JP5888267B2 - Method for producing high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvanized steel sheet - Google Patents

Description

  The present invention is a method for producing a high-strength hot-dip galvanized steel sheet with excellent plating appearance, corrosion resistance, high-peeling peel-off resistance and workability using a high-strength steel sheet containing Si and Mn, and high-strength melting. It relates to a galvanized steel sheet.

  In recent years, in the fields of automobiles, home appliances, building materials and the like, surface-treated steel sheets imparted with rust resistance to raw steel sheets, particularly hot-dip galvanized steel sheets and galvannealed steel sheets have been widely used. In addition, from the viewpoint of improving the fuel efficiency of automobiles and improving the collision safety of automobiles, there is an increasing demand for reducing the thickness of the vehicle body by increasing the strength of the vehicle body material and reducing the weight of the vehicle body. Therefore, application of high-strength steel sheets to automobiles is being promoted.

  In general, a hot dip galvanized steel sheet uses a thin steel sheet obtained by hot rolling or cold rolling a slab as a base material. Manufactured by recrystallization annealing and hot dip galvanizing. In the case of an alloyed hot-dip galvanized steel sheet, it is manufactured after further hot-dip galvanizing treatment.

  Here, there are DFF type (direct flame type), NOF type (non-oxidation type), all radiant tube type, etc. as the heating furnace type of CGL annealing furnace. The construction of CGLs equipped with an all-radiant tube type heating furnace is increasing for the reason that it is possible to produce high-quality plated steel sheets at low cost due to difficulty in carrying out the process. However, unlike the DFF type (direct flame type) and NOF type (non-oxidation type), the all-radiant tube type heating furnace does not have an oxidation step immediately before annealing, so a steel plate containing an easily oxidizable element such as Si or Mn. Is disadvantageous in terms of securing plating properties.

  As a method for producing a hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of Si and Mn as a base material, Patent Document 1 discloses a technique for annealing and plating at a recrystallization temperature of -900 ° C. Patent Document 2 discloses a technique for annealing and plating at 750 to 900 ° C. Patent Document 3 discloses a technique of annealing and plating at 800 to 850 ° C. However, in the case of a high-strength steel sheet containing a large amount of Si and Mn, when annealing is performed at a high temperature exceeding 750 ° C., Si and Mn in the steel are selectively oxidized, and an oxide is formed on the surface of the steel sheet. There is a concern that defects such as non-plating occur.

  Furthermore, Patent Document 4 and Patent Document 5 disclose a technique for internally oxidizing the surface layer of the ground iron by defining the heating temperature in the reduction furnace with an expression represented by the partial pressure of water vapor and increasing the dew point. However, since the area for controlling the dew point is premised on the entire inside of the furnace, it is difficult to control the dew point, and stable operation is difficult. In addition, the production of alloyed hot-dip galvanized steel sheets under unstable dew point control has been observed in the distribution of internal oxides formed on the base steel sheet, and is plated in the longitudinal and width directions of the steel strip. There are concerns that defects such as wettability and uneven alloying may occur.

In Patent Document 6, not only the oxidizing gases H ₂ O and O _2, but also the CO ₂ concentration is simultaneously defined, so that the surface layer immediately before plating is internally oxidized to suppress external oxidation and plating. A technique for improving the appearance is disclosed. However, in Patent Document 6, cracks are likely to occur during processing due to the presence of the internal oxide, and the plating peel resistance deteriorates. Moreover, deterioration of corrosion resistance is also recognized. Furthermore, there is a concern that CO ₂ may cause problems such as in-furnace contamination and carburizing on the steel sheet surface, resulting in changes in mechanical properties.

  Furthermore, recently, the application of high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets to places where processing is severe has progressed, and the anti-plating resistance characteristics during high processing have become important. Yes. Specifically, it is required to suppress plating peeling at the processed part when the plated steel sheet is bent at an angle of more than 90 ° and bent at an acute angle or when the steel sheet is processed by impact.

