JP2005146317A - Hot-rolled steel plate superior in chemical conversion treatability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel plate which contains more Si and uniformly acquires a chemical conversion coating on the whole area of the steel plate as an undercoat for painting, and to provide a method for manufacturing the steel plate, which uses a conventional manufacturing line and facilitates quality control. <P>SOLUTION: This steel plate includes, by mass%, 0.03-0.15% C, 0.8-3.0% Si, 0.5-3.0% Mn, 0.07% or less P, 0.01% or less S, 0.015-0.1% Al, 0.001-0.008% N, and Ti and Nb as needed; and has regions of oxides on the steel plate surface, which contain 3.5 mass% or less Si and 3.5 mass% or less Mn, and dispersedly occupy 20% or more of all the surface by an area rate when viewed from the surface. The manufacturing method includes scrubbing the plate surface with a flexible material containing abrasive grains. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet excellent in chemical conversion property and a method for producing the same, which can form a chemical conversion film uniformly on the entire surface of the steel sheet when chemical conversion is performed as a coating base treatment of the steel sheet.

When coating metal surfaces, such as electrodeposition coating on automobile bodies, chemical conversion treatment is performed as a base treatment. In the chemical conversion treatment, the metal surface is covered with an inert chemical conversion coating, thereby improving the adhesion and corrosion resistance of the coating applied thereon.
In addition, high strength thin steel sheets are used from the viewpoint of weight reduction and safety of automobiles, and hot rolled steel sheets that are cheaper than cold rolled steel sheets are used for underbody members.
A hot-rolled steel sheet is manufactured through hot rolling and a pickling process. In the pickling process, the oxidized scale on the surface of the steel sheet is removed by hydrochloric acid pickling.

Regarding the high-strength hot-rolled steel sheet with improved chemical conversion properties, there are the following known documents.
Patent Document 1 discloses a high-strength hot-rolled steel sheet that solves the problem of chemical conversion degradation and the resulting corrosion resistance degradation after painting by setting the Si concentration ratio between the steel sheet surface and the interior to 1.3 or less. Has been. As a means for setting the Si concentration ratio as described above, it has been shown that Si oxide existing on the surface is reduced by grinding the hot-rolled steel sheet after pickling.

Patent Document 2 discloses a high-strength hot-rolled steel sheet having improved chemical conversion property and workability by setting the ratio between the surface layer of the steel sheet and the internal Vickers hardness to 0.95 or less. The object is Ti-added steel, and the hardness ratio is set as described above by replacing the precipitate on the surface of the steel sheet with a carbide instead of the Ti oxide TiO ₂ that deteriorates the chemical conversion property. TiO ₂ becomes consistent fine precipitates and increases the hardness of the steel sheet, whereas TiC is non-aligned and decreases the hardness of the steel sheet, and the means shows hot rolling conditions. Yes.
Japanese Patent Laid-Open No. 11-50187 Japanese Patent Laid-Open No. 10-1748

  When a chemical conversion treatment is performed as a coating base treatment of a hot-rolled steel sheet, particularly in steel with a high Si content, a portion called “ske” where a chemical conversion film is not generated may be observed by microscopic observation. In such a part, generation of rust is recognized by visual observation, and even if rust is not observed, problems such as peeling of the coating film occur with time after coating.

  The technology of Patent Document 1 improves the chemical conversion processability by limiting the Si concentration ratio between the surface of the steel sheet and the interior, and the technology of Patent Document 2 limits the hardness ratio of the surface and the interior to a specific range. . For this reason, when these techniques are applied to a hot-rolled steel sheet production line, it is necessary to measure the inside of the steel sheet, causing problems in quality control measurement. Incidentally, in the former, the measured value at the position ground 0.5 mm from the surface is the internal Si concentration, and in the latter, the measured value at the depth position of 1/4 of the thickness from the surface is the internal hardness.

