature with respect to the heating time in Example 1 of this invention. It is a graph of the test piece temperature with respect to the heating time in Example 2 of this invention.

  First, an overview of how the present invention has been achieved will be described.

  When manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using Si-added steel, oxidation-reduction annealing is performed to obtain a plating material. Oxidation-reduction annealing is an oxidation process in which an annealed material is heated in an oxidizing atmosphere to form an oxidized Fe film on the surface of the annealed material, and an annealed material on which the oxidized Fe film is formed is heated in a reducing atmosphere to produce oxidized Fe It is heat processing which has the reduction process which reduces a membrane | film | coat, and forms a reduced Fe layer, and the cooling process which obtains an annealing material by cooling the annealing raw material in which the reduced Fe layer was formed.

  When oxidation-reduction annealing is performed on the annealing material of the Si-added steel, Si inside the annealing material is prevented from being selectively oxidized on the surface of the annealing material by the barrier action of the oxidized Fe film in the reduction process. Therefore, when the hot-dip galvanized layer is formed by immersing the annealed material of Si-added steel subjected to oxidation-reduction annealing in a hot-dip galvanizing bath, the occurrence of non-plating is suppressed.

  Further, in order to soften the hot rolled material and reduce the load on the subsequent cold rolling equipment, the coiling temperature after hot rolling is set to a high temperature (for example, 600 to 700 ° C.). Thereby, an oxide of Si is formed at the crystal grain boundary in the surface layer portion of the annealed material. The reason will be described with reference to FIGS.

  FIG. 1 is a schematic cross-sectional view showing a distribution state of solute Si in the thickness direction of a hot-rolled material of Si-added steel. In FIG. 1, (a) represents the case where the winding temperature immediately after hot rolling is low, and (b) represents the case where the winding temperature immediately after hot rolling is high.

  When the coiling temperature is set to a low temperature (for example, 500 ° C. or lower), oxygen in the Fe oxide (so-called hot rolled scale) formed on the steel sheet surface does not diffuse in the cooling process after winding. Therefore, Si oxide is not formed in the surface layer portion, and as shown in FIG. 1A, a hot rolled material in which solute Si is uniformly distributed from the central portion in the plate thickness direction to the surface layer portion is obtained.

  On the other hand, when the coiling temperature is set to a high temperature (for example, 600 to 700 ° C.), oxygen in the Fe oxide (so-called hot rolled scale) formed on the steel sheet surface diffuses during the cooling process after coiling. To do. Thereby, Si oxide is formed in the surface layer portion, and grain boundary oxidation proceeds on the inner side in the thickness direction. As a result, in the hot-rolled material cooled to room temperature, as shown in FIG. 1B, solute Si is relatively less in the surface layer portion than in the central portion in the plate thickness direction. This is used as an annealing material.

  FIG. 2 is a schematic cross-sectional view of an oxidation process, a reduction process, and a hot dipping process of an annealed material of Si-added steel wound at a high temperature after hot rolling. In FIG. 2, the symbol Si schematically represents the distribution state of solute Si. As shown in the annealed material in FIG. 2, almost no solid solution Si is distributed in the surface layer portion.

  When oxidation-reduction annealing is performed on the annealed material, as shown in the oxidation step of FIG. 2, first, Fe in the surface layer portion is oxidized to form a sufficient oxidized Fe film. Next, as shown in the reduction step of FIG. 2, the oxidized Fe film is reduced from the surface layer side to form a reduced Fe layer. At that time, as described above, the selective oxidation of Si is suppressed by the barrier action of the oxidized Fe film. In the reduction step, the reduction reaction continues until the Fe oxide film is completely reduced, and an annealed material is obtained.

  When this annealed material is immersed in a hot dip galvanizing bath, a hot dip galvanized steel sheet in which a plating layer is uniformly formed is obtained as shown in the hot dip galvanizing process of FIG. This is because no Si oxide is formed on the surface of the annealed material. Moreover, when the plating layer of this hot dip galvanized steel sheet is alloyed, an alloyed hot dip galvanized steel sheet in which the alloy layer is uniformly formed is obtained.

  However, when the present inventors performed pickling and cold rolling on the hot-rolled material obtained by winding the Si-added steel at a high temperature immediately after hot rolling, and then subjected to oxidation-reduction annealing, it was partially unsatisfactory. Uneven plating and alloying occurred. As a result of examining the cause, the time taken to reach the annealing temperature from room temperature is longer at the end in the plate width direction than at the center in the plate width direction. It was found that the film was not sufficiently formed. Such a phenomenon was assumed to have occurred for the following reasons.

