JP2013124381A - High-strength steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet that exhibits excellent chemical treatment properties and corrosion resistance after electrodeposition coating, and to provide a manufacturing method therefor.SOLUTION: The steel sheet contains, by mass, 0.01 to 0.18% C, 0.4 to 2.0% Si, 1.0 to 3.0% Mn, 0.001 to 1.0% Al, 0.005 to 0.060% P, and 0.01% or less S, with the remainder comprising Fe and unavoidable impurities. During a heating step when performing continuous annealing on the steel sheet, the atmospheric hydrogen concentration is set to be 26 vol% or more at a heating-furnace temperature range of 600°C or more and A°C or less, the rate of temperature rise is set to be 8°C/s or higher, and the atmospheric dew point is set to be -8°C or higher at a heating-furnace temperature range of higher than A°C and B°C or lower, wherein the range of A is 680≤A≤780, and the range of B is 800≤B≤900.

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 prior art for improving the chemical conversion processability of a high Si content 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, 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.

  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 Si by 0.1% by mass and / or Mn by 1.0% or more by mass%, the surface of the steel sheet 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.

  In the past, the dew point or oxygen concentration was raised by simply increasing the partial pressure of water vapor or oxygen in the entire annealing furnace to excessively oxidize the inside or outside of the steel sheet. Various problems have occurred, such as a problem in the properties, unevenness in the chemical conversion treatment property, and deterioration in corrosion resistance after electrodeposition coating.

  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, a high-strength steel sheet with excellent corrosion resistance after electrodeposition coating can be obtained by performing more advanced control over the structure and structure of the steel sheet surface layer that may become the starting point of corrosion resistance degradation after electrodeposition coating. I found out. Furthermore, it has been found that chemical conversion processability is also improved by performing more advanced control on the structure and structure of the steel sheet surface layer as described above.

  Specifically, in a heating furnace temperature in the heating process: 600 ° C. or more and A ° C. or less (A: 680 ≦ A ≦ 780), the hydrogen concentration in the atmosphere is 26 vol% or more, and the heating rate is 8 Annealing by controlling the dew point in the atmosphere to be −8 ° C. or higher in a temperature range of A ° C./s or higher and the temperature inside the heating furnace: A ° C. to B ° C. or lower (B: 800 ≦ B ≦ 900) Then, chemical conversion treatment is performed. By performing such treatment, selective surface oxidation can be suppressed, surface concentration can be suppressed, and a high-strength steel sheet excellent in chemical conversion treatment properties and corrosion resistance after electrodeposition coating can be obtained.

The high-strength steel sheet obtained by the above method is selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni on the steel sheet surface layer within 100 μm from the steel sheet surface. At least one kind of oxide is formed on one side of 0.020 to 0.200 g / m ² , and in a region within 10 μm from the steel plate surface, crystalline Si, Mn-based oxidation is within grains within 1 μm from the grain boundary of the steel plate. It becomes the organization and structure where things exist. This prevents deterioration of corrosion resistance after electrodeposition coating and is excellent in chemical conversion treatment.

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 performing continuous annealing on a steel plate consisting of Fe and inevitable impurities, the heating furnace temperature: Temperature range of 600 ° C or higher and A ° C or lower with hydrogen concentration in the atmosphere of 26 vol% or higher, heating rate of 8 ° C / s or higher, furnace temperature: temperature range of A ° C to B ° C or lower Dew point: A method for producing a high-strength steel sheet, which is carried out at -8 ° C or higher.
However, A: 680 ≦ A ≦ 780 and B: 800 ≦ B ≦ 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 according to the above [1], comprising at least a seed element.
[3] The method for producing a high-strength steel sheet 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, Cr are produced by the production method according to any one of [1] to [3] above the steel sheet surface layer within 100 μm from the steel sheet surface. One or more oxides selected from Mo, Cu, and Ni are formed at 0.020 to 0.200 g / m ² per side, and from the grain boundary of the steel sheet in a region within 10 μm from the steel sheet surface. A high-strength steel sheet characterized by the presence of crystalline Si and Mn-based oxides in grains within 1 μm.

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 the heating process in the heating furnace, the temperature in the heating furnace: 600 ° C. or more and A ° C. or less, the hydrogen concentration in the atmosphere is 26 vol% or more, the heating rate is 8 ° C./s or more, and the heating furnace temperature : A temperature range of A ° C to B ° C is performed at a dew point in the atmosphere of -8 ° C or higher.