  In order to satisfy such characteristics, not only does a large amount of Si be added to the steel to ensure the desired steel sheet structure, but also the iron core directly under the plating layer, which may be the starting point of cracking during high processing. More advanced control of the structure and structure of the surface layer is required. However, such control is difficult with the prior art, and hot dip galvanization with excellent anti-plating properties at the time of high processing using Si-containing high-strength steel sheet as a base material in CGL with an all-radiant tube type heating furnace in the annealing furnace. A steel plate could not be produced.

JP 2009-287114 A JP 2008-24980 A JP 2010-150660 A JP 2004-323970 A JP 2004-315960 A JP 2006-233333 A

  The present invention has been made in view of such circumstances, and uses a steel sheet containing Si and Mn as a base material, and has high strength melting excellent in plating appearance, corrosion resistance, plating peeling resistance during high processing and workability. An object of the present invention is to provide a method for producing a galvanized steel sheet and a high-strength hot-dip galvanized steel sheet.

Conventionally, Fe has been actively oxidized or internally oxidized for the purpose of improving plating properties. However, at the same time, corrosion resistance and workability deteriorate. Therefore, the present inventors have studied a method for solving the problem by a new method not confined to the conventional idea. As a result, by appropriately controlling the atmosphere and temperature of the annealing process, the formation of internal oxidation is suppressed in the surface layer of the steel sheet directly under the plating layer, and the excellent plating appearance, higher corrosion resistance and good resistance to high processing. It has been found that plating peelability can be obtained. Specifically, the steel plate maximum temperature in the annealing furnace is 600 ° C. or more and 700 ° C. or less, the steel plate passage time in the temperature range of 600 ° C. or more and 700 ° C. or less is 30 seconds or more and 10 minutes or less, hydrogen in the atmosphere Annealing and hot dip galvanizing are performed while controlling the concentration to be 20 vol% or more. By making the maximum steel plate temperature in the annealing furnace 600 ° C. or higher and 700 ° C. or lower, and the hydrogen concentration in the atmosphere in the temperature range of 600 ° C. or higher and 700 ° C. or lower being 20 vol% or higher, The oxygen potential at the interface is lowered, and internal surface oxidation is prevented as much as possible (without forming oxide as much as possible), and selective surface diffusion and oxidation (hereinafter referred to as surface concentration) such as Si and Mn are suppressed.
By controlling the annealing temperature and the hydrogen concentration in the atmosphere in this way, internal oxidation is prevented, surface concentration is suppressed as much as possible, no plating is eliminated, plating appearance, corrosion resistance, and excellent plating peeling resistance during high processing are excellent. A high-strength hot-dip galvanized steel sheet will be obtained. In addition, having excellent plating appearance means having an appearance in which non-plating and alloying unevenness are not recognized.
And the high-strength hot-dip galvanized steel sheet obtained by the above method is Fe, Si, Mn, Al, P, and B, in the steel sheet surface layer portion within 100 μm from the surface of the underlying steel sheet, directly under the galvanized layer. The formation of one or more oxides selected from Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V is suppressed, and the total formation amount is 0.030 g / side. It is suppressed to less than m ^2. Thereby, the plating appearance is excellent, the corrosion resistance is remarkably improved, the crack prevention at the bending process in the surface layer of the base metal is realized, and the plating peeling resistance at the high processing is excellent.

The present invention is based on the above findings, and features are as follows.
[1] By mass%, C: 0.03 to 0.35%, Si: 0.01 to 0.50%, Mn: 3.6 to 8.0%, Al: 0.001 to 1.000% , P ≦ 0.10%, S ≦ 0.010%, and a zinc plating layer having a plating adhesion amount of 20 to 120 g / m ^{2 on} one side on the surface of the steel plate, the balance being Fe and inevitable impurities. A method for producing a high-strength hot-dip galvanized steel sheet having a steel sheet that reaches the maximum temperature in an annealing furnace when the steel sheet is subjected to annealing and hot-dip galvanizing treatment in a continuous hot-dip galvanizing facility. A high-strength hot-dip galvanized steel sheet characterized in that the steel sheet passage time in the temperature range of 600 ° C. or higher and 700 ° C. or lower is 30 seconds or longer and within 10 minutes, and the hydrogen concentration in the atmosphere is 20 vol% or higher. Production method.
[2] In the above [1], the steel sheet has a component composition in mass%, and further B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005. ~ 0.050%, Cr: 0.001-1.000%, Mo: 0.05-1.00%, Cu: 0.05-1.00%, Ni: 0.05-1.0%, Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10%, V: 0.001 A method for producing a high-strength hot-dip galvanized steel sheet, comprising one or more elements selected from 0.10%.
[3] In the above [1] or [2], after the hot dip galvanizing treatment, the steel plate is further heated to a temperature of 450 ° C. or higher and 600 ° C. or lower to perform alloying treatment, and the Fe content of the plating layer is 8 to A method for producing a high-strength hot-dip galvanized steel sheet, characterized by being in the range of 14% by mass.
[4] Fe, Si, Mn, Al produced by the manufacturing method according to any one of [1] to [3], and formed in a steel plate surface layer portion within 100 μm from the surface of the underlying steel plate immediately below the galvanized layer. At least one oxide selected from P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V is less than 0.030 g / m ² per side in total. A high-strength hot-dip galvanized steel sheet characterized by

