JP5233346B2 - High-strength cold-rolled steel sheet excellent in chemical conversion treatment and post-coating corrosion resistance and method for producing the same - Google Patents

Description

  The present invention is a high-strength cold-rolled steel sheet suitable for use as a strength member or reinforcing member of an automobile body, having a tensile strength of 780 MPa or more, excellent workability, and excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating. And its manufacturing method.

  In recent years, from the viewpoint of protecting the global environment, it has been required to reduce the weight of an automobile body and improve fuel efficiency. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, it is required to improve the strength of the automobile body. In order to achieve both weight reduction and strength improvement of an automobile body, it is effective to increase the strength of the material to make it thinner and lighter. Therefore, the adoption of high-strength steel sheets with reduced plate thickness has been actively promoted for automobile bodies. Recently, high-strength thin sheets with a tensile strength of 780 MPa or more are used for the strength members and reinforcing members of automobile bodies. Steel plates are starting to be used.

  Since a strength member or the like of an automobile body is generally manufactured by a molding process such as a press process, the material is required to have excellent workability in addition to high strength. In order to increase the strength of a steel sheet as a raw material, a method of adding a solution element such as Si or Mn to strengthen the solution or refining crystal grains, or a precipitate forming element such as Nb, Ti, or V is used. A method of adding and strengthening precipitation and a method of strengthening by generating a hard transformation structure such as a martensite phase are effective, and have already been put into practical use.

  In general, when an alloy element is added to increase the strength of a steel sheet, the workability usually decreases. However, Si is known to be an alloy element useful for achieving both the strength and workability of a steel sheet because it can increase the strength while suppressing a decrease in workability. However, since Si is an easily oxidizable element, when the content is increased, Si is preferentially oxidized during annealing in a reducing atmosphere that is performed in a normal cold-rolled steel sheet manufacturing process. It is also known to concentrate near the surface of the steel sheet to reduce the chemical conversion processability and the corrosion resistance after chemical electrodeposition coating, which are one of the characteristics required for cold rolled steel sheets for automobiles. For this reason, application of steel plates containing a large amount of Si to automobile bodies has been limited to sites where corrosion resistance after chemical electrodeposition coating is not required.

  Conventionally, many proposals have been made on methods for improving the chemical conversion property of a Si-containing steel sheet, particularly a cold-rolled steel sheet containing a high Si content of 0.8 mass% or more and the corrosion resistance after chemical electrodeposition coating. These technologies are considered to be due to the following two reasons for the reason that the chemical conversion processability of the Si-containing steel and the corrosion resistance after chemical electrodeposition coating are inferior, and different improvement measures have been proposed for each. Yes.

First, the first possible reason is that the oxide mainly composed of Si coats the surface of the steel sheet, thereby preventing the formation of chemical crystals on the surface of the steel sheet, resulting in the formation of scale crystals in the chemical conversion crystal. The adhesion of the film is lowered. As an improvement measure against this, for example, in Patent Document 1, the Si / Mn ratio of the steel sheet surface layer oxide is controlled to 1 or less, and the chemical conversion treatment is performed by reducing the ratio of the Si oxide that adversely affects the chemical conversion treatment property. Techniques for improving the performance have been proposed. In addition, in Patent Documents 2 to 4, in addition to the ratio of Si and Mn of the steel sheet surface layer oxide, by controlling the presence state (size, density, coverage) of the oxide, chemical conversion processability, coating film Techniques for improving adhesion have been proposed. Patent Document 5 discloses a steel plate made of Si oxide by controlling the dew point during continuous annealing to 0 ° C. to −20 ° C. and removing Si oxide on the surface layer with concentrated hydrochloric acid or concentrated sulfuric acid after continuous annealing. There has been proposed a technique for improving the chemical conversion treatment property by controlling the surface coverage and the size of the Si oxide. Patent Document 6 discloses that Si or the like is concentrated on the surface of the steel sheet by forming an oxide such as Si in the grain boundary and / or in the crystal grain from the steel sheet surface to a depth of 0.1 to 100 μm. A technique has been proposed that suppresses the above and improves the chemical conversion processability. Further, Patent Document 7 discloses that after continuous annealing, the steel plate surface is ground at 2.0 g / m ² or more and then pickled with hydrochloric acid, or thereafter, further 0.1 to 0.3 g / m ² is ground. Techniques for improving the processability have been proposed. Furthermore, in Patent Document 8, pickling treatment is performed at a pH of 0 to 4 at a temperature of 10 to 100 ° C. for 5 to 150 seconds after continuous annealing, and then at a pH of 10 to 14 at a temperature of 10 to 100 ° C. A technique for improving the chemical conversion processability by performing an alkali treatment for 2 to 50 seconds is proposed. All of these techniques are intended to improve the chemical conversion processability and improve the corrosion resistance after chemical conversion electrodeposition by appropriately controlling the composition and presence state of the oxide on the surface layer of the steel sheet.

