TWI491741B - High strength steel sheet and method for manufacturing the same - Google Patents

Description

High-strength steel plate and manufacturing method thereof

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

In recent years, from the viewpoint of improving the fuel cost of automobiles and improving the safety of conflicts between automobiles, the use of the strength of the vehicle body material has been sought to reduce the thickness of the vehicle body, and the demand for lightweight and high strength of the vehicle body has been increasing. . Therefore, the application of high-strength steel sheets to automobiles is being promoted.

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

In order to increase the strength and ductility of the steel sheet, the addition of Si is effective. However, in the case of continuous annealing, even in the case where the oxidation of Fe does not occur (reduction of Fe oxide) in the environment of reducing N ₂ + H ₂ gas, Si is oxidized, and Si is formed on the outermost layer of the steel sheet. Oxide (SiO ₂ ). Since this SiO ₂ inhibits the formation reaction of the chemical conversion film in the chemical conversion treatment, a minute region (hereinafter also referred to as a void) in which the chemical conversion film is not formed is formed, and the chemical conversion processability is lowered.

As a conventional technique for improving the chemical conversion treatment property of a high-Si 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 by an electroplating method. However, this method requires an additional plating apparatus, and there are problems in that steps are increased and costs are also increased.

Further, in Patent Document 2, the Mn/Si ratio is defined, and in Patent Document 3, the phosphoric acid treatability is improved by the addition of Ni. However, the effect depends on the Si content in the steel sheet, and further improvement is required for a steel sheet having a high Si content.

Further, Patent Document 4 discloses a method of forming an internal oxide layer composed of a Si-containing oxide by setting the dew point during annealing to -25 to 0 ° C to a depth of 1 μm on the surface of the steel sheet substrate. The proportion of the Si-containing oxide in the surface length of the steel sheet of 10 μm was 80% or less. However, in the case of the method described in Patent Document 4, it is assumed that the region where the dew point is controlled is the entire furnace, and therefore the controllability of the dew point is difficult, and it is difficult to perform stable operation. Further, when the annealing is performed under the control of the unstable dew point, the distribution of the internal oxide formed in the steel sheet is observed to be uneven, and the processing property in the longitudinal direction or the width direction of the steel sheet may be uneven (integral or Part of it has a hole). Further, in the case where the chemical conversion treatment property is improved, since the Si-containing oxide is present directly under the chemical conversion treatment film, there is a problem that the corrosion resistance after the electrocoating is poor.

Further, Patent Document 5 describes a method in which an steel sheet is heated to a recrystallization temperature in a reducing environment by cooling the steel sheet to an evaporation temperature of 350 to 650 ° C in an oxidizing atmosphere. However, in this method, the thickness of the oxide film formed on the surface of the steel sheet differs depending on the method of oxidation, and oxidation may not be sufficiently performed, or the oxide film may become too thick, and may occur in annealing in a subsequent reducing environment. The surface of the oxide film is deteriorated or peeled off. In the examples, the technique of performing oxidation in the atmosphere is described, but there is a problem that oxidation in the atmosphere generates a thick oxide to make subsequent reduction difficult, or a reduction environment having a high hydrogen concentration is required.

Further, Patent Document 6 describes a method of using a cold-rolled steel sheet containing 0.1% or more of Si and/or 1.0% or more of Mn by mass%, at a steel sheet temperature of 400 ° C or more and an oxidizing atmosphere of iron on the surface of the steel sheet. An oxide film is formed, and then the oxide film on the surface of the steel sheet is reduced in a reducing atmosphere of iron. Specifically, at 400 ° C or higher, the Fe on the surface of the steel sheet is oxidized using a direct fire furnace having an air ratio of 0.93 or more and 1.10 or less, and then annealed in a N ₂ +H ₂ gas atmosphere in which Fe oxide is reduced, thereby suppressing the reaction. A method of forming oxidized layer of Fe on the outermost surface by oxidizing SiO _{2 which} is deteriorated in handleability to the outermost surface. Patent Document 6 does not specifically describe the heating temperature of the direct fire furnace. However, when Si is contained in a large amount (about 0.6% or more), the amount of oxidation of Si which is more likely to be oxidized than Fe is increased, and oxidation of Fe is suppressed, or Fe is suppressed. The oxidation itself will become too small. As a result, the formation of the surface Fe reduction layer after reduction may be insufficient, or SiO ₂ may be present on the surface of the reduced steel sheet to form a pore of the film.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 5-320952

