JP2007084868A - Cold-rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet having excellent properties of preventing rust initiation even when used in such a harsh environment that a dry atmosphere and a moist atmosphere are repeated in the presence of chlorine ions; and to provide a manufacturing method therefor. <P>SOLUTION: The cold-rolled steel sheet includes Mn of 0.05 mass% or more and S of 0.020 mass% or less; and has a film containing P on at least one surface. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a cold-rolled steel sheet suitable for automobiles, home appliances, building materials, and the like, and particularly in an oil-free state, rust prevention in the period from production immediately to processing such as painting at the customer, that is, initial rust prevention The present invention relates to a cold-rolled steel sheet having excellent properties and a method for producing the same.

Cold-rolled steel sheets are usually coated with about 1 to ² g / m ² of rust-preventive oil per side of the steel sheet to prevent rust (hereinafter referred to as initial rust) from immediately after production until use by customers. Is done. The rust-preventing oil may be intended for lubrication in press molding or the like in addition to rust-prevention, but is generally removed by degreasing treatment prior to processing such as painting by the customer. Since the omission of this degreasing treatment is effective for environmental protection and cost reduction, it is desirable to provide a steel sheet that does not generate initial rust without using rust preventive oil.

  As a technology related to cold-rolled steel sheets in which the occurrence of initial rust is suppressed, that is, excellent in initial rust prevention, for example, Patent Document 1 includes 0.5 to 10 vol of steel in which the amount of components such as C and S is specified. A method is disclosed in which continuous annealing is performed in a reducing gas atmosphere containing.% Hydrogen and the balance being nitrogen. It is presumed that by gradually segregating components such as C and S on the surface layer, an extremely thin dense carbide or sulfide layer is formed to improve the initial rust prevention. Patent Document 2 discloses a method of regenerating an oxide film having an automatic reduction time of 40 seconds or more by washing with water after removing the oxide film on the steel sheet surface by, for example, pickling, on the downstream side of the continuous annealing process. Has been.

  Further, Patent Document 3 and Patent Document 4 describe that for automobiles having excellent perforation resistance even in extremely severe environments in which dry and wet conditions are repeated in the presence of chlorine ions due to snowmelt salt that is sown in winter in North America and Northern Europe. A steel plate and a manufacturing method thereof are disclosed.

Furthermore, a method for improving the corrosion resistance by forming a barrier film on the surface of the steel sheet has also been proposed. For example, in Patent Document 5, an aqueous solution mainly composed of lithium silicate is applied to the surface of the steel sheet and dried to form a rust preventive film. In addition, Patent Document 6 discloses a method in which phosphate ions or phosphate compounds are applied to a surface-treated steel sheet and dried to form a chemical conversion film.
Japanese Patent Laid-Open No. 2001-348621 JP 2004-269919 A JP 11-293346 A JP 2000-34544 A Japanese Examined Patent Publication No. 4-69235 JP 2000-144444 A

  However, in the methods described in Patent Document 1 and Patent Document 2, it is said that there is no rust even when left indoors for 10 to 14 days in an oil-free state. In an environment where a relatively large amount of deposits are present, sufficient initial rust resistance can be obtained with carbides, sulfides and oxides formed on the surface layer due to the effects of maintaining the moist state due to the adhering salt and the oxide destruction due to chloride ions. I can't.

  In addition, in the steel sheet described in Patent Document 3 and Patent Document 4 and its manufacturing method, both regulate the components in the steel and densify rust formed on the surface layer of the steel to suppress pitting corrosion, The effect of suppressing initial rust is small.

  Furthermore, in the anticorrosive film described in Patent Document 5, the initial rust generation can be delayed, but it is necessary to increase the film thickness in order to obtain a sufficient effect. The film is easily peeled off and the initial rust resistance is lowered. This is probably due to the progress of corrosion starting from a local missing part. Even if the phosphate ion or phosphate compound described in Patent Document 6 is applied directly to the surface of the steel sheet and dried, the film thickness is increased to obtain a sufficient effect as in the case of the anticorrosive film described in Patent Document 5. When the film thickness is increased, the adhesion of the film is lowered, and a problem arises in the water resistance and peel resistance of the film.