Next, the second possible reason is that the oxide containing Si generated at the grain boundary near the steel sheet surface is removed by pickling or the like, so that fine irregularities or cracks are generated in the surface layer, In the interior, formation of chemical crystals is unlikely to occur, so that corrosion is accelerated at this portion and the corrosion resistance deteriorates. As an improvement measure against this, for example, in Patent Documents 2 to 4 and Patent Document 9, by controlling the presence of grain boundary oxides and surface layer cracks within a range of about 10 μm from the surface of the steel sheet, coating film adhesion Techniques for improving the performance have been proposed.
Japanese Patent Laid-Open No. 04-276060 JP 2005-187863 A JP 2005-290440 A JP 2006-283130 A JP 2004-323969 A Japanese Patent No. 3386657 JP 2003-226920 A JP 2007-000269 A JP 2004-244698 A

  However, since the methods described in Patent Documents 1 to 4 are techniques for controlling the ratio and existence state of Si and Mn in the surface layer oxide, the ratio of Si and Mn contained in the steel is inevitably limited. The degree of freedom of the additive element is reduced, and it is difficult to obtain sufficient strength and workability. In addition, the composition and presence state of the surface oxide are easily affected by the dew point during annealing, the hydrogen concentration, and the like, and it is difficult to stably obtain excellent chemical conversion properties. Moreover, the method of improving the coating-film adhesiveness of patent documents 2-4, and aiming at the improvement of the corrosion resistance after a chemical conversion electrodeposition is that the coiling temperature of hot rolling is 500 degrees C or less, and the dew point at the time of continuous annealing is Since manufacturing conditions are extremely limited, such as −40 ° C. or lower and a quenching start temperature during continuous annealing of 550 ° C. or lower, there is a problem that it is difficult to obtain desired strength and workability.

  In the method described in Patent Document 5, the chemical conversion treatment property is improved to some extent by reducing the Si oxide in the surface layer, but no investigation is made on the corrosion resistance after coating. Similarly, in the method described in Patent Document 6, the chemical conversion property is improved by suppressing the concentration of Si or the like on the surface layer, but the corrosion resistance after chemical electrodeposition coating has not been improved. Moreover, since the method described in Patent Document 7 needs to perform mechanical grinding twice, the manufacturing cost increases. Moreover, the object to be controlled is limited to the steel sheet surface layer, and no mention is made of the corrosion resistance after painting. In addition, the method described in Patent Document 8 requires an alkali treatment after the pickling treatment, resulting in an increase in cost. In the alkali treatment, the Si-containing oxide on the surface of the steel sheet can be dissolved, but is present inside the steel sheet. Since it is impossible to remove the Si-containing oxide, it is difficult to stably obtain excellent post-coating corrosion resistance. In addition, the method described in Patent Document 9 says that the corrosion resistance failure is caused by the linear compounds derived from Si and O due to oxidation during the transfer from the annealing furnace to the water quenching water tank, and improves the corrosion resistance. In this case, it is necessary to enclose the water tank with a chamber filled with non-oxidizing gas, which increases the equipment cost. Furthermore, as described later, the methods described in Patent Documents 2 to 4 and Patent Document 9 are difficult to stably obtain excellent corrosion resistance even if the presence state of grain boundary oxides or surface layer cracks is controlled. there were.

  As described above, in order to solve the problems that the chemical conversion processability of Si-containing steel sheets, particularly high-strength cold-rolled steel sheets containing 0.8 mass% or more of Si and the corrosion resistance after chemical electrodeposition coating are inferior, However, a technology that drastically solves the above problems has not been found so far.

  Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, high strength cold rolled steel sheet having high strength and excellent workability, and excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating, and its manufacturing method Is to propose.

  In order to solve the above-mentioned problems, the inventors variously in the range of C: 0.03-3.0 mass%, Si: 0.1-3.0 mass%, Mn: 1.0-3.5 mass%. The changed steel is melted in the actual machine, continuously cast into a steel slab, hot-rolled, cold-rolled, and then finish-annealed under various annealing conditions and cold-rolled with a thickness of 1.6 mm As the annealed plate, the cold rolled annealed plate was examined for mechanical properties, chemical conversion treatment, and corrosion resistance after chemical electrodeposition coating (hereinafter also referred to as “corrosion resistance after coating”) under the conditions of the examples described later.