Moreover, in patent document 1, as a means for making steel sheet surface and internal Si density | concentration ratio 1.3 or less, it performs a high-pressure deske between rough rolling and finish rolling, a shot between winding and pickling. Although blasting, tension leveler, skin pass rolling, and grinding after pickling are described, specifically, only an example of lightly grinding a steel sheet surface with an abrasive nylon brush is shown. .
The technique of Patent Document 2 is a special technique that controls Ti-added steel having a Si content of 0.8% by mass or less and controls the state of precipitates by hot rolling conditions.

  Therefore, the problem to be solved by the present invention is that a high-strength hot-rolled steel sheet with an increased Si content can be uniformly formed on the entire surface of the steel sheet by applying a coating base treatment, and applied to a conventional production line in steel sheet production. It is possible to facilitate quality control.

The present invention for solving the above problems is a steel plate produced through hot rolling and pickling processes, the component composition is in mass%,
C: 0.03-0.15%, Si: 0.8-3.0%,
Mn: 0.5 to 3.0%, P: 0.07% or less,
S: 0.01% or less, Al: 0.015-0.1%,
N: 0.001 to 0.008%
The balance is made of Fe and inevitable impurities, and the oxide on the surface of the steel sheet has an area ratio of 20% or more at a mass ratio of Si concentration 3.5% or less and Mn concentration 3.5% or less. It is a hot-rolled steel sheet excellent in chemical conversion treatment, characterized by being dispersed as seen from the surface.

Moreover, it is a steel plate manufactured through a hot rolling and pickling process, the component composition is in mass%,
C: 0.03-0.15%, Si: 0.8-3.0%,
Mn: 0.5 to 3.0%, P: 0.07% or less,
S: 0.01% or less, Al: 0.015-0.1%,
N: 0.001 to 0.008%
In addition,
One or both of Ti: 0.02-0.3% and Nb: 0.01-0.5%;
Cu: 0.2-1.8% and Ni: 0.1-2.0%,
Mo: 0.05-0.5%
B: 0.0002 to 0.006%,
Ca: 0.0005 to 0.005%
Are contained alone or in combination, and the balance consists of Fe and inevitable impurities, and the oxide on the surface of the steel sheet has an area ratio of Si concentration 3.5% or less and Mn concentration 3.5% or less in mass%. It is a hot-rolled steel sheet excellent in chemical conversion treatment, characterized by being present in a dispersed state as viewed from the surface at 20% or more.

  In addition, the present invention method for solving the above-mentioned problem is that the steel plate surface after pickling comprising the above component composition is scraped with a soft material having abrasive grains, so that the oxide on the steel plate surface is reduced to mass%. Manufacturing a hot-rolled steel sheet excellent in chemical conversion treatment, characterized in that parts having an Si concentration of 3.5% or less and an Mn concentration of 3.5% or less are dispersed as viewed from the surface at an area ratio of 20% or more. Is the method.

  Since the present invention does not require reduction of the Si content as in the prior art in order to improve chemical conversion processability, the high strength and high workability hot rolling used for reducing the weight and ensuring safety of automobiles, etc. In the steel sheet, the strength and workability are not impaired even if other additive elements are not used. Moreover, in a normal pickling process, it can manufacture by providing a brush roll and a rotating belt after pickling. By observing and measuring the surface state of the steel sheet with EPMA, quality control can be easily performed.

  In the present invention, the component composition of the steel sheet is limited to the above range so as to have high strength and high workability that can be used for an undercarriage member of an automobile, and excellent chemical conversion treatment. The reasons for the limitation are as follows. The ratio of each element is all mass%.

If C is less than 0.03%, the elongation is low, and if it exceeds 0.15%, the corrosion resistance decreases.
If Si is less than 0.8%, the strength and elongation are low, and if it exceeds 3.0%, the pickling property decreases.
If Mn is less than 0.5%, the elongation is low, and if it exceeds 3.0%, the pickling property is lowered.