  In the present invention, the high strength hot dip galvanized steel sheet has a tensile strength TS of 590 MPa or more. The high-strength hot-dip galvanized steel sheet of the present invention includes a plated steel sheet (hereinafter sometimes referred to as GI) that is not subjected to alloying after the hot-dip galvanizing process, and a plated steel sheet (hereinafter referred to as GA) that is subjected to the alloying process. In some cases).

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength hot-dip galvanized steel plate excellent in plating external appearance, corrosion resistance, the plating peeling resistance at the time of high processing, and workability is obtained.

  Hereinafter, the present invention will be specifically described. In the following description, the content of each element of the steel component composition and the unit of the content of each element of the plating layer component composition are all “mass%”, and hereinafter, simply “%” unless otherwise specified. Show.

First, the annealing condition for determining the structure of the surface of the underlying steel sheet immediately below the plating layer, which is the most important requirement in the present invention, will be described.
In high-strength hot-dip galvanized steel sheets with a large amount of Si and Mn added to the steel, in order to satisfy corrosion resistance and anti-plating resistance during high processing, it may be the starting point for corrosion and cracking during high processing. Therefore, it is required to minimize internal oxidation of the surface layer of the railway just below the plating layer.

  Although it is possible to improve the plateability by oxidizing Fe or promoting internal oxidation of Si or Mn, this leads to deterioration of corrosion resistance and workability. For this reason, it is necessary to improve corrosion resistance and workability by suppressing internal oxidation while maintaining good plating properties, other than a method of promoting internal oxidation of Si or Mn. As a result of the examination, in the present invention, in order to ensure plating properties, the oxygen potential is lowered in the annealing step, and the activity in the surface layer portion of the easily oxidizable elements such as Si and Mn is reduced. And the external oxidation of these elements is suppressed and, as a result, the platability is improved. And the internal oxidation formed in a surface iron surface layer part is also suppressed, and corrosion resistance and workability will be improved.

  Such effects are as follows. When performing annealing and hot dip galvanizing treatment in a continuous hot dip galvanizing facility, the steel plate maximum temperature in the annealing furnace is 600 ° C. or higher and 700 ° C. or lower, and the steel plate temperature is 600 ° C. or higher and 700 ° C. or lower. It can be obtained by controlling the steel plate passage time in the temperature range from 30 seconds to 10 minutes and the hydrogen concentration in the atmosphere to 20 vol% or more. By controlling in this way, the oxygen potential at the interface between the steel sheet and the atmosphere is lowered, and selective surface diffusion and surface concentration of Si, Mn, etc. are suppressed without internal oxidation. And higher corrosion resistance without unplating and good plating peeling resistance at the time of high processing will be obtained.

  The reason why the maximum temperature of the steel sheet in the annealing furnace is 600 ° C. or more and 700 ° C. or less is as follows. In the temperature range below 600 ° C, surface enrichment and internal oxidation that cause problems such as non-plating, deterioration of corrosion resistance and deterioration of plating peeling resistance do not occur, but good materials can be obtained at temperatures below 600 ° C. Absent. Therefore, the temperature range in which the effects of the present invention are manifested is 600 ° C. or higher. On the other hand, in a temperature range exceeding 700 ° C., surface concentration becomes remarkable, and non-plating occurs, corrosion resistance deteriorates, plating peel resistance deteriorates, and the like. Further, from the viewpoint of material, in the temperature range where both TS and El exceed 700 ° C., the effect of balance between strength and ductility is saturated. From the above, the maximum temperature reached by the steel sheet is 600 ° C. or more and 700 ° C. or less.