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-323969

[Patent Document 3] Japanese Patent Laid-Open No. Hei 6-10096

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2003-113441

[Patent Document 5] Japanese Patent Laid-Open No. 55-145122

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2006-45615

In view of the above, it is an object of the present invention to provide a high-strength steel sheet having excellent chemical conversion treatability and corrosion resistance after electrocoating even when the Si content is large, and a method for producing the same.

In the prior art, the dew point or the oxygen concentration is increased by simply increasing the partial pressure of water vapor or the partial pressure of oxygen in the annealing furnace to excessively oxidize the inside of the steel sheet. Therefore, as described above, there is a dew point or The problem of oxidation control property may cause various problems such as unevenness in chemical conversion treatment, deterioration of corrosion resistance after electrocoating, and the like. Therefore, the present inventors reviewed a new method that was not taken in the conventional idea to solve the problem. As a result, it has been found that high-strength steel sheets having excellent chemical conversion properties and corrosion resistance after electrocoating can be obtained by highly controlling the structure and structure of the surface layer of the steel sheet which may become the starting point of corrosion resistance after electrocoating. Specifically, in the heating process, the temperature in the heating furnace is limited to a temperature range of A ° C or more and B ° C or less (A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900), and the dew point of the environment becomes Controlled by -10 ° C or more, and annealed to carry out a chemical conversion treatment. By performing such a treatment, surface oxidation can be selectively controlled, and surface concentration can be suppressed, and a high-strength steel sheet excellent in chemical conversion treatability and corrosion resistance after electrocoating can be obtained. Further, the excellent chemical conversion property means an appearance which has no voids and is uneven after the chemical conversion treatment.

In addition, the high-strength steel sheet obtained by the above method is formed from the surface layer portion of the steel sheet within 100 μm from the surface of the steel sheet, and is formed of Fe, Si, Mn, Al, and P at a ratio of 0.010 to 0.50 g/m ² per one side. B, Nb, Ti, Cr, Mo, Cu, and Ni are at least one or more oxides selected from the surface of the steel sheet, and crystallinity is precipitated in the base iron particles within 1 μm from the grain boundary. The structure and structure of Si and Mn-based oxides. Thereby, deterioration of corrosion resistance after electrocoating can be achieved, and the chemical conversion treatability is excellent.

The present invention has been completed based on the above findings and is characterized as follows.

[1] A method for producing a high-strength steel sheet, comprising C: 0.01 to 0.18%, Si: 0.4 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0%, and P: 0.005 in mass%. 0.060%, S≦0.01%, and the rest of the steel sheet consisting of Fe and unavoidable impurities is subjected to continuous annealing. During the heating process, the temperature in the heating furnace is above A°C and below B°C. Dew point: above -10 °C. Among them, A: 600≦A≦780, B:800≦B≦900.

[2] The method for producing a high-strength steel sheet according to the above [1], wherein the component composition of the steel sheet further contains one or more elements selected from the following elements in mass%; 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%, and Ni: 0.05 to 1.0%.

[3] The method for producing a high-strength steel sheet according to [1] or [2] above, wherein after the continuous annealing, electrolytic pickling is performed in an aqueous solution containing sulfuric acid.