When P exceeds 0.07%, the hole-expanding property is lowered, and mechanical properties such as elongation are lowered. When S exceeds 0.01%, the corrosion resistance decreases.
If Al is less than 0.015%, Si and Mn oxides are likely to be formed on the steel sheet surface, and the chemical conversion treatment property is lowered. If it exceeds 0.1%, the corrosion resistance is lowered.
If N is less than 0.001%, the chemical conversion property is lowered, and if it exceeds 0.008%, the elongation is lowered.

In addition to the above components, the steel sheet of the present invention may contain the following components alone or in combination, if necessary.
In order to further improve the strength, one or both of Ti and Nb can be added. In that case, if Ti is less than 0.02%, the effect of improving strength by forming carbonitride is small, and the effect of improving mechanical strength by addition cannot be ensured. Even if added over 0.3%, the effect of increasing the strength is saturated. If Nb is less than 0.01%, the effect of improving the strength is small, and the effect of improving the mechanical strength by addition cannot be ensured. Even if added over 0.5%, the effect of increasing the strength is saturated.

  In order to further improve the strength, Cu can be added, and if necessary, heat treatment can be performed by heating to a temperature of about 450 to 650 ° C. In that case, if Cu is less than 0.2%, the effect is small, and even if added over 1.8%, the effect is saturated. When adding Cu, in order to prevent the crack of the steel plate at the time of hot working, Ni is added together. The effect of Ni is exhibited at 0.1% or more and is saturated at 2.0%.

  In order to further improve the strength, Mo can also be added. In that case, if Mo is less than 0.05%, the effect of improving the strength due to carbide formation is small, and the effect of improving the mechanical strength due to the addition cannot be ensured. Even if added over 0.5%, the effect of increasing the strength is saturated.

Moreover, B can also be added in order to reduce the aging by nitrogen and to improve hole expansibility. The effect is exhibited when B is added in an amount of 0.0002% or more, and is saturated at 0.006%.
Further, Ca can be added in order to prevent a decrease in hole expanding property due to MnS formation. The effect is exhibited when Ca is added in an amount of 0.0005% or more, and is saturated at 0.005%.

In the hot-rolled steel sheet of the present invention, the surface oxide of the steel sheet having such a component composition has a surface area with an area ratio of 20% or more at an Si concentration of 3.5% by mass or less and an Mn concentration of 3.5% by mass or less. It exists in a distributed manner.
Hot-rolled steel sheets manufactured through hot rolling and pickling processes have surface oxide scales pickled and removed, but in steel sheets with a high Si content, even if the oxide scales are entirely removed, they are partially removed. Oxide remains on the surface. The present invention solves the problem of chemical conversion treatment by bringing this oxide into the above state.

  The chemical conversion treatment is performed by removing oil adhering to the steel sheet surface by degreasing and then immersing it in a chemical conversion treatment solution for a predetermined time. In this treatment, Fe ions are eluted from the steel sheet into the treatment solution and react with the components of the solution, and a large number of chemical conversion crystal nuclei composed of a compound containing Fe, Zn, P, 0, etc. are generated. Thus, the coating covers the entire surface of the steel plate. At this time, it is necessary to uniformly attach fine chemical conversion grains of 10 μm or less over the entire surface, and when this adhesion state is poor and there is a non-adhesion site called “skew”, poor adhesion of the coating film during coating In addition, problems such as a decrease in corrosion resistance after painting occur.

  When the Si content of the steel plate increases, the oxide having a high Si content increases on the surface scale after hot rolling, and it tends to remain on the steel plate surface by ordinary hydrochloric acid pickling. When a steel sheet in which an oxide having a high Si content remains is subjected to chemical conversion treatment, a non-attached portion called “ske” is likely to be generated. From this phenomenon, it is considered that in the remaining part of the oxide having a high Si content, the elution of Fe ions is delayed during the chemical conversion treatment, and the formation reaction of the chemical conversion grains is delayed, resulting in the above-described scaling. Similarly, even if an oxide having a high Mn content remains, a skelton is likely to occur.

