JP6123754B2 - Si-containing hot-rolled steel sheet having excellent chemical conversion property and method for producing the same - Google Patents

Description

  The present invention relates to a Si-containing steel sheet and a method for producing the same, and more particularly to a Si-containing hot rolled steel sheet suitable for automobile frame parts and the like and excellent in chemical conversion treatment and a method for producing the same.

  In recent years, high-strength steel sheets, particularly high-strength hot-rolled steel sheets, are being frequently used for automobile frame parts and undercarriage parts in order to reduce the weight of automobile bodies. Here, as a component composition of high-strength steel sheets, it is required to use rare alloy elements as much as possible in view of global procurement of raw materials, and a high-strength steel that uses inexpensive solid solution strengthening elements such as Si and Mn is required. Strength steel plates have been developed.

  On the other hand, by increasing the strength of the steel sheet and reducing the thickness of the steel sheet, the demand for corrosion resistance is higher than ever before. In particular, hot-rolled steel sheets applied to automobile frame parts and undercarriage parts are subjected to chemical conversion treatment and then applied by cationic electrodeposition coating or the like, and therefore, excellent chemical conversion treatment properties are required. Here, the chemical conversion treatment applied to the hot-rolled steel sheet for automobiles involves cleaning the steel sheet coated with rust-preventing oil after degreasing, for example, by forming a chemical conversion crystal mainly composed of phosphophyllite by immersion-type zinc phosphate treatment. Precipitation is performed to form a zinc phosphate coating (hereinafter also referred to as a chemical conversion coating).

  However, when Si, Mn, or the like is added, the chemical conversion processability is likely to deteriorate due to the concentration of these elements on the steel sheet surface. In particular, Si is an element that can increase the strength without greatly impairing the formability. However, Si-containing oxides are formed on the surface of the steel sheet to significantly reduce the chemical conversion property, thereby improving the corrosion resistance after coating. It is known to greatly reduce.

  As a technique for improving the chemical conversion property of the hot-rolled steel sheet containing a solid solution strengthening element such as Si and Mn and the corrosion resistance after coating, for example, Patent Document 1 discloses that the strengthening element is oxidized from iron such as Si and Mn. A hot rolled steel material containing a total of 0.5 mass% or more of easily oxidizable elements (hereinafter referred to as easily oxidizable elements), the concentration of the easily oxidizable elements on the steel material surface and the covering state of the oxide layer existing on the steel material surface A technology relating to a steel material that has been prescribed and improved in corrosion resistance after chemical conversion and painting is disclosed. In the technique of Patent Document 1, the concentration of the easily oxidizable element on the steel surface layer does not exceed the concentration in the steel and the coverage of the surface oxide layer covering the grains of the steel is increased, or the steel surface The density of the easily oxidizable elements is densely generated in a region higher than the concentration in steel, and by increasing the coverage of the surface oxide layer covering the grains of the steel, the formation of local anodes is reduced, It has been proposed to ensure corrosion resistance after painting.

  In Patent Document 2, a hot-rolled steel sheet containing 0.5 to 3.0 mass% of Si is appropriately pickled after hot rolling, and after removing the Si-containing oxide layer, the steel sheet is further pickled again. A technique for ensuring chemical conversion treatment by reducing the surface coverage of a surface iron-based oxide is disclosed. In this technology, the hot-rolled steel sheet surface is strongly pickled with nitric acid or the like, and the Si-containing oxide layer formed on the steel sheet surface layer is completely removed, and the iron-based oxide produced on the steel sheet surface by the strong pickling is used. By suppressing the formation, the reaction between the steel sheet surface and the chemical conversion treatment liquid is enhanced, and even when a low-temperature chemical conversion treatment liquid is used, a Si-containing steel sheet is obtained that is excellent in chemical conversion treatment properties and excellent in corrosion resistance after coating. It has been proposed.

JP 2006-265586 A JP 2012-172181 A

  However, in the technique of Patent Document 1, it is said that hot rolling / pickling conditions different from conventional ones are necessary to realize a predetermined surface state, and descaling conditions, hot rolling conditions, and pickling conditions are disclosed. However, in actual machine manufacturing, it is not easy to achieve the proposed manufacturing conditions and stably ensure the surface structure over the entire surface of the steel sheet. Moreover, the technique of patent document 2 is a technique which requires two picklings, and it cannot avoid an increase in production cost, and also with this technique, it can ensure an ideal surface state stably. It's not easy.