[4] A high-strength steel sheet produced by the production method according to any one of the above [1] to [3], wherein the surface layer portion of the steel sheet within 100 μm from the surface of the steel sheet is 0.010 to 0.50 g/m per one side. ⁽²⁾ forming at least one or more oxides selected from the group consisting of Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni, and further forming a region within 10 μm of the surface of the steel sheet. Crystalline Si and Mn-based oxides are present in the crystal grains within 1 μm of the crystal grain boundary of the steel sheet.

Further, in the present invention, the high strength means that the tensile strength TS is 340 MPa or more. Moreover, the high-strength steel sheet according to the present invention includes either a cold-rolled steel sheet or a hot-rolled steel sheet.

According to the present invention, even when the Si content is large, a high-strength steel sheet having excellent chemical conversion properties and corrosion resistance after electrocoating can be obtained.

Hereinafter, the present invention will be specifically described. In addition, in the following description, the content unit of each element of the steel component composition is "% by mass", and unless otherwise specified, it is represented by "%".

First, the annealing environment conditions for determining the surface structure of the steel sheet, which are the most important requirements of the present invention, will be described.

In the heating process in the annealing furnace, in the temperature range of the heating furnace: A ° C or more and B ° C or less (A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900), the ambient dew point is In the case of -10 ° C or more, the film is continuously annealed and then subjected to a chemical conversion treatment, whereby an oxide such as an easily oxidizable element (Si, Mn) is present in an amount of 10 μm or less in the surface layer of the steel sheet (hereinafter also referred to as internal Oxidation) suppresses the selective surface oxidation of Si, Mn, or the like in the steel sheet surface layer in the steel which deteriorates the processability after annealing (hereinafter also referred to as surface concentration).

The reason why the lower limit temperature A is set to 600 ≦ A ≦ 780 is as follows. In the temperature range of less than 600 ° C, even if the dew point control is not performed and internal oxidation is not formed, since it is originally a temperature region where the surface concentration is small, the formation processability is not inhibited. Further, when the dew point is not controlled and the temperature is raised to a temperature exceeding 780 ° C, since the surface is concentrated, the internal diffusion of oxygen is inhibited, and internal oxidation is less likely to occur. Therefore, it is necessary to control the dew point of at least -10 ° C from at least a temperature region of 780 ° C or less. From the above, the allowable range of A is A: 600 ≦ A ≦ 780. For the above reasons, A is preferably as low as possible within this range.

The reason why the upper limit temperature B is set to 800 ≦ B ≦ 900 is as follows. By forming internal oxidation, a region (hereinafter referred to as a lacking layer) which reduces the amount of solid solution of an easily oxidizable element (Si, Mn, etc.) within 10 μm of the surface layer of the steel sheet is formed, and the oxidizable element is suppressed from the steel to the surface. diffusion. In order to form the internal oxidation and form a lacking layer sufficient to suppress surface enrichment, B must be set at 800 ≦ B ≦ 900. When it is lower than 800 ° C, internal oxidation cannot be sufficiently formed. Moreover, when it exceeds 900 °C, the amount of formation of internal oxidation is excessive, and it becomes a starting point of deterioration of corrosion resistance after electrocoating.

The reason why the dew point of the temperature range of A ° C or more and B ° C or less is set to -10 ° C or more is as follows. By raising the dew point, the O _{2 position} generated by the decomposition of H ₂ O can be increased to promote internal oxidation. In the temperature region below -10 ° C, the amount of internal oxidation formed is small. Further, the upper limit of the dew point is not particularly limited. However, if it exceeds 90 ° C, the amount of oxidation of Fe increases, and the annealing furnace wall or the roll deteriorates. Therefore, it is preferably 90 ° C or lower.

Next, the steel component composition of the high-strength steel sheet to which the present invention is applied will be described.

C: 0.01~0.18%

The C system forms a granulated iron or the like as a steel structure, thereby improving workability. Therefore, 0.01% or more is required. On the other hand, when it exceeds 0.18%, the elongation is lowered, the material is deteriorated, and the weldability is deteriorated. Therefore, the amount of C is set to be 0.01% or more and 0.18% or less.