  Even if an oxide remains on the surface after pickling, the steel sheet of the present invention may be in a state in which parts with low Si concentration and Mn concentration are dispersed and present at an area ratio of 20% or more. In the low concentration region where the Si concentration is 3.5% by mass or less and the Mn concentration is 3.5% by mass or less, the elution of Fe ions in the chemical conversion treatment is not delayed, and a chemical conversion film is formed to the same extent as the region without oxide.

  If such Si, Mn low concentration sites are dispersed in the oxide at an area ratio of 20% or more as viewed from the surface, the nuclei generated in each low concentration site grow to a fine size of 10 μm or less. As a result, the surface of the entire oxide is covered, and a chemical conversion film composed of fine chemical crystal grains is uniformly deposited on the entire surface of the steel sheet, thereby preventing the occurrence of scaling.

  If the area ratio of the low-concentration site is less than 20%, a portion where the chemical conversion film is not generated is generated in the oxide, and if the low-concentration site is not dispersed, the chemical conversion film is generated unevenly. As an example of a good dispersion state, it is sufficient that the distance between the centers of the low-concentration portions is 10 μm or less, and when the oxide surface is divided by a grid having a side of 10 μm, at least one low density is obtained in all the grids. There may be a state where there is a concentration site.

  The oxide state of the steel sheet of the present invention is determined by EPMA from the oxygen distribution on the steel sheet surface, etc., and the distribution state of the low concentration portion whose Si concentration and Mn concentration are 3.5% by mass or less is observed. It can be determined by determining the rate. The analysis of Si and Mn on the steel surface by EPMA is usually performed by applying an acceleration voltage of 15 kV. In this case, the concentration from the outermost surface of the steel plate to a depth of about 3 μm is detected.

  However, even under these conditions, depending on the thickness and surface roughness of the surface oxide layer, information deeper than 3 μm may be detected, and there may be cases where Si and Mn of the ground iron are included. In the present invention, the analysis values of Si and Mn by EPMA at an acceleration voltage of 15 kV may be 3.5% by mass or less, respectively, and may not be the concentration of oxide alone. It has been confirmed that the chemical conversion processability is good if the parts having such a concentration are dispersed as described above.

  Next, the method of the present invention is a method for producing the steel plate of the present invention by scraping the steel plate surface after pickling with a soft material having abrasive grains. As the soft material, a plastic brush such as nylon or a non-woven fabric can be used, and the abrasive grains only need to be embedded or attached to the hair. Hard materials such as grinders and emery paper are not suitable.

The scraping is scraping, and a method of pressing while rotating a brush roll on the surface of a running steel plate, a method of pressing while rotating a belt attached with a nonwoven fabric, or the like can be employed.
In the case of using a brush roll, by setting the rotation speed of the brush according to the plate passing speed so that the brush hits the same spot on the steel sheet surface more than 100 times, the oxides on the steel sheet surface are subjected to the above Si, Mn low concentration sites. Can be present in an excellent dispersion state with an area ratio of 20% or more.

  The hot-rolled steel sheets having the components shown in Table 1 were pickled with hydrochloric acid and scraped under various conditions, and then the chemical conversion property was determined. In the comparative example of Table 1, the components marked with * are out of the scope of the present invention. The slab heating temperature in hot rolling was 1200 ° C., the hot rolling finishing temperature was 880 ° C., cooled to 390 ° C. with a hot run table, wound up at 390 ° C., and cooled to room temperature. The pickling was performed under the condition that the steel strip was passed through the pickling tank while the coil was rewound and immersed in a 7 mass% HCl aqueous solution at 90 ° C. for 20 seconds.