Next, the reason why the steel plate passage time in the temperature range of 600 ° C. or more and 700 ° C. or less is 30 seconds or more and 10 minutes or less is as follows. If it is less than 30 seconds, the target material (TS, El) cannot be obtained. On the other hand, if it exceeds 10 minutes, the effect of balance between strength and ductility is saturated.
The reason why the hydrogen concentration in the temperature range where the steel sheet temperature is 600 ° C. or more and 700 ° C. or less is 20 vol% or more is as follows. It is the hydrogen concentration: 20 vol% or more that the effect of suppressing the surface concentration is recognized. The upper limit of the hydrogen concentration is not particularly set, but if it exceeds 80 vol%, the effect is saturated and disadvantageous in terms of cost, so 80 vol% or less is desirable. In addition, about the hydrogen concentration in the temperature range whose steel plate temperature is less than 600 degreeC, it was based on 10 vol%.

Next, the steel component composition of the high-strength hot-dip galvanized steel sheet that is the subject of the present invention will be described.
C: 0.03-0.35%
C improves workability by forming martensite or the like as a steel structure. For that purpose, 0.03% or more is necessary. On the other hand, if it exceeds 0.35%, the weldability deteriorates. Therefore, the C content is 0.03% or more and 0.35% or less.

Si: 0.01 to 0.50%
Si is an element effective for strengthening steel and obtaining a good material, but it is an easily oxidizable element, which is disadvantageous for plating properties and should be avoided as much as possible. However, about 0.01% is inevitably contained in the steel, and in order to reduce to less than this, the cost increases, so 0.01% is made the lower limit. On the other hand, when it exceeds 0.50%, it becomes difficult to improve the plating peel resistance at the time of high processing. Therefore, the Si amount is set to 0.01% or more and 0.50% or less.

Mn: 3.6-8.0%
Mn is an element effective for increasing the strength of steel. In order to ensure mechanical properties and strength, it is necessary to contain 3.6% or more. On the other hand, if it exceeds 8.0%, it becomes difficult to ensure weldability and plating adhesion, and to ensure a balance between strength and ductility. Therefore, the Mn content is 3.6% or more and 8.0% or less.

Al: 0.001-1.000%
Al is added for the purpose of deoxidizing molten steel, but if the content is less than 0.001%, the purpose is not achieved. The effect of deoxidation of molten steel is obtained at 0.001% or more. On the other hand, if it exceeds 1.000%, the cost increases. Therefore, the Al amount is set to 0.001% or more and 1.000% or less.

P ≦ 0.10%
P is one of the elements inevitably contained, and in order to make it less than 0.005%, there is a concern about an increase in cost, so 0.005% or more is desirable. On the other hand, if P exceeds 0.10%, weldability deteriorates. Furthermore, the surface quality deteriorates. Also, the plating adhesion deteriorates during non-alloying treatment, and the desired degree of alloying cannot be achieved unless the alloying treatment temperature is increased during alloying treatment. Also, if the alloying temperature is raised to achieve the desired degree of alloying, the ductility deteriorates and at the same time the adhesion of the alloyed plating film deteriorates, so the desired degree of alloying, good ductility, and alloyed plating film Cannot be achieved. Therefore, the P content is preferably 0.10% or less, and the lower limit is preferably 0.005% or more.

S ≦ 0.010%
S is one of the elements inevitably contained. The lower limit is not specified, but if it is contained in a large amount, weldability deteriorates, so the content is made 0.010% or less.

In order to control the balance between strength and ductility, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, Cr: 0.001 One or more elements selected from -1.000%, Mo: 0.05-1.00%, Cu: 0.05-1.00%, Ni: 0.05-1.00% are required It may be added depending on.
The reason for limiting the appropriate addition amount in the case of adding these elements is as follows.