  An object of the present invention is to provide a cold-rolled steel sheet having excellent initial rust preventive properties without applying rust preventive oil and a method for producing the same even under severe environments such as dry and wet conditions in the presence of chlorine ions. To do.

As a result of studying a method capable of suppressing the occurrence of initial rust in a harsh environment where chlorine ions are present, the present inventors have found the following.
(1) It is not sufficient to form a compound layer by segregating steel components on the surface of a steel sheet or to form an oxide, but it is also effective to form a barrier film containing P.
(2) As mentioned above, increasing the thickness of the barrier film causes problems such as peeling, so it is effective to control the amount of Mn and S in the steel to improve the initial rust resistance with a thin film. is there.

  The present invention has been made on the basis of such findings, contains Mn: 0.05 mass% or more and S: 0.020 mass% or less, and has a film containing P on at least one surface, preferably on both surfaces. A cold-rolled steel sheet is provided.

  At this time, the thickness of the film containing P is preferably 5 to 1000 nm.

  Moreover, it is preferable to provide a reaction layer at the interface between the film and steel.

  The cold-rolled steel sheet of the present invention, after cold rolling steel containing Mn: 0.05 mass% or more and S: 0.020 mass% or less into a cold-rolled steel sheet, the cold-rolled steel sheet contains hydrogen and the balance from nitrogen The dew point: a cold-rolled steel sheet, which is subjected to continuous annealing in an atmosphere of −60 to −30 ° C., and then a solution containing P is brought into contact with at least one surface of the cold-rolled steel sheet after the continuous annealing. It can manufacture with the manufacturing method of.

  Further, after cold rolling a steel containing Mn: 0.05 mass% or more and S: 0.020 mass% or less into a cold-rolled steel sheet, the cold-rolled steel sheet contains hydrogen, and the dew point consisting of nitrogen as a balance: -60 to Continuous annealing is performed in an atmosphere of −30 ° C., and then at least one surface of the cold-rolled steel sheet after the continuous annealing is contacted with an acidic aqueous solution, washed with water, and then contacted with a solution containing P. If it manufactures with the manufacturing method of a cold-rolled steel plate, the more excellent initial stage rust prevention property will be acquired.

  According to the present invention, it has become possible to produce a cold-rolled steel sheet having excellent initial rust preventive properties without applying rust-preventing oil even in a harsh environment where drying and wetting are repeated in the presence of chlorine ions.

  When salt is present on the steel sheet, the electrical conductivity of condensed water and surface water film due to salt increases, the promotion of iron ion diffusion due to complex formation in the water film, the destruction of the oxide film due to adsorption of chlorine ions, chemical The steel is easily corroded by a combined effect such as the retention of moisture (condensation) due to the condensation action. In particular, as in coastal areas, corrosion easily occurs in an extremely short time in an environment where salt is easily attached due to the influence of incoming sea salt, and drying and wetting are repeated due to changes in the temperature of day and night.

  In order to eliminate the influence of such adhering salinity, the formation of a barrier film described in Patent Documents 5 and 6 is prevented, which prevents the attack of chlorine ions on the extreme surface layer of the steel sheet and is stable even if chlorine ions are present. Is effective. In particular, when the film has a rectifying action against electron transfer or an insulating film, it is considered that the corrosion reaction can be suppressed and the occurrence of initial rust can be extremely effectively suppressed. However, if even a portion of the defect exists in the film, the defective part is selectively corroded, so that the film needs to be uniform and dense, and for this purpose, a thinner film is advantageous.

  Therefore, the present inventors examined a method that can effectively suppress the occurrence of initial rust even in a thin film, as described above, after controlling the amount of Mn and S in the steel, a film containing P It was found that the formation was effective. The details will be described below.