  That is, in the prior art, it has been difficult to ensure excellent chemical conversion properties required for hot rolled steel sheets for automobiles and to ensure corrosion resistance after painting.

  An object of the present invention is to solve such problems of the prior art and to provide a Si-containing hot-rolled steel sheet that can ensure excellent chemical conversion properties and an advantageous production method thereof.

  In general, when producing a Si-containing hot-rolled steel sheet, a strong oxide film (hereinafter also referred to as scale) is generated on the surface of the steel sheet in the hot rolling process. For this reason, sufficient pickling is required to sufficiently descale the scale (primary scale) generated during heating in hot rolling and to remove the scale (secondary scale) generated during subsequent rolling. There is a need to do. In addition, in such a portion where the scale generated during hot rolling remains, the crystals constituting the chemical conversion coating (hereinafter also referred to as chemical conversion crystals) do not nucleate during chemical conversion, and the corrosion resistance after coating deteriorates. This phenomenon has been confirmed.

  The inventors of the present invention have made extensive studies on the surface structure of the hot-rolled steel sheet after pickling and the structure of the chemical conversion coating film, which greatly affect the chemical conversion processability, with respect to the Si-containing hot-rolled steel sheet that is prone to chemical conversion treatment defects. As described above, pickling is performed to remove the secondary scale, and after the pickling, water washing and then drying are performed. The present inventors have found that the characteristic surface structure observed on the steel sheet surface after such pickling, then water washing and drying greatly affects the nucleation of chemical crystals.

  In FIG. 1, the surface of the Si-containing hot-rolled steel sheet (Si content: 0.7% by mass) after hot-rolling, pickling and then water-washing and drying is scanned with a scanning electron microscope (SEM). An observed example is shown. As shown in FIG. 1, the surface of the Si-containing hot-rolled steel sheet after pickling, washing with water, and drying is characterized by a crystal crystal plane of the steel crystal structure and a porous film-like part. Surface structure. FIG. 2 shows the result of observing the state of formation of chemical conversion nuclei, with the chemical conversion treatment that normally requires treatment for 90 to 120 seconds being performed in a short time of 10 seconds. It was found that chemical conversion crystals nucleate easily in this region.

  In the Si-containing hot-rolled steel sheet, the present inventors considered that the porous film as described above after pickling, washing with water, and drying is a corrosion product formed at the time of pickling. Various studies were conducted. As a result, the excess water washing reduces the proportion of the porous film-like area, that is, the area ratio of the portion covered with the corrosion product, resulting in a decrease in the chemical conversion treatment property, and consequently the corrosion resistance after coating. Found that the decline.

  Generally, in order to improve the chemical conversion of a Si-containing steel sheet, it is essential to clean the surface by removing the scale by pickling. On the other hand, as described above, increasing the exposed surface of the base crystal suppresses the chemical conversion reaction, and in order to improve the chemical conversion treatment, it is important to optimize the water washing conditions after pickling. The inventors of the present invention stably stabilize the surface structure in which chemical conversion crystals are likely to nucleate by optimizing the water washing conditions after pickling, even if it is a Si-containing hot-rolled steel sheet that is difficult to obtain stable chemical conversion treatment. It was found that it is possible to stably obtain a Si-containing hot-rolled steel sheet that can be obtained and has excellent chemical conversion properties.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  [1] It has a composition having a Si content of 0.5% by mass or more, and is excellent in chemical conversion treatment, characterized in that the steel sheet surface is coated with a porous corrosion product by 50% or more by area ratio. Si-containing hot-rolled steel sheet.

  [2] By mass%, C: 0.05 to 0.10%, Si: 0.5 to 1.0%, Mn: 1.0 to 1.5%, P: 0.010% or less, S: The chemical conversion according to the above [1], comprising 0.004% or less, Al: 0.05% or less, N: 0.0045% or less, with the balance being composed of Fe and inevitable impurities. Si-containing hot-rolled steel sheet with excellent processability.