B: 0.001 to 0.005%
When B is less than 0.001%, it is difficult to obtain an effect of promoting quenching. On the other hand, if it exceeds 0.005%, the plating adhesion deteriorates. Therefore, when it contains, B amount shall be 0.001% or more and 0.005% or less.

Nb: 0.005 to 0.050%
If Nb is less than 0.005%, it is difficult to obtain the effect of adjusting the strength and the effect of improving the plating adhesion at the time of composite addition with Mo. On the other hand, if it exceeds 0.050%, cost increases. Therefore, when it contains, Nb amount shall be 0.005% or more and 0.050% or less.

Ti: 0.005 to 0.050%
If Ti is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if it exceeds 0.050%, the plating adhesion deteriorates. Therefore, when it contains, Ti amount shall be 0.005% or more and 0.050% or less.

Cr: 0.001-1.000%
When Cr is less than 0.001%, it is difficult to obtain a hardenability effect. On the other hand, if it exceeds 1.000%, the surface of Cr is concentrated, so that the plating adhesion and weldability deteriorate. Therefore, when it contains, Cr content shall be 0.001% or more and 1.000% or less.

Mo: 0.05-1.00%
If Mo is less than 0.05%, it is difficult to obtain the effect of adjusting the strength and the effect of improving the plating adhesion at the time of composite addition with Nb, Ni or Cu. On the other hand, if it exceeds 1.00%, cost increases. Therefore, when it contains, Mo amount shall be 0.05% or more and 1.00% or less.

Cu: 0.05-1.00%
If Cu is less than 0.05%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion when combined with Ni or Mo. On the other hand, if it exceeds 1.00%, cost increases. Therefore, when contained, the Cu content is 0.05% or more and 1.00% or less.

Ni: 0.05-1.00%
When Ni is less than 0.05%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion upon the combined addition of Cu and Mo. On the other hand, if it exceeds 1.00%, cost increases. Therefore, when contained, the Ni content is 0.05% or more and 1.00% or less.

Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%
Sn or Sb can be contained from the viewpoint of suppressing decarburization in the region of several tens of microns on the surface of the steel sheet caused by nitriding, oxidation, or oxidation of the steel sheet surface. By suppressing such nitriding and oxidation, a reduction in the amount of martensite produced on the steel sheet surface is prevented, and fatigue characteristics and surface quality are improved. From the viewpoint of suppressing nitriding and oxidation, when Sn or Sb is contained, the content is made 0.001% or more. If it exceeds 0.20%, the toughness is deteriorated. Therefore, the content is preferably 0.20% or less.

Ta: 0.001 to 0.10%
Ta contributes to high strength and high yield ratio (YR) by forming C and N together with carbides and carbonitrides. Furthermore, Ta has the effect of refining the hot-rolled sheet structure, and since the ferrite grain size after cold rolling and annealing is refined, the increase in the amount of C segregation at the grain boundary accompanying the increase in grain boundary area A high seizure hardening amount (BH amount) can be obtained. From such a viewpoint, 0.001% or more of Ta can be contained. On the other hand, the content of excess Ta exceeding 0.10% not only increases the raw material cost but also may hinder the formation of martensite in the cooling process after annealing, and further precipitates in the hot-rolled sheet. TaC increases the deformation resistance during cold rolling and may make it difficult to produce a stable actual machine. When TaC is contained, the content is made 0.10% or less.

W: 0.001% to 0.10%
By adding W in combination with Si and Mn, there is an effect of suppressing the formation of a Γ phase and improving plating adhesion. Such an effect is recognized by containing W: 0.001% or more. On the other hand, even if it contains exceeding 0.10%, an effect will be saturated and the effect corresponding to content cannot be expected, but it becomes economically disadvantageous. As mentioned above, when it contains W, it is set as 0.001% or more and 0.10% or less.

V: 0.001 to 0.10%
V is an element that forms carbonitrides and has the effect of increasing the strength of steel by the precipitation effect, and can be contained as necessary. Such an effect is recognized when the content of V is 0.001% or more. On the other hand, when it contains exceeding 0.10%, it will become high strength too much and ductility will deteriorate. From the above, when V is contained, the content is made 0.001% or more and 0.10% or less.

