JP2012072497A - Hot-rolled steel sheet for chemical conversion treatment, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet for chemical conversion treatment inexpensively, and to provide a method for manufacturing the same.SOLUTION: The hot-rolled steel sheet for chemical conversion treatment includes, by mass, 0.01-0.10% C, 0.01-2.00% Si, 0.15-2.00% Mn, 0.020% or less P, 0.010% or less S, 0.050% or less Al, and the balance Fe except the inevitable impurities, wherein the dislocation density in a surface layer part is 2×10/mor more.

Description

  The present invention relates to a hot-rolled steel sheet for surface treatment, in particular, a hot-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.

  Hot-rolled steel sheets used for automobiles, home appliances, furniture, building materials, and the like are widely subjected to painting 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 discloses an invention in which phosphate treatment is improved by adsorbing 0.01 to 500 mg / m ² with a sulfur content of a specific sulfur-containing compound such as methylbutane on the surface of a cold-rolled steel sheet. 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 above problems related to the prior art, and proposes a means for improving the chemical conversion property of a hot-rolled steel sheet that can be applied to a hot-rolled steel sheet and can be implemented at a low cost. The purpose is that.

The hot-rolled steel sheet for chemical conversion treatment according to the present invention is, by mass ratio, C: 0.01 to 0.10%, Si: 0.01 to 2.00%, Mn: 0.15 to 2.00%, 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 dislocation density of the surface layer portion is 2 × 10 ¹³ / m ² or more. Is. The dislocation density is desirably 5 × 10 ¹³ / m ² or more and 5 × 10 ¹⁵ / m ² or less.

  The hot-rolled steel sheet for chemical conversion treatment according to the above invention can further contain Cr: 0.05 to 1.00%.

In the hot-rolled steel sheet for chemical conversion treatment according to the present invention, the dislocation introduction operation is performed on the hot-rolled steel sheet having the above composition so that the dislocation density of the surface layer portion is 2 × 10 ¹³ / m ² or more after annealing and pickling. It can be manufactured by doing.

  According to the present invention, it is possible to provide a hot-rolled steel sheet having excellent chemical conversion properties, and it is possible 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.

It is a graph which shows the relationship between the dislocation density when a chemical conversion process is performed with respect to a hot-rolled steel sheet, and the state of a chemical conversion treatment film (the generation | occurrence | production state with "suke" or without "suke").

  The raw sheet of the chemical conversion hot-rolled steel sheet according to the present invention has a mass ratio of C: 0.01 to 0.10%, Si: 0.01 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, it is preferable to make a low-carbon / low-alloy steel made of Fe. 05-1.00 can also be contained. In addition, it is not necessary to provide the structure | tissue and a characteristic restriction | limiting in particular in the said hot-rolled steel plate, 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 present invention, it is important that the dislocation density in the surface layer portion of the steel sheet 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 the dislocation density and chemical conversion coating when the hot rolled steel sheet having the composition shown in Table 1 is processed under the conditions shown in Table 2 to introduce dislocations on the surface of the hot rolled steel sheet and perform chemical conversion treatment. It is a graph which shows the relationship of the state (occurrence state with "skew" or without "skew"). The hot-rolled steel sheet is obtained by hot rolling a slab having the composition shown in Table 1 to the final thickness and then pickling.

  The introduction of dislocations on the surface of the steel sheet was performed by applying a strain to the hot-rolled steel sheet obtained above by shot blasting. The specific contents of these strain imparting processes are as described in Table 2.

  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 steel sheet surface layer portion is 2 × 10 ¹³ / m ² or more, “suke” is not recognized in the hot-rolled steel sheet subjected to chemical conversion treatment. 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.

Claims (4)

By mass ratio, C: 0.01 to 0.10%, Si: 0.01 to 2.00%, Mn: 0.15 to 2.00%, P: 0.020% or less, S: 0.010 %, Al: 0.050% or less, made of Fe except for the remaining inevitable impurities, and having a dislocation density of 2 × 10 ¹³ / m ² or more in the surface layer portion, a hot-rolled steel sheet for chemical conversion treatment. The hot rolled steel sheet for chemical conversion treatment according to claim 1, wherein the dislocation density is 5 × 10 ¹³ / m ² or more and 5 × 10 ¹⁵ / m ² or less.   Furthermore, Cr: 0.05-1.00% is contained, The hot-rolled steel plate for chemical conversion treatment of Claim 1 or 2 characterized by the above-mentioned. The hot rolled steel sheet having the composition according to claim 1 or 3 is characterized in that after annealing and pickling, a dislocation introduction operation is performed so that a dislocation density of a surface layer portion is 2 × 10 ¹³ / m ² or more. A method for producing a hot-rolled steel sheet for chemical conversion treatment.