  [3] Furthermore, it has a composition containing at least one of Ti: 0.04 to 0.10%, V: 0.05 to 0.12%, and Nb: 0.005% or less by mass%. The Si-containing hot-rolled steel sheet having excellent chemical conversion properties as described in [2] above.

  [4] Hot-rolled steel sheet by subjecting a steel sheet after hot rolling having a composition having a Si content of 0.5% by mass or more to a pickling process that is a process of pickling, washing and drying. In the manufacturing method of a steel plate, the pickling treatment in the pickling step is performed at a temperature of 70 to 90 ° C. and a hydrochloric acid concentration of 5 to 12 vol. The water washing conditions are adjusted so that the hot-rolled steel sheet after washing and drying is covered with a porous corrosion product by 50% or more by area ratio. A method for producing a Si-containing hot-rolled steel sheet.

  [5] The steel sheet is mass%, C: 0.05 to 0.10%, Si: 0.5 to 1.0%, Mn: 1.0 to 1.5%, P: 0.010% [4], wherein S: 0.004% or less, Al: 0.05% or less, N: 0.0045% or less, with the balance being composed of Fe and inevitable impurities The manufacturing method of Si containing hot-rolled steel plate as described in 2.

  [6] The steel sheet further contains at least one of Ti: 0.04 to 0.10%, V: 0.05 to 0.12%, and Nb: 0.005% or less in mass%. The method for producing a Si-containing hot-rolled steel sheet having excellent chemical conversion properties as described in [5], wherein the composition has the following composition:

  According to the present invention, it has become possible to stably provide a Si-containing hot-rolled steel sheet having excellent chemical conversion properties. Since the Si-containing hot-rolled steel sheet of the present invention contains 0.5% by mass or more of Si, it has high strength and excellent corrosion resistance after coating applied after chemical conversion treatment. It contributes to the weight reduction of the automobile body.

It is a SEM photograph which shows the example of observation of the Si-containing hot-rolled steel sheet surface after pickling, water washing, and drying. It is a SEM photograph which shows the example of observation of the Si containing hot-rolled steel plate surface which carried out chemical conversion treatment for a short time (10 seconds).

  Hereinafter, the Si-containing hot-rolled steel sheet excellent in chemical conversion treatment and the manufacturing method thereof according to the present invention will be described in detail. Note that “%”, which is a unit of component content, means “% by mass” unless otherwise specified.

  The Si-containing hot-rolled steel sheet of the present invention has a steel composition having a Si content of 0.5% or more. When the Si content is less than 0.5%, it is difficult to obtain a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more. For this reason, Si content in a steel plate shall be 0.5% or more. In addition, since it will become a chemical conversion defect when Si content exceeds 1.0%, it is preferable to set it as 1.0% or less.

  In the Si-containing hot-rolled steel sheet of the present invention, the steel sheet surface is covered with a porous corrosion product by 50% or more by area ratio. Here, as described above, the porous corrosion product is observed as a porous film-like portion observed on the surface of the Si-containing hot-rolled steel sheet after hot rolling, pickling, washing with water, and drying. The As shown in FIG. 2, the portion covered with the porous corrosion product becomes a nucleation site of chemical conversion crystals during chemical conversion treatment using an aqueous zinc phosphate solution. On the steel sheet surface of the Si-containing hot-rolled steel sheet, 50% or more of the area ratio with respect to the entire steel sheet surface is covered with a porous corrosion product, so that during the chemical conversion treatment using the zinc phosphate aqueous solution, Nucleation can be promoted, and the chemical conversion crystals are densely deposited. Specifically, a chemical conversion film having a film porosity (P) represented by the following formula (1) of 0.3 or less is formed. Thus, a chemical conversion film having excellent corrosion resistance after coating can be obtained stably.

Film porosity (P) = 1−C / (t × d) (1)
Here, C is the amount of the chemical conversion coating (g / m ² ), t is the thickness (m) of the chemical conversion coating obtained by observing the cross section in the thickness direction of the steel sheet, and d is Zn ₃ (PO ₄ ) ₂ . Density (g / m ³ ).
When the area ratio of the portion covered with the porous corrosion product is less than 50% in terms of the area ratio with respect to the entire surface of the hot-rolled steel, the nucleation of chemical crystals is delayed during the chemical conversion treatment using the zinc phosphate aqueous solution. However, it is difficult to stably obtain a chemical conversion treatment film in which chemical conversion crystals are densely deposited and excellent in corrosion resistance after coating.

