JP5834869B2 - High-strength steel sheet with excellent chemical conversion and process for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the Si content is large, and a method for producing the same.

  In recent years, 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. For this reason, application of high-strength steel sheets to automobiles is being promoted.

  In general, steel plates for automobiles are used after being coated, and as a pretreatment for the coating, a chemical conversion treatment called a phosphate treatment is performed. The chemical conversion treatment of the steel sheet is one of the important treatments for ensuring the corrosion resistance after painting.

In order to increase the strength and ductility of the steel plate, addition of Si is effective. However, during continuous annealing, Si is oxidized even in a reducing N ₂ + H ₂ gas atmosphere in which no oxidation of Fe occurs (which reduces Fe oxide), and Si oxide (SiO ₂ ) is formed on the outermost surface of the steel sheet. To do. In general, the continuous annealing time is as long as 100 s or more in a high temperature range of 700 ° C. or higher, so the amount of SiO ₂ formed increases, and this SiO ₂ inhibits the formation reaction of the chemical film during the chemical conversion treatment, so that a chemical conversion film is formed. A small region (hereinafter sometimes referred to as “ske”) that is not formed is formed, and the chemical conversion processability is lowered.

As a conventional technique for improving the chemical conversion property of a high Si-containing steel sheet, Patent Document 1 discloses a method of forming an iron coating layer of ^{20 to} 1500 mg / m ² on a steel sheet using an electroplating method. However, this method has a problem in that an electroplating facility is separately required, and the number of processes increases and the cost also increases.

  Moreover, in patent document 2, the Mn / Si ratio is prescribed | regulated, and patent document 3 is improving the phosphate processability by adding Ni, respectively. However, the effect depends on the Si content in the steel sheet, and it is considered that further improvement is necessary for the steel sheet having a high Si content.

  Furthermore, in patent document 4, the dew point at the time of annealing is set to −25 to 0 ° C., thereby forming an internal oxide layer made of an Si-containing oxide within a depth of 1 μm from the surface of the steel sheet, and the steel sheet surface length is 10 μm. A method is disclosed in which the proportion of the Si-containing oxide is 80% or less. However, in the case of the method described in Patent Document 4, 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, when annealing was performed under unstable dew point control, variations were observed in the distribution of internal oxides formed on the steel sheet, and chemical conversion treatment unevenness in the longitudinal direction and width direction of the steel sheet (overall or There is a concern that some scales may occur. Furthermore, even when the chemical conversion treatment property is improved, there is a problem that the corrosion resistance after electrodeposition coating is poor because the Si-containing oxide is present directly under the chemical conversion treatment film.

  Patent Document 5 describes a method in which a steel sheet temperature reaches 350 to 650 ° C. in an oxidizing atmosphere to form an oxide film on the steel sheet surface, and then heated to a recrystallization temperature and cooled in a reducing atmosphere. However, in this method, there is a difference in the thickness of the oxide film formed on the surface of the steel sheet due to the oxidation method, and sufficient oxidation does not occur, or the oxide film becomes too thick, and in subsequent annealing in a reducing atmosphere. Oxide film may remain or peel off, and surface properties may deteriorate. In the examples, a technique for oxidizing in the air is described, but oxidation in the air generates a thick oxide and subsequent reduction is difficult, or a reducing atmosphere with a high hydrogen concentration is required. There are problems such as.

Further, in Patent Document 6, a cold-rolled steel sheet containing 0.1% or more of Si and / or 1.0% or more of Mn by mass%, the steel sheet surface in an iron oxidizing atmosphere at a steel sheet temperature of 400 ° C. or more. Describes a method in which an oxide film is formed, and then the oxide film on the surface of the steel sheet is reduced in an iron reducing atmosphere. Specifically, after oxidizing Fe on the surface of the steel sheet using a direct fire burner at 400 ° C. or higher and an air ratio of 0.93 or higher and 1.10 or lower, annealing is performed in an N ₂ + H ₂ gas atmosphere that reduces Fe oxide. This is a method of suppressing oxidation at the outermost surface of Si, which deteriorates the chemical conversion processability, and forming an Fe oxide layer on the outermost surface. Patent Document 6 does not specifically describe the heating temperature of an open flame burner, but when it contains a large amount of Si (approximately 0.6% or more), the amount of oxidation of Si that is easier to oxidize than Fe. As a result, the oxidation of Fe is suppressed, or the oxidation of Fe itself is too little. As a result, formation of the surface Fe reduction layer after reduction was insufficient, or SiO ₂ was present on the steel plate surface after reduction, and there was a case where the conversion film was scaled.

