JP2007138212A - Cold-rolled steel sheet superior in chemical conversion treatment property and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet containing Si, which has satisfactory chemical conversion treatment properties. <P>SOLUTION: This manufacturing method comprises the steps of: forming an Mn-containing film (excluding a phosphate film) on the surface of the cold-rolled steel sheet containing 0.1 mass% or more Si; annealing the treated cold-rolled steel sheet; and making the annealed film contact with an aqueous acid solution. The Mn-containing film (excluding the phosphate film) contains Mn of 0.1 to 10,000 mg/m<SP>2</SP>. The steel sheet contains 0.3 mass% or more Si and contains Mn so as to satisfy the expression; Si content/Mn content≥0.4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cold-rolled steel sheet having excellent chemical conversion properties, for example, a cold-rolled steel plate having excellent chemical conversion properties used as a material for automobiles.

  Cold-rolled steel sheets are inexpensive metal materials and are widely used in fields such as automobiles, home appliances, and building materials. In particular, in the automotive field, cold-rolled steel sheets are still the mainstream of automotive materials because they have superior press formability and chemical conversion properties compared to other metal materials. In recent years, in the automobile industry, the weight of automobiles has been reduced from the viewpoint of improving fuel consumption and reducing exhaust gas, and the use of high-strength cold-rolled steel sheets has been rapidly increasing in conjunction with the need for improved collision safety.

  A high-strength cold-rolled steel sheet is a steel sheet to which Si, Mn, etc. are added as elements in the steel, and it has been conventionally known that the Si oxide that is concentrated on the surface during annealing particularly deteriorates the chemical conversion processability. The development of high-strength cold-rolled steel sheets with excellent processability has long been desired.

  As a technique for improving the chemical conversion processability of a high-strength cold-rolled steel sheet, for example, in Patent Document 1, the Si oxide concentrated on the steel sheet surface by pickling using hydrochloric acid or sulfuric acid is removed to a specific coverage or less. Techniques to do this are disclosed. However, since Si oxide does not dissolve in common acids such as hydrochloric acid and sulfuric acid, removal of Si oxide by this method is not practical at all. In addition, even if it is below a specific coverage, the remaining Si oxide has a significant adverse effect on the chemical conversion treatment performance, so that, for example, when chemical conversion treatment is performed under severe conditions, good chemical conversion treatment performance is ensured. It is extremely difficult.

  For example, Patent Literature 2, Patent Literature 3 and the like are disclosed as techniques aimed at improving the chemical conversion property and the resistance to galling of cold-rolled steel sheets.

  Patent Document 2 is a technique for discontinuously depositing one or more metals of Ni, Mn, Co, Mo, and Cu on the surface of a cold-rolled steel sheet. However, even if this technique is applied to a cold-rolled steel sheet containing Si, the chemical conversion processability is poor because Si oxide remains on the surface of the steel sheet. Furthermore, since elements such as Mo and Cu have an adverse effect on the chemical conversion treatment property, there is also a problem that the chemical conversion treatment property deteriorates due to elution during the chemical conversion treatment.

  Patent Document 3 discloses an oxide of a second element group consisting of an ultrathin film mainly composed of zero-valent zinc on the surface of a cold-rolled steel sheet, and one or more of divalent zinc, P, B, and Si as an upper layer. This is a technique for forming a multi-layered amorphous film. However, even if this technique is applied to a cold-rolled steel sheet containing Si, the chemical conversion processability is poor because Si oxide remains on the surface of the steel sheet.

  On the other hand, for example, Patent Document 4 and Patent Document 5 disclose techniques aiming to improve chemical conversion property and mold galling resistance by subjecting a cold-rolled steel sheet before annealing to surface treatment.

  In Patent Document 4, after applying a compound containing Ni, Co, Al, Zn, Cr, Ti, Sb, Bi to the surface of the cold-rolled steel sheet, annealing is performed, thereby adding a metal oxide or a metal to the surface of the cold-rolled steel sheet. It is a technique aimed at improving the chemical conversion property by forming it and using it as a crystal nucleus during the chemical conversion reaction. However, even if these compounds are applied to the surface of the Si-containing cold-rolled steel sheet before annealing, it is not possible to suppress the surface concentration of Si during annealing, and Si oxide is formed on the surface of the steel sheet after annealing. Therefore, good chemical conversion processability cannot be obtained.