  That is, the hot-rolled coil obtained by winding the Si-added steel at a high temperature immediately after hot rolling is cooled in an air atmosphere. At this time, since the central portion in the plate width direction of the hot-rolled coil is gradually cooled, the Si oxide grows deep along the grain boundary in the plate thickness direction. On the other hand, the plate width direction end portion of the hot-rolled coil is cooled relatively quickly, so that the growth along the grain boundary of the Si oxide stays at a shallow portion on the surface layer side in the plate thickness direction. As a result, a difference occurs in the degree of Si grain boundary oxidation between the central portion in the plate width direction and the end portion in the plate width direction. And it was speculated that there was a difference in the heating rate of the steel sheet according to the variation in grain boundary oxidation of Si.

  The inventors diligently studied that the temperature rising rate at the end portion in the plate width direction is approximately the same as the temperature rising rate at the center portion in the plate width direction during annealing heating. Then, paying attention to the fact that the emissivity of the plate width direction end is smaller than the emissivity of the plate width direction center, so that the emissivity of the plate width direction end approaches the emissivity of the plate width direction center, The plate width direction end was first oxidized, and then oxidized over the entire plate width direction. As a result, it has been found that the temperature rising rate at the end in the plate width direction is substantially the same as the temperature rising rate at the center in the plate width direction, and the present invention has been completed.

  In this specification, the rate of temperature increase refers to a value obtained by dividing the temperature difference between the heating target temperature and the heating start temperature by the time from the heating start time to the time when the heating target temperature is reached. For example, when it takes 40 seconds to heat from a room temperature of 20 ° C. to a target temperature of 900 ° C., the rate of temperature increase is 22 ° C./second.

  Next, the manufacturing method of the hot dip galvanized steel sheet which is one aspect of the present invention will be described.

  In the manufacturing method of the hot dip galvanized steel sheet according to the present invention, a hot rolled material is obtained by hot rolling a steel material containing 0.8 to 2.7% by mass of Si and winding at 600 ° C. or higher. Annealing material is obtained by pickling the rolled material, oxidizing the width end of the annealing material, then oxidizing the entire width of the annealing material to form an oxide film, reducing the oxide film, A hot dip galvanized steel sheet is obtained by hot dip galvanizing.

  Hereinafter, the reason for this definition will be described.

1. Steel material The steel material used in the method for producing a hot-dip galvanized steel sheet according to the present invention refers to a steel piece to be subjected to hot rolling. For example, the steel slab obtained by casting the molten steel melted with the converter can be mentioned as a steel raw material. This steel material contains Fe as a main component and 0.8 to 2.7% by mass of Si. This steel material also contains inevitable impurities. The reason why this steel material contains 0.8 to 2.7% by mass of Si will be described below.

  Si is an element that has a large solid solution strengthening ability and increases the strength without reducing the ductility of the steel sheet. From the viewpoint of ensuring mechanical properties, the Si content is 0.8 mass% or more, preferably 0.9 mass% or more, more preferably 1.0 mass% or more. However, when the Si content is excessive, the strength becomes too high and the rolling load increases. And Si scale is formed in the case of hot rolling, and the surface property of a hot-rolled material is deteriorated. Therefore, the Si content is 2.7% by mass or less, preferably 2.5% by mass or less, and more preferably 2.3% by mass or less.

  From the viewpoint of ensuring excellent mechanical properties, the steel material may have the following contents of C and Mn.

C: 0.05 mass%-0.5 mass%
C is an element that increases the strength of the steel sheet. When used as a high-strength steel plate, the C content is preferably 0.05% by mass or more. On the other hand, when the C content is excessive, the weldability is lowered. Therefore, the C content is preferably 0.5% by mass or less.

Mn: 1.6% by mass to 4.0% by mass
Mn is an element that increases the strength of the steel sheet and promotes the formation of retained austenite to improve workability. When used as a high-strength steel plate, the Mn content is preferably 1.6% by mass or more. On the other hand, when the Mn content is excessive, ductility and weldability deteriorate. Therefore, the Mn content is preferably 4.0% by mass or less.

  Moreover, the said steel raw material may further contain the following 1 or more types of element, for example.

Al: more than 0% by mass and 0.5% by mass or less Al is an element that acts as a deoxidizer when melting a steel material. When deoxidizing with Al, the Al content is preferably 0.010% by mass or more in order to effectively exhibit the effect. On the other hand, when the Al content is excessive, many inclusions such as alumina are produced in the steel sheet, which may deteriorate the workability. Therefore, the Al content is preferably 0.5% by mass or less.