JP-A-5-320952 Japanese Patent No. 4319559 Japanese Patent No. 2951480 Japanese Patent No. 3840392 JP 55-145122 A JP 2006-45615 A

  The present invention has been made in view of such circumstances. Even when the Si content is high and the continuous annealing time at a high temperature (≧ 700 ° C.) is long (≧ 100 s), excellent chemical conversion treatment and electrodeposition are possible. It aims at providing the high-strength steel plate which has the corrosion resistance after coating, and its manufacturing method.

Conventionally, the inside of a steel plate has been actively oxidized for the purpose of improving chemical conversion properties. However, at the same time, the corrosion resistance after electrodeposition coating deteriorates.
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, it was found that by appropriately controlling the atmosphere and temperature in the annealing process, formation of internal oxidation was suppressed in the surface layer portion of the steel sheet, and excellent chemical conversion property and higher corrosion resistance were obtained.

  Specifically, in the temperature raising process, in the temperature range of the annealing furnace temperature: 600 ° C. or more and A ° C. or less (A: 700 ≦ A ≦ 900), the temperature raising rate is 8 ° C./s or more and Annealing is performed by controlling the hydrogen concentration to be 22 vol% or more, and a chemical conversion treatment is performed. Annealing furnace temperature: Selective surface oxidation of easily oxidizable elements (Si, Mn, etc.) by increasing the rate of temperature rise in the temperature range from 600 ° C. to A ° C. at 8 ° C./s (hereinafter referred to as surface enrichment) Called) as much as possible. Furthermore, by setting the hydrogen concentration in the atmosphere to 22 vol% or more in the temperature range of 600 ° C. or more and A ° C. or less in the annealing furnace temperature, the oxygen potential at the interface between the steel sheet and the atmosphere is lowered, and internal oxidation is not formed. , Suppress the surface thickening. By controlling the hydrogen concentration in a limited region in the atmosphere in this way, internal oxides are not formed, surface concentration is suppressed as much as possible, there is no scum or unevenness, chemical conversion treatment properties and after electrodeposition coating A high-strength steel sheet having excellent corrosion resistance will be obtained.

The high-strength steel plate obtained by the above method is selected from the group consisting of Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni in the steel plate surface layer within 100 μm from the steel plate surface. The formation of one or more selected oxides is suppressed, and the amount formed is suppressed to 0.050 g / m ² or less per side in total. Thereby, it is excellent in chemical conversion property and the corrosion resistance after electrodeposition coating improves remarkably.

The present invention is based on the above findings, and features are as follows.
[1] By mass%, C: 0.01 to 0.18%, Si: 0.4 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0% , P: 0.005 to 0.060%, S ≦ 0.01%, and when continuously annealing a steel plate made of Fe and inevitable impurities, the temperature in the annealing furnace is 600 in the temperature rising process. A method for producing a high-strength steel sheet excellent in chemical conversion treatment, characterized in that, in a temperature range of from 0 ° C. to A ° C., the rate of temperature rise is 8 ° C./s or more and the hydrogen concentration in the atmosphere is 22 vol% or more .
However, A: 700 ≦ A ≦ 900.
[2] 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: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0% The method for producing a high-strength steel sheet excellent in chemical conversion treatment as described in [1] above, comprising at least a seed element.
[3] The method for producing a high-strength steel sheet excellent in chemical conversion treatment according to [1] or [2], wherein after the continuous annealing, electrolytic pickling is performed in an aqueous solution containing sulfuric acid. .
[4] Fe, Si, Mn, Al, P, B, Nb, Ti produced by the production method according to any one of [1] to [3] and formed on the steel sheet surface layer within 100 μm from the steel sheet surface. A high-strength steel sheet excellent in chemical conversion property, characterized in that one or more oxides selected from Cr, Mo, Cu, and Ni are 0.050 g / m ² or less per side.

In the present invention, the high strength means that the tensile strength TS is 340 MPa or more. Moreover, being excellent in chemical conversion treatment property means having a non-scaling and uneven appearance after chemical conversion treatment.
The high-strength steel sheet of the present invention includes both cold-rolled steel sheets and hot-rolled steel sheets.

  According to the present invention, a high-strength steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the Si content is high and the continuous annealing time in a high temperature range (≧ 700 ° C.) is long (≧ 100 s). Is obtained.

  Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element of the steel component composition is “mass%”, and hereinafter, it is simply indicated by “%” unless otherwise specified.

First, the annealing atmosphere conditions that determine the structure of the steel sheet surface layer, which is the most important requirement in the present invention, will be described.
In a high-strength steel sheet in which a large amount of Si and Mn is added to the steel, in order to satisfy the corrosion resistance, it is required to minimize the internal oxidation of the steel sheet surface layer that may be a starting point of corrosion.
Although it is possible to improve the chemical conversion treatment 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 suppress the internal oxidation and improve the corrosion resistance while maintaining good chemical conversion properties other than the method of promoting the internal oxidation of Si and Mn.
As a result of the examination, in the present invention, in order to ensure chemical conversion treatment, the oxygen potential is lowered in the annealing process, and the activity in the steel sheet surface layer portion such as Si or Mn, which is an easily oxidizable element, is lowered. And the surface concentration of these elements is suppressed, and as a result, chemical conversion property is improved. And the internal oxidation formed in a steel plate surface layer part is also suppressed, and the corrosion resistance after electrodeposition coating will improve.

  Such effects are obtained when annealing is performed in a continuous annealing facility, in the temperature raising process, in the temperature range of the annealing furnace: 600 ° C. to A ° C., the temperature raising rate is 8 ° C./s or more, and the atmosphere It can be obtained by controlling the hydrogen concentration in it to be 22 vol% or more. Annealing furnace temperature: In the temperature range of 600 ° C. or more and A ° C. or less, the temperature rising rate is controlled to be 8 ° C./s or more, thereby suppressing the generation of surface concentrate as much as possible. Furthermore, in the temperature range of annealing furnace temperature: 600 ° C. or higher and A ° C. or lower, by controlling the hydrogen concentration in the atmosphere to be 22 vol% or higher, the oxygen potential at the interface between the steel sheet and the atmosphere is lowered, and internal oxidation is performed. The selective surface diffusion and surface enrichment of Si, Mn, etc. are suppressed without forming. And the outstanding chemical conversion processing property without a scale and unevenness and higher corrosion resistance after electrodeposition coating will be obtained.

The reason why the temperature range for controlling the rate of temperature rise and the hydrogen concentration is 600 ° C. or higher is as follows. In the temperature range below 600 ° C., surface enrichment and internal oxidation that cause problems such as scaling, unevenness, and deterioration of corrosion resistance do not occur. Therefore, the temperature is set to 600 ° C. or higher, which is a temperature range in which the effect of the present invention is exhibited.
The reason why the temperature range is set to A ° C. or lower (A: 700 ≦ A ≦ 900) is as follows. In the temperature range below 700 ° C., the time for controlling the temperature rising rate to 8 ° C./s or more is short, and the effect of the present invention is small. Even if the hydrogen concentration is set to 22 vol% or more, the effect of suppressing surface concentration is not sufficient. For this reason, it is set as 700 degreeC or more. Moreover, when it exceeds 900 degreeC, there is no problem in the effect of this invention, but it becomes disadvantageous from a viewpoint of deterioration of the equipment (roll etc.) in an annealing furnace, and cost increase. Therefore, it shall be 900 degrees C or less.

  The reason why the rate of temperature rise is 8 ° C./s or more and the hydrogen concentration is 22 vol% or more is as follows. The effect of suppressing the surface concentration begins to be recognized when the heating rate is 8 ° C./s or more and the hydrogen concentration is 22 vol% or more. The upper limit of the heating rate is not particularly set, but if it exceeds 500 ° C./s, the effect is saturated and disadvantageous in terms of cost, so 500 ° C./s or less is desirable. The upper limit of the hydrogen concentration is not particularly set, but if it exceeds 75 vol%, the effect is saturated and disadvantageous in terms of cost, so 75 vol% or less is preferable. In addition, in order to make a temperature increase rate 8 degrees C / s or more, heating, such as a radiant tube and an induction heater, can be applied.

Next, the steel component composition of the high-strength 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 elongation is reduced, the material is deteriorated, and the weldability is further deteriorated. Therefore, the C content is 0.01% or more and 0.18% or less.