Patent Document 5 discloses that a cold-rolled steel sheet is coated with a water-soluble non-metallic phosphate and an organic acid salt such as Na, Ca, Mg, Mn, Fe, Sn, Al, and Co, and then annealed. This technique aims to improve the anti-galling property by forming a phosphate film on the surface of the rolled steel sheet. However, according to this technique, although some improvement in mold galling resistance is recognized, ensuring of good chemical conversion treatment is not considered at all, and the chemical conversion treatment film is formed on the upper layer of the formed phosphate film. Little formed. Furthermore, even if these compounds are applied to the surface of the Si-containing cold-rolled steel sheet before annealing, the surface concentration of Si during annealing cannot be suppressed, and Si oxide is formed on the surface of the steel sheet after annealing. Therefore, the chemical conversion processability is poor.
JP 2004-323969 A JP-A-3-236491 JP-A-10-158858 Japanese Patent Laid-Open No. 55-14854 JP 52-63831 A

  As described above, the conventional technology has not established a technology that sufficiently satisfies the chemical conversion processability of a cold-rolled steel sheet containing Si, and particularly satisfies the chemical conversion processability of a high-strength cold-rolled steel sheet containing Si. There was no technology to make it happen.

  In view of such circumstances, an object of the present invention is to provide a technique that satisfies the chemical conversion properties of a cold-rolled steel sheet containing Si, particularly used as a steel sheet for automobiles. Furthermore, recent high-strength cold-rolled steel sheets have become more difficult to ensure good chemical conversion properties because of the addition of a large amount of elements such as Si, and high-strength cold-rolled steel sheets containing Si. It aims at providing the steel plate which can satisfy the chemical conversion property of.

  The manufacturing method of the cold-rolled steel sheet in the present invention for solving the above-mentioned problem is that a Mn-containing film (excluding the phosphate film) is applied to the surface of the cold-rolled steel sheet having a Si content of ≧ 0.1% by mass. It is the manufacturing method of the cold-rolled steel plate excellent in the chemical conversion property characterized by performing an annealing process after forming and then performing a contact process with acidic aqueous solution.

Moreover, in the said manufacturing method, it is preferable that a Mn containing membrane | film | coat (except a phosphate membrane | film | coat) contains 0.1-10000 mg / m < ² > Mn.

  Further, in the above manufacturing method, it is preferable that the steel sheet has an Si content ≧ 0.3 mass% and an Si content / Mn content ≧ 0.4 because the effects of the present invention are more remarkably exhibited.

  Moreover, the cold-rolled steel sheet in this invention for solving the said subject is a cold-rolled steel sheet excellent in chemical conversion property characterized by being manufactured by one of the said manufacturing methods.

  The present invention provides a Si-containing cold-rolled steel sheet and a manufacturing method that are excellent in chemical conversion processability, and is an extremely effective technique that remarkably improves the chemical conversion processability of a high-strength cold-rolled steel sheet containing Si. It is extremely industrially valuable.

  Hereinafter, the present invention will be described together with the background to the invention.

  The present inventors diligently studied various surface modification treatments for the purpose of improving the chemical conversion treatment properties of high-strength cold-rolled steel sheets containing Si. As a result, in order to improve the chemical conversion property of the high-strength cold-rolled steel sheet containing Si to the same level as that of the mild steel sheet, Si oxide existing on the surface of the steel sheet after annealing can be almost completely eliminated. In order to do so, it has been proposed to suppress the Si surface concentration by applying chemicals before annealing, etc., or to perform acid pickling treatment after annealing to dissolve and remove Si oxides. It was found that this method is difficult to achieve.

Therefore, the present inventors tried to examine a chemical conversion treatment improvement measure based on the idea of reversal, which is different from the conventional idea of suppressing or removing Si surface enrichment, and the surface enrichment of Si was actively generated. In contrast, the Mn-containing coating (excluding the phosphate coating) previously applied to the surface of the cold-rolled steel sheet is compounded by waiting for Si to be concentrated on the surface when annealed. By forming a Si-Mn composite oxide such as MnSiO ₃ or Mn ₂ SiO ₄ that is soluble in water and almost completely disappearing by contacting it with an acidic aqueous solution, the acid solubilization modification treatment of the surface concentrate The present inventors have found a chemical conversion treatment improving method and have reached the present invention.

  Hereinafter, the present invention will be described in more detail.