Ti: more than 0% by mass and 0.2% by mass or less Ti is an element that improves the strength of the steel sheet by forming carbides and nitrides. Further, by forming Ti nitride, it is also an element for reducing the N content in the steel to suppress the formation of B nitride and effectively utilizing the hardenability of solute B. In order to effectively exhibit such an effect, the Ti content is preferably 0.005% by mass or more. On the other hand, when the Ti content is excessive, Ti carbide and Ti nitride are excessive, which deteriorates ductility, stretch flangeability, and stretch workability. Therefore, the Ti content is preferably 0.2% by mass or less.

Nb: more than 0% by mass and 0.1% by mass or less Nb is an element that refines the structure and improves the strength and toughness of the steel sheet. In order to effectively exhibit this effect, the Nb content is preferably 0.005% by mass or more. On the other hand, when the Nb content is excessive, workability is deteriorated. Therefore, the Nb content is preferably 0.1% by mass or less.

V: more than 0% by mass and 0.3% by mass or less V is an element that refines the structure and improves the strength and toughness of the steel sheet. In order to effectively exhibit this effect, the V content is preferably 0.005% by mass or more. On the other hand, if the V content is excessive, the above effect is saturated and only the cost is increased. Therefore, the V content is preferably 0.3% by mass or less.

B: more than 0% by mass and 0.01% by mass or less B is an element that contributes to increasing the strength of the steel sheet by improving the hardenability. In order to effectively exhibit this effect, the B content is preferably 0.0005% by mass or more. On the other hand, if the B content is excessive, the above effect is saturated and only the cost is increased. Therefore, the B content is preferably 0.01% by mass or less.

Mo: more than 0% by mass and 0.5% by mass or less Mo is an element that suppresses ferrite generated during cooling from a high temperature range. In order to effectively exhibit this effect, the Mo content is preferably 0.01% by mass or more. On the other hand, if the Mo content is excessive, the above effect is saturated and only the cost is increased. Therefore, the Mo content is preferably 0.5% by mass or less.

Ca: more than 0% by mass and 0.005% by mass or less Ca is an element effective for improving the stretch flangeability by spheroidizing sulfides in steel. In order to effectively exhibit this effect, the Ca content is preferably 0.0005% by mass or more. On the other hand, when the Ca content is excessive, the above effect is saturated and only the cost is increased. Therefore, the Ca content is preferably 0.005% by mass or less.

Cu: more than 0% by mass and 1.0% by mass or less Cu is an element effective for improving the corrosion resistance of a steel sheet. In order to effectively exhibit this effect, the Cu content is preferably 0.01% by mass or more. On the other hand, if the Cu content is excessive, the above effect is saturated and only the cost is increased. Therefore, the Cu content is preferably 1.0% by mass or less.

Ni: more than 0% by mass and 1.0% by mass or less Ni is an element effective for improving the corrosion resistance of a steel sheet. In order to effectively exhibit this effect, the Ni content is preferably 0.01% by mass or more. On the other hand, when the Ni content is excessive, the above effect is saturated and only the cost is increased. Therefore, the Ni content is preferably 1.0% by mass or less.

Cr: more than 0% by mass and 0.3% by mass or less Cr is an element that suppresses oxidation. When the steel material contains Cr, the amount of Si oxide generated at the grain boundary is reduced, and the amount of solute Si is increased. Both solute Si and Cr act as oxidation-inhibiting elements and prevent rapid progress of oxidation in the oxidation process. In order to exert such effects, the Cr content is preferably 0.1% by mass or more. On the other hand, if the steel material contains excessive Cr, the progress of oxidation is greatly suppressed, resulting in insufficient oxidation. Therefore, the Cr content is preferably 0.3% by mass or less.

  The inevitable impurities refer to elements brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. In addition to P and S shown below, for example, N, O, Pb, Bi, Sb, Sn, etc. Can be mentioned.

  P: more than 0% by mass and 0.03% by mass or less P is an element contained in the steel material as an inevitable impurity. P not only segregates at the grain boundary and promotes grain boundary embrittlement, but also deteriorates hole expansibility. Therefore, the P content is as low as possible, for example, 0.03% by mass or less is preferable.

S: More than 0% by mass and 0.01% by mass or less S, like P, is an element contained in the steel material as an inevitable impurity. S produces inclusions in the steel sheet and degrades workability. Therefore, the S content is as low as possible, and is preferably 0.01% by mass or less, for example.

2. Hot-rolled material In the manufacturing method of the hot dip galvanized steel sheet of the present invention, a hot-rolled material is obtained by hot rolling a steel material satisfying the above chemical composition and winding it at 600 ° C or higher.