Si: 0.4~2.0%

The Si system strengthens the steel and increases the elongation, and is used to obtain an effective element of a good material, and 0.4% or more is required in order to obtain the strength of the object of the present invention. If Si is less than 0.4%, the strength of the application range of the present invention cannot be obtained, and the chemical conversion processability is not a problem. On the other hand, if it exceeds 2.0%, the steel reinforcing ability or the elongation improving effect is saturated. In addition, improvement in chemical conversion process becomes difficult. Therefore, the amount of Si is set to be 0.4% or more and 2.0% or less.

Mn: 1.0~3.0%

An effective element for increasing the strength of Mn-based steel. In order to ensure mechanical properties and strength, it must be 1.0% or more. On the other hand, when it exceeds 3.0%, it is difficult to ensure the balance of weldability or strength and ductility. Therefore, the amount of Mn is set to be 1.0% or more and 3.0% or less.

Al: 0.001~1.0%

Al is added for the purpose of deacidification of molten steel. The deacidification effect of the molten steel can be obtained by 0.001% or more. On the other hand, if it exceeds 1.0%, the cost will increase. Further, the surface concentration of Al is increased, and it becomes difficult to improve the processability. Therefore, the amount of Al is set to be 0.001% or more and 1.0% or less.

P: 0.005~0.060% or less

P is one of the elements inevitably contained, and if it is intended to be less than 0.005%, there is a concern that the cost increases, so it is 0.005% or more. On the other hand, when P contains more than 0.060%, the weldability may deteriorate. Further, the deterioration of the chemical conversion treatment property is fierce, and even with the present invention, it is difficult to improve the chemical conversion processability. Therefore, the amount of P is set to be 0.005% or more and 0.060% or less.

S≦0.01%

The S system inevitably contains one of the elements. The lower limit is not specified. When it is contained in a large amount, the weldability and the corrosion resistance are deteriorated, so that it is made 0.01% or less.

In addition, in order to control the balance between strength and ductility, it is also possible to add from 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%, and Ni: 0.05 to 1.0% of one or more selected elements.

The reason for the appropriate addition amount in the case of adding these elements is as follows.

B: 0.001~0.005%

If B is less than 0.001%, it is difficult to obtain a quenching promoting effect. On the other hand, when it exceeds 0.005%, the chemical conversion processability will deteriorate. Therefore, in the case of being contained, the amount of B is set to be 0.001% or more and 0.005% or less.

Nb: 0.005~0.05%

If Nb is less than 0.005%, it is difficult to obtain the effect of strength adjustment. On the other hand, if it exceeds 0.05%, the cost will increase. Therefore, in the case of being contained, the amount of Nb is set to be 0.005% or more and 0.05% or less.

Ti: 0.005~0.05%

If Ti is less than 0.005%, it is difficult to obtain the effect of strength adjustment. On the other hand, when it exceeds 0.05%, the chemical conversion treatability deteriorates. Therefore, in the case of being contained, the amount of Ti is set to be 0.005% or more and 0.05% or less.

Cr: 0.001~1.0%

If Cr is less than 0.001%, it is difficult to obtain a quenching promoting effect. On the other hand, when it exceeds 1.0%, surface-concentration of Cr will occur, and the weldability will deteriorate. Therefore, in the case of being contained, the amount of Cr is set to 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 strength adjustment. On the other hand, if it exceeds 1.0%, the cost will increase. Therefore, in the case of being contained, the amount of Mo is set to be 0.05% or more and 1.0% or less.

Cu: 0.05~1.0%

When Cu is less than 0.05%, it is difficult to obtain a residual γ phase formation promoting effect. On the other hand, if it exceeds 1.0%, the cost will increase. Therefore, in the case of being contained, the amount of Cu is set to be 0.05% or more and 1.0% or less.