  Although not particularly limited, if the area ratio of the portion covered with the porous corrosion product on the steel sheet surface is too large, the surface of the hot-rolled steel sheet is homogenized, and on the contrary, the chemical conversion treatment property is improved. Since there is a concern about the decrease, the area ratio of the portion covered with the porous corrosion product on the surface of the Si-containing hot rolled steel sheet is preferably 80% or less.

  As described above, the Si-containing hot-rolled steel sheet of the present invention has a Si content of 0.5% or more, and the steel sheet composition other than Si is not particularly limited. The Si-containing hot rolled steel sheet of the present invention has C: 0.05 to 0.10%, Si: 0.5 to 1.0%, Mn: 1.0 to 1.5%, P: 0.010. % Or less, S: 0.004% or less, Al: 0.05% or less, N: 0.0045% or less, preferably having a steel composition comprising Fe and inevitable impurities, or further Ti The steel composition preferably contains at least one of 0.04 to 0.10%, V: 0.05 to 0.12%, and Nb: 0.005% or less.

  Hereinafter, a preferable component composition will be described.

C: 0.05-0.10%
C is an element effective for increasing the strength of steel, and by making the C content 0.05% or more, TS> 590 MPa can be achieved. Therefore, the C content is preferably 0.05% or more. On the other hand, if the C content exceeds 0.10%, the strength becomes excessive. For this reason, the C content is preferably 0.10% or less.

Si: 0.5 to 1.0%
As described above, Si is an element that increases the strength without significantly impairing the formability. In the present invention, the Si content needs to be 0.5% or more in order to increase the strength. On the other hand, if the Si content exceeds 1.0%, chemical conversion is likely to be poor, so the Si content is preferably 1.0% or less.

Mn: 1.0 to 1.5%
Mn is an element effective for solid solution strengthening of steel, and can be (TS> 590 MPa) by setting the C content to 1.0% or more. Therefore, the Mn content is preferably 1.0% or more. On the other hand, if the Mn content exceeds 1.5%, the hardenability increases and the strength becomes excessive, so the Mn content is preferably 1.5% or less.

P: 0.010% or less Since P is a typical impurity element of steel, the P content is preferably 0.010% or less.

S: 0.004% or less Since S is an impurity that causes hot brittleness during rolling, the S content is preferably 0.004% or less.

Al: 0.05% or less Al is an element added as a deoxidizing material in the steel making process, and is an element effective for separating non-metallic inclusions as slag. It is preferable to set it to 02% or more. However, since excessive addition induces cracking due to coarse agglomeration of Al ₂ O ₃ , the Al content is preferably 0.05% or less.

N: 0.0045% or less Since N is an element that deteriorates the aging resistance of steel, the N content is preferably 0.0045% or less.

Any one or more of Ti: 0.04 to 0.10%, V: 0.05 to 0.12%, Nb: 0.005% or less Ti, V, and Nb are extremely strong carbonitride forming elements. Yes, in order to obtain such an effect, Ti is 0.04% or more, V is 0.05% or more, Nb is 0.02% or more, and any one of Ti, V, and Nb is used. It is preferable to contain 1 or more types. On the other hand, these elements are rare and cause an increase in raw material cost. Therefore, when Ti is contained, the content is 0.10% or less for Ti, 0.12% or less for V, and 0.005% or less for Nb. It is preferable.

  In the Si-containing hot rolled steel sheet of the present invention, the balance other than the above is preferably Fe and inevitable impurities.

  Next, the preferable manufacturing method of the Si containing hot-rolled steel sheet of this invention is demonstrated.

  The Si-containing hot-rolled steel sheet of the present invention is subjected to a pickling process that is a process of pickling, washing, and drying the steel sheet after hot rolling with a Si content of 0.5% or more. In the method for producing a hot-rolled steel sheet, the pickling treatment in the pickling step is performed at a temperature (bath temperature) of 70 to 90 ° C. and a hydrochloric acid concentration of 5 to 12 vol. The condition of washing with water is adjusted so that the hot-rolled steel sheet after washing and drying is covered with a porous corrosion product by 50% or more by area ratio. Can be manufactured.