The hot rolling method is not particularly limited, and a known method can be adopted. For example, the steel material may be heated to 1100 ° C. or higher, and hot rolling may be completed at a temperature of Ar ₃ points or higher.

  The coiling temperature after completion of hot rolling is set to 600 ° C. or higher. By winding at 600 ° C. or higher, the hot-rolled material becomes relatively soft, and when cold rolling is performed, the load on the cold rolling equipment can be reduced. The upper limit value of the coiling temperature is not particularly limited, but is preferably 750 ° C. or less, for example, due to equipment restrictions.

  The hot rolled material wound up at the above temperature is naturally cooled by a known method. For example, it may be allowed to cool in an air atmosphere. Thereby, oxygen is supplied from the Fe oxide (that is, hot-rolled scale) formed on the surface layer portion of the hot-pressed material to the inner side of the hot-rolled material, and the grain boundary oxidation of Si is performed in the lower layer of the Fe oxide. Progresses.

3. Annealing material In the manufacturing method of the hot dip galvanized steel sheet of the present invention, pickling material is obtained by pickling the hot-rolled material.

  The pickling method is not particularly limited, and a known method can be employed. For example, hydrochloric acid may be used to completely remove the Fe oxide (that is, the hot rolling scale) on the surface layer of the hot rolled material.

  When the hot-dip galvanized steel sheet is a hot-rolled steel sheet, the pickling material is used as an annealing material. On the other hand, when the original sheet of the hot-dip galvanized steel sheet is a cold-rolled steel sheet, a cold-rolled material obtained by subjecting the pickling material to cold rolling is used as an annealing material.

  The cold rolling method is not particularly limited, and a known method can be adopted. For example, the cold rolling rate may be 30 to 80%.

4). Oxidation-reduction annealing In the manufacturing method of the hot-dip galvanized steel sheet of the present invention, an annealing material is obtained by performing oxidation-reduction annealing on the annealing material. More specifically, with respect to the annealing material, a step (oxidation step) of forming an oxide film by oxidizing the width end portion of the annealing material and then oxidizing the entire width of the annealing material, and the oxide film By performing an annealing process including a step (reduction step) of reducing a reduced Fe layer and a step (cooling step) of cooling the annealing material after completion of the reduction reaction to obtain a desired metal structure (melting step). An annealing material (that is, a plating material) to be immersed in the galvanizing bath is obtained.

  The oxidation-reduction annealing is not particularly limited including a continuous type and a batch type as long as one or a plurality of facilities capable of performing the oxidation step, the reduction step and the cooling step are used. However, from the viewpoint of ensuring production efficiency and product quality, for example, it is preferable to use a continuous hot dip galvanizing line (CGL: Continuous Galvanizing Line).

  A schematic diagram of a continuous hot dip galvanizing line is shown in FIG. This line is equipped with pre-tropical zone, oxidation zone, reduction zone, plating bath and alloying furnace in the order of the passing direction of annealing material, oxidation-reduction annealing, hot dip galvanization, and alloying of plating layer as necessary Can be performed continuously.

  Hereinafter, the case where the oxidation-reduction annealing in the manufacturing method of the hot-dip galvanized steel sheet of the present invention is performed using the above-described continuous hot-dip galvanizing line will be described separately for the oxidation step, the reduction step, and the cooling step.

4-1. Oxidation Step In the oxidation step, first, the end portion in the plate width direction (that is, the width end portion) of the annealing material is oxidized under heating to form an oxide on the steel plate surface at the end portion in the plate width direction (hereinafter referred to as pre-processing). Also called oxidation). At this time, it is preferable that the central portion in the plate width direction is not oxidized (that is, only the end portion in the plate width direction is oxidized), but the end portion in the plate width direction is oxidized more than the central portion in the plate width direction. If it is such a configuration, the central part of the plate width may be slightly oxidized. By pre-oxidation, the emissivity at the end portion in the plate width direction of the annealing material being heated is brought closer to the emissivity at the center portion in the plate width direction, and the temperature increase rate at the end portion in the plate width direction is improved. And after forming an oxide in the board width direction edge part of an annealing raw material by pre-oxidation, an oxidation reaction is advanced in the whole board width direction of the said annealing raw material under heating, and an oxide film is formed over the whole width (below) Also referred to as total oxidation). Through the overall oxidation, the thickness of the oxide film formed at various locations in the plate width direction of the annealed material is made uniform.