Ni: 0.05~1.0%

If Ni is less than 0.05%, it is difficult to obtain a residual γ phase formation promoting effect. On the other hand, if it exceeds 1.0%, the cost will increase. Therefore, in the case of being contained, the amount of Ni is set to be 0.05% or more and 1.0% or less.

The rest of the above is Fe and unavoidable impurities.

Next, the method for producing the high-strength steel sheet of the present invention and the reasons for its limitation will be described.

For example, after the steel having the above chemical composition is hot rolled, cold rolling is performed, followed by annealing in a continuous annealing apparatus, and then chemical conversion treatment is performed. Further, at this time, in the heating process during annealing in the present invention, the temperature in the heating furnace is a temperature range of A ° C or more and B ° C or less (A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900). Take the environment dew point: -10 °C or more. This is the most important requirement in the present invention. By controlling the dew point (that is, the oxygen partial pressure in the environment) in the annealing step to increase the oxygen potential energy, the internal oxidation is performed in advance before the chemical treatment of the Si or Mn which is an oxidizable element is performed. The activity of Si and Mn in the surface layer is lowered. Then, external oxidation of the elements is suppressed, and as a result, chemical conversion treatability can be improved. Further, in the above, there is a case where the cold rolling is performed without performing cold rolling after the completion of the hot rolling.

[hot rolling]

It can be carried out under normal conditions.

[pickling]

It is preferred to carry out a pickling treatment after hot rolling. The black rust generated on the surface is removed by the pickling step, followed by cold rolling. Further, the pickling conditions are not particularly limited.

[Cold rolling]

It is preferably carried out at a reduction ratio of 40% or more and 80% or less. When the rolling reduction ratio is less than 40%, the recrystallization temperature is lowered, and thus the mechanical properties are easily deteriorated. On the other hand, when the rolling reduction ratio exceeds 80%, it belongs to a high-strength steel sheet, so that not only the rolling cost but also the surface concentration during annealing is increased, and thus the chemical conversion property is deteriorated.

The cold-rolled steel sheet or the hot-rolled steel sheet is annealed, followed by a chemical conversion treatment.

In the annealing furnace, a heating step of heating the steel sheet to a predetermined temperature is carried out in the previous stage, and a soaking step of maintaining the tempering temperature in the subsequent stage at a predetermined temperature for a predetermined time is performed. Next, a cooling step is performed.

Further, as described above, in the temperature range of the heating furnace: A ° C or more and B ° C or less (A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900), the dew point of the environment becomes -10 ° C or more. Control is performed and annealing is performed. The dew point of the atmosphere in the annealing furnace other than the region of A ° C or more and B ° C or less is not particularly limited, but is preferably -50 ° C to -10 ° C.

The gas component in the annealing furnace is composed of nitrogen, hydrogen, and unavoidable impurities. Any gas component may be contained as long as it does not impair the effects of the present invention. When the hydrogen concentration in the atmosphere in the annealing furnace is less than 1% by volume, the activation effect by reduction cannot be obtained, and the chemical conversion treatment property is deteriorated. The upper limit is not specifically defined, and if it exceeds 50% by volume, the cost is increased and the effect is saturated. Therefore, the hydrogen concentration is preferably 1% by volume or more and 50% by volume or less. Further, the gas component in the annealing furnace is composed of nitrogen gas and an unavoidable impurity gas in addition to hydrogen gas. Any gas component may be contained as long as it does not impair the effects of the present invention.

After cooling from a temperature range of 750 ° C or higher, quenching and tempering may be performed as needed. The conditions are not particularly limited, but the tempering is preferably carried out at a temperature of from 150 to 400 °C. If the temperature is less than 150 ° C, the elongation tends to deteriorate, and if it exceeds 400 ° C, the hardness tends to decrease.

In the present invention, even if electrolytic pickling is not performed, good chemical conversion treatability can be ensured, but it is preferred to remove a small amount of surface concentrate which is inevitably generated during annealing, and it is preferable to ensure better chemical conversion treatability. It is subjected to electrolytic pickling.