  The steel plate after hot rolling to be subjected to the pickling process is a steel plate having a Si content of 0.5% or more, and preferably a steel plate having the above-described preferred component composition. The Si-containing hot-rolled steel sheet of the present invention is manufactured by subjecting a steel sheet hot-rolled in a hot rolling process to a pickling process.

  In the hot rolling step, the steel, which is a hot rolled material containing 0.5% or more of Si, preferably having the same composition as that of the above-described hot rolled steel sheet, is heated after heating according to a conventional method. It is subjected to hot rough rolling and hot finish rolling to form a steel plate, and then wound into a coil shape. Here, since the steel which is a raw material of hot rolling contains 0.5% or more of Si, a strong primary oxide scale called a red scale is easily formed. For this reason, it is preferable to implement a descaling process with high-pressure washing water after hot rough rolling.

  In addition, after the hot finish rolling is performed, it is preferable to perform winding at a low temperature in order to suppress generation of secondary scale, and for example, the winding temperature is preferably set to about 400 to 500 ° C. When the coiling temperature exceeds 500 ° C., the secondary scale increases, it becomes difficult to remove the scale in the subsequent pickling process, the scale remains without being completely removed on the surface of the steel sheet, and the chemical conversion property tends to deteriorate.

  As for the steel plate hot-rolled as described above, the scale is removed in the pickling process. In the pickling process, the hot-rolled steel sheet is washed with water and dried after the pickling treatment. The pickling treatment is performed at a temperature of 70 to 90 ° C. and a hydrochloric acid concentration of 5 to 12 vol. It is preferable to set it as the process immersed in a pickling bath of 40% or more for 40%. In the pickling bath, the scale is sufficiently removed so as not to adversely affect the chemical conversion treatment. For this reason, the temperature (bath temperature) of the pickling bath is set to 70 ° C. or more, and the hydrochloric acid concentration is 5 vol. % Hydrochloric acid bath, and a treatment of immersing in the hydrochloric acid bath for 40 seconds or more is preferable. When the temperature of the pickling bath is less than 70 ° C., the hydrochloric acid concentration is less than 5%, or the immersion time is less than 40 seconds, the chemical conversion treatment property may be deteriorated due to the remaining scale. On the other hand, when the temperature of the pickling bath exceeds 90 ° C., the pickling bath is preferably at a temperature of 90 ° C. or lower because it is per-acid pickling. In addition, even if the hydrochloric acid concentration in the pickling bath exceeds 15%, the pickling bath tends to be peracid washed, so the hydrochloric acid concentration in the pickling bath is preferably 15% or less. The immersion time is preferably 60 seconds or less in order to improve productivity.

  The steel sheet after the pickling treatment is washed and dried. In the present invention, the water washing conditions after the pickling treatment are important, and the water washing conditions are adjusted so that the hot rolled steel sheet after the water washing and drying is covered with a porous corrosion product by 50% or more by area ratio. To do. Although pickling has a large secondary scale removal effect by dissolving the base steel plate, the eluted Fe or the like forms a porous corrosion product as described above and covers the steel plate surface. Excess water washing removes corrosion products that are nucleation sites of chemical conversion crystals, leading to deterioration in chemical conversion treatment performance. In the present invention, in the hot-rolled steel sheet after washing and drying, 50% or more of the area ratio of the steel sheet surface is covered with the porous corrosion product. The washing condition is not particularly limited as long as the area ratio of the porous corrosion product on the surface of the hot rolled steel sheet after washing and drying is 50% or more. For example, the area of the porous corrosion product on the surface of the hot-rolled steel sheet after drying is usually set at least once and at most about 3 times after passing through the spray-type water washing bath after pickling. The rate can be 50% or more. Moreover, what is necessary is just to perform drying according to a conventional method. The hot-rolled steel sheet after pickling which has been washed with water under appropriate conditions is covered with a porous Fe corrosion product in an area ratio exceeding 50%, and the remainder has a structure in which the underlying crystal structure is exposed. It becomes a steel plate.

