JP5552859B2 - High-strength hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in workability using a high-strength steel sheet containing Si and Mn as a base material and a method for producing the same.

  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, and the base steel sheet is used in an annealing furnace of a continuous hot dip galvanizing line (hereinafter referred to as CGL). 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 and Patent Document 2 define a heating temperature in a reduction furnace by an expression expressed by a partial pressure of water vapor. A technique for internally oxidizing the surface layer of the railway by increasing the dew point is disclosed. 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, in the manufacture of galvannealed steel sheets under unstable dew point control, variations in the distribution of internal oxides formed on the base steel sheet were observed, and wetting in the longitudinal and width directions of the steel sheet was observed. There is a concern that defects such as unevenness in properties and unevenness in alloying may occur.

In Patent Document 3, not only the oxidizing gases H ₂ O and O ₂ but also the CO ₂ concentration are simultaneously defined, so that the surface layer immediately before plating is internally oxidized to suppress external oxidation. A technique for improving the appearance is disclosed. However, similarly to Patent Documents 1 and 2, also in Patent Document 3, 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. Further, there is a concern that CO ₂ may cause problems such as in-furnace contamination, carburizing on the steel sheet surface, and mechanical characteristics changing.

  Furthermore, recently, the application of high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets to places where machining is severe has progressed, and the anti-plating resistance characteristics at the time of 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 high processing using CGL with Si-containing high-strength steel plate as a base material with an all-radiant tube type heating furnace in the annealing furnace. A steel plate could not be produced.

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 a high-strength hot-dip galvanized steel sheet excellent in plating appearance, corrosion resistance, and plating peeling resistance during high processing, and It aims at providing the manufacturing method.

  Conventionally, the inside of a steel plate has been actively oxidized for the purpose of improving the plating property. 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, in the soaking process, the temperature range in the annealing furnace: 820 ° C. or more and 1000 ° C. or less is the dew point of the atmosphere: −45 ° C. or less, and in the cooling process, the temperature in the annealing furnace: temperature range of 750 ° C. or more. Is controlled such that the dew point of the atmosphere is −45 ° C. or lower, and annealing and hot dip galvanizing are performed. By performing such treatment, the reducing ability in the atmosphere is increased, and oxides of oxidizable elements such as Si and Mn that are selectively surface oxidized (hereinafter referred to as surface concentration) on the steel sheet surface are reduced. Can do.

Until now, no attempt has been made to carry out hot dip galvanization of high-strength steel sheets containing Si and Mn through annealing in an atmosphere of −45 ° C. or lower. The reason is that there is a common knowledge in the art that selective oxidation of Si and Mn in the furnace cannot be prevented in a dew point atmosphere that can be industrially implemented. In Reference 1 (7th International Conference on Zinc and Zinc Alloy Coated Steel Sheet, Galvatech 2007, Proceedings p404), when the oxygen potential is converted into a dew point from thermodynamic data of the oxidation reaction of Si and Mn, 800 ° C., N ₂ -5% It has been shown that in the presence of H ₂ , oxidation cannot be prevented and the oxide once formed cannot be reduced unless the dew point is set to −80 ° C. or lower for Mn and −60 ° C. or lower for Mn. Therefore, when annealing a high-strength steel sheet containing Si and Mn, it has been considered that even if the hydrogen concentration is increased, surface concentration cannot be prevented unless the dew point is at least −80 ° C. or lower. Therefore, no attempt was made to galvanize after annealing at a dew point of −45 to −70 ° C.

  However, the present inventors dared to examine it. As a result, the present invention has been achieved.

  Usually, since the dew point of the annealing atmosphere of the steel sheet is higher than −40 ° C., moisture in the annealing atmosphere must be removed in order to obtain a dew point of −45 ° C. or lower, and the atmosphere of the entire annealing furnace is −45 ° C. However, in the present invention, the temperature in the annealing furnace in the soaking process is 820 ° C. or more and 1000 ° C. or less, and the temperature in the annealing furnace in the cooling process is 750 ° C. or more. Since predetermined characteristics can be obtained by controlling the temperature range so that the dew point of the atmosphere is −45 ° C. or lower, there is a feature that equipment costs and operation costs can be reduced.