The conditions of the electrolytic pickling are not particularly limited, and it is preferably an alternating current having a current density of 1 A/dm ² or more in order to efficiently remove oxides of Si or Mn which are inevitably formed on the surface after annealing. The reason for the alternating current is that if the steel sheet is maintained at the cathode, the pickling effect is small. On the other hand, if the steel sheet is maintained at the anode, the Fe eluted during the electrolysis is accumulated in the pickling liquid, and the Fe concentration in the pickling liquid is increased. If it is attached to the surface of the steel sheet, problems such as drying of the dirt may occur.

Further, the pickling liquid used for electrolytic pickling is not particularly limited, and nitric acid and hydrofluoric acid are highly corrosive to equipment, and must be handled with particular care, and thus are not preferable. Also, hydrochloric acid may generate chlorine gas from the cathode, which is less preferred. Therefore, in consideration of corrosiveness and environment, it is preferred to use sulfuric acid. The sulfuric acid concentration is preferably 5% by mass or more and 20% by mass or less. When the sulfuric acid concentration is less than 5% by mass, the electrical conductivity is lowered. Therefore, the cell voltage at the time of electrolysis is increased, and the power supply load is increased. On the other hand, in the case of more than 20% by mass, the loss due to the drag-out is large, and there is a problem in cost.

The temperature of the electrolytic solution is preferably 40 ° C or more and 70 ° C or less. Since the heat generated by continuous electrolysis causes the bath temperature to rise, if the temperature is less than 40 ° C, the pickling effect may be lowered. Moreover, it is difficult to maintain the temperature below 40 °C. Further, from the viewpoint of the durability of the inner liner of the electrolytic cell, the temperature exceeding 70 ° C is not preferable.

The high strength steel sheet of the present invention can be obtained from the above.

In addition, as described below, the structure of the surface of the steel sheet has characteristics.

In the surface layer portion of the steel sheet within 100 μm from the surface of the steel sheet, a total of 0.010 to 0.50 g/m ² is formed from Fe, Si, Mn, Al, P, further B, Nb, Ti, Cr, Mo, Cu. At least one or more oxides selected from Ni. Further, in the region from the surface of the steel sheet of 10 μm, crystalline Si or Mn-based composite oxide is present in the base iron particles within 1 μm from the grain boundary.

In the high-strength steel sheet in which Si and a large amount of Mn are added to the steel, in order to satisfy the corrosion resistance after electrocoating, it is necessary to more highly control the structure and structure of the surface layer of the steel sheet which may become a starting point of corrosion cracking or the like. Therefore, in the present invention, first, in order to ensure chemical conversion treatability, dew point control is performed in order to increase the oxygen level in the annealing step as described above. As a result, by increasing the oxygen potential energy, Si or Mn which is an oxidizable element is internally oxidized before the chemical conversion treatment, and the activity of Si and Mn in the surface layer portion of the steel sheet is lowered. Then, the external oxidation of the elements is suppressed, and as a result, the chemical conversion treatability and the corrosion resistance after electrocoating are improved. In addition, in the surface layer portion of the steel sheet within 100 μm from the surface of the steel sheet, there are 0.010 g/m ² or more of Fe, Si, Mn, Al, P, further B, Nb, Ti, Cr per one side. At least one or more oxides selected from the group consisting of Mo, Cu, and Ni. On the other hand, if more than 0.50 g/m ^{2 is present} , the effect is saturated, so the upper limit is 0.50 g/m ² .

Further, when the internal oxide exists only in the grain boundary and does not exist in the grain, the grain boundary diffusion of the oxidizable element in the steel can be suppressed, but the intragranular diffusion cannot be sufficiently suppressed. Therefore, in the present invention, as described above, in the temperature range of the heating furnace: A ° C or more and B ° C or less (A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900), the environmental dew point Controlled at a temperature above -10 ° C, internal oxidation occurs not only in the grain boundaries but also in the grains. Specifically, in the region of the steel sheet surface layer within 10 μm, the crystalline Si and Mn-based composite oxide are present in the base iron particles within 1 μm of the grain boundary. Due to the presence of oxides in the base iron particles, the amount of solid solution Si and Mn in the base iron particles in the vicinity of the oxide is reduced. As a result, it is possible to suppress the concentration of the surface to be diffused by the intragranular diffusion of Si and Mn.

