JP5044952B2 - Cold-rolled steel sheet for chemical conversion treatment and production method thereof - Google Patents

Description

The present invention relates to a steel sheet for surface treatment, in particular, a cold-rolled steel sheet for chemical conversion treatment that is performed prior to coating of automobiles, electric machines, can materials, and the like, and a method for manufacturing the same.

  Steel sheets used for automobiles, home appliances, furniture, building materials, and the like are widely subjected to coating treatment at the stage of raw steel sheets or at the stage of press processing in order to impart corrosion resistance and decorativeness. As a pretreatment for such a coating treatment, for example, a chemical conversion treatment such as a phosphoric acid treatment or a zinc phosphate treatment is performed in order to increase the adhesion of the paint to the steel plate and prevent the occurrence of coating defects. Therefore, the chemical conversion treatment needs to be performed as completely as possible so that defects such as so-called soot do not occur.

  However, the increase in the tensile strength of the steel sheet, which is carried out for the purpose of reducing the weight of automobiles, requires the addition of various alloy elements, and therefore causes a reduction in the chemical conversion processability. For example, although Cr added for increasing the tensile strength of steel depends on the amount of addition, the chemical conversion property is greatly deteriorated and the time required for the chemical conversion treatment is prolonged. In addition, the types and combinations of strengthening elements added to increase the tensile strength of steel are extremely diverse depending on the requirements of the mechanical properties and formability of the steel sheet, and are therefore used, for example, in the assembly of automobiles. Even with steel plates, the chemical conversion processability differs for each part, complicating the management of the chemical conversion process.

In order to solve such a problem, Patent Document 1 discloses that galling resistance of a high-strength steel sheet is projected by projecting solid particles having an average particle diameter of 30 to 300 μm on a steel sheet containing 0.1 mass% or more of Si. Means for improving the property and the chemical conversion property are disclosed. Patent Document 2 describes that the phosphate treatment property is improved by adsorbing 0.01 to 500 mg / m ² with a sulfur content of a specific sulfur-containing compound such as methylbutanes on the surface of a cold-rolled steel sheet. The invention is disclosed. Patent Document 3 discloses an invention in which phosphate treatment properties are improved by attaching a metal oxide or metal to a steel sheet surface. Furthermore, Patent Document 4 states that surface treatment properties including chemical conversion treatment properties are improved by providing an AlN precipitation layer in the surface layer portion of steel containing 0.1% or more and less than 3% of Al by mass%. The invention is disclosed.

JP-A-2005-240148 Japanese Patent Publication No. 61-41990 Japanese Patent Publication No.57-61114 JP 2004-162163 A

  However, the invention disclosed in Patent Document 1 is effective as means for improving chemical conversion treatment when Si is contained and the steel sheet surface is inactivated by Si-based oxides. For example, when the chemical conversion treatment property is inhibited by Cr, there is no effect. On the other hand, in the inventions disclosed in Patent Documents 2 and 3, it is required to use special chemicals, and it is necessary to provide facilities such as an electrolytic cell, which may increase the cost. The invention disclosed in Patent Document 4 can be applied only to steel containing Al, and it is necessary to perform a special heat treatment for the precipitation of AlN. As with the invention disclosed in Patent Document 2, the cost is high. May be incurred.

The present invention aims to solve the problems related to the above prior art, and can be applied to a cold-rolled steel sheet and proposes a chemical conversion treatment improving means that can be carried out at a low cost. Yes.

The cold-rolled steel sheet for chemical conversion treatment according to the present invention is, by mass ratio, C: 0.005 to 0.150% (excluding C: 0.0001 to 0.0050%) , Si: 0.02 to 2 0.05%, Mn: 0.015 to 1.91%, P: 0.020% or less, S: 0.010% or less, Al: 0.050% or less, made of Fe except for the remaining inevitable impurities, and the surface layer portion Is a dislocation density of 2 × 10 ¹³ / m ² or more. The dislocation density is desirably 5 × 10 ¹³ / m ² or more and 5 × 10 ¹⁵ / m ² or less.

