JP2006299351A - Steel sheet having excellent galling resistance and chemical conversion treatability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet in which galling resistance and chemical conversion treatability are highly made consistent, and which is producible at a low cost. <P>SOLUTION: The surface of a steel sheet is provided with a galvanizing film in which coating weight is 10 to 2,000 mg/m<SP>2</SP>, and the rate of orientation R(00i2) in the (00i2) face defined by following formula is ≥0.5: R(00i2)=[I(00i2)/Is(00i2)]/Σ[I(hkil)/Is(hkil)]; wherein, I(hkil) is the diffraction peak intensity (cps) of each crystal face (hkil) in the galvanizing film obtained by X-ray diffraction measurement; Is(hkil) is the diffraction peak intensity (cps) of each crystal face (hkil) in standard zinc powder; and Σ[I(hkil)/Is(hkil)] is the total of the values of I(hkil)/Is(hkil)in which (hkil) is each crystal face of (00i2), (10i0), (10i1), (10i2), (10i3), (11i0), (11i2), (20i1), (10i4) and (20i3). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel sheet excellent in mold galling resistance and chemical conversion treatment, for example, a cold-rolled steel sheet or a hot rolled steel sheet excellent in mold galling resistance and chemical conversion treatment used as a material for automobiles.

  Since steel plates are inexpensive metal materials, they are widely used in fields such as automobiles, home appliances, and building materials. In particular, in the automotive field, steel sheets are still the mainstream of automotive materials because they have superior press formability and chemical conversion properties compared to other metal materials. In recent years, in the automobile industry, the weight of automobiles has been reduced from the viewpoint of improving fuel efficiency and reducing exhaust gas, and the use of high-strength steel sheets has been rapidly increasing in conjunction with the need for improved collision safety.

  A high-strength steel plate is a steel plate to which Si, Mn and the like are added as elements in the steel, and it has been conventionally known that chemical conversion properties are remarkably deteriorated when these elements are distributed on the surface of the steel plate. On the other hand, when high-strength steel sheets are press-formed, there is a problem that not only the forming load increases but also the local high surface pressure part causes mold galling. The development of high-strength steel sheets with excellent processability has been desired.

  As a technique for improving the anti-galling property and chemical conversion treatment property of a steel sheet, for example, in Patent Document 1, the lower layer is an ultrathin film mainly composed of zero-valent zinc, and the upper layer is one kind of divalent zinc and P, B, or Si. A technique for forming a multi-layered amorphous film made of an oxide of a second element group composed of two or more kinds is disclosed.

  This technology aims to form a hard, glassy amorphous film by bringing the surface of soft metallic zinc into contact with a specific aqueous solution, and to improve the lubricity by this amorphous film. is there. However, with this technique, the anti-mold galling resistance is insufficient because the anti-mold galling resistance improvement effect is reduced by the presence of the hard upper layer film. In addition, the orientation of the underlying metal zinc-based film is not controlled at all, and a stable mold galling resistance has not been secured. On the other hand, the upper amorphous film is a film that is hardly dissolved by the alkaline degreasing before the chemical conversion treatment and is almost completely dissolved in the chemical conversion solution. For this reason, not only the chemical conversion solution is deteriorated by changing the concentration balance of each component in the chemical conversion solution, but also the dissolution of the amorphous film and the formation of the chemical conversion crystal occur at the same time. Unlike this, there is a problem that normal chemical conversion treatment crystals are not formed. Furthermore, in order to form such a film, for example, it is necessary to provide a contact treatment step with an aqueous solution after the galvanization step, and the production cost is high because the treatment step is complicated.

  On the other hand, it is different from the technology related to the cold-rolled steel plate or hot-rolled steel plate intended in the present invention, but in the galvanized steel plate, the technology for improving the quality by controlling the orientation of the galvanized layer. For example, Patent Document 2 discloses a technique for improving the color tone and conductivity by setting the orientation of the (00 · 2) plane and other crystal planes within a specific range. .

However, these techniques are techniques for a galvanized steel sheet which is a rust-proof steel sheet, and the adhesion amount of the galvanized layer is usually about 20 g / m ^{2 in} order to ensure corrosion resistance even when the amount is small. m ² or more adhesion amount have required, the cold-rolled steel sheets and hot-rolled steel sheets as an object of the present invention first place art is different. Naturally, it is not intended to improve die galling resistance and chemical conversion treatment, which is a problem with cold-rolled steel sheets and hot-rolled steel sheets, but also about the relationship between the orientation of the galvanized layer and die galling resistance and chemical conversion treatment properties. It is not revealed at all.
JP-A-10-158858 Japanese Patent No. 3354142

  As described above, the conventional technology has not established a technology that achieves a high level of mold galling resistance and chemical conversion treatment of cold-rolled steel sheets and hot-rolled steel sheets. There was no technology that satisfied the processability.