Si: 0.4-2.0%
Si is an element effective for strengthening steel and improving elongation to obtain a good material, and 0.4% or more is necessary to obtain the intended strength of the present invention. If Si is less than 0.4%, the strength within the scope of the present invention cannot be obtained, and there is no particular problem with chemical conversion treatment. On the other hand, when it exceeds 2.0%, the steel strengthening ability and the effect of improving elongation become saturated. Furthermore, it becomes difficult to improve the chemical conversion processability. Therefore, the Si amount is set to 0.4% 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 secure weldability and the 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. Furthermore, the surface concentration of Al increases and it becomes difficult to improve chemical conversion properties. 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 chemical conversion processability is greatly deteriorated, and even with the present invention, it is difficult to improve the chemical conversion processability. 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 and corrosion resistance deteriorate, so the content is made 0.01% or less.

For the following reasons, B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0% One or more elements selected from Mo: 0.05-1.0%, Cu: 0.05-1.0%, Ni: 0.05-1.0% are added as necessary. May be.
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%, chemical conversion processability deteriorates. Therefore, when it contains, B amount shall be 0.001% or more and 0.005% or less. In addition, when it is judged that it is not necessary to add for the purpose of improving a mechanical characteristic, it is not necessary to add.

Nb: 0.005 to 0.05%
If Nb 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 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%, chemical conversion processability is deteriorated. 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 an effect of promoting quenching. On the other hand, if it exceeds 1.0%, the surface of Cr is concentrated, so that the weldability is deteriorated. 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%, the effect of adjusting the strength is difficult to obtain. 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. 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%
If Ni is less than 0.05%, the effect of promoting the formation of residual γ phase is difficult to obtain. 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 steel plate of the present invention and the reason for limitation will be described.

  The steel having the above chemical components is hot-rolled, cold-rolled to obtain a steel plate, and then subjected to chemical conversion treatment after annealing in a continuous annealing facility. In addition, after completion | finish of hot rolling, continuous annealing may be performed as it is, without performing cold rolling. At this time, in the present invention, in the temperature raising process during annealing, in the temperature range of annealing furnace temperature: 600 ° C. or more and A ° C. or less, the temperature raising rate is 8 ° C./s or more and the hydrogen concentration in the atmosphere is set. It controls so that it may become 22 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 sheet, so that not only the rolling cost is increased, but also the surface concentration during annealing is increased, so that the chemical conversion treatment property is deteriorated.

A cold-rolled steel plate or a hot-rolled steel plate is preferably subjected to an electrolytic pickling treatment after continuous annealing. Next, chemical conversion treatment is performed.
In the annealing furnace, a heating process is performed in which the temperature of the steel sheet is raised to a predetermined temperature in the preceding heating zone, and a soaking process is performed in which the temperature is maintained at a predetermined temperature for a predetermined time in the subsequent soaking zone.
As described above, in the temperature raising step (temperature raising process), the temperature inside the annealing furnace: in the temperature range of 600 ° C. or higher and A ° C. or lower, the temperature rising rate is 8 ° C./s or more, and the hydrogen concentration in the atmosphere Is controlled to be 22 vol% or more, and annealing is performed to perform chemical conversion treatment. However, A: 700 ≦ A ≦ 900. The hydrogen concentration in the atmosphere in the heating furnace other than the region where the hydrogen concentration is controlled is not particularly limited. A concentration lower than 22 vol% may be used. A normal hydrogen concentration of 5 to 10 vol% is not a problem. However, if it is less than 1 vol%, the activation effect due to reduction cannot be obtained, and chemical conversion processability may deteriorate. If it exceeds 50 vol%, the cost increases and the effect is saturated. Therefore, the hydrogen concentration is preferably 1 vol% or more and 50 vol% or less.
In addition, the gaseous component in an annealing furnace consists of nitrogen, hydrogen, and an unavoidable impurity. Other gas components may be included as long as the effects of the present invention are not impaired.
There is no particular limitation on the temperature increase rate other than the region where the temperature increase rate is controlled, but if it is 1 ° C./s or less, it takes too much time to increase the production efficiency. If it exceeds 120 ° C./s, the effect may be saturated and the cost may be increased.

  After cooling, quenching and tempering may be performed as necessary. This condition is not particularly limited, but tempering is preferably performed at a temperature of 150 to 400 ° C. This is because the elongation tends to deteriorate when the temperature is less than 150 ° C., and the hardness tends to decrease when the temperature exceeds 400 ° C.