  The end in the plate width direction refers to a portion where the grain boundary oxidation of Si is hardly formed, and varies depending on the cooling condition of the hot-rolled material, but is located, for example, in a range of up to about 100 mm from each end in the plate width direction. .

  The form of the oxide formed by pre-oxidation is not particularly limited, and the emissivity at the end in the plate width direction is close to the emissivity at the center in the plate width direction, and the temperature rise rate at the end in the plate width direction is improved. If it is. Therefore, the oxide formed by pre-oxidation may be different from the oxide generated in the process of winding and cooling the hot-rolled material. In pre-oxidation, Fe oxides such as wustite, magnetite, and hematite are formed on the steel sheet surface in order to increase the emissivity. In that case, Fe-Si complex oxides, such as a firelite, Si oxide, etc. may be formed simultaneously with Fe oxide.

The pre-oxidation heating atmosphere is not particularly limited as long as it is an oxidation atmosphere in which the Fe oxide is formed on the steel sheet surface. As heating atmosphere of the pre-oxidized, for example, exhaust gas of the burner _{(CO, CO 2, H 2} O, N 2 , etc.) mixed gas and the _{O 2,} and the like 20 _vol% H 2 O-80 vol% _{N 2} it can. Alternatively, it may be an air atmosphere as will be described later.

  On the other hand, the heating atmosphere for the entire oxidation is not particularly limited as long as it is an oxidizing atmosphere in which an oxide film of Fe oxide is formed in the entire region of the annealing material in the plate width direction. For example, the same atmosphere as the pre-oxidation heating atmosphere may be used.

  When the pre-oxidation and the overall oxidation proceed, the atmospheric gas can be convectively circulated to suppress variations in oxide formation in the longitudinal direction of the annealed material.

  In the pre-oxidation, the plate width direction end of the annealed material may be set to 500 ° C. or higher, preferably 550 ° C. or higher, more preferably 600 ° C. or higher. However, if the temperature at the time of pre-oxidation becomes too high, the emissivity becomes high due to peroxidation. More preferably, it is 750 degrees C or less.

On the other hand, in the total oxidation, the entire annealing material may be set to Ac ₁ point or higher, preferably 800 ° C. or higher, more preferably 850 ° C. or higher.

  The pre-oxidation time is a time during which oxidation proceeds until the emissivity at the end in the plate width direction approaches the emissivity at the center in the plate width direction, and is preferably 150 seconds or less, and more preferably 15 to 30 seconds. .

  On the other hand, the total oxidation time is a time until the thickness of the oxide film grows substantially uniformly over the entire plate width direction, and is preferably 120 seconds or less, and more preferably 15 to 30 seconds.

  Pre-oxidation can be performed, for example, in the pre-tropical zone of the continuous hot dip galvanizing line.

  When pre-oxidation is performed in the pre-tropical zone, for example, only the end in the plate width direction may be heated to about 600 ° C. using an edge burner. At this time, the temperature of the central part in the plate width direction not heated by the burner is, for example, about 400 ° C. Due to this temperature difference, the oxidation reaction at the end in the plate width direction preferentially proceeds to form an oxide. The heating atmosphere can be realized by setting the air ratio of the edge burner in the pre-tropical zone to a value larger than 1.0. Pre-oxidation is performed in the pre-tropical zone, and then the plate width direction end portion and the plate width direction center portion are set to approximately the same temperature (for example, a difference of 15% or less in Celsius temperature), and then total oxidation is performed in the oxidation zone. At this time, by setting the air ratio of the burner in the oxidation zone to a value larger than 1.0, the same heating atmosphere as that in the pre-tropical zone can be realized.

  In addition, instead of performing pre-oxidation in the pre-tropical zone, it may be performed on the leading end side of the oxidizing zone, or on both the pre-tropical zone and the leading end side of the oxidizing zone. Alternatively, a separate atmospheric heating oxidation facility should be provided upstream of the pre-tropical zone, and before introducing the annealing material into the continuous hot dip galvanizing line, only an edge heater in the plate width direction should be used in the atmospheric heating oxidation facility. And heated to about 800 ° C. At this time, the pre-oxidation heating atmosphere is an air atmosphere.

  By performing pre-oxidation as described above, the rate of temperature increase during the overall oxidation becomes approximately the same at the end portion and the center portion in the plate width direction, and an approximately uniform oxide film can be formed throughout the plate width direction. It becomes possible.

4-2. Reduction process In the reduction process, the oxide film formed in the entire width of the annealing material in the oxidation process is reduced under heating to form a reduced Fe layer. Thereby, the annealing material in which Si is not exposed to the surface layer can be obtained.

