JP2005240148A - High-tensile steel plate having excellent scuffing resistance and chemical conversion property, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-tensile steel plate capable of compatibly realizing both characteristics of die scuffing resistance and chemical conversion property and having excellent die scuffing resistance and chemical conversion property, and a method for manufacturing the same. <P>SOLUTION: In the method for manufacturing a high-tensile steel plate having excellent die scuffing resistance and chemical conversion property, solid particles of average grain size of 30-300 μm are blasted against the steel plate containing ≥ 0.1 mass% Si. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel plate excellent in mold galling resistance and chemical conversion treatment suitable for automobile steel plates and the like, and a method for producing the same.

In recent years, automobile body weight has been reduced as an effort to improve the fuel efficiency of automobiles for the purpose of reducing CO ₂ , and as the approach for reducing body weight, higher tension of steel plates constituting the body has been advanced. Yes. In high-tensile steel sheets, Si, Mn, C, and the like are usually added in order to achieve both improvement in material strength and workability. Of these, Si and Mn are easily oxidizable elements, and are easily concentrated as oxides on the surface layer in a normal reduction annealing atmosphere. Here, since these oxides concentrated on the surface layer are inactive, the Fe etching reaction does not occur uniformly in the phosphate treatment process which is the base treatment of the automobile painting process. Moreover, depending on conditions, a phosphate crystal may not grow sufficiently. As a result, problems such as paint adhesion after coating and swelling in a corrosive environment are likely to occur.

  On the other hand, when press-molding a high-tensile steel plate, the surface pressure at the contact portion with the mold is higher than in the case of mild steel. As a result, there is a problem that mold galling is likely to occur during press molding. In general, when the limit load capacity at which the surface pressure is changed and die galling is compared, the steel sheet with a high strength level has a large limit load capacity. In addition, the presence of an oxide layer made of an easily oxidizable component formed on the surface prevents direct contact between the mold and the steel sheet, making it difficult for adhesion to occur. It is good compared.

  In order to improve chemical conversion property, it is effective to increase the activity of the surface. However, when the oxide layer is removed by acid treatment, brush roll, or the like, the good mold galling resistance inherent to the high-tensile steel sheet as described above is lowered.

  Therefore, it is difficult to achieve both the chemical conversion processability and the galling resistance of the high-tensile steel plate simply by controlling the surface oxide layer.

  As a technique for improving the chemical conversion processability of a high-strength steel sheet, publicly known document 1 discloses a technique for controlling the annealing conditions and optimizing the presence of Si oxide in the surface layer. However, the present technology has not only limitations on the annealing conditions, but also has a drawback in that the mold galling resistance is reduced because the surface oxide layer is small.

Patent Document 2 discloses a technique for forming a zinc oxide-based ultrathin film containing P, B, Si, etc. on the metal zinc layer and its surface layer as a technique for achieving both chemical conversion treatment and mold galling resistance. Has been. According to the present technology, the mold galling resistance is improved by employing the surface layer oxide. However, in the case of a high-strength steel sheet containing an easily oxidizable substance such as Si and Mn, there is a drawback that the effect of the oxide layer formed on the surface layer cannot be eliminated and the chemical conversion processability cannot be obtained sufficiently. Further, in the case of the present technology, since it is necessary to form a zinc layer by electrolysis, it is necessary to pass through the electrolysis equipment again, which causes a process problem.
JP 2003-113441 A JP-A-10-158858

  The present invention has been made in consideration of the above circumstances, and provides a high-tensile steel sheet excellent in mold galling resistance and chemical conversion processability, and a method for producing the same, which can achieve both the characteristics of mold galling resistance and chemical conversion processability The purpose is to provide.

The technology of the present invention is characterized by the following points.
1) Production of a high-tensile steel plate excellent in mold galling resistance and chemical conversion treatment, characterized by projecting solid particles having an average particle size of 30 to 300 μm to a steel plate containing 0.1 mass% or more of Si. Method.
2) The production method according to 1) above, wherein a projection distance is within 700 mm, a solid particle initial projection speed is 60 m / s or more, a solid particle density is 2 g / cm ³ or more, and a projection density is 0.2 to A method for producing a high-tensile steel sheet excellent in mold galling resistance and chemical conversion treatment, characterized by being 50 kg / m ² .

  3) A steel plate containing 0.1 mass% or more of Si obtained by the production method described in 1) or 2) above, having dimple-like surface irregularities formed by projection of solid particles, and a surface layer The hardness in the range of 20 μm or less in the depth direction is 1.1 times or more the hardness of the region in the depth direction of 50 μm or more. Tensile steel plate.

  According to the present invention, it is possible to provide a high-strength steel sheet excellent in mold galling resistance and chemical conversion treatment, which can satisfy both characteristics of mold galling and chemical conversion treatment, and a method for producing the same.