  The balance other than the above is Fe and inevitable impurities. Even if elements other than the elements described above are contained, the present invention is not adversely affected, and the upper limit is made 0.1%.

  Next, the manufacturing method of the high-strength hot-dip galvanized steel sheet of the present invention and the reason for limitation will be described.

  The steel having the above chemical components is hot-rolled and then cold-rolled to obtain a steel plate, and then subjected to annealing and hot-dip galvanizing treatment in a continuous hot-dip galvanizing facility. At this time, in the present invention, the maximum reached temperature of the steel plate in the annealing furnace is 600 ° C. or more and 700 ° C. or less, and the steel plate passage time in the temperature range of 600 ° C. or more and 700 ° C. or less is 30 seconds or more and 10 Within a minute, the hydrogen concentration in the atmosphere is 20 vol% or more. This is the most important requirement in the present invention.

Hot rolling Usually, it can be performed on the conditions performed.

It is preferable to perform a pickling treatment after hot pickling. The black scale formed on the surface in the pickling process is removed, and then cold-rolled. The pickling conditions are not particularly limited.

Cold rolling is preferably performed at a rolling reduction of 40% to 80%. If the rolling reduction is less than 40%, the recrystallization temperature is lowered, and the mechanical characteristics are likely to deteriorate. On the other hand, when the rolling reduction exceeds 80%, the steel sheet is a high-strength steel plate, so that not only the rolling cost is increased, but also the surface concentration during annealing is increased, so that the plating characteristics are deteriorated.

The cold-rolled steel sheet is annealed and then hot dip galvanized.
In the annealing furnace, a heating process is performed in which the steel sheet is heated to a predetermined temperature in a preceding heating zone, and a soaking process is performed in which the temperature is maintained at a predetermined temperature for a predetermined time in a subsequent soaking zone.
And as above-mentioned, the steel plate highest ultimate temperature in an annealing furnace is 600 degreeC or more and 700 degrees C or less, and the steel plate passage time in the temperature range whose steel plate temperature is 600 degreeC or more and 700 degrees C or less is 30 seconds or more and less than 10 minutes, Annealing and hot dip galvanizing are performed by controlling the hydrogen concentration in the atmosphere to 20 vol% or more. The balance consists of N ₂ and inevitable impurity gas. Other gas components such as H ₂ O, CO ₂ and CO may be contained as long as the effects of the present invention are not impaired.
The hot dip galvanizing treatment can be performed by a conventional method.

Next, an alloying treatment is performed as necessary.
When the alloying treatment is performed subsequent to the hot dip galvanizing treatment, the hot dip galvanizing treatment is performed, and then the steel plate is heated to 450 ° C. or higher and 600 ° C. or lower to perform the alloying treatment, and the Fe content of the plating layer is 8 to 14%. It is preferable to do so. If it is less than 8%, uneven alloying and flaking resistance deteriorate. On the other hand, if it exceeds 14%, the plating peel resistance deteriorates.

As described above, the high-strength hot-dip galvanized steel sheet of the present invention is obtained. The high-strength hot-dip galvanized steel sheet of the present invention has a galvanized layer having a plating adhesion amount of 20 to 120 g / m ^{2 on} one surface of the steel sheet. If it is less than 20 g / m ² , it becomes difficult to ensure corrosion resistance. On the other hand, when it exceeds 120 g / m ² , the plating peel resistance deteriorates.

And it has the characteristic in the structure of the base steel plate surface just under a plating layer as follows.
In the steel sheet surface layer portion within 100 μm from the surface of the underlying steel sheet immediately below the galvanized layer, Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, The formation of one or more oxides selected from Ta, W, and V is suppressed to less than 0.030 g / m ² per side in total.