  As a result, in order to achieve both the desired strength (tensile strength: 780 MPa or more) and workability (strength-ductility balance (TS × El ≧ 18000 MPa ·%)), it is necessary to add 0.8 mass% or more of Si. I found out that However, when the Si content is 0.8 mass% or more, as schematically shown in FIGS. 1A to 1C, an oxide containing Si is present on the steel sheet surface and / or in the vicinity of the steel sheet. (Hereinafter, also referred to as “Si-containing oxide”) is inevitably present, and it has been found that the chemical conversion treatment property and the post-coating corrosion resistance are reduced.

  In addition, in order to improve the chemical conversion property and the corrosion resistance after coating, as shown in FIG. 1 (c), even if only the Si oxide on the surface of the steel sheet is completely removed, the scale of the chemical conversion crystal can be improved, but the coating can be improved. It was found that post-corrosion resistance may deteriorate. Furthermore, it has been found that even after the presence of grain boundary oxides and surface layer cracks in the steel sheet is appropriately controlled in accordance with the prior art, excellent post-coating corrosion resistance may not be obtained stably.

  Therefore, the surface layer cross section of each steel plate was observed using a scanning electron microscope (SEM), and a detailed investigation was conducted on the cause of the decrease in corrosion resistance after coating. As a result, even if there is no oxide mainly composed of Si on the surface of the steel sheet, if there is an oxide containing Si in any form inside the steel sheet in the vicinity of the surface, the corrosion resistance after coating is reduced. Therefore, as shown in FIG. 1 (d), stable and excellent chemical conversion treatment can be achieved only by completely removing the Si-containing oxide present in the steel sheet surface and in the vicinity of the steel sheet. And found that corrosion resistance after painting can be obtained, and the present invention has been completed.

That is, the present invention is C: 0.05-0.30 mass%, Si: 0.8-3.0 mass%, Mn: 1.5-3.0 mass%, P: 0.10 mass% or less, S: 0 .01 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being composed of Fe and inevitable impurities, with a steel plate surface and a depth of 1 μm from the surface. high strength cold maximum P ₂ Si concentration in the steel inside the range of, excellent in chemical conversion treatability and corrosion resistance after coating, characterized in that is less than 1.3 times the Si concentration P ₁ in the sheet thickness 1/4 It is a rolled steel sheet.

  In the present invention, C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0 .01 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, and a slab having a composition composed of Fe and unavoidable impurities in the remainder is hot-rolled and cold-rolled In the method of producing a cold-rolled steel sheet by continuous annealing, the steel sheet is pickled after the continuous annealing, and the steel sheet surface is removed by 1 μm or more per side. A method for producing rolled steel sheets is proposed.

  In the above production method of the present invention, 1-5 mass% nitric acid aqueous solution, 2-20 mass% nitric acid and 0.1-5 mass% hydrochloric acid aqueous solution, 0.5-5 mass% nitric acid and 0 are used for pickling. Any one of a mixed acid aqueous solution comprising 1 to 5 mass% hydrofluoric acid is used.

  According to the present invention, it is possible to stably produce a high-strength cold-rolled steel sheet that not only has high strength and excellent workability but also has excellent chemical conversion properties and post-coating corrosion resistance. Therefore, the high-strength cold-rolled steel sheet of the present invention satisfies all requirements for strength, workability, chemical conversion property and post-coating corrosion resistance required for materials such as high-strength members of automobiles. Can be suitably used.

  First, the high-strength cold-rolled steel sheet of the present invention is intended for those having a tensile strength of 780 MPa or more. This is because the steel sheet of the present invention is intended for application to a strength member or reinforcing member of an automobile body that requires high strength, a skeleton part of an automobile seat, or the like. In addition, if the tensile strength is less than 780 MPa, a steel sheet having characteristics having both high strength and workability can be easily obtained without adding Si by 0.8 mass% or more. Note that excellent workability in the present invention means excellent elongation characteristics, but from the balance with strength, the strength-ductility balance (TS × El), which is the product of tensile strength TS and elongation El, is 18000 MPa.・ It means% or more.

Next, the component composition that the steel sheet of the present invention should have will be described.
C: 0.05-0.30 mass%
C is an element necessary for securing a desired steel plate strength. In the present invention, 0.05 mass% or more is required to obtain a tensile strength of 780 MPa or more while ensuring workability. . On the other hand, addition exceeding 0.30 mass% significantly deteriorates weldability. Therefore, C is added in the range of 0.05 to 0.30 mass%. When more excellent weldability is required, C is preferably 0.20 mass% or less, and more preferably 0.15 mass% or less.

Si: 0.8-3.0 mass%
Si is an element that increases the strength of the steel sheet by solid solution strengthening, and is particularly an effective element that can increase the strength while suppressing a decrease in workability. However, as described above, Si impairs chemical conversion properties and post-coating corrosion resistance, but in the present invention, the adverse effects can be avoided by completely removing the Si oxide layer on the steel sheet surface. Therefore, in the present invention, Si is added in an amount of 0.8 mass% or more in order to ensure desired strength and workability. However, the addition exceeding 3.0 mass% saturates the above effect and, on the other hand, causes a decrease in workability. Preferably, it is in the range of 1.0 to 2.5 mass%.