Hereinafter, the high-tensile steel plate of the present invention will be described.
The high-tensile steel plate in the present invention has a tensile strength of 340 N / mm ² or more, and may be either a cold-rolled steel plate or a hot-rolled steel plate. In addition to C, the steel may contain Si, Mn, P, Ti, Nb, V, etc. in order to obtain a predetermined mechanical property value. Among these, in order to obtain a predetermined mechanical property value of the high-tensile steel plate, the content of Si and Mn is particularly large, and both or one of the elements is added in an amount of 0.1 mass% or more.

  Usually, steel sheets are annealed after hot rolling or cold rolling. Annealing is a reducing atmosphere for Fe, but Si, Mn, and the like are oxidizable atmospheres, so these elements are concentrated on the steel sheet surface to form oxides. Of the Si and Mn-based oxides formed on the surface layer, the Mn oxide is relatively easily removed by pickling with sulfuric acid or hydrochloric acid after annealing, but the Si oxide tends to remain.

  By the way, the Si oxide remaining in the surface layer inactivates the surface of the steel sheet. As a result, in the chemical conversion treatment that is processed as an automobile coating base, there is a problem in that the steel is difficult to be etched in the phosphate aqueous solution, and the phosphate crystals are difficult to form densely. On the other hand, a method of mechanically removing these surface oxides with a brush roll or the like is also possible. However, when the surface layer oxide is removed, it is disadvantageous for mold galling resistance as described above.

Therefore, in the present invention, in order to achieve both the chemical conversion processability and the anti-galling property of the high-strength steel plate containing 0.1 mass% or more of Si, solid particles having an average particle size of 30 to 300 μm are added to the steel plate. Project. Hereinafter, the present invention will be described in detail.
First, the projection of solid particles destroys the Si-based oxide on the surface of the steel sheet, improving the reactivity between the phosphating solution and the iron ground. Furthermore, due to the presence of dimple-like surface irregularities formed by projection of solid particles, press oil is held in the concave portions even under high surface pressure, and good lubricity is maintained. Here, the “dimple shape” is a form in which the shape of the surface depression is mainly composed of a curved surface, and for example, a large number of crater-like depressions formed by collision of a spherical object with the surface. On the surface obtained by projecting solid particles, the size and distribution of the depressions, that is, the recesses, are determined by the hardness of the steel sheet to be projected, the particle diameter, density, initial velocity, projection distance, and projection density of the solid particles. Here, in the dimple shape effective for the galling property of the steel plate, it is important that the concave portions where oil is held are densely present.

  On the other hand, since the surface layer portion is locally deformed by projecting solid particles under appropriate conditions, only the surface layer is cured. As a result, even during sliding at a high surface pressure, surface deformation is unlikely to occur, and galling due to tool digging during sliding is less likely to occur. Furthermore, since the true contact area is unlikely to increase, the direct contact portion between the mold and the steel plate due to running out of oil is reduced, adhesion is unlikely to occur, and galling due to accumulation of adhesion is suppressed.

  On the other hand, in order to ensure chemical conversion processability, it is important that solid particles are projected onto the steel sheet with sufficient energy to destroy the Si-based oxide.

In view of the above, in a high-strength steel sheet containing 0.1 mass% of Si, in order to achieve both mold galling resistance by an appropriate oil retaining action and chemical conversion treatment by destruction of Si-based oxide, the average particle size is 30. It is preferable to project solid particles of ˜300 μm within a projection distance of 700 mm. Furthermore, it is possible to obtain a more effective surface state that the initial velocity of solid particles is 60 m / s or more, the density of solid particles is 2 g / cm ³ or more, and the projection density is 0.2 to 50 kg / m ^2. It is.

  In the present invention, when the average particle size of the solid particles is less than 30 μm, the speed of the particles is greatly reduced during projection, and sufficient energy cannot be imparted at the time of collision with the steel plate. In addition, when the particle diameter is small, it becomes difficult in terms of handling such as particle conveyance and recovery. On the other hand, when the particle diameter exceeds 300 μm, the indentation becomes large and dense irregularities are not formed. As a surface form effective against mold galling, it has been found that it is effective to have dense unevenness, and the following range is displayed in the roughness parameter.

  As the roughness Ra, an average roughness of 0.3 to 3 μm (the average roughness Ra is a center line average roughness defined in JIS B0601) and a peak count PPI of 150 to 600 are preferable. A more preferred PPI is 200 to 400. Here, the “peak count PPI” is the number of uneven peaks per inch as defined in the SAE 911 standard. The peak count PPI is represented by a value at a count level of ± 0.625 μm.