The cold-rolled steel sheet for chemical conversion treatment according to the invention further includes Mo: 0.05 to 0.30%, Ni: 0.05 to 0.50%, Cu: 0.05 to 0.50%, Cr: 0. 0.05 to 0.50%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.05% and B: 0.0002 to 0.001% It can contain elements.

The cold-rolled steel sheet for chemical conversion treatment according to the present invention has a dislocation density of 2 × 10 ¹³ / m ² or more after annealing / pickling and temper rolling after the cold-rolled steel sheet having the above composition. Can be produced by performing a dislocation introducing operation.

  According to the present invention, it is possible to provide a steel plate having excellent chemical conversion properties regardless of the steel type, and to improve the efficiency of the coating process for automobiles, electric machines, can materials, and the like. Moreover, the manufacturing cost can be reduced compared with the conventional proposal.

  The raw sheet of the chemical conversion treated steel sheet according to the present invention may be either a cold rolled steel sheet or a hot rolled steel sheet. Moreover, it is not necessary to provide restrictions on the composition, structure, or characteristics of these cold-rolled steel sheets or hot-rolled steel sheets. What is necessary is just to select a composition and a structure | tissue according to intensity | strength, workability, etc. according to the use of a steel plate.

  In the case of a cold-rolled steel sheet, by mass ratio, C: 0.001 to 0.20%, Si: 0.02 to 2.00%, Mn: 0.15 to 2.00%, P: 0.020% or less , S: 0.010% or less, Al: 0.050% or less, except for the remaining inevitable impurities, a cold-rolled steel sheet made of Fe can be typically selected, and for the purpose of improving strength and workability Furthermore, Mo: 0.05 to 0.30%, Ni: 0.05 to 0.50%, Cu: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ti: 0 One or two or more elements selected from 0.03 to 0.20%, Nb: 0.01 to 0.05% and B: 0.0002 to 0.001% can be contained.

  In the case of a hot-rolled steel sheet, by mass ratio, C: 0.01 to 0.10%, Si: 0.01 to 2.00%, Mn: 0.15 to 2.00%, P: 0.020% Hereinafter, S: 0.010% or less, Al: 0.050% or less, except for the remaining inevitable impurities, it is good to be a low carbon, low alloy steel made of Fe, Cr: 0.05-1.00 is added to this It can also be contained.

In the present invention, in any of the above steel types, it is important that the dislocation density of the steel sheet surface layer portion is 2 × 10 ¹³ / m ² or more. As will be described later, when the chemical conversion treatment is performed when the dislocation density is 2 × 10 ¹³ / m ² or more, defects such as “suke” are not recognized on the chemical conversion treated steel sheet surface.

Here, the dislocation density of the steel sheet surface layer part means the dislocation density existing in the steel sheet surface layer part immediately before being subjected to the chemical conversion treatment, and the adjustment method of the sample for the measurement and the determination method of the dislocation density are as follows. is there.
(1) Measurement sample adjustment: A sample was cut out from the position of the steel plate 1/4 and degreased. (2) Dislocation density determination method: The surface of the steel plate surface was distorted by the X-ray diffractometer for the sample adjusted as described above. Was measured. The X-ray diffractometer is used for the measurement, and the diffraction intensity of the (110) plane, (211) plane, and (220) plane of the α iron of the steel sheet surface layer is measured using CoKα rays. The half width of the peak value of the reflection intensity of the surface is obtained, and the local strain ε applied to the steel sheet surface is determined by the following formulas (1) and (2).
βcos θ / λ = 0.9 / D + 2ε'sin θ / λ (1)
here,
β: half width of peak value (however, the value corrected by equation (2) is used)
θ: diffraction angle λ: wavelength of Cokα ray (0.1791 nm)
D: Crystallite size (dislocation cell, crystal grain size)
ε ′: local strain β ² = β _m ² −β ₀ ² (2)
here,
β _m : half width of peak of sample for measuring dislocation density β ₀ : half width of peak of sample without strain.
By plotting β cos θ / λ against sin θ / λ, b and ε ′ are obtained from the slope and intercept. The dislocation density ρ is determined from the obtained local strain ε ′ by the following equation (3).
ρ = 14.4ε ′ ² / b ² (3)
here,
b: Burgers vector (8.25 nm)