  In view of such a situation, the present invention provides a technology that can achieve both high-level galling resistance and chemical conversion treatment of cold-rolled steel sheets and hot-rolled steel sheets used as automotive steel sheets, and can be manufactured at low cost. For the purpose. Furthermore, recent high-strength steel sheets contain a large amount of elements such as Si and Mn, making it difficult to secure good chemical conversion treatment. Also, mold galling occurs as the strength increases. Therefore, an object of the present invention is to provide a steel sheet that can satisfy both the anti-galling property and the chemical conversion property of a high-strength steel sheet.

  The inventors diligently studied the effects of various coatings coated on the surface of the steel sheet for the purpose of improving the mold galling resistance of the high-strength steel sheet. As a result, even if a galvanized film is simply formed on a steel sheet, a stable mold galling resistance improvement effect cannot be obtained, but the adhesion amount of the galvanized film and the orientation of the (00 · 2) plane are within a predetermined range. It has been newly found that by controlling it to a very high level, the effect of improving the resistance to mold galling is exhibited and the chemical conversion treatment property is also excellent.

This invention is made | formed based on this knowledge, The means of this invention which solves the said subject is 10-2000 mg / m < ² > of adhesion amounts to the steel plate surface, and is defined by following formula (1). It is a steel plate excellent in mold galling resistance and chemical conversion property, characterized by having a galvanized film having an orientation ratio R (00 · 2) of (00 · 2) plane of 0.5 or more.

In the formula (1), I (hk · l) is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the galvanized film obtained by X-ray diffraction measurement, and Is (hk · l) Is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the standard zinc powder, and Σ [I (hk · l) / Is (hk · l)] is (hk · l) is (00・ 2), (10.0), (10.1), (10.2), (10.3), (11.0), (11.2), (20.1), (10.4) ), (20 · 3) is the total of I (hk · l) / Is (hk · l) for each crystal plane.

  The present invention provides a steel sheet that is highly compatible with mold galling resistance and chemical conversion treatment, and is an extremely effective technique for achieving both mold galling resistance and chemical conversion treatment characteristics of a high-strength steel sheet. Since the technology can be manufactured at low cost, it is extremely valuable industrially.

  The present invention will be described below together with the background to the invention.

  The inventors first conducted intensive studies on the effects of various coatings coated on the surface of the steel sheet for the purpose of improving the mold galling resistance of the high-strength steel sheet. As a result, even if a galvanized film is simply formed on a steel sheet, a stable mold galling resistance improvement effect cannot be obtained, but the adhesion amount of the galvanized film and the orientation of the (00 · 2) plane are within a predetermined range. It has been newly found out that an extremely remarkable effect of improving the galling resistance is manifested by controlling to the above.

  The mold galling resistance aimed by the present inventors meets an extremely strict requirement level such that adhesion between the steel sheet and the mold does not occur even under a high surface pressure condition of, for example, 500 MPa or more. In order to achieve this, the method of applying a hard coating on the outermost surface of a steel sheet as disclosed in Patent Document 1 is rather counterproductive, and the entire coating applied to the steel sheet surface is soft. It is necessary. The reason for this is that under extremely severe high surface pressure conditions such as 500 MPa or more as described above, the hardness of the coating itself applied to the steel sheet surface is an important factor, and the entire coating is soft and susceptible to shear deformation. The mold galling resistance becomes better, and when a hard layer is present, this layer itself requires a great deal of energy when subjected to shear deformation, so it not only becomes a starting point for mold galling, but also when this hard layer is destroyed This is because a part of the hard coating remains adheres to the mold and digs up the surface, which deteriorates the resistance to mold galling.

  Furthermore, the present inventors have newly found that controlling the orientation of the (00 · 2) plane of the galvanized film is an important factor for stably improving mold galling resistance. . The reason for this is that the galvanized film has a shape in which hexagonal plate crystals are laminated, but the higher the orientation ratio of the (00 · 2) plane that is the basal plane of the hexagonal plate crystals, It is considered that the film itself is liable to slip inside during press forming, and as a result, the shear resistance of the galvanized film itself is further reduced.