  The cold-rolled steel sheet used in the present invention is a cold-rolled steel sheet having an Si content of ≧ 0.1% by mass. Since Si is an element effective for increasing the strength of a steel sheet, it is an effective means in the present invention to contain Si, particularly when producing a high-strength cold-rolled steel sheet. Even if the production method of the present invention is applied to a steel sheet having a Si content of <0.1% by mass, there is no problem, but a steel plate having a Si content of <0.1% by mass originally has a great problem with chemical conversion treatment. In the present invention, Si-Mn composite oxide is obtained by compounding Si in steel with a Mn-containing film (excluding a phosphate film). Since the basic idea is to form it, the present invention is intended for cold-rolled steel sheets having a Si content of ≧ 0.1% by mass. The upper limit of the Si content is not limited at all, but if the Si content is excessively high, production problems such as slab cracking may occur, so the Si content is 5.0 mass%. Preferably, the Si content is 3.0% by mass or less.

  Moreover, since the element which has the most bad influence on chemical conversion property is Si, content of elements other than Si is not limited at all. Elements other than Si may be appropriately contained according to desired properties, for example, C: 0.0005 to 0.5 mass%, Mn: 0.05 to 3.5 mass%, P: 0.005. 0.2 mass%, S: 0.05 mass% or less, Al: 0.005-1.5 mass%, N: 0.001-0.1 mass%, Ti: 0.1 mass% or less, Nb: A steel sheet containing elements such as 0.05% by mass or less, V: 0.10% by mass or less, B: 0.005% by mass or less, Mo: 0.5% by mass or less, Cr: 0.5% by mass or less. Illustrated.

  If the cold-rolled steel sheet used in the present invention is Si content ≧ 0.3 mass% and Si content / Mn content ≧ 0.4, it is particularly preferable because the effect of the present invention is more remarkably exhibited. This is because the higher the Si content and the higher the Si content / Mn content ratio, the higher the ratio of the Si single oxide produced during annealing, and the lower the ratio of the Si-Mn composite oxide. In particular, in the region where the Si content ≧ 0.3 mass% and the Si content / Mn content ≧ 0.4, the chemical conversion processability becomes extremely poor. It is particularly preferable to apply the production method of the present invention to such a steel sheet, since the effect of improving the chemical conversion treatment property is more remarkably exhibited.

In the present invention, it is an essential requirement to form a Mn-containing film (excluding the phosphate film) on the surface of the steel sheet before annealing. Mn-containing coating (excluding phosphate film) is by complexing the Si oxide surface segregation during annealing, the Si-Mn composite oxide soluble form to the acid, conversion treatment properties harmful to a SiO ₂ The formation of Si single oxide such as is suppressed. For this reason, it is desirable that the above-described Mn-containing film is formed in an amount necessary for converting all of the Si oxide to be concentrated to a Si—Mn composite oxide. On the contrary, even if the Mn-containing film is excessively formed, there is no problem because the Mn-containing film is soluble in acid and does not adversely affect the chemical conversion property.

The amount of Mn-containing coating described above cannot be unconditionally specified because the surface concentration of Si varies depending on various conditions such as the Si content in steel, the annealing atmosphere, the annealing temperature, and the annealing time. However, if Mn is in the range of 0.1 to 10,000 mg / m ² , the object can be achieved in most cases, which is preferable. When the adhesion amount of the Mn-containing film is less than 0.1 mg / m ² as Mn, it may be difficult to convert all Si that is surface-concentrated into a complex oxide particularly when annealing conditions are severe, Chemical conversion processability may be slightly insufficient. Even if the adhesion amount of the Mn-containing coating exceeds 10,000 mg / m ² as Mn, the chemical conversion property is not deteriorated, but the effect is saturated, which is economically disadvantageous. The adhesion amount of the Mn-containing film is more preferably in the range of 0.5 to 5000 mg / m ² as Mn.

  The type of the Mn-containing film is not limited at all, and Mn may be contained as an essential element. For example, from metal Mn film, Mn oxide, Mn hydroxide, Mn nitrate, Mn sulfate, Mn-containing inorganic compound film such as potassium perMn acid, Mn-containing organic compound film such as Mn acetate, and two or more of these Mn-containing films are exemplified (except for those containing a phosphate film).

  In addition, in the Mn-containing film of the present invention, the reason for removing the phosphate film is that when the Mn phosphate film is formed on the surface of the steel sheet before annealing, the Mn phosphate film is not decomposed even in the annealing process after annealing. This is because the Mn phosphate film remains, and a chemical conversion treatment film is hardly formed on the upper layer of the Mn phosphate film, so that the chemical conversion treatment performance is significantly deteriorated.