Temperature in the heating furnace: Control the hydrogen concentration in the atmosphere at a temperature range of 600 ° C. or higher and A ° C. or lower to 26 vol% or higher, the temperature rising rate to 8 ° C./s or higher, and the hydrogen concentration in the controlled atmosphere to 26 vol% or higher. As a result, the oxygen potential on the surface of the steel sheet is lowered, and selective surface oxidation (surface enrichment) can be suppressed. However, there is a case where surface concentration cannot be suppressed to the extent that generation of scale and unevenness at the time of chemical conversion treatment can be completely suppressed only by a decrease in oxygen potential. For this reason, in addition to controlling the hydrogen concentration in the atmosphere to 26 vol% or more, the temperature increase rate is further controlled to 8 ° C./s or more. In this way, by controlling the hydrogen concentration in the atmosphere and the rate of temperature increase, the temperature enrichment surface is passed as quickly as possible, and the surface enrichment is further suppressed to the extent that no scaling or unevenness occurs during the chemical conversion treatment. Is possible. In order to set the temperature rising rate to 8 ° C./s or more, heating such as a radiant tube or an induction heater can be applied. The upper limit of the hydrogen concentration is not particularly set, but if it exceeds 75 vol%, the effect is saturated and the cost increases, so 75 vol% or less is desirable. The upper limit of the rate of temperature rise is not particularly set, but if it exceeds 500 ° C./s, the effect is saturated and the cost increases.

  The reason why the temperature is 600 ° C. or more and A ° C. or less (A: 680 ≦ A ≦ 780) is as follows. In the temperature range below 600 ° C., the amount of easily oxidizable elements (Si, Mn, etc.) that diffuse on the surface is small due to the low temperature. Further, the temperature concentration is originally in a low temperature range, and the chemical conversion treatment property is not hindered. Therefore, it shall be 600 degreeC or more. On the other hand, the reason why the upper limit temperature is set to A ° C is that, as described later, in a temperature range exceeding A ° C, by setting the dew point in the atmosphere to -8 ° C or higher, internal oxidation is promoted and surface concentration is almost complete. This is because it will not happen.

Temperature in the heating furnace: A temperature range of A ° C. to B ° C. or less Dew point in the atmosphere: −8 ° C. or more Control heating furnace temperature: A ° C. to B ° C. or less (A: 680 ≦ A ≦ 780, B: 800 ≦ In a limited temperature range of B ≦ 900), the dew point in the atmosphere is controlled to be −8 ° C. or higher, and annealing is performed, so that an easily oxidizable element (Si, Mn) is present within 10 μm from the steel plate surface. And the like (hereinafter referred to as internal oxide) in an appropriate amount, and selective oxidation of the surface of the steel plate with easily oxidizable elements (Si, Mn, etc.) in steel that degrade the chemical conversion property after annealing (Si, Mn, etc.) Henceforth, it is possible to suppress surface thickening).

  The reason why the lower limit temperature A is set to 680 ≦ A ≦ 780 is as follows. In the temperature range lower than 680 ° C., even if the dew point is controlled to be −8 ° C. or higher, almost no internal oxide is formed. Internal oxidation begins to occur above 680 ° C. Further, when the temperature is raised to a temperature exceeding 780 ° C. without controlling the dew point, the surface is heavily concentrated, so that the inward diffusion of oxygen is inhibited and internal oxidation is less likely to occur. Therefore, the dew point must be controlled to at least -8 ° C from a temperature range of 780 ° C or lower. From the above, the allowable range of A is 680 ≦ A ≦ 780, and for the reasons described above, it is desirable that A be as low as possible within this range.

  The reason why the upper limit temperature B is set to 800 ≦ B ≦ 900 is as follows. The mechanism for suppressing surface concentration is as follows. By forming an internal oxide, a region (hereinafter referred to as a deficient layer) in which the solid solution amount of an easily oxidizable element (Si, Mn, etc.) within 10 μm from the steel sheet surface is reduced is formed. Suppresses the surface diffusion of oxidizable elements (Si, Mn, etc.) from In order to form an internal oxide and to form a deficient layer sufficient to suppress surface concentration, B needs to satisfy 800 ≦ B ≦ 900. When the temperature is lower than 800 ° C., the internal oxide is not sufficiently formed. On the other hand, if the temperature exceeds 900 ° C., the amount of internal oxidation formed becomes excessive, which becomes a starting point for corrosion resistance deterioration after electrodeposition coating.

The reason why the dew point in the temperature range from A ° C. to B ° C. is -8 ° C. or higher is as follows. By increasing the dew point, it is possible to increase the O ₂ potential resulting from the decomposition of H ₂ O and promote internal oxidation. In the temperature range below -8 ° C, the amount of internal oxidation formed is small. Further, the upper limit of the dew point is not particularly defined, but if it exceeds 80 ° C., the amount of Fe oxidation increases, and there is concern about deterioration of the heating furnace wall and roll.