  Table 2 shows the abrasion conditions and results. The scraping was performed by press-contacting the steel plate in the plate at 90 m / min while rotating the nylon brush roll containing abrasive grains. Table 2 shows the diameter, density, and rotational speed of the brush roll of the nylon in which the abrasive grains were embedded. The rotation direction of the brush roll is opposite to the plate passing direction of the steel plate. The Si, Mn low-concentration site after scraping shows the concentration, area ratio, and distribution state analyzed by EPMA at an acceleration voltage of 15 kV. When the oxide was divided by a grid of 10 μm squares, the distribution was indicated by ◯ when there was a low-concentration site in each cell, and by x when there was a cell without a low-concentration site. * Mark in Table 2 indicates that the condition of the present invention is not satisfied.

  The steel plate after scraping was subjected to chemical conversion treatment in the same manner as the actual chemical conversion treatment. That is, after degreasing, it was immersed in a surface conditioning solution for 30 seconds, then immersed in a chemical conversion treatment solution (PBWL35 manufactured by Nippon Parkerizing Co., Ltd.) for 120 seconds, washed with water and dried. The chemical conversion property was determined based on the presence or absence of staining by SEM observation and the presence or absence of rusting by visual observation immediately after drying.

In Table 2, Nos. 1 to 7 and Nos. 9 to 24 of the examples of the present invention were all free of scum and rust after the chemical conversion treatment, and excellent chemical conversion treatment properties were obtained.
In Comparative Example No. 8, since the area ratio of the Si, Mn low concentration portion was less than 20% and was not dispersed, there was skein and rust generation.

In Comparative Example No. 25, the amount of C was high and rust was generated. No. 26 has a high Si content and the Si concentration in the Si and Mn low concentration region exceeds 3.5%, and No. 27 has a high Mn content and the Mn concentration in the Si and Mn low concentration region has 3.5%. In both cases, the area ratio of the low-concentration site was 0, and there was scaling and rusting.
No. 28 had a high amount of S, a low area ratio of Si and Mn low-concentration sites, a poor dispersion, and scumming and rusting. No. 29 has a low Al content, No. 30 has a low N content, and No. 31 has a high P content. Occurrence occurred.

Claims (3)

It is a steel plate manufactured through hot rolling and pickling processes, and the component composition is in mass%,
C: 0.03-0.15%, Si: 0.8-3.0%,
Mn: 0.5 to 3.0%, P: 0.07% or less,
S: 0.01% or less, Al: 0.015-0.1%,
N: 0.001 to 0.008%
And the balance consists of Fe and inevitable impurities,
The oxide on the surface of the steel sheet is characterized in that the parts having a Si concentration of 3.5% or less and a Mn concentration of 3.5% or less are present in a mass% dispersed and viewed from the surface with an area ratio of 20% or more. Hot-rolled steel sheet with excellent chemical conversion. It is a steel plate manufactured through hot rolling and pickling processes, and the component composition is in mass%,
C: 0.03-0.15%, Si: 0.8-3.0%,
Mn: 0.5 to 3.0%, P: 0.07% or less,
S: 0.01% or less, Al: 0.015-0.1%,
N: 0.001 to 0.008%
In addition,
One or both of Ti: 0.02-0.3% and Nb: 0.01-0.5%;
Cu: 0.2-1.8% and Ni: 0.1-2.0%,
Mo: 0.05-0.5%
B: 0.0002 to 0.006%,
Ca: 0.0005 to 0.005%
Singly or in combination, the balance consisting of Fe and inevitable impurities,
The oxide on the surface of the steel sheet is characterized in that the parts having a Si concentration of 3.5% or less and a Mn concentration of 3.5% or less are present in a mass% dispersed and viewed from the surface with an area ratio of 20% or more. Hot-rolled steel sheet with excellent chemical conversion. By scraping the steel plate surface after pickling comprising the component composition according to claim 1 or 2 with a soft material having abrasive grains, the Si concentration is 3.5% by mass in the oxide on the steel plate surface. %, And a Mn concentration of 3.5% or less is present in an area ratio of 20% or more in a dispersed manner when viewed from the surface. A method for producing a hot-rolled steel sheet excellent in chemical conversion treatment.