  Since the oxygen potential in the atmosphere is very low, little internal oxidation occurs. By controlling the dew point of the atmosphere in this way, a high-strength hot-dip galvanized steel sheet that reduces surface enrichment without forming internal oxidation and is excellent in plating appearance, corrosion resistance, and plating peeling resistance during high processing. Will be obtained. In addition, having excellent plating appearance means having an appearance in which non-plating and alloying unevenness are not recognized.

  Here, the dew point other than the region where the dew point is −45 ° C. or lower may be higher than −45 ° C. The normal dew point may be above −40 ° C. to −10 ° C.

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 (except for Fe only) selected from Nb, Ti, Cr, Mo, Cu, and Ni is suppressed, and the total amount formed is 0.060 g / m ² or less per side. It is suppressed. 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: 0.01 to 0.18%, Si: 0.02 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0% , P: 0.005 to 0.060%, S ≦ 0.01%, with the balance being Fe and unavoidable impurities on the surface of the steel sheet, the plating adhesion amount per side is 20 to 120 g / m ² A method for producing a high-strength hot-dip galvanized steel sheet having a galvanized layer, and when the steel sheet is subjected to annealing and hot-dip galvanizing treatment in a continuous hot-dip galvanizing facility, the temperature in the annealing furnace: 820 ° C. or higher in the soaking process A temperature range of 1000 ° C. or lower is set to a dew point of the atmosphere: −45 ° C. or lower, and a temperature range of 750 ° C. or higher in the annealing furnace is set to a dew point of the atmosphere: −45 ° C. or lower in the cooling process. A method for producing high-strength hot-dip galvanized steel sheets.
[2] In the above [1], the steel sheet is in mass% as a component composition, and further B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005. -0.05%, Cr: 0.001-1.0%, Mo: 0.05-1.0%, Cu: 0.05-1.0%, Ni: 0.05-1.0% A method for producing a high-strength hot-dip galvanized steel sheet, comprising one or more elements selected from the inside.
[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 galvanized layer is set to 7 A method for producing a high-strength hot-dip galvanized steel sheet, characterized by being in the range of ˜15 mass%.
[4] Fe, Si, Mn, Al produced by the production 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. One or more oxides selected from P, B, Nb, Ti, Cr, Mo, Cu, and Ni are 0.060 g / m ² or less per side, and a high-strength hot-dip galvanized steel sheet .

  In the present invention, the high strength means that the tensile strength TS is 340 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, and the plating peeling resistance at the time of high processing 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, an annealing atmosphere condition that determines 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 promoting the internal oxidation of Si and 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, first, in order to ensure the plating property, the surface concentrate such as Si and Mn formed in the annealing heating process is reduced by a relatively high temperature soaking process, and the oxygen potential in the initial stage of cooling is reduced. Lowering prevents oxidation, reduces oxides on the steel sheet surface, and improves plating properties. And since almost no internal oxidation is formed in the surface layer portion of the iron base, the corrosion resistance and the high workability are improved.

  Such an effect is obtained by performing annealing and hot dip galvanizing treatment in a continuous hot dip galvanizing facility. In the soaking process, the temperature in the annealing furnace is 820 ° C to 1000 ° C, and the dew point of the atmosphere is -45 ° C or lower. In the cooling process, the temperature in the annealing furnace: 750 ° C. or higher is controlled by controlling the dew point of the atmosphere to be −45 ° C. or lower. By controlling in this way, the surface concentrate formed in the heating process is reduced, and the oxide on the surface layer of the steel sheet is reduced. In addition, since the annealing atmosphere has a low oxygen potential, it is possible to obtain higher corrosion resistance without plating and good plating peeling resistance during high processing without almost forming internal oxidation.

  The reason why the temperature range of the annealing furnace temperature in the soaking process is 820 ° C. or more and 1000 ° C. or less is as follows. In the temperature range below 820 ° C., even if the dew point is lowered to −45 ° C. or less and the reduction ability is increased, the surface concentrate such as Si and Mn cannot be sufficiently reduced. Moreover, the reason for setting it as 1000 degrees C or less becomes disadvantageous from a viewpoint of deterioration by the oxidation of the equipment (roll etc.) in an annealing furnace, and a cost increase when it exceeds 1000 degreeC.