In hot-dip galvanized steel sheets with Si and a large amount of Mn added to the steel, in order to satisfy the corrosion resistance and anti-plating resistance during high processing, there is a possibility of starting from corrosion and cracking during high processing. It is required to minimize the internal oxidation of the surface layer of the ground metal directly below a certain plating layer. Therefore, in the present invention, first, the activity in the surface layer portion of the iron base such as Si or Mn, which is an easily oxidizable element, is reduced by lowering the oxygen potential in the annealing process in order to ensure the plating property. And the external oxidation of these elements is suppressed and, as a result, the platability is improved. Furthermore, internal oxidation formed in the surface layer portion of the ground iron is also suppressed, and corrosion resistance and high workability are improved. Such an effect is obtained by applying Fe, Si, Mn, Al, P, and B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta on the steel sheet surface layer within 100 μm from the surface of the base steel sheet. , W, and V, the formation amount of at least one oxide selected from the group consisting of less than 0.030 g / m ² is recognized. When the total oxide formation amount (hereinafter referred to as internal oxidation amount) is 0.030 g / m ² or more, the corrosion resistance and the high workability deteriorate. Even if the internal oxidation amount is suppressed to less than 0.0001 g / m ² , the effect of improving corrosion resistance and high workability is saturated, so the lower limit of the internal oxidation amount is preferably 0.0001 g / m ² or more.

  Furthermore, in addition to the above, in the present invention, in order to improve the plating peel resistance, the base iron structure on which the Si and Mn-based composite oxide grows is preferably a soft and rich workability ferrite phase.

Hereinafter, the present invention will be specifically described based on examples.
After pickling the hot rolled steel sheet having the steel composition shown in Table 1 and removing the black scale, it was cold rolled under the conditions shown in Tables 2 and 3 to obtain a cold rolled steel sheet having a thickness of 1.0 mm. .

Next, the cold-rolled steel sheet obtained above was charged into a CGL equipped with an all-radiant tube type heating furnace in an annealing furnace. In CGL, as shown in Table 2, the steel plate temperature in the annealing furnace is controlled to pass through the hydrogen concentration and the steel plate passage time in the temperature range of 600 ° C to 700 ° C as shown in Table 2, After annealing, hot dip galvanizing treatment was performed in an Al-containing Zn bath at 460 ° C.
Note that the control of the dew point in the atmosphere, and previously installed separately N ₂ gas flows pipe humidified by heating water tank was placed in N _2, introducing H ₂ gas to N ₂ gas humidified The dew point in the atmosphere was controlled by introducing the mixture into the furnace.
In addition, GA used a 0.14% Al-containing Zn bath, and GI used a 0.18% Al-containing Zn bath. The adhesion amount was adjusted by gas wiping, and GA was alloyed.

  The hot-dip galvanized steel sheets (GA and GI) obtained as described above were examined for appearance (plating appearance), corrosion resistance, plating peeling resistance during high processing, and workability. Further, the amount of oxide (internal oxidation amount) present in the surface layer portion of the steel sheet up to 100 μm immediately below the plating layer was measured. The measurement method and evaluation criteria are shown below.

<Appearance (plating appearance)>
Appearance was judged as good appearance (symbol ◯) when there was no appearance defect such as non-plating or alloying unevenness, and when it was present, it was judged as poor appearance (symbol x).

<Corrosion resistance>
A salt spray test based on JIS Z 2371 (2000) is performed on an alloyed hot-dip galvanized steel sheet having dimensions of 70 mm × 150 mm for 3 days, and the corrosion product is used for 1 minute using chromic acid (concentration 200 g / L, 80 ° C.). After washing and removing, the plating corrosion weight loss (g / m ² · day) before and after the test per one side was measured by a weight method and evaluated according to the following criteria.
○ (Good): Less than 20 g / m ² · day × (Bad): 20 g / m ² · day or more <Plating resistance>
With regard to the plating peel resistance at the time of high processing, in the case of a plated steel sheet in GA, it is required to suppress plating peeling at the bent portion when bent to an acute angle exceeding 90 °. In this example, the cellophane tape (registered trademark) is pressed against the bending part when it is bent at 120 ° to transfer the peeled material to the cellophane tape (registered trademark), and the amount of peeled material on the cellophane tape (registered trademark) is counted as Zn. The number was obtained by fluorescent X-ray method. At this time, the mask diameter is 30 mm, the fluorescent X-ray acceleration voltage is 50 kV, the acceleration current is 50 mA, and the measurement time is 20 seconds. In light of the following criteria, those with ranks 1 and 2 were evaluated to have good plating peel resistance (symbol ◯), and those with three or more were evaluated to have poor plating peel resistance (symbol x).
Fluorescent X-ray Zn count number Rank 0 to less than 500: 1 (good)
500 or more and less than 1000: 2
1000 or more and less than −2000: 3
2000 or more and less than −3000: 4
3000 or more: 5 (poor)
In GI, resistance to plating peeling during an impact test is required. A ball impact test was performed, the processed part was peeled off with tape, and the presence or absence of peeling of the plating layer was visually determined. Ball impact conditions are a ball weight of 1000 g and a drop height of 100 cm.
○: Plating layer is not peeled ×: Plating layer is peeled <Processability>
For workability, a JIS No. 5 tensile test piece was sampled from the sample in a 90 ° direction with respect to the rolling direction, a tensile test was performed at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241, and the tensile strength (TS / MPa) and elongation (El%) were measured, and those with TS × El ≧ 24000 were good and those with TS × El <24000 were bad.