Mn: 1.5 to 3.0 mass%
Mn contributes to improving the strength of the steel sheet by solid solution strengthening, and also has an effect of improving the hardenability, so that Mn is an effective element for ensuring the strength stably. In order to obtain such an effect, addition of 1.5 mass% or more is necessary. On the other hand, the addition exceeding 3.0 mass% causes a decrease in workability. Therefore, Mn is added in the range of 1.5 to 3.0 mass%. Preferably, it is in the range of 1.8 to 2.5 mass%.

P: 0.10 mass% or less P has an effect of strengthening steel, and can be added according to the strength level of the steel sheet. However, if it exceeds 0.10 mass%, weldability and toughness are lowered. Therefore, P is 0.10 mass% or less. When more excellent weldability and toughness are required, it is preferably 0.05 mass% or less.

S: 0.01 mass% or less S is a harmful element that exists as sulfide inclusions in steel and reduces stretch flangeability. Therefore, S is preferably reduced as much as possible, and needs to be 0.01 mass% or less. When more excellent stretch flangeability is required, the content is preferably 0.005 mass% or less.

Al: 0.01-0.1 mass%
Al is added as a deoxidizing element for steel and has the effect of improving the cleanliness of the steel and also has the effect of refining the steel structure. Therefore, in the present invention, Al is added in an amount of 0.01 mass% or more. . However, excessive addition of Al causes deterioration of the surface properties, so the upper limit is made 0.1 mass%.

N: 0.005 mass% or less The content of N exceeding 0.005 mass% causes variation in strength. Therefore, N is limited to 0.005 mass% or less.

  The steel sheet of the present invention can obtain the intended strength and workability by satisfying the above component composition, but depending on the required strength and the like, in addition to the above components, Ti: 0.005 0.1 mass%, Nb: 0.005-0.1 mass%, V: 0.005-0.1 mass%, Cr: 0.01-0.5 mass%, Mo: 0.01-0.5 mass%, and B : One or more selected from 0.0005 to 0.005 mass% can be added.

  Ti, Nb, and V all have the effect of increasing strength by forming carbides in the steel, strengthening precipitation, and refining crystal grains. In order to acquire such an effect, it is necessary to add 0.005 mass% or more, respectively. On the other hand, the addition exceeding 0.1 mass% significantly reduces workability. Therefore, Ti, Nb, and V are preferably added in the range of 0.005 to 0.1 mass%, respectively. More preferably, it is the range of 0.01-0.05 mass%.

  Further, Cr, Mo and B have the effect of increasing the hardenability and increasing the strength, and can be added as necessary. In order to obtain such an effect, it is necessary to add Cr and Mo in an amount of 0.01 mass% or more and B in an amount of 0.0005 mass% or more. On the other hand, the addition of a large amount of these elements decreases the workability. Therefore, Cr and Mo are preferably added in the range of 0.01 to 0.5 mass% and B in the range of 0.0005 to 0.005 mass%. More preferably, Cr and Mo are in the range of 0.05 to 0.4 mass%, and B is in the range of 0.001 to 0.004 mass%.

  In the steel sheet of the present invention, the balance other than the above components consists of Fe and inevitable impurities. Examples of the inevitable impurities include Ni, Cu, Mg, Ca, Zr, and REM in addition to Ti, Nb, V, Cr, Mo, and B, which are the selective additive elements described above. In general, elements are not rejected as long as they are within the range of impurities contained and do not impair the effects of the present invention. Other inevitable impurities include impurities such as Sb, Sn, Zn, and Co. These are Sb: 0.01 mass% or less, Sn: 0.1 mass% or less, Zn: 0.01 mass% or less. Co: 0.1 mass% or less is acceptable.

Next, the surface of the high-strength cold-rolled steel sheet of the present invention and the inside of the steel sheet near the surface will be described.
The high-strength cold-rolled steel sheet of the present invention has high strength and excellent workability, as well as excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating. For this purpose, the steel sheet of the present invention has the above-described component composition, and it is necessary that substantially no Si-containing oxide is present in the steel sheet surface and in the steel sheet having a depth of 1 μm from the surface. . Here, “the oxide containing Si substantially does not exist” means that the inside of the steel sheet within a range of 1 μm in depth from the steel sheet surface and the surface when the steel sheet surface is measured by a glow discharge spectrometer (GDS). maximum value _{P 2} of Si intensity at it is meant that 1.3 times the Si intensity _{P 1} of grinding the steel sheet surface below _(P 2 / _{P 1} ≦ 1.3) to a thickness of 1/4. In addition, the said Si intensity means Si density | concentration.