  FIG. 1 shows a cold-rolled steel sheet having the composition shown in Table 1 and a hot-rolled steel sheet having the composition shown in Table 2 under the conditions shown in Table 3, introducing dislocations on the steel sheet surface, and performing chemical conversion treatment. 5 is a graph showing the relationship between the dislocation density and the state of the chemical conversion coating (occurrence state with or without “suke”). The cold-rolled steel sheet is obtained by hot rolling a slab having the composition shown in Table 1, pickling, cold rolling to a final thickness, further annealing and pickling, and 0.3% temper rolling. The hot-rolled steel sheet is obtained by hot rolling a slab having the composition shown in Table 2 to the final thickness and then pickling.

  Introduction of dislocations on the steel sheet surface was performed on the cold-rolled steel sheet or hot-rolled steel sheet obtained as described above by shot blasting or strain imparting with a high-strength brush. Specific contents of these strain imparting processes are as described in Tables 3 and 4.

  The chemical conversion treatment was performed at a liquid temperature of 43 ° C. using Nippon Paint Surf Fine 5N-10 as the surface conditioning liquid and Nippon Paint Surf Dyne SD2500 as the chemical conversion liquid.

As can be seen from FIG. 1, when the dislocation density of the surface layer portion of the steel sheet is 2 × 10 ¹³ / m ² or more, “suke” is not recognized in the chemically treated steel sheet regardless of the steel type. “Suke” is recognized. In order to reduce the crystal size of zinc phosphate and generate dense crystals, the dislocation density of the steel sheet surface layer portion is preferably 5 × 10 ¹³ / m ² or more, but the dislocation density is 5 × 10 5. ^If it exceeds ¹⁵ / m ² , the formability of the steel sheet decreases, so the upper limit is preferably set to 5 × 10 ¹⁵ / m ² . Here, “suke” is a portion where no zinc phosphate crystals are deposited, and the presence or absence is determined by visual observation of three 350 × photographs taken with a SEM and visually.

It is a graph which shows the relationship between the dislocation density when a chemical conversion treatment is performed on a cold-rolled steel plate and a hot-rolled steel plate, and the state of the chemical conversion coating (the occurrence state with or without “suke”).

Claims (4)

By mass ratio, C: 0.005 to 0.150% (excluding C: 0.0001 to 0.0050%) , Si: 0.02 to 2.05%, Mn: 0.015 to 1.50%. 91%, P: 0.020% or less, S: 0.010% or less, Al: 0.050% or less, composed of Fe except for the remaining inevitable impurities, and the dislocation density of the surface layer portion being 2 × 10 ¹³ / m ² or more A cold-rolled steel sheet for chemical conversion treatment, characterized by   Furthermore, Mo: 0.05 to 0.30%, Ni: 0.05 to 0.50%, Cu: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ti: 0.00. It contains one or more elements selected from 01 to 0.20%, Nb: 0.01 to 0.05% and B: 0.0002 to 0.001%. Cold-rolled steel sheet for chemical conversion treatment as described. The dislocation density is 5 × 10 ¹³ / m ² or more and 5 × 10 ¹⁵ / m ² or less, the cold-rolled steel sheet for chemical conversion treatment according to claim 1 or 2. The cold-rolled steel sheet having the composition according to claim 1 or 2 is subjected to dislocation introduction operation so that the dislocation density of the surface layer portion is 2 × 10 ¹³ / m ² or more after temper rolling after annealing and pickling. The manufacturing method of the cold-rolled steel plate for chemical conversion treatment characterized by performing.

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