In the present invention, good chemical conversion treatment can be ensured without carrying out electrolytic pickling, but a small amount of surface condensate inevitably generated during annealing is removed to ensure better chemical conversion treatment. For the purpose, it is preferable to perform electrolytic pickling.
The conditions of the electrolytic pickling are not particularly limited, but in order to efficiently remove the inevitably surface-enriched Si and Mn oxides formed after annealing, an alternating electrolysis with a current density of 1 A / dm ² or more is used. It is preferable. The reason for alternating electrolysis is that the pickling effect is small when the steel plate is held at the cathode, and conversely, Fe that is eluted during electrolysis accumulates in the pickling solution while the steel plate is held at the anode. This is because if the Fe concentration increases and adheres to the surface of the steel sheet, problems such as dry dirt occur. Furthermore, the pickling solution used for the electrolytic pickling is not particularly limited, but nitric acid and hydrofluoric acid are not preferable because they are highly corrosive to equipment and require careful handling. Hydrochloric acid is not preferred because it may generate chlorine gas from the cathode. For this reason, use of sulfuric acid is preferable in consideration of corrosivity and environment. The sulfuric acid concentration is preferably 5% by mass or more and 20% by mass or less. If the sulfuric acid concentration is less than 5% by mass, the electrical conductivity will be low, so that the bath voltage during electrolysis will rise and the power load may become large. On the other hand, if it exceeds 20% by mass, a loss due to drag-out is large, which causes a problem in cost. The temperature of the electrolytic solution is preferably 40 ° C. or higher and 70 ° C. or lower. Since the bath temperature rises due to heat generated by continuous electrolysis, the pickling effect may be reduced at less than 40 ° C. Moreover, it may be difficult to maintain the temperature below 40 ° C. Moreover, it is not preferable that temperature exceeds 70 degreeC from a durable viewpoint of the lining of an electrolytic cell.

As described above, the high-strength steel sheet of the present invention is manufactured. And it has the characteristics in a steel plate surface layer structure as follows.
In the steel plate surface layer portion within 100 μm from the steel plate surface, the formation of one or more oxides selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni is total. It is suppressed to 0.050 g / m ² or less per one side.
In a high-strength steel sheet in which Si and a large amount of Mn are added to steel, in order to satisfy the corrosion resistance, it is required to minimize the internal oxidation of the steel sheet surface layer that may be a starting point of corrosion. 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 reducing the oxygen potential in the annealing process in order to ensure chemical conversion treatment. And the surface concentration of these elements is suppressed, and as a result, chemical conversion property is improved. Furthermore, the internal oxidation formed in the steel plate surface layer portion is also suppressed, and the corrosion resistance is improved. Such an effect is obtained by applying one or more kinds of oxidation selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni to the steel sheet surface layer portion within 100 μm from the surface of the base steel plate. It is recognized by suppressing the formation amount of the product to 0.050 g / m ² or less in total. When the total oxide formation amount (hereinafter referred to as internal oxidation amount) exceeds 0.050 g / m ² , the corrosion resistance deteriorates. Moreover, even if the amount of internal oxidation is suppressed to less than 0.0001 g / m ² , the corrosion resistance improving effect is saturated, so the lower limit of the amount of internal oxidation is preferably 0.0001 g / m ² .

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 after removing the black scale, it was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. In addition, some did not implement cold rolling, but the thing with the hot-rolled steel plate (thickness 2.0mm) after black scale removal was also prepared.

  Next, the cold-rolled steel plate and hot-rolled steel plate obtained above were charged into a continuous annealing facility. In the annealing equipment, as shown in Table 2, the temperature rising rate and hydrogen concentration of 600 ° C. or more and A ° C. or less (A: 700 ≦ A ≦ 900) are controlled and annealed by passing through, and then water quenching and 300 ° C. × 140 s. Temporary tempering was performed. Subsequently, electrolytic pickling was performed at 40 ° C. in a 5% by mass sulfuric acid aqueous solution under the current density conditions shown in Table 2 to obtain a test material. The electrolytic pickling was performed by alternating electrolysis for 3 seconds each in the order of the anode and the cathode.

  Note that the hydrogen concentration in the atmosphere other than the region where the hydrogen concentration was controlled was 10 vol%. Moreover, the gaseous component of atmosphere consists of nitrogen gas, hydrogen gas, and unavoidable impurity gas. In the annealing furnace, heating by an all radiant tube and heating by an induction heater were used in combination, so that the heating rate was 8 ° C./s or more.

  Tensile strength (TS) and elongation (El) were measured according to the JIS Z 2241 metal material tensile test method for the specimens obtained as described above.