  As described above, when an oxide mainly composed of Si is present on the surface of the steel sheet, the conversion crystal can be scaled and the adhesion of the electrodeposition coating film is lowered. In addition, when an oxide containing Si is present inside the steel sheet having a depth of 1 μm from the steel sheet surface, sufficient corrosion resistance after coating cannot be obtained even when no oxide containing Si is present on the steel sheet surface. . The reason why the corrosion resistance after painting decreases when Si-containing oxide is present inside the steel sheet has not been fully elucidated, but if the steel sheet surface after painting is scratched, it contains Si exposed on the scratched surface The inventors consider that this is because corrosion of the ground iron is promoted by the potential difference between the oxide and the ground iron. Therefore, in order to stably obtain excellent chemical conversion treatment properties and corrosion resistance after coating, it is necessary that substantially no oxide containing Si is present in the steel sheet surface and in the steel sheet within a range of 1 μm depth from the surface. is there.

Next, the manufacturing method of the high intensity | strength cold-rolled steel plate excellent in the chemical conversion property of this invention and the corrosion resistance after coating is demonstrated.
The steel sheet of the present invention is a slab obtained by melting a steel having a composition suitable for the present invention described above into a slab by a conventional steelmaking process. The slab is hot-rolled, cold-rolled, and continuously annealed. Manufactured.

  The slab manufacturing method may use either a continuous casting method or an ingot-bundling rolling method. From the viewpoint of preventing macro segregation in the slab and stabilizing the material, the slab is manufactured by a continuous casting method. Is preferred. Further, a thin slab casting method may be used.

  The subsequent hot rolling is usually performed after reheating the slab once cooled to room temperature to a temperature of 1000 ° C. or higher in a heating furnace, but without reheating after slab production (after continuous casting). An energy saving process in which rolling is performed after direct heating rolling (direct rolling) or by charging in a heating furnace in a warm piece state without cooling to room temperature and performing light heating or heat retention may be employed.

  When reheating the slab, the slab heating temperature is preferably 1000 ° C. or higher. The upper limit is not particularly limited, but if it exceeds 1300 ° C., scale loss increases as the oxidized weight increases or surface defects occur, and therefore, the upper limit is preferably 1300 ° C. Moreover, also when employ | adopting the said energy saving process, it is preferable that slab temperature shall be 1000 degreeC or more.

  In the hot rolling, it is preferable that hot rolling is performed by performing rough rolling as necessary and then finishing rolling with a finish rolling finishing temperature of 800 ° C. or higher. When the finish rolling finish temperature is lower than 800 ° C., the steel sheet structure becomes non-uniform and the workability is lowered. On the other hand, the upper limit of the finish rolling finish temperature is not particularly limited, but if it is rolled at an excessively high temperature, it causes surface defects such as scale marks. After hot rolling, it is preferable to wind at a temperature of 650 ° C. or lower. When the winding temperature exceeds 650 ° C., a large amount of scale is generated after winding, and the pickling load before cold rolling becomes large.

  Next, the hot-rolled sheet obtained as described above is pickled, descaled, and then cold-rolled. The cold rolling is not particularly limited as long as a cold-rolled sheet having a desired size and shape can be obtained. From the viewpoint of the flatness of the surface and the uniformity of the structure, rolling with a rolling reduction of 20% or more may be performed. preferable. In addition, pickling before cold rolling can be omitted when the surface scale of the hot-rolled sheet is extremely thin.

  The cold-rolled sheet after cold rolling is then annealed in a continuous annealing line in order to impart desired strength and workability. The annealing in this continuous annealing is preferably heated and held in a temperature range of 750 to 900 ° C. When the heating and holding temperature is less than 750 ° C., recrystallization does not occur sufficiently, and workability decreases. On the other hand, if it exceeds 900 ° C., the structure becomes coarse and the strength-ductility balance decreases. The time for maintaining the temperature is preferably 30 seconds or longer, and more preferably 60 seconds or longer in order to make the steel plate material uniform. More preferably, it is 120 sec or more. On the other hand, the upper limit of the holding time is preferably 600 sec or less in consideration of productivity. Moreover, it is preferable that the cooling after heating is rapidly cooled to 300 ° C. or less at an average cooling rate of 30 ° C./sec or more. If the cooling rate is less than 30 ° C./sec, the strength cannot be improved by the transformation structure. Therefore, in order to obtain a desired strength, it is necessary to add a large amount of alloy elements, and the raw material cost increases. Moreover, it is preferable to leave it as it is after the rapid cooling or to reheat it after cooling it to near room temperature and hold it at a temperature of 100 to 450 ° C. for 3 to 30 minutes.