The heating atmosphere in the reduction process is not particularly limited as long as it is a reducing atmosphere in which the reduced Fe layer is formed by reducing the oxide film formed in the oxidation process. For example, a mixed gas containing 15 to 25% by volume of H ₂ gas and the balance being an inert gas such as N ₂ can be used. At that time, the dew point is controlled to -50 to 0 ° C.

In the reduction process, the annealing material may be set to Ac ₁ point or more, preferably Ac ₃ point or more, and may be appropriately set to, for example, a soaking temperature of 850 to 970 ° C. according to the chemical composition of the steel material. At that time, the time for holding at the soaking temperature may be set to 30 to 70 seconds, for example, and the reduction reaction may proceed.

4-3. Cooling step In the cooling step, the annealed material after completion of the reduction reaction is appropriately cooled according to the target cooling pattern. Thereby, a desired metal structure can be obtained.

  The cooling pattern varies depending on the desired metal structure. For example, in the case of obtaining a metal structure mainly composed of tempered martensite and bainite, with the balance being composed of ferrite and retained austenite, a cooling rate of 1 to 50 ° C./s from a soaking temperature to a cooling stop temperature of 460 to 550 ° C. And is kept at the cooling stop temperature for 20 to 100 seconds, followed by plating.

5. Hot dip galvanizing In the manufacturing method of the hot dip galvanized steel sheet of the present invention, a hot dip galvanized steel sheet is obtained by performing hot dip galvanizing on the annealed material.

The hot dip galvanizing method is not particularly limited, and a known method can be adopted. For example, when the Al content in the hot dip galvanizing bath is 0.08 to 0.12% by mass, the bath temperature may be adjusted to 460 to 480 ° C. Moreover, what is necessary is just to let the adhesion amount of a hot dip galvanization layer be a desired adhesion amount, for example, to control it to about 45-65 g / m < ² > using gas wiping.

  Next, the manufacturing method of the galvannealed steel plate which is another one aspect | mode of this invention is demonstrated.

  In the method for producing an alloyed hot-dip galvanized steel sheet according to the present invention, after the hot-dip galvanized steel sheet is obtained according to the method for producing a hot-dip galvanized steel sheet, an alloy is formed by alloying the plating layer of the hot-dip galvanized steel sheet. A galvannealed steel sheet is obtained.

  Hereinafter, alloying of the plating layer will be described.

  The method for alloying the plating layer is not particularly limited, and a known method can be employed. For example, what is necessary is just to control the alloying temperature in an alloying furnace to about 450-600 degreeC. When the alloying temperature is less than 450 ° C., the alloying speed is extremely slow, and productivity may not be ensured. On the other hand, when the alloying temperature exceeds 600 ° C., alloying proceeds too much, and a Γ phase is formed at the interface between the plating layer and the base steel sheet, which may deteriorate the powdering resistance.

  As described above, according to one aspect of the present invention, a hot-rolled material is obtained by hot-rolling a steel material containing 0.8 to 2.7% by mass of Si and winding at 600 ° C. or higher. Annealing material is obtained by pickling the rolled material, the width end portion of the annealing material is oxidized, and then an oxide film is formed by oxidizing the entire width of the annealing material, and the oxide film is reduced, Subsequently, it is a manufacturing method of the hot dip galvanized steel plate which obtains a hot dip galvanized steel plate by performing hot dip galvanization.

  According to this configuration, the width direction end of the annealed material is first oxidized to form an oxide, the emissivity of the width direction end increases, and the emissivity of the width direction center and the width direction end are close. Become. By heating in this state, the temperature in the center in the width direction and the end in the width direction are raised substantially uniformly. As a result, the oxidation reaction proceeds substantially uniformly over the entire width of the annealed material, and the oxide grows substantially uniformly over the entire width direction. Since the oxidized Fe film is substantially uniform over the entire width, the subsequent reduction reaction proceeds substantially uniformly over the entire width. Therefore, the plating quality in the plate width direction is stabilized, and the occurrence of non-plating is prevented.

  In the manufacturing method of the hot dip galvanized steel sheet according to the present invention, the annealed material can be cold-rolled before the width end portion of the annealed material is oxidized. With this configuration, a hot-dip galvanized steel sheet using a cold-rolled steel sheet as the original sheet can be obtained.

  In the manufacturing method of the hot dip galvanized steel sheet according to the present invention, the width end portion of the annealing material can be oxidized using a pre-tropical edge burner. With this configuration, it is possible to perform pre-oxidation in the pre-tropical zone of the continuous hot dip galvanizing line, thereby improving the production efficiency and stabilizing the quality in the plate width direction of the hot dip galvanized steel sheet.

