JP2007138211A - Cold-rolled steel sheet superior in chemical conversion treatment property and galling resistance, 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 both of excellent chemical conversion treatment properties and galling resistance. <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; and annealing the treated cold-rolled steel sheet. The Mn-containing film (excluding the phosphate film) contains Mn of 0.5 to 5,000 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 excellent in chemical conversion treatment and mold galling resistance, for example, to a cold-rolled steel sheet excellent in chemical conversion treatment and mold galling resistance 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 efficiency 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, and the like are added as elements in the steel, and it has been conventionally known that Si oxide that is concentrated on the surface particularly during annealing significantly deteriorates the chemical conversion property. On the other hand, when high-strength cold-rolled steel sheets are press-formed, there is a problem that not only the forming load is increased, but also that a local high surface pressure portion is generated, which causes mold galling. The development of a high-strength cold-rolled steel sheet excellent in mold galling resistance has 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 for improving both the chemical conversion property and the resistance to galling of a cold-rolled steel sheet.

  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. In addition, with this technology, metal is deposited discontinuously with the aim of improving chemical conversion properties, but mold galling occurs between the exposed portion of the steel plate and the mold, so that there is no problem with respect to mold galling resistance. It is enough.

  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. Further, since the upper amorphous film is hard, the mold may be worn particularly during sliding under high surface pressure, and the mold galling resistance is insufficient.

  On the other hand, for example, Patent Document 4 and Patent Document 5 disclose techniques for improving 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 achieves a high degree of compatibility between the chemical conversion property of the cold-rolled steel sheet and the galling resistance, and in particular, the chemical conversion property of the high-strength cold-rolled steel sheet containing Si. There was no technology that satisfied mold galling resistance.

  In view of such circumstances, an object of the present invention is to provide a technology that achieves a high degree of compatibility between the chemical conversion property and the anti-galling property 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 secure good chemical conversion properties due to the addition of a large amount of elements such as Si, and die galling also occurs as the strength increases. Therefore, an object of the present invention is to provide a steel sheet capable of satisfying the chemical conversion treatment property and the anti-galling property of a high-strength cold-rolled steel sheet containing Si.

  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 treatment property and the die-proof galling property characterized by performing an annealing process after forming.

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 > of 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 the chemical conversion property and the mold-proofing property characterized by being manufactured by one of the said manufacturing methods.

  The present invention provides a cold-rolled steel sheet containing Si and a method for producing the same, in which the chemical conversion property and mold galling resistance are both highly compatible, and in particular, the chemical conversion property of a high-strength cold-rolled steel sheet containing Si and Since it is an extremely effective technique that achieves both mold galling resistance, it is extremely valuable industrially.

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

The inventors of the present invention first made extensive studies on 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 of high-strength cold-rolled steel sheets containing Si to a level equivalent to that of mild steel sheets, the generation of Si single oxides such as SiO _{2 on} the steel sheet surface after annealing is almost complete. In order to achieve this, it is necessary to avoid the Si surface concentration by applying chemicals before annealing, or by pickling treatment after annealing, and so on. It has been found that it is difficult to achieve this by the method of dissolving and removing the.

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 film (excluding the phosphate film) previously applied to the surface of the cold-rolled steel sheet is compounded by waiting for Si to be concentrated at the time of annealing, and MnSiO Forming Si-Mn composite oxides such as ₃ and Mn ₂ SiO ₄ , and at the same time almost completely avoiding the formation of Si single oxides such as SiO ₂ I came to find a way.

The reason why the chemical conversion treatment performance is remarkably improved by this method is that Si single oxide such as SiO ₂ is insoluble in acid, whereas Si—Mn composite oxide such as MnSiO ₃ and Mn ₂ SiO ₄ is acid. Because it is soluble in the acid, it does not only allow the chemical conversion reaction to proceed by dissolving in an acidic chemical conversion solution, but it also activates the nucleation of chemical conversion crystals by providing a potential heterogeneous part. It is thought to make it.

Furthermore, the present inventors have found that Si—Mn composite oxides such as MnSiO ₃ and Mn ₂ SiO ₄ are more effective in improving the galling resistance than Si single oxides such as SiO _2, and before annealing. The present inventors have found that it is possible to significantly improve the mold galling resistance of a cold-rolled steel sheet by applying a Mn-containing film (excluding a phosphate film) to the present invention. The reason why the Si-Mn composite oxide is superior to the Si single oxide in improving the galling resistance is that the Si-Mn composite oxide is softer than the Si single oxide. This is considered to further reduce the shear resistance during sliding between the molds.