  Moreover, in this invention, it is preferable that the dew point during the heating holding in the said continuous annealing shall be -20 degrees C or less. When the dew point exceeds −20 ° C., decarburization in the steel sheet surface layer becomes remarkable, which adversely affects the material. More preferably, it is −25 ° C. or lower.

  Furthermore, in the present invention, in order to provide the excellent chemical conversion treatment property and post-coating corrosion resistance of the present invention, the steel plate after the continuous annealing is pickled at least 1 μm per side, and the steel plate surface and the steel plate surface are in the range of 1 μm. It is necessary to perform a pickling treatment that completely removes the Si oxide inside the steel plate. If the removal amount by pickling is less than 1 μm, excellent chemical conversion properties and corrosion resistance after coating cannot be stably obtained. Preferably it is 1.5 micrometers or more. The upper limit is preferably 3 μm or less from the viewpoint of steel plate yield.

The reason why the removal amount is limited to the above range by pickling is as follows.
In continuous annealing in the production method of the present invention, as described above, it is preferable to perform annealing at a temperature of 750 to 900 ° C. for 30 to 600 seconds in a reducing atmosphere having a dew point of −20 ° C. or less. The depth of the Si-containing oxide layer formed inside the steel sheet under conditions is usually about 0.5 μm, and at most 1 μm. Therefore, in the present invention, the removal amount of the Si-containing oxide layer by pickling is set to 1 μm or more per one surface of the steel plate. In addition, the measurement depth of the Si concentration distribution in GDS should be 1 μm, which is the maximum depth at which the Si-containing oxide layer is present in the steel sheet after annealing, considering that there is a pickling removal amount. is there.

  Next, in the production method of the present invention, the acid used for the pickling is preferably a nitric acid aqueous solution or a mixed acid aqueous solution containing nitric acid such as nitric hydrochloric acid or nitric hydrofluoric acid from the viewpoint of productivity. In order to perform pickling removal of 1 μm or more per side within a time period that does not impede industrial productivity or economy (approximately within 30 sec), the concentration rate of concentrated hydrochloric acid, concentrated sulfuric acid, etc. is low, so pickling treatment It is not economical because it is necessary to lengthen the tank or slow down the line speed. Further, when the Si oxide is dissolved only with hydrofluoric acid or concentrated alkali, it is possible to remove the Si oxide on the extreme surface layer, but it is difficult to remove the Si oxide inside the steel sheet.

  The acid used for the pickling preferably has a concentration in the range of 1 to 5 mass% when only a nitric acid aqueous solution is used. If the concentration is less than 1 mass%, it is difficult to obtain a desired removal amount within 30 seconds. On the other hand, if it exceeds 5 mass%, the dissolution rate becomes too high and the removal amount becomes difficult to control.

  From the viewpoint of controlling the removal amount, the acid used for the pickling is preferably a mixed acid aqueous solution of nitric acid and hydrochloric acid. By adding hydrochloric acid to the aqueous nitric acid solution, the pickling speed is reduced, the controllability of the removal amount is improved, and surface roughness of the steel sheet due to pickling can be suppressed. In this case, the concentration of nitric acid and hydrochloric acid is preferably in the range of 2 to 20 mass% for nitric acid and 0.1 to 5 mass% for hydrochloric acid. Since the pickling speed of nitric acid is reduced by adding hydrochloric acid as compared with the case of nitric acid alone, the lower limit of the nitric acid concentration is 2 mass%. On the other hand, if the nitric acid concentration exceeds 20 mass%, even when mixed acid is used, the dissolution rate becomes too high and it becomes difficult to control the pickling rate. Also, if the hydrochloric acid concentration is less than 0.1 mass%, the effect of reducing the dissolution rate of nitric acid cannot be obtained. On the other hand, if the concentration exceeds 5 mass%, the dissolution rate is too low and the desired removal amount can be reduced within 30 seconds. Can't get.

  Further, from the viewpoint of removal of Si oxide, it is preferable to use a mixed acid aqueous solution of nitric acid and hydrofluoric acid. Since hydrofluoric acid can dissolve the Si oxide itself, it can be removed more efficiently than nitric acid alone. When a mixed acid aqueous solution of nitric acid and hydrofluoric acid is used, nitric acid is preferably in the range of 0.5 to 5 mass%, and hydrofluoric acid is preferably in the range of 0.1 to 5 mass%. If the nitric acid concentration is less than 0.5 mass%, a desired removal amount cannot be obtained within 30 seconds. On the other hand, if it exceeds 5 mass%, the dissolution rate becomes too high and the removal amount becomes difficult to control. If the hydrofluoric acid concentration is less than 0.1 mass%, a sufficient Si oxide removal efficiency improvement effect cannot be obtained. On the other hand, if it exceeds 5 mass%, the effect is saturated and the economy is deteriorated.