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 decreases, the material deteriorates, and further the weldability deteriorates. 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. 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, when 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, and then annealed in a continuous annealing facility, followed by chemical conversion treatment. 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 heating process during annealing, the temperature in the heating furnace: 600 ° C. or more and A ° C. or less (A: 680 ≦ A ≦ 780), the hydrogen concentration in the atmosphere is 26 vol% or more, and The heating rate is 8 ° C./s or higher, and the temperature in the heating furnace is higher than A ° C. and lower than B ° C. (B: 800 ≦ B ≦ 900) at a dew point in the atmosphere of −8 ° C. or higher. . This is the most important requirement in the present invention. In this way, by controlling the dew point, that is, the oxygen partial pressure in the atmosphere in the annealing process, the oxygen potential is increased and the easily oxidizable elements such as Si and Mn are internally oxidized in advance immediately before the chemical conversion treatment, and Si and Mn in the steel sheet surface layer portion. The activity of is reduced. And the surface concentration of these elements is suppressed, and as a result, chemical conversion property improves.

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.

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 heating (annealing) furnace, a heating process is performed in which the steel sheet is heated 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. Next, a cooling process is performed.
As described above, in the temperature range of the heating furnace temperature: 600 ° C. or more and A ° C. or less (A: 680 ≦ A ≦ 780), the hydrogen concentration in the atmosphere is 26 vol% or more, and the heating rate is 8 ° C. in a temperature range of A ° C. to B ° C. or less (B: 800 ≦ B ≦ 900), so that the dew point in the atmosphere is −8 ° C. or higher. Control and perform annealing. The hydrogen concentration in the atmosphere in the heating furnace other than the region where the hydrogen concentration is controlled is not particularly limited, but if it is less than 1 vol%, the activation effect due to the reduction cannot be obtained and the chemical conversion treatment performance deteriorates. If it exceeds 75 vol%, the cost increases and the effect is saturated. Therefore, the hydrogen concentration is preferably 1 vol% or more and 75 vol% or less. The dew point in the atmosphere in the heating furnace other than the region where the dew point is controlled is not particularly limited. Preferably it is the range of -50 degreeC--10 degreeC. In addition, since a normal dew point is about -40 to -30 degreeC, the dew point of -8 degreeC or more is realizable by replenishing moisture in a furnace. Moreover, the gaseous component in a heating 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, the temperature increase takes too much time and the production efficiency may decrease. If it exceeds 120 ° C./s, the effect may be saturated and the cost may increase.

  Further, when compared under the same annealing conditions, the surface enrichment amount of Si and Mn increases in proportion to the amount of Si and Mn in the steel. In the case of the same steel type, in a relatively high oxygen potential atmosphere, since Si and Mn in the steel move to internal oxidation, the amount of surface enrichment decreases as the oxygen potential in the atmosphere increases. Therefore, when the amount of Si and Mn in steel is large, it is necessary to increase the oxygen potential in the atmosphere by increasing the dew point.

  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 lower 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 for 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, the loss due to drag-out may be large, which may cause 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, 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.
One or more oxides selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni are added to the surface of the steel plate within 100 μm from the surface of the steel sheet. It is formed at 0.020-0.200 g / m ² per unit. Further, in a region within 10 μm from the steel plate surface, crystalline Si and Mn-based oxides exist in grains within 1 μm from the steel plate crystal grain boundary. The crystalline Si and Mn-based oxides are determined from the intensity ratio of TEM-EDX to determine whether they are Si and Mn-based oxides, and whether they are crystalline are confirmed by electron diffraction.
In high-strength steel sheets with Si and a large amount of Mn added to the steel, in order to satisfy the corrosion resistance after electrodeposition coating, the structure and structure of the steel sheet surface layer, which may be the starting point of corrosion cracking, is improved. Need to control. Therefore, in the present invention, first, the dew point control is performed as described above in order to increase the oxygen potential in the annealing process in order to ensure chemical conversion treatment. As a result, by increasing the oxygen potential, easily oxidizable elements such as Si and Mn are internally oxidized in advance immediately before the chemical conversion treatment, and the activities of Si and Mn in the steel sheet surface layer portion are lowered. And the surface concentration of these elements is suppressed, and as a result, chemical conversion property and corrosion resistance after electrodeposition coating are improved. Furthermore, this improvement effect is achieved by at least one oxidation selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni on the steel sheet surface layer portion within 100 μm from the steel sheet surface. A thing will exist 0.020 g / m < ² > or more per one side. On the other hand, if it exceeds 0.200 g / m ² , there is a concern that it may become a starting point of corrosion cracking, and the effect of improving chemical conversion treatment is saturated, so the upper limit is made 0.200 g / m ² .