  On the other hand, in the steel sheet to which the galvanized film in the adhesion amount range of the present invention is applied, the chemical conversion treatment reaction is promoted compared to the case of the untreated steel sheet, and a uniform chemical conversion treatment crystal without scallops and unevenness can be obtained. . This is considered to be because microcells are formed between the adhesion portion of the galvanized film and the exposed portion of the steel plate, and the chemical conversion reaction proceeds more actively and more uniformly. Even if the galvanized film covers the entire surface of the steel sheet, there is no problem in performance because a film of the same quality as the zinc phosphate film formed on the galvanized steel sheet is formed. If a zinc phosphate coating of the same quality as that of a cold-rolled steel plate or a hot-rolled steel plate is desired, the amount of the galvanized coating may be controlled to be small so that a steel plate exposed portion is formed.

  Further, the present invention is a technique developed by controlling the adhesion amount and orientation of a galvanized film that has conventionally been put to practical use within a predetermined range. For example, it is disclosed in Patent Document 1 described above. Since no special elements such as P, B, and Si are contained, the concentration balance of each component in the chemical conversion liquid is not changed, and an abnormal chemical conversion crystal is not formed. Furthermore, since there is no post-processing step after the galvanizing step, the manufacturing cost is low.

  Hereinafter, the present invention will be described in more detail.

  Although it does not specifically limit as a steel plate used by this invention, Steel plates, such as a hot rolled steel plate and a cold rolled steel plate, are illustrated. Among these, a hot-rolled steel sheet that has been frequently used as a material for automobiles, a hot-rolled steel sheet from which black skin has been removed by pickling treatment, and a cold-rolled steel sheet whose material has been adjusted by annealing treatment are preferably used. Further, the strength level of the steel plate is not limited, and the steel plate can be applied to all steel plates ranging from a mild steel plate having a tensile strength of 300 MPa or less to a high strength steel plate having a tensile strength exceeding 1000 MPa. The plate thickness of the steel plate is not limited at all. For example, a steel plate having a thickness of about 0.2 to 5 mm is applicable.

In this invention, the adhesion amount of a galvanization film shall be 10-2000 mg / m < ² >. If the amount of galvanized coating is less than 10 mg / m ² , both mold galling resistance and chemical conversion property are insufficient. The reason why mold galling resistance is insufficient is that a soft galvanized film has a small effect of reducing the shear resistance during press molding. The reason why the chemical conversion property is inadequate is that the effect of promoting the chemical conversion reaction by the microcell formed between the galvanized adhesion part and the exposed part of the steel sheet is small, and therefore, the number of nucleation points of the chemical conversion crystal is small and uniform. This is because it is difficult to obtain a chemical conversion treatment crystal. On the other hand, when the adhesion amount of the galvanized film exceeds 2000 mg / m ² , the mold galling resistance deteriorates. The reason for this is that the shear resistance of the galvanized film itself is so large that it cannot be ignored. As a result, smooth lubrication cannot be performed during press molding, and the galvanized film itself becomes the starting point for mold galling or broken zinc. This is because a part of the plating film may become hard and cause mold galling.

In the case where the adhesion amount of the zinc plating film was blanketing the entire steel sheet surface increases, the crystals formed during the chemical conversion treatment, phosphophyllite _{(Zn 2 Fe (PO 4)} 2 · 4H 2 O) It changes from a main body to a hopite (Zn ₃ (PO ₄ ) ₂ .4H ₂ O) main body. However, hopite is a zinc phosphate crystal formed when a chemical conversion treatment is performed on a zinc-based plated steel sheet, and there is no performance problem. Conventionally, it is known that the higher the P ratio (P / (P + H) value when the strength of phosphophyllite is P and the strength of the phosphite is H), the better the corrosion resistance after coating, In recent years, chemical conversion treatment chemicals and electrodeposition coatings have been rapidly improved, and the actual situation is that the influence of the P ratio on the performance after coating is so small that it hardly becomes a problem.

If a high P ratio is necessary due to insufficient chemical conversion treatment chemicals or electrodeposition paint performance, the amount of deposit is controlled so that the galvanized film does not cover the entire surface of the steel sheet. For example, when it is desired to set the P ratio to 0.50 or more, the adhesion amount of the galvanized film is approximately 800 mg / m ² or less, and when it is desired to set the P ratio to 0.85 or more, approximately 600 mg / m ² or less. It may be controlled to.

  In the present invention, R (00 · 2), which is the orientation ratio of the (00 · 2) plane of the galvanized film, is 0.5 or more. R (00 · 2) is defined by the following formula (1).