  The reason why the dew point control temperature range of the annealing furnace internal temperature during the cooling process is set to 750 ° C. or higher is as follows. In the temperature range of 750 ° C. or higher, the surface concentration of components in the steel starts to start. If the dew point of the atmosphere is not controlled to be −45 ° C. or lower in this temperature range, surface concentration of the components in the steel occurs. Further, after the cooling zone, even if the dew point of the atmosphere is raised, if the temperature is low, the surface concentrate cannot be reduced. Therefore, the temperature range (dew point control region) of the annealing furnace temperature in the cooling process is set to 750 ° C. or higher.

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: 0.01 to 0.18%
C improves workability by forming martensite or the like as a steel structure. For that purpose, 0.01% or more is necessary. On the other hand, if it exceeds 0.18%, the weldability deteriorates. Therefore, the C content is 0.01% or more and 0.18% or less.

Si: 0.02 to 2.0%
Si is an element effective for strengthening steel to obtain a good material, and 0.02% or more is necessary to obtain the intended strength of the present invention. If Si is less than 0.02%, the strength within the scope of application of the present invention cannot be obtained, and there is no particular problem with respect to resistance to plating peeling during high processing. On the other hand, if it exceeds 2.0%, it becomes difficult to improve the plating peel resistance at the time of high processing. Therefore, the Si content is 0.02% or more and 2.0% or less.

Mn: 1.0-3.0%
Mn is an element effective for increasing the strength of steel. In order to ensure mechanical properties and strength, it is necessary to contain 1.0% or more. On the other hand, if it exceeds 3.0%, it becomes difficult to ensure weldability and plating adhesion, and to ensure a balance between strength and ductility. Therefore, the Mn content is 1.0% or more and 3.0% or less.

Al: 0.001 to 1.0%
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 1.0%, the cost increases. Therefore, the Al content is 0.001% or more and 1.0% or less.

P: 0.005 to 0.060% or less 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 And On the other hand, if P exceeds 0.060%, weldability deteriorates. Furthermore, the surface quality deteriorates. Further, when the alloying treatment is not performed, the plating adhesion is deteriorated, and when the alloying treatment is performed, the alloying treatment temperature cannot be increased unless the alloying treatment temperature is increased. 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 0.005% or more and 0.060% or less.

S ≦ 0.01%
S is one of the elements inevitably contained. The lower limit is not specified, but if it is contained in a large amount, the weldability deteriorates, so the content is made 0.01% or less.

In order to control the balance between strength and ductility, B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 One or more elements selected from -1.0%, Mo: 0.05-1.0%, Cu: 0.05-1.0%, Ni: 0.05-1.0% 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. Needless to say, however, it is not necessary to add when it is judged that it is not necessary to improve the mechanical properties.

Nb: 0.005 to 0.05%
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 0.05%, the cost increases. Therefore, when it contains, Nb amount shall be 0.005% or more and 0.05% or less.

Ti: 0.005 to 0.05%
If Ti is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if it exceeds 0.05%, the plating adhesion deteriorates. Therefore, when it contains, Ti amount shall be 0.005% or more and 0.05% or less.

Cr: 0.001 to 1.0%
When Cr is less than 0.001%, it is difficult to obtain a hardenability effect. On the other hand, if it exceeds 1.0%, the surface of Cr is concentrated, so that the plating adhesion and weldability deteriorate. Therefore, when it contains, Cr amount shall be 0.001% or more and 1.0% or less.

Mo: 0.05-1.0%
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 1.0%, cost increases. Therefore, when contained, the Mo content is 0.05% or more and 1.0% or less.

Cu: 0.05 to 1.0%
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 1.0%, cost increases. Therefore, when contained, the Cu content is 0.05% or more and 1.0% or less.

Ni: 0.05-1.0%
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 1.0%, cost increases. Therefore, when it contains, Ni amount shall be 0.05% or more and 1.0% or less.

  The balance other than the above is Fe and inevitable impurities.

  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, in the soaking process, the annealing furnace temperature: 820 ° C. or more and 1000 ° C. or less is set to the dew point of the atmosphere: −45 ° C. or less, and in the cooling process, the annealing furnace temperature: 750 A temperature range of ℃ or higher is performed at the dew point of the atmosphere: -45 ℃ or lower. 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, if 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 may be deteriorated.