  Another aspect of the present invention is a method for producing an alloyed hot-dip galvanized steel sheet, in which an alloyed hot-dip galvanized steel sheet is obtained by alloying the plating layer of the hot-dip galvanized steel sheet obtained by the above production method. .

  According to this configuration, in the oxidation process of oxidation-reduction annealing, the rate of temperature rise at the end in the plate width direction of the annealed material and the central portion in the plate width direction are approximately the same as in the method for manufacturing the hot dip galvanized steel sheet. As a result, it is possible to obtain an alloyed hot-dip galvanized steel sheet in which the plating quality in the plate width direction is stabilized and the occurrence of non-plating and alloy unevenness is prevented.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention. .

Example 1
There is provided a hot-rolled steel sheet in which no grain boundary oxidation of Si exists at both edge portions from both ends in the plate width direction to 70 mm, and in the center portion, which is a portion excluding the edge portion in the plate width direction, Si grain boundary oxidation exists. As a sample, the following laboratory heating test was conducted to evaluate the rate of temperature rise.

  In addition, this test material contains C: 0.095 mass%, Si: 1.8 mass%, and Mn: 2.1 mass%, and the steel slab which has the chemical composition which the remainder consists of Fe and an unavoidable impurity. This is an annealed material that has been hot-rolled, wound at 650 ° C., allowed to cool in an air atmosphere, pickled and cold-rolled. The plate thickness of this test material is 1.4 mm. Moreover, the soaking target temperature in the oxidation-reduction annealing of this test material is 900 ° C.

[Lab heating test]
First, specimens each having a length of 140 mm and a width of 70 mm were collected from the edge part and the center part of the specimen.

Then, one heating test piece (test No. 1) with a thermocouple attached to the specimen taken from the center of the specimen, and a thermocouple attached to the specimen taken from the edge of the specimen The three heated test pieces (Test Nos. 2-1 to 2-3) were placed in an atmosphere-regulating heating furnace, heated to a target temperature of 950 ° C., and the temperature of the heated test piece every 10 seconds from the start of heating. It was measured. The measurement results are shown in Table 1. The heating atmosphere is 20 volume% H ₂ O-80 volume% N ₂ simulating the oxidation zone of a continuous hot dip galvanizing line. Moreover, it was confirmed that oxides were formed during the heating process in any of the heating test pieces.

On the other hand, for three other heating test pieces (test Nos. 3-1 to 3-3) in which a thermocouple is attached to a test piece taken from the edge portion of the test material, first, a continuous hot dip galvanizing line In the pre-tropics, heating was simulated to simulate pre-oxidation using an edge burner. In this heating, the furnace temperature was set to 600 ° C., and the heated test piece was held in the furnace for 150 seconds. Heating atmosphere is 20 _vol% H 2 O-80 vol% _{N 2.} It was confirmed that an oxide was formed by this heating. Subsequently, for three heating test pieces (test Nos. 3-1 to 3-3) subjected to heating simulating pre-oxidation, test nos. 1 and test no. Under the same conditions as in 2-1 to 2-3, heating was performed simulating the overall oxidation in the oxidation zone of the continuous hot dip galvanizing line. And the temperature of the heating test piece was measured every 10 seconds from the start of the heating which simulated the whole oxidation. The measurement results are also shown in Table 1. In addition, it confirmed that the oxide film grew by this heating.

[Evaluation of heating rate]
Whether or not the temperature rising rate at the edge portion is comparable to the temperature rising rate at the center portion was evaluated from the degree of deviation of the temperature rising rate at the edge portion from the temperature rising rate at the center portion. Specifically, the evaluation was performed by the following method.

  Since the soaking target temperature of the test material is 900 ° C., each edge portion at the time when the center portion (test No. 1) reaches 904 ° C. (that is, when 60 seconds have elapsed from the start of heating) The percentage R of the temperature 904 ° C. of the center portion between the test Nos. 2-1 to 3-3) and the temperature difference Δt between the center portions was calculated. Table 1 shows the temperature difference Δt and the percentage R of each edge part (Test Nos. 2-1 to 3-3). A case where the percentage R was within ± 5% was considered acceptable, and a case where the percentage R exceeded ± 5% was regarded as unacceptable.

  Test No. 3-1 to 3-3 are examples which satisfy each condition prescribed | regulated with the manufacturing method of the hot dip galvanized steel plate of this invention. These indicated that the percentage R was within ± 5%, and that the rate of temperature rise at the edge was the same as the rate of temperature rise at the center.