I) Mn, S amount In the surface layer of the steel sheet, sulfides such as MnS tend to be concentrated unevenly, and therefore promote the generation of initial rust, so the S amount needs to be 0.020 mass% or less. In order to suppress the occurrence of initial rust, the smaller the amount of S, the better. However, if it is extremely reduced, a significant increase in cost is caused for desulfurization, so 0.001 mass% or more is preferable. On the other hand, if the amount of Mn is less than 0.05 mass%, S does not precipitate as a sulfide, and it concentrates unevenly on the surface layer and lowers the initial rust resistance. Therefore, the amount of Mn needs to be 0.05 mass% or more, but if it exceeds 2.0 mass%, it becomes easy to produce Mn-based oxides on the steel sheet surface layer, and it becomes impossible to maintain the homogeneity of the surface layer, so 2.0 mass% or less It is preferable that

  In addition, component elements other than Mn and S are not specified, but since the influence of oxides and carbides concentrated on the steel sheet surface layer cannot be ignored, C: 0.15 mass% or less, Si: 1.5 mass% or less Is preferred. Moreover, in order to fully demonstrate the effect of this invention, it is preferable to set it as P: 0.05 mass% or less. Such component elements other than Mn and S are defined in view of the mechanical properties necessary for the material. Hereinafter, when the preferable lower limit value and upper limit value of the steel components applied in the present invention are listed, as mass%, C: 0.001 to 0.15%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, P: 0.01 to 0.05%, Al: 0.01 to 0.06%, N: 30 ppm or less.

II) Coating containing P
The film containing P is formed by bringing a solution containing P into contact with the steel sheet surface. For example, it is an amorphous film obtained by applying an aqueous aluminum phosphate solution and drying it without washing with water. Cationic species for phosphate ions are not particularly limited, and besides Al, Ti, Zr, V, Mo, Co, Fe, Ni, Mg, La, Ce, etc. may be used, and these elements are two types. The above may be combined. The added cation can be mixed with an aqueous phosphoric acid solution in the state of carbonate, oxyacid ion, oxide sol, chelating agent, or the like. The phosphate ions also include condensed phosphate ions such as pyrophosphate, polyphosphate, tripolyphosphate, and ultraphosphate. Furthermore, water-soluble organic phosphonic acid may be contained, and these may be mainly used.

  These solutions are dried after being brought into contact with the steel plate surface by coating, spraying, dipping, etc., but before drying, the liquid film thickness can be controlled by a process such as roll drawing or gas drawing, if necessary. . When the solution is insolubilized by the reaction, the soluble component may be washed with water before drying.

Since the film containing P is composed of O, H and other composite components other than P, the amount of P adhesion is not particularly defined, but is preferably ¹ to 500 mg / m ² .

  In addition, by increasing the chemical stability of the film containing P, the period until the initial rust can be further increased, but for that, Ti, Zr, V, Mo, Co, Fe, Ni, Zn, It is effective to combine Al, Mg, P, La, Ce and the like.

  The thickness of the film containing P is preferably 5 to 1000 nm. If it is less than 5 nm, sufficient corrosion resistance cannot be obtained. If it exceeds 1000 nm, the initial rust resistance is degraded because the film tends to be locally lost. In addition, when the film is thick, there is a possibility that it may adversely affect the conductivity, paintability, etc. during use at the customer. Therefore, the thinner the film, the better.

  The aqueous solutions containing phosphate ions are all acidic, and a reaction layer containing Fe is formed at the interface with the steel sheet, so that the initial rust resistance is improved. In order to form such a reaction layer more effectively, at least one selected from nitrate ions, chlorate ions, fluorine ions and the like may be added to an aqueous solution containing phosphate ions. At this time, the amount to be added is not particularly defined, but it is preferable to add so that the pH of the aqueous solution is 1 to 4 because the film containing P formed becomes non-uniform if the reactivity of the treatment liquid is excessively increased. .

III) Manufacturing conditions of cold-rolled steel sheet before film formation containing P The cold-rolled steel sheet of the present invention is as described above after continuously annealing a cold-rolled steel sheet containing Mn: 0.05 mass% or more and S: 0.020 mass% or less. In addition, it may be brought into contact with a solution containing P, but it is preferable to appropriately control the surface state of the steel sheet in order to exhibit excellent initial rust prevention properties even with a thin film containing P. Therefore, the present inventors have made the following investigations on the influence of the dew point of the atmosphere during continuous annealing.