  Steels having the composition shown in Table 1 were hot-rolled in a hot rolling process to obtain steel plates. In the hot rolling step, hot rough rolling and then hot finish rolling were performed according to a conventional method, and wound at a winding temperature of 450 ° C. The obtained steel sheet after hot rolling was subjected to a tensile test under the following conditions to investigate the tensile strength (TS). The results are shown in Table 2.

<Tensile test>
From a steel plate obtained by hot rolling, a JIS No. 5 tensile test piece with the rolling direction as the tensile direction was sampled and subjected to a tensile test in accordance with the provisions of JISZ2241, to determine the tensile strength TS.

  Next, the steel sheet after hot rolling obtained as described above was subjected to pickling treatment, water washing and drying in order in the pickling process, and the scale on the surface of the hot rolled steel sheet was removed. The pickling treatment was performed by dipping in a hydrochloric acid bath under the pickling conditions shown in Table 2. Moreover, the washing was performed by passing the pickled steel sheet through a spray-type washing bath, and changing the number of times the spray-type washing bath was passed (number of washing baths to be passed) to change the washing conditions. In the past, the number of times of passing through the spray-type water washing bath was 3 times or more. Drying was performed with dry air. Table 2 shows pickling conditions (concentration of hydrochloric acid, pickling bath temperature, immersion time, line speed) and washing conditions (number of times of washing).

  Test specimens were collected from the hot-rolled steel sheet produced by the above pickling process, and the hot-rolled steel sheet surface was observed with a SEM at five times with 2000 views, and the area ratio of the portion covered with the porous corrosion product (Porous film coverage) was determined. The observation conditions were an acceleration voltage of 5 kV so that the corrosion product on the outermost layer could be observed, and the average of the area ratios obtained for the five visual fields was defined as the porous film coverage. The results are shown in Table 3.

  Moreover, after taking a test piece from the hot-rolled steel sheet manufactured by performing the above-mentioned pickling process and performing alkaline degreasing (FC-E2011 manufactured by Nihon Parkerizing Co., Ltd.), it is subjected to chemical conversion treatment under the following conditions, and hot-rolled after chemical conversion The steel sheet was evaluated for the film porosity (P) as described below. Table 3 shows the adhesion amount of the chemical conversion film, the film thickness of the chemical conversion film, and the film porosity.

  Moreover, about the produced chemical conversion treatment steel plate, it applied by electrodeposition coating so that it might become a film thickness of 25 micrometers using (Nippon Paint) company make (V-50), the following salt spray test was performed as a corrosion test, and coating Post-corrosion resistance was evaluated. The results are shown in Table 3.

<Chemical conversion treatment>
The surface treatment with Ti-based colloidal solution is performed using PL-X manufactured by Nihon Parkerizing Co., Ltd., and the chemical conversion treatment liquid (PB-L3065) is used, and the chemical treatment liquid temperature is 40 ° C. In the meantime, it was immersed in a chemical conversion solution and subjected to chemical conversion treatment.

<Coating porosity (P)>
A test piece was collected from the hot-rolled steel sheet after the chemical conversion treatment, and the amount of the chemical conversion coating film deposited was determined by weight comparison. Moreover, a cross-sectional SEM observation sample is extract | collected from the hot-rolled steel plate after chemical conversion treatment, and the cross-section thickness direction cross-sectional observation is performed by 1000 time, the average film thickness of a chemical conversion treatment film is calculated | required, and this is taken as the film thickness of a chemical conversion treatment film did. Then, from the obtained adhesion amount of the chemical conversion treatment film, the film thickness of the chemical conversion treatment film, and the density of Zn ₃ (PO ₄ ) ₂ : ₄ × 10 ⁶ (g / m ³ ), The rate (P) was determined.

Film porosity (P) = 1−C / (t × d) (1)
Here, C is the amount of the chemical conversion coating (g / m ² ), t is the thickness (m) of the chemical conversion coating obtained by observing the cross section in the thickness direction of the steel sheet, and d is Zn ₃ (PO ₄ ) ₂ . Density (g / m ³ ).
When the film porosity (P) calculated | required as mentioned above is 0.3 or less, it was judged that it was excellent in chemical conversion treatment property.