  The method for forming the Mn-containing film is not limited in any way, and an aqueous solution and / or water in which one or more of the aforementioned metal Mn, Mn-containing inorganic compound, and Mn-containing organic compound are dissolved and / or dispersed. Using the dispersion, it is possible to form a Mn-containing film by electrolytic treatment, reactive treatment by dipping or spraying, or coating treatment by roll coater or Ringer drawing. Moreover, it is also possible to form a Mn-containing film by performing vapor deposition treatment using metal Mn, an Mn-containing inorganic compound, and an Mn-containing organic compound as they are. Furthermore, it is also possible to form a Mn-containing film by combining these plural methods.

In the case of forming a Mn-containing film by electrolytic treatment, for example, an electrolytic solution containing 10 to 100 g / l of manganese nitrate hexahydrate and having a bath temperature of 20 to 80 ° C. is used, and a current density of 0.5 to 100 A / A method of electrodepositing a mixture of metal Mn and Mn hydroxide by performing an electrolytic treatment at dm ² for 0.001 to 300 seconds is exemplified, but it is not limited thereto.

  Further, when forming a Mn-containing film by immersion type treatment, for example, a treatment liquid containing 10-100 g / l of manganese nitrate hexahydrate and having a bath temperature of 20-80 ° C. is used, and the steel plate is placed for 1-120 seconds. Although the method of performing the process by immersing and precipitating the mixture of Mn oxide and Mn hydroxide is illustrated, it is not limited to this.

  Also in the case of spray type treatment, a method of depositing a mixture of Mn oxide and Mn hydroxide by performing treatment by spraying the treatment liquid on the steel sheet for 1 to 120 seconds using the same treatment liquid as the above immersion type treatment liquid However, the present invention is not limited to this.

  In the case of coating-type treatment, one or two of Mn-containing inorganic compounds such as manganese nitrate hexahydrate, manganese sulfate pentahydrate, potassium permanganate, and manganese acetate tetrahydrate are used. An example is a method in which an aqueous solution or an aqueous dispersion containing 1 to 50% by mass of seeds or more is prepared, attached to the steel sheet with a roll coater or the like, and then dried to attach these Mn compounds to the steel sheet surface. However, the present invention is not limited to this.

  In the present invention, after forming a Mn-containing film (excluding the phosphate film) on the surface of the steel sheet before annealing, annealing treatment is performed. In this step, simultaneously with the adjustment of the material of the steel sheet, Si in the surface-enriched steel is taken into the Mn-containing film to form an acid-soluble Si—Mn composite oxide. In addition, excess oxygen and unnecessary elements such as C, H, N, S, and P contained in the Mn-containing film disappear by being reduced or burned in this annealing process, Si-Mn composite oxide and Mn oxide derived from excess Mn remain on the surface.

  The annealing conditions are not limited in any way. For example, in a nitrogen atmosphere containing 0.3 to 30% hydrogen, the dew point is -60 to 15 ° C, the soaking temperature is 700 to 950 ° C, and the soaking time is 15 to 15 ° C. Recrystallization annealing may be performed under conditions such as 500 seconds. Prior to the recrystallization annealing in the reducing atmosphere, heat treatment in an oxidizing atmosphere may be performed as necessary. Further, following the recrystallization annealing, quenching, tempering, etc. You may implement the process of.

Next, in this invention, the contact process with acidic aqueous solution is performed with respect to the steel plate after an annealing process. By this treatment, the acid-soluble Si—Mn composite oxide and Mn oxide are dissolved and removed, and the surface concentrate is almost lost on the surface of the steel sheet, and the chemical conversion treatment property is improved.
The liquid composition, pH, liquid temperature, treatment time, etc. of the acidic aqueous solution are not limited at all, but in order to efficiently remove the oxide, for example, an aqueous solution of sulfuric acid acid, hydrochloric acid acid, nitric acid acid, etc. The pH may be 0.5 to 5, the liquid temperature may be 10 to 90 ° C., and the treatment time may be about 0.5 to 60 seconds.

  Hereinafter, the present invention will be described in more detail based on examples.

  Table 1 shows the components in the steel of the cold-rolled steel sheet used before annealing (the remaining components in Table 1 are Fe and inevitable impurities). In addition, all board thickness was 1.2 mm.