  In addition, it is preferable that the temperature of the said acid aqueous solution used for pickling is the range of 30-80 degreeC. If the temperature is below 30 ° C, it is necessary to excessively cool the steel sheet after continuous annealing in order to maintain the liquid temperature. On the other hand, if the temperature exceeds 80 ° C, the effect of promoting the dissolution rate is not only saturated, but also the steel plate surface. Roughness becomes remarkable.

Moreover, in the manufacturing method of this invention, it is preferable to wash with water and to neutralize the steel plate after the said pickling process. Furthermore, in order to further improve the chemical conversion treatment property and the corrosion resistance after coating, 5-50 mg / m ^{2 of} Ni or the like, which has the effect of promoting the formation and growth of chemical conversion crystals and uniform refinement after the above pickling treatment. You may perform the process made to adhere to some extent. Further, the annealed steel sheet may be subjected to temper rolling with an elongation of 5% or less in order to adjust the shape correction, surface roughness, or control the material.

  A to I steels having the composition shown in Table 1 were melted by a conventional steelmaking process, continuously cast into a slab, and then the slab was reheated to 1250 ° C., and the finish rolling finish temperature was 850. C. and hot rolled to a coiling temperature of 600.degree. C. to obtain a hot rolled sheet having a thickness of 3.0 mm. This hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled sheet having a thickness of 1.6 mm, and then in an atmosphere controlled at a dew point of −50 to −20 ° C. to a temperature of 780 to 860 ° C. for 300 sec. After annealing, the steel was gradually cooled to 550 to 720 ° C., cooled to room temperature, and subjected to continuous annealing for tempering at 150 to 400 ° C. for 300 to 1200 seconds. Next, after performing the pickling process under the conditions shown in Table 2 to remove the Si-containing oxide layer on the steel sheet surface, or without performing the pickling process, temper rolling with an elongation of 0.3% was performed. A high-strength cold-rolled steel sheet was used. In addition, the amount of surface removal (sheet thickness reduction) by pickling after continuous annealing in each example was measured by pickling reduction under each pickling condition by a laboratory experiment conducted in advance, and the reduction value was determined as the surface area of the test piece. It was estimated by calculating the relationship between the pickling condition and the plate thickness reduction by dividing by the iron density and converting to the plate thickness.

For the various cold-rolled steel sheets obtained as described above, the mechanical properties, the formation status of Si-containing oxides on the steel sheet surface and surface layer, the chemical conversion treatment property and the corrosion resistance after chemical electrodeposition coating were investigated in the following manner. did.
<Mechanical properties>
From each cold-rolled steel sheet after pickling, a JIS No. 5 tensile test piece is taken from the direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with the provisions of JIS Z2241, tensile strength (TS: MPa), elongation. (El:%) was measured, and a strength-ductility balance (TS × El: MPa ·%) was determined. In this example, TS: 780 MPa or more and TS × El of 18000 MPa ·% or more were evaluated as having good mechanical properties.
<Generation of Si-containing oxides on steel sheet surface and surface layer>
The surface of each cold-rolled steel sheet and the cross-section of the steel sheet after pickling (the steel sheet surface and the inside of the steel sheet within a range of 1 μm in depth from the steel sheet surface) are observed 1000 times using SEM, and five elements are analyzed by EDX. The presence or absence of Si-containing oxide was examined. The presence or absence of the Si-containing oxide was determined to be “None” when no Si-containing oxide in which Si and O were detected by analysis by EDX was observed, and “Yes” when even one was observed.
In addition, the Si concentration distribution in the depth direction in the range of 1 μm depth from the steel sheet surface was measured using GDS, and the Si strength (P ₁ ) of the sample surface ground to a thickness of ¼, and 1 μm from the surface The ratio (P ₂ / P ₁ ) with the maximum value (P ₂ ) of the Si intensity in the range was determined.
<Chemical conversion processability>
Each cold-rolled steel sheet after the pickling is subjected to chemical conversion treatment using a commercially available chemical conversion chemical (Nippon Parkerizing Co., Ltd., Palbond PB-L3020) under conditions of bath temperature: 43 ° C. and processing time: 120 sec. The treated steel sheet surface is observed with 5 fields of view at 500 times magnification using SEM, and the chemical conversion processability is good when the area ratio is 95% or more and uniform conversion crystals are formed in all 5 fields. The chemical conversion treatment property was evaluated as “x” when the scale with an area ratio exceeding 5% was observed even in the visual field.
<Corrosion resistance after chemical conversion electrodeposition>
Two pieces of 150 mm × 70 mm test pieces were sampled from each of the steel sheets after the chemical conversion treatment, and a coating thickness was obtained using a commercially available electrodeposition paint (manufactured by Kansai Paint Co., Ltd., GT-10HT). Was subjected to electrodeposition coating so as to be 20 to 25 μm, followed by baking at 170 ° C. for 20 minutes. Next, two notches having a depth of about 20 μm and a length of 100 mm were put in an X shape on the surface of the test piece with a cutter knife, and then immersed in a solution of 5 mass% NaCl at 50 ° C. for 240 hours. Thereafter, an adhesive tape is applied on the cut of the test piece, and a film peel test is performed to measure the peel width of the paint film when peeled off. If the maximum peel width is 5.0 mm or less in all the test pieces, For example, if the corrosion resistance after chemical electrodeposition coating is good “◯”, even if one sheet exceeds 5.0 mm, the corrosion resistance after chemical electrodeposition coating was evaluated as “x”.