  Moreover, when an internal oxide exists only in a steel plate crystal grain boundary and does not exist in a grain, the grain boundary diffusion of an easily oxidizable element in steel can be suppressed, but the intragranular diffusion may not be sufficiently suppressed. Therefore, in the present invention, as described above, in the heating furnace temperature: 600 ° C. or more and A ° C. or less (A: 680 ≦ A ≦ 780), the hydrogen concentration in the atmosphere is 26 vol% or more, and the rate of temperature increase : By controlling the temperature range of 8 ° C./s or more and the temperature in the heating furnace: A ° C. to B ° C. or less (B: 800 ≦ B ≦ 900) so that the dew point in the atmosphere is −8 ° C. or more, Internal oxidation is performed not only in the grain boundaries of the steel sheet but also in the grains. Specifically, in a region within 10 μm from the steel sheet surface layer, crystalline Si and Mn-based oxides exist in grains within 1 μm from the steel plate crystal grain boundary. The presence of oxide in the steel plate crystal grains reduces the amount of solute Si and Mn in the steel plate crystal grains near the oxide. As a result, concentration on the surface due to intragranular diffusion of Si and Mn can be suppressed.

  The structure of the steel plate surface layer of the high-strength steel plate obtained by the production method of the present invention is as described above. For example, there is no problem even if the oxide grows in a region exceeding 100 μm from the steel plate surface. . Further, in the region exceeding 10 μm from the steel plate surface, there is no problem even if crystalline Si and Mn-based oxides exist in grains within 1 μm from the steel plate crystal grain boundary.

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 inside the heating furnace, the hydrogen concentration, the heating rate and the dew point were controlled and passed through the plate, and then tempered between 300 ° C. and 140 s after water quenching. 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.

  In addition, the dew point of the heating furnace other than the region where the dew point was controlled was -35 ° C. The atmospheric gas components are nitrogen gas, hydrogen gas, and unavoidable impurity gas, and the dew point is controlled to be -8 ° C or higher. Was separately installed, hydrogen gas was introduced into and mixed with humidified nitrogen gas, and this was introduced into a heating furnace to control the dew point of the atmosphere. The hydrogen concentration in the atmosphere other than the region where the hydrogen concentration is controlled was 10 vol%.

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

<Chemical conversion processability>
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 liquid preparen Z (registered trademark) manufactured by Nippon 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 ×: Over 10% <Corrosion resistance after electrodeposition coating>
A test piece having a size of 70 mm × 150 mm was cut out from the sample for chemical conversion treatment obtained by the above method, and was subjected to cationic electrodeposition coating with PN-150G (registered trademark) manufactured by Nippon Paint Co., Ltd. (baking conditions: 170 ° C. × 20 minutes, film thickness 25 μm). 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 ○: peeling width is less than 2.5 mm on one side ×: peeling width is 2.5 mm or more on one side <workability>
As for workability, a JIS No. 5 tensile test piece was sampled from the test material in the direction of 90 ° with respect to the rolling direction, and a tensile test was conducted at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241. (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 steel sheet surface layer up to 100 μm>
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 specimen before annealing), in the present invention, the surface layer portions on both sides of the specimen after continuous annealing are 100 μm or more, directly below the steel sheet surface. In the case of the amount of internal oxide up to 10 μm, the oxygen concentration in the steel is measured by polishing 10 μm or more, the measured value is the amount of oxygen OH contained in the material, and the thickness of the test material after continuous annealing The oxygen concentration in the steel in the entire direction was measured, and the measured value was defined as the oxygen amount OI after internal oxidation. The difference between OI and OH (= OI-OH) was calculated using the oxygen amount OI after the internal oxidation of the test material thus obtained and the oxygen amount OH contained in the material, and further a 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 treatment 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 (4)

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 steel sheet containing 0.005 to 0.060%, S ≦ 0.01%, the balance being Fe and inevitable impurities, is subjected to continuous annealing,
In the heating process, the temperature in the heating furnace: 600 ° C. or more and A ° C. or less, the hydrogen concentration in the atmosphere is 26 vol% or more, the heating rate is 8 ° C./s or more, and the heating furnace temperature is over A ° C. A method for producing a high-strength steel sheet, wherein a temperature range of B ° C. or lower is performed at a dew point in the atmosphere of −8 ° C. or higher.
However, A: 680 ≦ A ≦ 780 and B: 800 ≦ B ≦ 900.   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 strength steel plate of Claim 1 characterized by the above-mentioned.   3. The method for producing a high-strength steel sheet according to claim 1, wherein after the continuous annealing, electrolytic pickling is performed in an aqueous solution containing sulfuric acid. It manufactures by the manufacturing method as described in any one of Claims 1-3, and Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu is formed in the steel plate surface layer part within 100 micrometers from the steel plate surface. , At least one oxide selected from Ni is formed at 0.020 to 0.200 g / m ² per side, and in a region within 10 μm from the surface of the steel plate, grains within 1 μm from the grain boundary of the steel plate A high-strength steel sheet characterized by the presence of crystalline Si and Mn-based oxides.