These steel sheets were first subjected to a treatment for forming a Mn-containing film. As the treatment method, electrolytic, immersion, spray, and coating treatment methods were used. In the case of electrolytic treatment, an electrolytic solution containing 50 g / l of manganese nitrate hexahydrate and having a bath temperature of 50 ° C. is used, and an electrolytic treatment is performed at a current density of 2 A / dm ² for 0.006 to 140 seconds. A mixture of metal Mn and Mn hydroxide was electrodeposited. In the case of the immersion type treatment, the same treatment solution as the above electrolytic treatment solution was used, and the treatment was performed by immersing the steel sheet for 15 seconds to precipitate a mixture of Mn oxide and Mn hydroxide. Also in the case of the spray type treatment, the same treatment liquid as the above electrolytic treatment liquid was used, and the treatment was performed by spraying the treatment liquid on the steel sheet for 15 seconds to precipitate a mixture of Mn oxide and Mn hydroxide. In the case of the coating type treatment, 10% by mass aqueous solution of manganese nitrate hexahydrate, manganese acetate tetrahydrate, manganese sulfate pentahydrate, and potassium permanganate are prepared and coated on the steel sheet by a roll coater. Processing was performed by drying, and each Mn compound was made to adhere to the steel plate surface.

  The Mn adhesion amount on the steel sheet surface was calculated by preparing an aqueous solution in which a Mn-containing film was dissolved with 0.1N hydrochloric acid, and measuring the Mn concentration in this aqueous solution by ICP.

  Next, the steel sheet on which the Mn-containing film was formed was subjected to annealing treatment. The annealing conditions were an atmosphere of 5% hydrogen-nitrogen and a dew point of −20 ° C., an annealing temperature of 830 ° C., and an annealing time of 360 seconds.

  Next, a part of the steel plate after annealing was subjected to contact treatment with an acidic aqueous solution. As the acidic treatment liquid, sulfuric acid, hydrochloric acid, and nitric acid were used, and the pH was adjusted to 1.0 to 2.5 by changing the dilution ratio with water. The steel plate was immersed in these acidic treatment liquids for 1 to 5 seconds to carry out the treatment.

  Evaluation of chemical conversion property was performed about the cold-rolled steel plate produced in this way.

The chemical conversion treatment was evaluated using a commercially available chemical conversion treatment agent (Nippon Parkerizing Co., Ltd., Palbond PB-L3020 system) under the conditions of a bath temperature of 42 ° C. and a chemical conversion treatment time of 120 seconds. Uniformity was evaluated. The uniformity evaluation of the chemical conversion treatment crystal was judged according to the following criteria.
A: There is no scum or unevenness in the conversion crystal. O: There is no scent in the conversion crystal, but there is some unevenness. △: There is a part of the conversion crystal. ×: Scaling of the conversion crystal is remarkable.

  Table 2 shows the steel sheet used, the content of the treatment applied to the steel sheet, and the evaluation results of the chemical conversion treatment.

  The tensile strength of the steel plate finally obtained through the annealing step is as follows: Steel plate A is 270 MPa, Steel plate B is 340 MPa, Steel plates C, D, and E are 590 MPa, Steel plates F and G are 980 MPa, Steel plate H is 1180 MPa, Steel plate I was 1340 MPa.

  As shown in Table 2, all of the cold-rolled steel sheets manufactured by the manufacturing method of the present invention are excellent in chemical conversion treatment.

  The manufacturing method of the cold-rolled steel sheet of this invention can be utilized as a manufacturing method of the cold-rolled steel sheet excellent in chemical conversion property. The cold-rolled steel sheet produced by the production method of the present invention can be used as a cold-rolled steel sheet that is excellent in chemical conversion treatment and used in application fields such as the automobile field. Furthermore, the cold-rolled steel sheet produced by the production method of the present invention can be used as a high-strength cold-rolled steel sheet that is excellent in chemical conversion treatment and used in application fields such as the automobile field.

Claims (4)

After forming a Mn-containing film (excluding the phosphate film) on the surface of the cold-rolled steel sheet having a Si content of ≧ 0.1% by mass, an annealing treatment is performed, followed by a contact treatment with an acidic aqueous solution. The manufacturing method of the cold-rolled steel plate excellent in the chemical conversion processability characterized by this. The method for producing a cold-rolled steel sheet having excellent chemical conversion property according to claim 1, wherein the Mn-containing film (excluding the phosphate film) contains 0.1 to 10000 mg / m ² of Mn. The steel sheet has a Si content of ≧ 0.3% by mass and a Si content / Mn content of ≧ 0.4. The cold rolling excellent in chemical conversion treatment according to claim 1 or 2, A method of manufacturing a steel sheet. A cold-rolled steel sheet excellent in chemical conversion property, produced by the production method according to claim 1.