In the formula (1), I (hk · l) is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the galvanized film obtained by X-ray diffraction measurement, and Is (hk · l) Is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the standard zinc powder, and Σ [I (hk · l) / Is (hk · l)] is (hk · l) is (00・ 2), (10.0), (10.1), (10.2), (10.3), (11.0), (11.2), (20.1), (10.4) ), (20 · 3) is the total of I (hk · l) / Is (hk · l) for each crystal plane.

  When R (00 · 2), which is the orientation ratio of the (00 · 2) plane of the galvanized film, is less than 0.5, the mold galling resistance is insufficient. The reason for this is that even though the galvanized film is soft, smooth lubrication is not performed because the effect of reducing shear resistance due to sliding inside the galvanized film does not occur during press molding, and the galvanized film itself is This is because a part of the galvanized film that becomes a starting point or is broken may harden and cause mold galling. The (00 · 2) plane of the galvanized film is the basal plane of the hexagonal plate crystal of zinc. If R (00 · 2) is 0.5 or more, the sliding direction and the hexagonal plate crystal during press molding The frequency with which the sliding directions are the same is increased, and the shear resistance is remarkably reduced, so that the mold galling resistance is improved.

  R (00 · 2), which is the orientation ratio of the (00 · 2) plane of the galvanized film, is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the galvanized film obtained by X-ray diffraction measurement. Use and calculate according to equation (1).

  In the present invention, when calculating R (00 · 2), (00 · 2), (10 · 0), (10 · 1), (10 · 2), (10 · 3), (11 · The X-ray diffraction peak intensities of 10 crystal planes (0), (11.2), (20.1), (10.4), (20.3) are used. The reason for evaluating these 10 kinds of crystal planes is that, as described in Patent Document 2, the diffraction peaks obtained when the electrogalvanized film is measured by X-ray diffraction are almost in the 10 kinds of crystal faces. Concentration, and the true orientation of the galvanized film can be more accurately evaluated if the equivalent surfaces such as (00 · 2) and (00 · 4) are not evaluated twice. It is.

The method for producing the steel sheet of the present invention is not limited as long as the steel sheet that satisfies the constituent requirements of the present invention can be produced, but the electrogalvanization method is easy to control the adhesion amount and orientation. In addition, it is most suitable because it can be manufactured at low cost. The conditions for electrogalvanization are not limited in any way, and any conditions can be used as long as a steel sheet that satisfies the constituent requirements of the present invention can be produced, and known electrogalvanization conditions can also be suitably used. Examples of the bath composition of the galvanizing bath include an acidic bath such as a sulfuric acid bath and a chloride bath containing Zn ions at a concentration of 0.1 to 2 mol / liter, or an alkaline bath. The zinc plating bath may contain Ni ions, Fe ions, Sn ions, and the like as impurity ions, as well as contained in known zinc plating baths, and alkali metals, alkaline earth metals, etc. The conductive assistant may be contained. As is well known, additives such as organic compounds and inorganic compounds may be contained as a glossing agent for changing the glossiness or as a color tone adjusting agent for changing the whiteness. In order to change the adhesion amount of the galvanized film, it is only necessary to change the amount of electricity input by changing the current density and the energization time. R (00 · 2) which is the orientation ratio of the (00 · 2) plane In order to achieve 0.5 or more, for example, the current density is a low current density of 10 A / dm ² or less, or the amount of impurity ions contained in the galvanizing bath is adjusted, and the orientation ratio is in a desired range. What should be done. Prior to electrogalvanization, pretreatment for cleaning or activating the steel sheet surface may be performed, and any pretreatment using a known acid or alkali is applicable.

  Hereinafter, the present invention will be described in more detail based on examples.

  Table 1 shows the test steel plates used. As the hot-rolled steel plate, three types of steel plates having a tensile strength of 270 MPa class, 590 MPa class, and 980 MPa class were used, and steel sheets from which black skin was removed by pickling treatment were used. In addition, as the cold-rolled steel sheet, steel sheets after annealing with tensile strengths of 270 MPa class, 590 MPa class, and 980 MPa class were used. In addition, the plate thickness used the steel plate of 1.2 mm in all cases.