The cold-rolled steel sheet is annealed and then hot dip galvanized.
In the annealing furnace, a heating step of heating the steel sheet to a predetermined temperature is performed in the heating zone in the previous stage, a soaking step in which the steel plate is maintained at a predetermined temperature for a predetermined time in a soaking zone in the subsequent stage, and then a cooling step is performed.
As described above, in the soaking process, the temperature in the annealing furnace is set to 820 ° C. or more and 1000 ° C. or less, and the dew point of the atmosphere is −45 ° C. or less. In the cooling process, the annealing furnace temperature is 750 ° C. or more. Annealing and hot dip galvanizing are performed by controlling the temperature range so that the dew point of the atmosphere is −45 ° C. or lower. Since the normal dew point is higher than −40 ° C., the dew point of −45 ° C. or lower is obtained by absorbing and removing moisture in the furnace with an absorbent.

If the volume fraction of hydrogen gas in the atmosphere is less than 1 vol%, the activation effect due to reduction cannot be obtained and the plating peel resistance deteriorates. The upper limit is not particularly specified, but if it exceeds 50 vol%, the cost increases and the effect is saturated. Therefore, the volume fraction of hydrogen gas is preferably 1 vol% or more and 50 vol% or less. In addition, the gaseous component in an annealing furnace consists of nitrogen gas and an unavoidable impurity gas other than hydrogen gas. Other gas components may be included 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 more and 600 ° C. or less to perform the alloying treatment, and the Fe content of the plating layer is 7 to 15%. It is preferable to do so. If it is less than 7%, unevenness in alloying and flaking properties deteriorate. On the other hand, if it exceeds 15%, 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 plate surface layer portion within 100 μm from the surface of the underlying steel plate immediately below the galvanized layer, it is selected from Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, and Ni. The formation of one or more oxides is suppressed to 0.060 g / m ² or less 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 a kind selected from Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, Ni on the surface layer portion of the steel plate within 100 μm from the surface of the base steel plate. It is recognized by suppressing the total amount of oxides formed to 0.060 g / m ² or less in total. When the total amount of oxide formation (hereinafter referred to as internal oxidation amount) exceeds 0.060 g / m ² , corrosion resistance and high workability deteriorate. Even if the internal oxidation amount is suppressed to less than 0.0001 g / m ² , the corrosion resistance and the high workability improvement effect are saturated, so the lower limit of the internal oxidation amount is preferably 0.0001 g / m ² .

  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.
The hot-rolled steel sheet having the steel composition shown in Table 1 was pickled and the black scale removed, and then cold-rolled under the conditions shown in Table 2 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 dew point in the temperature range of 820 ° C. to 1000 ° C. in the annealing furnace in the soaking process and the temperature range of 750 ° C. in the annealing furnace in the cooling process is controlled and passed. After annealing, hot dip galvanizing treatment was performed in an Al-containing Zn bath at 460 ° C. The dew point of the annealing furnace other than the region where the dew point was controlled was basically -35 ° C.
The gas components in the atmosphere consisted of nitrogen gas, hydrogen gas, and inevitable impurity gas, and the dew point was controlled by absorbing and removing moisture in the atmosphere. The hydrogen concentration in the atmosphere was basically 10 vol%.
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>
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>
An alloyed hot-dip galvanized steel sheet with dimensions of 70 mm x 150 mm was subjected to a salt spray test based on JIS Z 2371 (2000) for 3 days, and the corrosion product was treated with chromic acid (concentration 200 g / L, 80 ° C) for 1 minute. 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 resistance to plating peeling at the time of high processing, in GA, it is required to suppress plating peeling at the bent portion when bent at an acute angle exceeding 90 °. In this example, the cellophane tape was pressed against the processed portion bent by 120 ° to transfer the peeled material to the cellophane tape, and the amount of the peeled material on the cellophane tape was determined by the fluorescent X-ray method as the Zn count number. 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, JIS No. 5 tensile test specimens were taken from the sample in the 90 ° direction with respect to the rolling direction, and subjected to a tensile test at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241. / MPa) and elongation (El%) were measured, and when TS was less than 650 MPa, TS × El ≧ 22000 was judged good, and TS × El <22000 was judged poor. When TS was 650 MPa or more and 900 MPa, TS × El ≧ 20000 was judged good, and TS × El <20000 was judged poor. When TS was 900 MPa or more, TS × El ≧ 18000 was judged good, and TS × El <18000 was judged poor.