The results of the above tests are summarized in Table 2.
From these results, the steel sheets of the inventive examples having the component composition suitable for the present invention and manufactured under the production conditions suitable for the present invention all have a tensile strength TS of 780 MPa or more and a strength ductility balance (TS × El ) Is 18000 MPa ·% or more, and both strength and workability are achieved. Furthermore, with respect to the Si strength (P ₁ ) at a thickness of 1/4, the maximum value (P ₂ ) of the Si strength in the range of 1 μm depth from the steel sheet surface and the surface is 1.3 times or less (P ₂ / P ₁ ≦ 1.3), and as a result, it can be seen that both the chemical conversion property and the corrosion resistance after coating are excellent.
On the other hand, No. in which Si content deviates from the lower limit of the present invention. The steel plate No. 3 has TS × El of less than 18000 MPa ·%, and the workability is not sufficient. No. Since the steel plates 7 to 11 were not pickled or were not pickled sufficiently, the Si-containing oxide layer remained on the steel plate surface, and P ₂ / P ₁ > 1.3. Processability and post-coating corrosion resistance do not meet the required characteristics. In addition, no pickling No. In Steel No. 13, in addition to the surface, the Si-containing oxide is also present inside the steel plate having a depth of 1 μm from the surface, and the chemical conversion property and the corrosion resistance after painting do not satisfy the required characteristics. Furthermore, no. The steel plates 14 and 16 are excellent in chemical conversion treatment because there is no Si-containing oxide on the surface, but they are not pickled or insufficiently pickled, so there are Si-containing oxides inside the steel plate. Therefore, it is inferior in corrosion resistance after painting. No. Steel plate No. 15 is an example in which Si-containing oxide removal treatment was performed with strong alkali after hydrochloric acid pickling, but the removal amount was less than 1 μm, and the Si-containing oxide generated inside the steel plate could not be removed. Inferior in corrosion resistance after painting.

  The steel sheet of the present invention has high strength and excellent workability, and also has excellent chemical conversion treatment and corrosion resistance after chemical electrodeposition coating, so that it is not only used for automobile bodies, for example, in home appliances and construction fields. It can also be suitably used in fields where similar characteristics are required.

It is a figure which illustrates typically the relation between the existence state of Si content oxide near the Si content steel plate surface and the steel plate surface, and the chemical conversion treatment nature and the corrosion resistance after chemical electrodeposition coating.

Claims (5)

C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% or less, Al : 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being a component composition consisting of Fe and inevitable impurities, in the steel sheet surface and in the steel sheet within a depth range of 1 μm from the surface maximum value P ₂ of the Si concentration is high strength cold rolled steel sheet excellent in chemical conversion treatability and corrosion resistance after coating, characterized in that is less than 1.3 times the Si concentration P ₁ in the sheet thickness 1/4. C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% or less, Al : A slab containing 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being composed of Fe and inevitable impurities, hot-rolled, cold-rolled, and continuously annealed In the method for producing a cold-rolled steel sheet, a method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion property and post-coating corrosion resistance, characterized in that after the above-described continuous annealing, pickling is performed to remove the steel sheet surface by 1 μm or more per side. The method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance according to claim 2, wherein a 1 to 5 mass% nitric acid aqueous solution is used for the pickling. The high-strength cold-rolled steel sheet having excellent chemical conversion property and post-coating corrosion resistance according to claim 2, wherein a mixed acid aqueous solution comprising 2 to 20 mass% nitric acid and 0.1 to 5 mass% hydrochloric acid is used for the pickling. Manufacturing method. The high strength excellent in chemical conversion treatment property and corrosion resistance after coating according to claim 2, wherein a mixed acid aqueous solution comprising 0.5 to 5 mass% nitric acid and 0.1 to 5 mass% hydrofluoric acid is used for the pickling. A method for producing a cold-rolled steel sheet.

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