At mass%, C: 0.0011%, Si: 0.004%, Mn: 0.1%, P: 0.007%, S: 0.007% and Cu: 0.01%, Ni: 0.01%, Cr: 0.02%, Al: 0.025%, Nb : Hot rolled steel containing 0.002%, Ti: 0.03%, pickled, cold rolled into a 0.8mm cold rolled steel sheet, dew point of -10 ~ in a reducing atmosphere consisting of hydrogen and nitrogen The temperature was changed to −40 ° C. and kept at 830 ° C. for 20 seconds for continuous annealing. The obtained steel plate was brought into contact with an acidic aqueous solution as necessary, washed with water, and then an aqueous aluminum phosphate solution (Al: 3.2 mass%, P ₂ O ₅ : 23.5 mass%) composed of phosphoric acid and aluminum ions was applied to the steel plate surface. And dried to form an amorphous film containing 300 nm thick P. Then, the following initial rust generation test was conducted to investigate the occurrence of initial rust in an environment where dry and wet conditions were repeated in the presence of chlorine ions.

Initial rust generation test:
The steel sheet was sheared to a size of 70 mm × 70 mm, the end face and one face were sealed, and a corrosion test was performed under the following conditions.
Artificial sea salt was dissolved in ion-exchanged water to prepare an aqueous solution with a concentration of 0.035 mass%, and this aqueous solution was sprayed onto the steel plate after sealing and dried to deposit 100 mg / m ^{2 of} salt. Subsequently, the steel sheet was put in an environmental test machine, and the corrosion test was repeated three times in the cycle of drying (temperature 60 ° C, relative humidity 35%) 3 hours → wet (temperature 40 ° C, relative humidity 95%) 3 hours. went. At this time, the transition time between wet and dry was 1 hour. Thereafter, the salt content of the steel sheet was washed with ion-exchanged water, dried, and the artificial sea salt was again adhered to 100 mg / m ² , and a similar corrosion test was performed.
The number of spot rust generated per 1 cm ² was determined and evaluated as follows.
◎: No rusting ○: 1 to less than 10 △: 10 to less than 30 ×: 30 to less than 50 × ×: 50 or more As a result, by controlling the dew point to -30 ℃ or less, Even when the film containing P was extremely thin, the rusting condition was 状況 or ○, and it was revealed that excellent initial rust prevention properties were obtained. The reason for the difference in the occurrence of initial rust depending on the dew point condition is not sufficiently clear, but is presumed as follows. That is, when the dew point is low, the reduction effect is high, and it is difficult for oxides to be concentrated on the surface layer to exist, and since sulfides and the like are relatively uniformly present on the surface layer, the surface properties are extremely uniform. As a result, the wettability between the treatment liquid for film formation and the steel is homogenized, and the formed film containing P is uniform with very few defective portions, and the occurrence of initial rust is suppressed. On the other hand, when the dew point is high, the homogeneity of the surface texture is not sufficient, the reactivity of the aqueous aluminum phosphate solution with a pH of about 2 and the steel varies depending on the location, a non-uniform film is formed, and initial rust is It tends to occur.

  The lower limit of the dew point of the atmosphere is not particularly limited, but a large equipment cost is required to make it below -60 ° C. If the dew point temperature is controlled, the hydrogen ion concentration is not particularly defined, but the hydrogen concentration is preferably 2 to 7 vol.%.

  When the annealed steel sheet is brought into contact with an acidic aqueous solution and then a film containing P is formed, a more excellent initial rust resistance can be obtained. For example, when immersed in a 50 ° C. sulfuric acid aqueous solution (98 mass% sulfuric acid 50 g / l) for several seconds, an extremely thin oxide layer is formed on the surface layer in the subsequent washing and drying process. The reactivity of the aqueous solution for forming a film containing P and steel is slightly reduced by the formation of the oxide layer, but the oxide layer is homogeneous, so the synergistic action of the oxide layer and the film containing P It is thought that initial rust prevention property improves.