<Salt spray test>
The salt spray test was performed according to JISZ2371. The surface of the hot-rolled steel sheet that has been subjected to electrodeposition coating is subjected to a cross-cut scissors with a length of 45 mm with a cutter, and then subjected to a salt spray test (SST test) at 35 ° C. with a 5 mass% NaCl aqueous solution for 720 hours. ) After the SST test, an adhesive tape was applied to the crosscut collar and then a tape peeling test was performed to measure the maximum total peel width of the left and right crosscut collars. Here, when the measured peeling width was 2 mm or less, it was judged that the SST test result was suitable and the corrosion resistance after coating was excellent. Moreover, when the peeling width | variety measured exceeded 2 mm, it was judged that the SST test result was incompatible and it was inferior to the corrosion resistance after coating.

  As shown in Table 2 and Table 3, the hot-rolled steel sheet of the example of the present invention has a tensile strength of 590 MPa or more, and the chemical conversion film has a porosity of 0.3 or less, excellent in chemical conversion treatment, and after coating. It can be seen that the corrosion resistance is excellent.

  On the other hand, steel plate No. which is a comparative example. 1-4 was inferior in corrosion resistance because the chemical conversion treatment time was too short.

  Steel plate No. 1-5, 2-2, 3-3 and 3-4 are all excessively washed with water after pickling, and the coverage of the corrosion product on the surface of the hot-rolled steel sheet is less than 50%, and the film porosity is Is over 0.3, indicating that the corrosion resistance after painting is poor.

Claims (6)

The Si content has a composition of 0.5% by mass or more, and the steel sheet surface is covered with a porous corrosion product by 50% or more by area ratio ,
When performing chemical conversion treatment with zinc phosphate aqueous solution, the following (1) chemical treatment coating porosity represented (P) is characterized Rukoto formed 0.3 following the chemical conversion coating by the formula Si-containing hot-rolled steel sheet with excellent properties.
Film porosity (P) = 1−C / (t × d) (1)
Here, C is the amount of the chemical conversion coating (g / m ² ), t is the thickness (m) of the chemical conversion coating obtained by observing the cross section in the thickness direction of the steel sheet, and d is Zn ₃ (PO ₄ ) ₂ . Density (g / m ³ ).   In mass%, C: 0.05 to 0.10%, Si: 0.5 to 1.0%, Mn: 1.0 to 1.5%, P: 0.010% or less, S: 0.004 2 or less, Al: 0.05% or less, N: 0.0045% or less, with the balance being composed of Fe and inevitable impurities, excellent chemical conversion treatability according to claim 1 Si-containing hot-rolled steel sheet.   Furthermore, it has a composition containing at least one of Ti: 0.04 to 0.10%, V: 0.05 to 0.12%, and Nb: 0.005% or less in mass%. The Si-containing hot-rolled steel sheet having excellent chemical conversion treatment properties according to claim 2. A method for producing a Si-containing hot-rolled steel sheet according to any one of claims 1 to 3,
Manufacturing a hot-rolled steel sheet by subjecting a steel sheet after hot rolling having a composition having a Si content of 0.5% by mass or more to a pickling process that is a process of pickling, washing, and drying to obtain a hot-rolled steel sheet In the method, the pickling treatment in the pickling step is performed at a temperature of 70 to 90 ° C. and a hydrochloric acid concentration of 5 to 12 Vol. The water washing conditions are adjusted so that the hot-rolled steel sheet after washing and drying is covered with a porous corrosion product by 50% or more by area ratio. A method for producing a Si-containing hot-rolled steel sheet.   The said steel plate is the mass%, C: 0.05-0.10%, Si: 0.5-1.0%, Mn: 1.0-1.5%, P: 0.010% or less, S The Si according to claim 4, comprising: 0.004% or less, Al: 0.05% or less, N: 0.0045% or less, with the balance being composed of Fe and inevitable impurities. Manufacturing method of containing hot-rolled steel sheet.   The steel sheet further contains, in mass%, at least one of Ti: 0.04 to 0.10%, V: 0.05 to 0.12%, and Nb: 0.005% or less. The method for producing a Si-containing hot-rolled steel sheet having excellent chemical conversion properties according to claim 5.

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