  On the other hand, test no. 2-1 to 2-3 are examples that do not satisfy the conditions (that is, pre-oxidation) that oxidize the edge portion specified in the manufacturing method of the hot-dip galvanized steel sheet of the present invention first. All of these indicate that the percentage R exceeds ± 5%, and the temperature rising rate at the edge portion is smaller than the temperature rising rate at the center portion.

  The graph which plotted the temperature with respect to the heating time of each heating test piece (test No.1,2-1 to 3-3) used as another evaluation method of a temperature increase rate is shown in FIG. In each heating time of FIG. 3-1 to 3-3 are test Nos. 1 is close to or partially overlapping, 2-1 to 2-3 are test Nos. It is shown that there is a significant departure from 1. From FIG. 4, it is clear that the temperature increase rate at the edge portion becomes approximately the same as the temperature increase rate at the center portion by performing pre-oxidation.

(Example 2)
A laboratory heating test is performed in the same manner as in Example 1 except that the specimen collected from the edge portion of the specimen is heated by simulating the case of pre-oxidation in an air atmosphere using an edge heater. The temperature rising rate was evaluated.

  As in Example 1, three other heating test pieces (test Nos. 4-1 to 4-3) in which a thermocouple was attached to the test piece collected from the edge portion of the test material were edged in the atmosphere. Heating was performed simulating the case of pre-oxidation using a heater. In this heating, the specimen taken from the edge portion of the specimen is heated to 800 ° C. in the air atmosphere, and cooling is started immediately after the temperature is raised to 800 ° C. (holding time is 0 second). Condition. It was confirmed that an oxide was formed by this heating.

  Subsequently, three test pieces (test Nos. 4-1 to 4-3) that were heated to simulate pre-oxidation were used for test No. 1 shown in Example 1. 1 and test no. Under the same conditions as in 2-1 to 2-3, heating was performed simulating the overall oxidation in the oxidation zone of the continuous hot dip galvanizing line.

  Then, the temperature of the heated test piece was measured every 10 seconds from the start of heating simulating the whole oxidation, and evaluated in the same manner as in Example 1. The measurement results are shown in Test No. 1 shown in Table 1 above. 1 and test no. It shows in Table 2 with the result of 2-1 to 2-3. In addition, it confirmed that the oxide film grew by this heating.

Test No. 4-1 to 4-3 are examples that satisfy each condition defined by the method for producing a hot-dip galvanized steel sheet of the present invention. These indicated that the percentage R was within ± 5%, and that the rate of temperature rise at the edge was the same as the rate of temperature rise at the center. In particular, test no. As for 4-1 to 4-3, the temperature difference (DELTA) t of each edge part and center part in the time of 60 second having passed from the heating start exists in the range of -12 degreeC-3 degreeC, and the effect by performing pre-oxidation is remarkable. It has become.

  FIG. 5 is a graph plotting the temperature against the heating time of each heating test piece (test Nos. 1, 2-1 to 2-3, 4-1 to 4-3), which is used as another evaluation method of the heating rate. Shown in In each heating time of FIG. 4-1 to 4-3 are test Nos. It can be seen that it is close to 1 or partially overlapping. From FIG. 5, it is clear that the temperature rise rate at the edge portion is approximately the same as the temperature rise rate at the center portion by performing pre-oxidation.

Claims (5)

A hot-rolled material is obtained by hot rolling a steel material containing 0.8 to 2.7% by mass of Si and winding at 600 ° C. or higher.
Obtain the annealing material by pickling the hot rolled material,
Oxidizing the width end of the annealed material, then oxidizing the entire width of the annealed material to form an oxide film,
The manufacturing method of the hot dip galvanized steel plate which obtains a hot dip galvanized steel plate by reducing the said oxide film and then performing hot dip galvanization.   The manufacturing method of the hot-dip galvanized steel sheet according to claim 1, wherein cold annealing is performed on the annealed material before oxidizing the width end portion of the annealed material.   The manufacturing method of the hot dip galvanized steel plate of Claim 1 or 2 which oxidizes the width | variety edge part of the said annealing raw material using a pre-tropical edge burner.   The manufacturing method of the hot dip galvanized steel sheet of Claim 1 or 2 which oxidizes the width | variety edge part of the said annealing raw material using an edge heater in air | atmosphere atmosphere.   Manufacture of an alloyed hot-dip galvanized steel sheet which obtains an alloyed hot-dip galvanized steel sheet by alloying the plating layer which the hot-dip galvanized steel sheet obtained by the manufacturing method as described in any one of Claims 1-4 has. Method.

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