  Furthermore, as the surface hardness effective for mold galling, it is preferable that the hardness within a range of 20 μm or less in the depth direction from the surface is 120 Hv or more in terms of Vickers hardness. As described above, in order to make it difficult to cause deformation of the surface layer during sliding, the hardness of a depth region of about several μm, which is affected by sliding with a tool, is the same as the surface roughness level. It is considered important. Furthermore, in order to withstand phenomena such as digging, the hardness at deeper positions is also important. The surface hardness of the steel sheet obtained by the present invention and excellent in resistance to mold galling is hardened in the region within 20 μm in the depth direction, so that the region that should have high hardness is within 20 μm in the depth direction. Stipulated.

  In addition, since the hardness is usually slightly different depending on the steel type, the hardness of the surface layer portion is 1.1 times or more of the hardness measured in a portion other than the surface layer portion, that is, in a depth direction of 50 μm or more. It is necessary. When the ratio is less than 1.1 times, sufficient resistance to mold galling cannot be obtained.

  Such hardness can be obtained by polishing the cross section of the steel sheet, changing the position, and measuring the hardness with micro Vickers or the like.

  In the technique of the present invention, it is also important to obtain a shape having a dimple shape and a dense unevenness as expressed by the roughness parameter. For this purpose, the particle diameter needs to be 300 μm or less. A predetermined dimple shape can be obtained by projecting solid particles having a particle diameter of 30 to 300 μm. However, in order to further destroy the surface layer Si oxide and make the surface layer have the above hardness, it is necessary to have sufficient energy when the solid particles are projected onto the steel plate. Therefore, in the case of the solid particles of the present invention, when a suitable projection distance was examined, it was found that it should be 700 mm or less. If it exceeds 700 mm, the kinetic energy of the particles when reaching the steel sheet is not sufficient, the oxide cannot be sufficiently destroyed, and the surface layer is not sufficiently cured.

Furthermore, for the same reason as described above, the initial projection speed of the solid particles should be 60 m / s or more, and the density of the solid particles should be 2 g / cm ³ or more. When the initial projection speed is less than 60 m / s or the density of solid particles is less than 2 g / cm ³ , a dimple shape having fine irregularities effective for mold galling resistance cannot be obtained even if the projection distance is 700 mm or less. Further, the destruction of the Si-based oxide effective for chemical conversion treatment and the hardening of the surface layer are not sufficiently performed.

  In the present invention, a metal material or a ceramic material is suitable as the solid particles. Examples of the metal-based material include iron-based materials such as carbon steel, stainless steel, high-speed tool steel (high speed), and cemented carbide such as tungsten carbide. Examples of the ceramic material include alumina and zirconia.

The upper limit of the initial velocity of the solid particles is not particularly defined, but is limited by the upper limit of the projection device. The projection density is determined by the area projected on the steel sheet, but in order to obtain a dimple shape having predetermined fine irregularities in a wider range of the steel sheet, an average projection density of 0.2 to 50 kg / m ² is suitable. More preferably, it is 5-25 kg / m < ² >. When the average projection density is less than 0.2 kg / m ² , the surface processed area is small and the effect cannot be obtained sufficiently. On the other hand, if the average projection density exceeds 50 kg / m ² , warpage or damage occurs on the steel sheet surface.

  In the present invention, the solid particle projection device is preferably a centrifugal device rather than a pneumatic device. In a pneumatic apparatus, since the nozzle which projects a particle is small, in order to project on a wide steel plate, it is necessary to arrange many nozzles. On the other hand, in addition to being able to project in a wide range, the centrifugal method is effective for imparting a sufficient initial velocity to the solid particles.

  As a manufacturing method in the present invention, for example, when applied to a cold rolled material, it is as follows. In other words, cold rolling, annealing, temper rolling is provided with equipment composed of a device that projects solid particles onto a steel plate, a device that collects solid particles, and a device that removes solid particles that adhere to or remain on the steel plate surface. Thereafter, the processing of the present invention is performed. Here, conditions, such as cold rolling, annealing, and temper rolling, are not specifically limited, A washing | cleaning process etc. may exist after each process. Further, an acidic aqueous solution treatment for removing the surface oxide may be performed after the annealing.

  Further, a zinc-based plating or an organic or inorganic / organic-inorganic composite layer may be further applied to the steel plate obtained by the production method of the present invention by coating, chemical treatment, electrolytic treatment, or the like.

The steel plate which is the subject of the present invention has a tensile strength of 340 N / mm ² or more, more preferably 440 N / mm ² or more. The Si concentration is preferably 0.1 mass% or more, but a more preferable range is 0.2 mass% or more. In addition, any of a hot rolled steel plate and a cold rolled steel plate can be applied.

(Example)
Examples of the present invention will be described below.
In this example, cold-rolled steel sheets A, B, and C shown in Table 1 below, which have undergone cold rolling, annealing, and temper rolling with a thickness of 1.2 mm, differing in tensile strength, C, Si, Mn, and P concentrations. , D were used. And the solid particle was projected on the steel plate on the conditions shown in following Table 2 with the centrifugal projection apparatus or the pneumatic projection apparatus. The average particle size of the solid particles was measured with a Coulter counter (trade name: Multisizer IIE type) manufactured by Pacman Coulter.