<Internal oxidation amount in the region of 100 μm directly under the plating layer>
The amount of internal oxidation was 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 as described above are shown in Table 2 together with the production conditions.

As is apparent from Table 2, GI and GA (examples of the present invention) 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. Excellent in workability and anti-plating resistance during high processing, and good plating appearance.
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.

Hereinafter, the present invention will be specifically described based on examples.
The hot-rolled steel sheet having the steel composition shown in Table 3 was pickled and the black scale removed, and then cold-rolled under the conditions shown in Table 4 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 4, the dew point in the temperature range of 800 ° C. to 1000 ° C. in the annealing furnace in the soaking process and the temperature range of 650 ° C. in the annealing furnace in the cooling process is controlled and passed. After annealing, hot dip galvanizing treatment was performed in an Al-containing Zn bath at 460 ° C. The dew point of the annealing furnace other than the region where the dew point was controlled was basically -35 ° C. The gas components in the atmosphere consisted of nitrogen gas, hydrogen gas, and inevitable impurity gas, and the dew point was controlled by absorbing and removing moisture in the atmosphere. The hydrogen concentration in the atmosphere was basically 10 vol%.
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>
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 a size 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 resistance to plating peeling at the time of high processing, in GA, it is required to suppress plating peeling at the bent portion when bent at an acute angle exceeding 90 °. In this example, the cellophane tape was pressed against the processed portion bent by 120 ° to transfer the peeled material to the cellophane tape, and the amount of the peeled material on the cellophane tape was determined by the fluorescent X-ray method as the Zn count number. 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 taken from the sample at a 90 ° direction with respect to the rolling direction, a tensile test was conducted at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241, and tensile strength (TS / MPa) and elongation (El%) were measured, and when TS was less than 650 MPa, TS × El ≧ 22000 was judged good, and TS × El <22000 was judged poor. When TS was 650 MPa or more and 900 MPa, TS × El ≧ 20000 was judged good, and TS × El <20000 was judged poor. When TS was 900 MPa or more, TS × El ≧ 18000 was judged good, and TS × El <18000 was judged poor.

<Internal oxidation amount in the region of 100 μm directly under the plating layer>
The amount of internal oxidation was 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 Table 4 together with the production conditions.

As is apparent from Table 4, GI and GA (examples of the present invention) 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. Excellent in workability and anti-plating resistance during high processing, and good plating appearance.
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 (4)

In mass%, C: 0.01 to 0.18%, Si: 0.02 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0%, P: A galvanized layer containing 0.005 to 0.060%, S ≦ 0.01%, the balance being Fe and unavoidable impurities on the surface of the steel sheet, and the coating amount per one side being 20 to 120 g / m ² Is a method of manufacturing a high-strength hot-dip galvanized steel sheet having an annealing furnace temperature of 820 ° C. or higher and 1000 ° C. or lower in a soaking process when the steel sheet is subjected to annealing and hot-dip galvanizing treatment in a continuous hot-dip galvanizing facility. temperature range only the dew point: the -45 ° C. or less, and the cooling process in the annealing furnace temperature: 750 ° C. or higher temperature range only a dew point: high, characterized by controlling the -45 ° C. or less A method for producing high-strength hot-dip galvanized steel sheets.   The steel sheet is in mass% as a component composition, and B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0 One or more elements selected from 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0% The manufacturing method of the high intensity | strength hot-dip galvanized steel plate of Claim 1 characterized by the above-mentioned.   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 galvanized layer is set in the range of 7 to 15% by mass. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate of Claim 1 or 2. Fe, Si, Mn, Al, P, B, Nb, produced by the production method according to any one of claims 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. A high-strength hot-dip galvanized steel sheet, wherein one or more oxides selected from Ti, Cr, Mo, Cu, and Ni are 0.060 g / m ² or less per side.