<Internal oxidation amount in the region of 100 μm directly under the plating layer>
The amount of internal oxidation is measured by “impulse furnace melting-infrared absorption method”. However, since it is necessary to subtract the amount of oxygen contained in the material (that is, the high-strength steel plate before annealing), in the present invention, the surface layer portions on both surfaces of the high-strength steel plate after continuous annealing are polished by 100 μm or more in the steel. Measure the oxygen concentration, set the measured value as the amount of oxygen OH contained in the material, measure the oxygen concentration in the steel in the entire thickness direction of the high-strength steel sheet after continuous annealing, and measure the measured value internally. The subsequent oxygen amount OI was used. The difference between OI and OH (= OI-OH) is calculated using the oxygen amount OI after internal oxidation of the high-strength steel plate thus obtained and the oxygen amount OH contained in the material, and further, single-sided unit area (i.e. 1 m ²⁾ value converted into the amount per (g / ^{m 2)} as an internal oxide amount.

  The results obtained above are shown in Tables 2 and 3 together with the production conditions.

As is apparent from Tables 2 and 3, GI and GA (invention examples) produced by the method of the present invention are high-strength steel sheets containing a large amount of oxidizable elements such as Si and Mn. In addition, corrosion resistance, workability, resistance to plating peeling during high processing, and plating appearance are also good.
On the other hand, in the comparative example, any one or more of plating appearance, corrosion resistance, workability, and plating peeling resistance during high processing is inferior.

  The high-strength hot-dip galvanized steel sheet according to the present invention is excellent in plating appearance, corrosion resistance, workability, and anti-plating resistance during high processing, and is used as a surface-treated steel sheet for reducing the weight and strength of an automobile body. be able to. In addition to automobiles, the steel sheet can be applied in a wide range of fields such as home appliances and building materials as a surface-treated steel sheet provided with rust prevention properties.

Claims (3)

In mass%, C: 0.03 to 0.35%, Si: 0.01 to 0.50%, Mn: 3.6 to 8.0%, Al: 0.001 to 1.000%, P ≦ High strength having a galvanized layer with a coating adhesion amount of 20 to 120 g / m ^{2 on} one surface on the surface of a steel plate containing 0.10%, S ≦ 0.010%, the balance being Fe and inevitable impurities A method for producing a hot dip galvanized steel sheet, when the steel sheet is subjected to annealing and hot dip galvanizing treatment in a continuous hot dip galvanizing facility, the highest steel sheet temperature in the annealing furnace is 600 ° C or higher and 700 ° C or lower, A method for producing a high-strength hot-dip galvanized steel sheet, characterized in that the steel sheet passage time in a temperature range of 600 ° C. to 700 ° C. is 30 seconds to 10 minutes and the hydrogen concentration in the atmosphere is 20 vol% or more.   The steel sheet is in mass% as a component composition, and B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, Cr: 0 0.001 to 1.000%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sn: 0.001 to 0.20 %, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10%, V: 0.001 to 0.10% The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, comprising at least one element. After the hot dip galvanizing treatment, the steel sheet is further heated to a temperature of 450 ° C. or higher and 600 ° C. or lower to perform alloying treatment, and the Fe content of the plating layer is in the range of 8 to 14% by mass. Item 3. A method for producing a high-strength hot-dip galvanized steel sheet according to Item 1 or 2 .