The following evaluation was performed using the steel plate after projection. For comparison, a steel plate without projection was also evaluated in the same manner. Further, in the examples, samples Nos. 9 and 10 were filled with resin and then polished, and the hardness of the polished surface was measured by micro Vickers while changing the position. The results are shown in Table 3 below.

1) Evaluation of mold galling
The surface pressure of the tool (SKD11) having the shape shown in FIG. 1 was changed by a flat plate sliding device shown in FIG. In FIG. 1, the symbol H indicates the height of the tool 10, the symbol L ₁ indicates the length of the bottom of the tool 10, and the symbol L ₂ indicates the lateral width of the tool 10. The tool 10 is attached to FIG. 2 in the state of FIG.

  2 mainly includes a first load cell 11, a sliding table 12, and a vertically movable sliding table support 14 having a roller 13 in contact with the sliding table 12. A sample stage 15, a tool 16, a second load cell 17, and a rail 18. The sample 19 is fixed to the sample table 15, and the sample table 15 is fixed to the upper surface of the sliding table 12 that can move horizontally.

  The sliding table support 14 is provided on the lower surface of the sliding table 12, and the first load cell 11 for measuring the pressing load N applied to the sample 19 by the tool 16 by pushing up the sliding table 12 includes the first load cell 11. The slide table support 14 is attached. The second load cell 17 is for measuring a sliding resistance force F for moving the sliding table 14 in the horizontal direction in a state where the pressing force is applied. Installed on. The case where the tool 10 and the material were severely galvanized and the pulling force exceeded 500 kgf set as the limit value of the apparatus was regarded as die galling.

2) Chemical conversion treatment Using a chemical conversion treatment solution (trade name: PB-L3080) manufactured by Nippon Parkerizing Co., Ltd., surface adjustment (trade name: preparen Z, Nippon Parkerizing Co., Ltd.) → phosphate treatment → water washing → drying In this order, the chemical conversion treatment for the automobile paint base was performed. The treatment time was 10 seconds and 30 seconds, the treatment was stopped in a time shorter than the normal treatment time (120 seconds or more), and the amount of the phosphate film formed was measured by the gravimetric method. In the gravimetric method, the sample was immersed in a chromic acid aqueous solution (CrO ₃ 200 g / l) at room temperature for 20 minutes to dissolve the phosphate, and the weight difference before and after dissolution was determined.

  As can be seen from the results in Table 2 above, compared to the case where solid particles are not projected (Comparative Example in Table 2: Sample Nos. 1, 7, 9, and 11), the steel plate on which the solid particles were projected under the conditions of the present invention ( In Examples in Table 2: Sample Nos. 2, 3, 5, 6, 8, 10, 12), the initial phosphate film amount during chemical conversion treatment is increased, and the initial reactivity is improved. It can be seen that the chemical conversion processability is improved. Further, the limit load capacity is increased by about 1.4 to 1.8 times compared to the case where solid particles are not projected (Sample Nos. 1, 7, 9, and 11 in Table 2). It can be seen that the property is improved.

  Moreover, from the said Table 3, in the case of the sample which projected the solid particle on the conditions in a present Example, the hardness of the range within 20 micrometers from the surface layer is 1.1 of the hardness in the position of 50 micrometers or more in the depth direction. It turns out that it is more than double.

The perspective view of the tool for performing mold | steeling_steeling evaluation. Explanatory drawing of the flat plate sliding apparatus for measuring a limit surface pressure.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Tool, 11, 17 ... Load cell, 12 ... Sliding table, 13 ... Roller, 14 ... Sliding table support stand, 15 ... Sample stand, 16 ... Tool, 18 ... Rail, 19 ... Sample.

Claims (3)

A method for producing a high-strength steel sheet excellent in mold galling resistance and chemical conversion property, characterized in that solid particles having an average particle diameter of 30 to 300 μm are projected onto a steel sheet containing 0.1 mass% or more of Si. The projection distance is within 700 mm, the initial projection speed of solid particles is 60 m / s or more, the density of solid particles is 2 g / cm ³ or more, and the projection density is 0.2 to 50 kg / m ^2. A method for producing a high-strength steel sheet excellent in mold galling resistance and chemical conversion treatment as described in 1. A steel plate containing 0.1 mass% or more of Si obtained by the production method according to claim 1, wherein the steel plate has dimple-like surface irregularities formed by projection of solid particles, and is deeper than the surface layer. A high-tensile steel plate excellent in mold galling resistance and chemical conversion property, characterized in that the hardness within a range of 20 μm or less in the direction is 1.1 times or more the hardness of a region of 50 μm or more in the depth direction.

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