In addition, although the surface structure of the steel sheet of the high-strength steel sheet obtained by the production method of the present invention is as described above, for example, the oxide is grown in a region exceeding 100 μm from the surface of the steel sheet, and there is no problem. In addition, in the region of more than 10 μm from the surface of the steel sheet, the crystalline Si or Mn-based composite oxide is present in the base iron particles having a grain boundary of 1 μm or more, and there is no problem.

[Example 1]

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 to remove black rust, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. In addition, some of them were not subjected to cold rolling, and were also prepared to remove hot-rolled steel sheets (thickness: 2.0 mm) after black rust.

Next, the cold-rolled steel sheet and the hot-rolled steel sheet obtained above were placed in a continuous annealing apparatus. In the annealing equipment, as shown in Table 2, the temperature and the dew point in the heating furnace were controlled and annealed through a plate, and then tempered at 300 ° C for 140 seconds after water quenching. Next, the sample was subjected to electrolytic pickling in the order of the anode and the cathode in the order of the anode and the cathode at 40 ° C and a 5 mass % sulfuric acid aqueous solution under the current density conditions shown in Table 2 to obtain a sample. Further, the dew point of the annealing furnace other than the above-mentioned area for controlling the dew point is based on -35 °C. Further, the environmental gas component is composed of nitrogen gas, hydrogen gas, and unavoidable impurity gas, and the dew point is controlled by dehumidifying or absorbing and removing moisture in the environment. The concentration of hydrogen in the environment is based on 10% by volume.

For the obtained sample, TS and El were measured in accordance with JIS Z 2241 Metallic Material Tensile Test Method. Further, the obtained samples were investigated for chemical conversion properties and corrosion resistance. The amount of oxide (internal oxidation amount) of the surface layer portion of the steel sheet which was present immediately below the surface layer of the steel sheet to 100 μm was measured. The measurement method and evaluation criteria are as follows.

(Chemical processing)

The evaluation method of the chemical conversion treatment is described below.

For the chemical conversion treatment liquid, a chemical conversion treatment liquid (Parbond L3080 (registered trademark)) manufactured by Japan PARKERIZING Co., Ltd. was used, and the chemical conversion treatment was carried out in the following manner.

After degreasing with the degreasing liquid Fine Cleaner (registered trademark) manufactured by Japan PARKERIZING Co., Ltd., it was washed with water, and then subjected to surface conditioning of Prepan Z (registered trademark) manufactured by Japan PARKERIZING Co., Ltd. for 30 seconds at 43 ° C. After immersing in the chemical conversion liquid (Parbond L3080) for 120 seconds, 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, and the area of the pores of the chemical conversion treatment film was measured by image processing, and the following evaluation was performed using the pore area ratio. ○ is a qualified level.

○: 10% or less

╳: More than 10%

(corrosion resistance after electrocoating)

A test piece having a size of 70 mm × 150 mm was cut out from the chemically-treated sample obtained by the above method, and cationic electrocoating was carried out using PN-150G (registered trademark) manufactured by Nippon Paint Co., Ltd. (hardening condition: 170 ° C × 20 Minutes, film thickness 25 μm). Thereafter, the end portion and the side on which the evaluation was not performed were sealed with an A1 tape, and the cutter was transversely cut to reach the base iron (transverse angle of 60°) as a sample.

Next, the sample was immersed in a 5% by mass aqueous NaCl solution (55 ° C) for 240 hours, and then taken out, washed with water, and dried. Then, the cross-section was subjected to tape peeling, and the peeling width was measured, and the following evaluation was performed. ○ is a qualified level.