  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. By applying the production method of the present invention to such a steel sheet, the effect of improving the chemical conversion treatment property is more remarkably exhibited, and at the same time, good mold galling resistance can be secured, which is particularly preferable.

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 surface-enriched into a Si—Mn composite oxide. On the contrary, even if the Mn-containing film is formed excessively, the Mn-containing film is soluble in an acid, so that the chemical conversion treatment property is not adversely affected, and the Mn-containing film is resistant to mold galling. There is no problem because the improvement effect is excellent.

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 film exceeds 10,000 mg / m ² as Mn, the chemical conversion treatment property and the mold galling resistance are 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. It is possible to form a Mn-containing film by electrolytic treatment using the dispersion, reactive treatment such as dipping or spraying, or coating treatment using a roll coater or Ringer squeezing. 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, and S contained in the Mn-containing film disappear as they are reduced or burned in this annealing process, and the surface of the steel sheet is lost. Remains mainly Si-Mn composite oxide and Mn oxide derived from excess Mn. Since these Si-Mn composite oxides and Mn oxides are both soluble in acid, they do not inhibit the chemical conversion reaction by dissolving in an acidic chemical conversion solution, and also nucleate the chemical conversion crystal. It has the effect | action which remarkably improves chemical conversion processability by activating. In addition, since both the Si-Mn composite oxide and the Mn oxide are softer than the Si single oxide, the effect of reducing the shear resistance during sliding is great, and the effect of significantly improving mold galling resistance is achieved. Have

  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.

  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 were prepared and coated on the steel sheet with 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.

  The cold-rolled steel sheet produced in this way was evaluated for chemical conversion treatment and mold galling resistance.

The evaluation of mold galling resistance was carried out using a sliding tester, the steel sheet was slid while raising the pressing load of the mold in increments of 100 MPa to 50 MPa, and no galling occurred on the steel sheet by visual observation. Evaluation was made by obtaining a limit load capacity which is a load value. The mold used for the sliding test is made of SKD11, the mold width is 10 mm, the contact length in the sliding direction between the mold and the steel plate is 4 mm, and the end of the mold in the sliding direction is The shape which gave R of 4.5 mm was used. The sliding conditions were a sliding speed of 1.0 m / min, a sliding distance of 100 mm, and sliding with respect to a steel plate coated with general rust preventive oil (Idemitsu Kosan Co., Ltd., Daphne Oil Coat (registered trademark) SK). A dynamic test was performed. By this test, mold galling resistance was determined according to the following criteria.
◎: Limit load capacity is 1000 MPa or more ○: Limit load resistance is 500 MPa or more and less than 1000 MPa Δ: Limit load resistance is 300 MPa or more and less than 500 MPa ×: Limit load resistance is less than 300 MPa

Evaluation of chemical conversion treatment is performed using a commercially available chemical conversion treatment agent (Nippon Parkerizing Co., Ltd., Palbond PB-L3020 system) under conditions of a bath temperature of 43 ° 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 sheets used, the details of the treatment applied to the steel sheets, and the evaluation results of the chemical conversion treatment property and the mold galling resistance.

  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 produced by the production method of the present invention are excellent in chemical conversion treatment properties and mold galling resistance.

  The method for producing a cold-rolled steel sheet according to the present invention can be used as a method for producing a cold-rolled steel sheet that is excellent in chemical conversion treatment and mold galling resistance. 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 property and mold galling resistance 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 properties and mold galling resistance 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, chemical conversion treatment resistance and resistance A method for producing cold-rolled steel sheets with excellent mold galling properties. The cold-rolled steel sheet having excellent chemical conversion property and anti-galling resistance according to claim 1, wherein the Mn-containing film (excluding the phosphate film) contains 0.1 to 10000 mg / m ² of Mn. Manufacturing method. The steel sheet has a Si content of ≧ 0.3% by mass and a Si content / Mn content of ≧ 0.4. A method for producing an excellent cold-rolled steel sheet. A cold-rolled steel sheet excellent in chemical conversion property and anti-mold galling property, characterized by being produced by the production method according to claim 1.