  As the acidic aqueous solution, besides sulfuric acid aqueous solution, hydrochloric acid, a mixed aqueous solution of nitric acid and hydrochloric acid, oxalic acid, chloric acid and the like are effective. In addition, a complexing agent for capturing Fe ions such as citric acid can be added. Furthermore, if an acidic aqueous solution containing phosphoric acid is used, a more homogeneous film containing P can be formed. In the acidic aqueous solution, metal ions such as Ti, Zr, V, Mo, Co, Fe, Ni, Zn, Al, Mg, P, La, and Ce may be present. In order to obtain a sufficient effect, the pH of the acidic aqueous solution is preferably 3 or less.

Since the surface can be homogenized and the initial rust resistance can be further improved by controlling the dew point of the steel component and annealing atmosphere described above, and further contacting with the acidic aqueous solution after annealing described above, the mechanism simply includes P. In addition to improving the homogeneity of the film, the surface layer of the cold-rolled steel sheet is formed with the following two layers of nanometer thickness. It is thought to be due to the high expression.
(i) Reaction layer formed at the interface between the film containing P and steel
(ii) Film containing P The cold-rolled steel sheet of the present invention is basically subjected to continuous annealing after cold rolling, overaging treatment as necessary, temper rolling, and if necessary with an acidic aqueous solution. Although it is produced in the order of contact, washing, drying, and subsequently forming a film containing P, it can be contacted with an acidic aqueous solution before temper rolling. It is also possible to form a film containing P directly upon contact with the acidic aqueous solution. After continuous annealing, a film containing P can be formed off-line in a coating line or the like, but it is preferable to form it in-line in view of productivity.

  A steel sheet after cold rolling made of steels A to E having the components shown in Table 1 and the balance Fe and inevitable impurities was continuously annealed under the annealing conditions shown in Table 4. After the annealed cold-rolled steel sheet was contacted with the acidic aqueous solution under the treatment conditions shown in Table 2, a film containing P was formed under the conditions shown in Table 3 to prepare Samples Nos. 1 to 18. Some samples are not subjected to contact treatment with an acidic aqueous solution, and some are not formed with a film containing P. Then, the thickness of the coating containing P, the amount of deposited P, the presence or absence of a reaction layer, and the occurrence of initial rust were investigated by the initial rust generation test.

Film thickness including P:
Three arbitrary sections of the cross section cut out by FIB processing were observed by TEM, the thickness was measured, and the average was obtained.

P adhesion amount:
The amount of adhesion was quantified by fluorescent X-ray analysis by comparison with a calibration plate obtained by wet analysis (dissolved with acid and P content in acid by ICP analysis [high frequency plasma emission analysis]) using a calibration curve method. .

Presence or absence of reaction layer:
When measuring the film thickness of the FIB processed sample by TEM, the presence or absence of Fe inside the film was confirmed by elemental analysis. If there is continuous Fe from the interface, the reaction layer is assumed to be formed. It was judged.

  The results are shown in Table 4. Samples Nos. 1 to 12, 16, and 17 according to the present invention have excellent initial rust prevention properties even when the film containing P is thin even if the rusting state is ◎ or ○. It can be seen that

Claims (5)

  A cold-rolled steel sheet comprising a film containing Mn: 0.05 mass% or more and S: 0.020 mass% or less and containing P on at least one surface.   2. The cold-rolled steel sheet according to claim 1, wherein the film containing P has a thickness of 5 to 1000 nm.   3. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a reaction layer at an interface between the film containing P and steel.   After cold rolling a steel containing Mn: 0.05 mass% or more and S: 0.020 mass% or less into a cold-rolled steel sheet, the dew point of the cold-rolled steel sheet containing hydrogen and the balance consisting of nitrogen: -60 to -30 A method for producing a cold-rolled steel sheet, comprising performing continuous annealing in an atmosphere of ° C and then bringing a solution containing P into contact with at least one surface of the cold-rolled steel sheet after the continuous annealing. After cold rolling a steel containing Mn: 0.05 mass% or more and S: 0.020 mass% or less into a cold-rolled steel sheet, the dew point of the cold-rolled steel sheet containing hydrogen and the balance consisting of nitrogen: -60 to -30 Cold annealing, characterized in that continuous annealing is performed in an atmosphere of ° C., then, at least one surface of the cold-rolled steel sheet after the continuous annealing is contacted with an acidic aqueous solution, washed with water, and then contacted with a solution containing P A method of manufacturing a steel sheet.