  In addition, chemical conversion properties and corrosion resistance after electrodeposition coating were investigated. The amount of oxide (internal oxidation amount) present in the steel sheet surface layer portion was measured up to 100 μm immediately below the steel sheet surface layer. The measurement method and evaluation criteria are shown below.

The chemical conversion property evaluation method of chemical conversion property is described below.
A chemical conversion treatment solution (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd. was used as the chemical conversion treatment solution, and the chemical conversion treatment was performed by the following method.
After degreasing with a degreasing liquid Fine Cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washing with water, and then adjusting the surface for 30 s with surface conditioning solution preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. After being immersed in a chemical conversion treatment solution (Palbond L3080) for 120 s, it was washed with water and dried with warm air.
The sample after the chemical conversion treatment was randomly observed with a scanning electron microscope (SEM) at a magnification of 500 times, the scale area ratio of the chemical conversion film was measured by image processing, and the following evaluation was made based on the scale area ratio. Went. ○ is an acceptable level.
○: 10% or less ×: More than 10% Corrosion resistance after electrodeposition coating A test piece having a size of 70 mm × 150 mm was cut out from the test material subjected to chemical conversion treatment obtained by the above method, and PN made by Nippon Paint Co., Ltd. Cationic electrodeposition coating (baking conditions: 170 ° C. × 20 minutes, film thickness 25 μm) was performed with −150 G (registered trademark). Thereafter, the end surface and the side not evaluated were sealed with Al tape, and a cross cut (cross angle 60 °) reaching the ground iron with a cutter knife was used as a test material.

Next, the test material was taken out after being immersed in a 5% by mass NaCl aqueous solution (55 ° C.) for 240 hours, washed with water and dried, and then the tape was peeled off, the peel width was measured, and the following evaluation was performed. ○ is acceptable level ○: Peel width is less than 2.5 mm on one side ×: Peel width is 2.5 mm or more on one side Workability is JIS No. 5 tensile test piece in 90 ° direction from test material to rolling direction In accordance with the provisions of JIS Z 2241, a tensile test is performed at a constant crosshead speed of 10 mm / min, tensile strength (TS / MPa) and elongation (El%) are measured, and when TS is less than 650 MPa TS × El ≧ 22000 was evaluated as good, and TS × El <22000 was evaluated as defective. When TS was 650 MPa or more and less than 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.

The amount of internal oxidation in the region of the steel sheet surface layer of up to 100 μm 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, the present invention example is a high-strength steel sheet containing a large amount of easily oxidizable elements such as Si and Mn, but chemical conversion treatment, corrosion resistance after electrodeposition coating, workability It turns out that it is excellent.
On the other hand, in the comparative example, any one or more of chemical conversion property, corrosion resistance after electrodeposition coating, and workability is inferior.

  The high-strength steel sheet of the present invention is excellent in chemical conversion treatment, corrosion resistance after electrodeposition coating, and workability, and can be used as a surface-treated steel sheet for reducing the weight and increasing the strength of the automobile body itself. 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 (5)

In mass%, C: 0.01 to 0.18%, Si: 0.4 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0%, P: When continuously annealing a steel sheet containing 0.005 to 0.060%, S ≦ 0.01%, the balance being Fe and inevitable impurities,
In the temperature raising process, the temperature inside the annealing furnace: in the temperature range of 600 ° C. or more and A ° C. or less, the temperature raising rate is 8 ° C./s or more, and the hydrogen concentration in the atmosphere is 22 vol% or more , A method for producing a high-strength steel sheet excellent in chemical conversion treatment without hot dip galvanization .
However, A: 700 ≦ A ≦ 900.   The method for producing a high-strength steel sheet excellent in chemical conversion treatment according to claim 1, wherein after the continuous annealing, quenching and tempering are performed, and the tempering is performed at a temperature of 150 to 400 ° C. 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: One or more 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 method for producing a high-strength steel sheet excellent in chemical conversion treatment according to claim 1 or 2 , comprising an element. The method for producing a high-strength steel sheet having excellent chemical conversion treatment properties according to any one of claims 1 to 3, wherein after the continuous annealing, electrolytic pickling is performed in an aqueous solution containing sulfuric acid. Fe produced in the steel sheet surface layer portion within 100μm from the steel plate surface, Si, Mn, Al, P , B, Nb, Ti, Cr, Mo, Cu, oxides of one or more kinds selected from among Ni is, per side It is 0.050 g / m < ² > or less, The manufacturing method of the high strength steel plate excellent in the chemical conversion property as described in any one of Claims 1-4 characterized by the above-mentioned .