The surface of these steel plates was cleaned and activated by alkaline electrolytic degreasing and sulfuric acid pickling, and then a galvanized film was applied by an electrogalvanizing method. As the zinc plating bath, a plating bath containing 430 g / l of zinc sulfate heptahydrate (ZnSO ₄ .7H ₂ O) and having a bath temperature of 60 ° C. and a pH of 1.6 was used. The concentration of nickel sulfate hexahydrate (NiSO ₄ · 6H ₂ O) in the zinc plating bath is used to change R (00 · 2), which is the orientation ratio of the (00 · 2) plane of the galvanized film. Was added while changing the concentration of ferrous sulfate heptahydrate (FeSO ₄ · 7H ₂ O) to 0 to 15 g / l. Moreover, R (00 * 2) was changed by changing also the current density at the time of plating to 1-120 A / dm < ² >. It should be noted that since the nickel sulfate hexahydrate concentration is higher, the ferrous sulfate heptahydrate concentration is higher, and the current density is higher, R (00 · 2) becomes a smaller value. Steel plates having galvanized films with different values of R (00 · 2) were produced by combining them.

  The amount of the galvanized film thus prepared was quantified by wet analysis. Further, X-ray diffraction measurement of the galvanized film was performed, and R (00 · 2), which is the orientation ratio of the (00 · 2) plane of the galvanized film, using the above formula (1) from the diffraction peak intensity of each crystal face. ) Was calculated.

The evaluation of mold galling resistance was carried out using a sliding tester, the steel sheet was slid while raising the pressing load of the mold in increments of 100 MPa to 50 MPa, and the galling did not occur on the steel sheet by visual observation. Evaluation was made by obtaining a limit load capacity which is a load value. The mold used for the sliding test is made of SKD11, the width of the mold is 10 mm, the contact length in the sliding direction between the mold and the steel plate is 3 mm, and the end of the mold in the sliding direction is The shape which gave R of 4.5 mm was used. The sliding conditions were a sliding speed of 1.0 m / min, a sliding distance of 100 mm, and a sliding test was performed on a steel plate coated with general rust preventive oil (Idemitsu Kosan Co., Ltd., Daphne Oil Coat SK). It was. By this test, mold galling resistance was determined according to the following criteria.
◎: Limit load capacity is 1000 MPa or more ○: Limit load resistance is 500 MPa or more and less than 1000 MPa Δ: Limit load resistance is 300 MPa or more and less than 500 MPa ×: Limit load resistance is less than 300 MPa

Evaluation of chemical conversion treatment was performed under standard conditions using a commercially available chemical conversion chemical (Nippon Parkerizing Co., Ltd., Palbond PB-L3020 system), and was measured by uniformity evaluation of chemical conversion treatment crystals by SEM and X-ray diffraction. Evaluation was made based on the P ratio (P / (P + H) value where P is the intensity of phosphophyllite and H is the intensity of the phosphite). The uniformity evaluation of the chemical conversion treatment crystal was judged according to the following criteria.
○: There is no scale in the conversion crystal. Δ: There is a partial scale in the conversion crystal. X: The scale of the conversion crystal is remarkable.

  Table 2 shows the evaluation results of the steel plate used, R (00 · 2), which is the adhesion amount of the galvanized film, and the orientation ratio of the (00 · 2) plane, and the resistance to mold galling and chemical conversion treatment.

  As shown in Table 2, all of the steel sheets of the present invention are excellent in mold galling resistance and chemical conversion treatment.

  The steel sheet of the present invention can be used as a cold-rolled steel sheet or a hot-rolled steel sheet excellent in mold galling resistance and post-coating corrosion resistance used in application fields such as the automobile field. Furthermore, the steel sheet of the present invention can be used as a high-strength steel sheet (high-strength hot-rolled steel sheet or high-strength cold-rolled steel sheet) excellent in mold galling resistance and post-coating corrosion resistance used in application fields such as the automobile field. .

Claims (1)

A galvanized film having an adhesion amount of 10 to 2000 mg / m ² on the steel sheet surface and an orientation ratio R (00 · 2) of (00 · 2) plane defined by the following formula (1) of 0.5 or more. A steel sheet excellent in mold galling resistance and chemical conversion treatment, characterized by having

In the formula (1), I (hk · l) is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the galvanized film obtained by X-ray diffraction measurement, and Is (hk · l) Is the diffraction peak intensity (cps) of each crystal plane (hk · l) of the standard zinc powder, and Σ [I (hk · l) / Is (hk · l)] is (hk · l) is (00・ 2), (10.0), (10.1), (10.2), (10.3), (11.0), (11.2), (20.1), (10.4) ), (20 · 3) is the sum of I (hk · l) / Is (hk · l) for each crystal plane.