○: peeling width is less than 2.5 mm on one side

╳: peeling width is more than 2.5mm on one side

(processability)

The workability was measured in the direction of 90° with respect to the rolling direction, and the JIS No. 5 tensile test piece was taken from the sample, and the tensile test was carried out at a crosshead speed of 10 mm/min in accordance with JIS Z 2241. Stretching strength (TS/MPa) and elongation (El%). When TS is less than 650 MPa, TS × El ≧ 22,000 is good, and TS × El < 2 2,000 is bad. When TS is 650 MPa or more and 900 MPa, TS × El ≧ 20000 is good, and TS × El < 2000 is bad. When TS is 900 MPa or more, TS × El ≧ 18000 is good, and TS × El < 18000 is bad.

(The amount of internal oxidation in the region up to 100 μm of the steel sheet surface layer)

The internal oxidation amount is measured by "pulse 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 sheet before annealing), in the present invention, the surface layer portions of both surfaces of the high-strength steel sheet after continuous annealing are polished to 100 μm or more, and the steel is measured. The oxygen concentration is determined by using the measured value as the oxygen amount OH contained in the material, and the oxygen concentration in the steel in the thickness direction of the high-strength steel sheet after continuous annealing is measured, and the measured value is taken as the oxygen amount OI after the internal oxidation. Using the oxygen content OI after internal oxidation of the high-strength steel sheet thus obtained and the amount of oxygen OH contained in the material, the difference between OI and OH (=OI-OH) is calculated, and further converted into a single-sided unit area (that is, 1 m). ² ) The amount of the amount (g/m ² ) is taken as the internal oxidation amount.

The results obtained above are shown in Table 2 together with the production conditions.

As can be seen from Table 2, the high-strength steel sheet produced by the method of the present invention is a high-strength steel sheet containing a large amount of easily oxidizable elements such as Si or Mn, but has excellent chemical conversion properties and corrosion resistance and workability after electrocoating. .

On the other hand, in the comparative example, the chemical conversion treatment property, the corrosion resistance after the electrocoating, and the workability are not preferable.

(industrial availability)

The high-strength steel sheet of the present invention is excellent in chemical conversion treatability, corrosion resistance, and workability, and can be used as a surface-treated steel sheet for reducing the weight and height of the automobile body itself. In addition to the automobile, the surface-treated steel sheet which imparts rust resistance to the material steel sheet can be used in a wide range of fields such as home appliances and building materials.

Claims (4)

A method for producing a high-strength steel sheet, comprising C: 0.01 to 0.18%, Si: 0.4 to 2.0%, Mn: 1.0 to 3.0%, Al: 0.001 to 1.0%, and P: 0.005 to 0.060% in mass%, When S≦0.01%, the rest of the steel sheet consisting of Fe and unavoidable impurities is subjected to continuous annealing. In the heating process, in the temperature range of the heating furnace: A°C or more and B°C or less, the ambient dew point: -10 ° C or more; wherein: A: 600 ≦ A ≦ 780, B: 800 ≦ B ≦ 900. The method of producing a high-strength steel sheet according to the first aspect of the invention, wherein the component composition of the steel sheet further contains one or more elements selected from the following elements in mass%; B: 0.001 to 0.005%, Nb: 0.005~0.05%, 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%. The method for producing a high-strength steel sheet according to claim 1 or 2, wherein after the continuous annealing, electrolytic pickling is performed in an aqueous solution containing sulfuric acid. A high-strength steel sheet produced by the manufacturing method according to any one of claims 1 to 3, wherein the surface layer portion of the steel sheet within 100 μm from the surface of the steel sheet is formed from 0.010 to 0.50 g/m ² per one side. At least one or more oxides selected from the group consisting of Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni, and further in a region within 10 μm of the surface of the steel sheet, in the steel grain of the steel sheet Crystalline Si and Mn-based oxides are present in the crystal grains within 1 μm.