JP2007162057A - High strength steel sheet having excellent phosphate treatability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Si-added high strength steel sheet having excellent phosphate treatability by controlling Si based oxides produced on the surface of the Si-added steel sheet having phosphate treatability remarkably inferior to that of soft steel. <P>SOLUTION: The content of Si based oxides in the surface of a high strength steel sheet is controlled to ≤20 mg/m<SP>2</SP>expressed in terms of SiO<SB>2</SB>. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel sheet that is optimal for automobile steel sheets and has excellent phosphatability.

  In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. In addition, from the viewpoint of occupant protection in the event of a collision, it is also required to improve the safety of the automobile body. For this reason, the application of high-strength steel sheets to automobile parts has been actively promoted for the purpose of reducing the weight and strengthening of automobile bodies.

  On the other hand, steel plates used as automotive parts are generally subjected to phosphate treatment as a coating ground treatment after press molding. High-strength steel sheets often contain Si as a strengthening element, and in this case, inferior phosphatability compared to mild steel is one of the major issues.

  Even in general mild steel, it is known that phosphate treatment performance deteriorates when cooling after continuous annealing is performed by water cooling or air-water cooling, etc. Many studies have been made on the following.

Patent Document 1 to Patent Document 10 describe a method of attaching a metal element other than Fe, for example, Ni, Co, Ti, Mn, Cu, Mo, W, etc. to the steel sheet surface, and Patent Document 11 discloses Fe— As a method of performing P plating, Patent Document 12 discloses a method of cooling a steel sheet with a coolant containing an organic or inorganic acid or a salt thereof, and Patent Document 13 discloses a cooling process as post-treatment after completion of cooling of continuous annealing. A method of bringing a boric acid aqueous solution into contact with a rolled steel sheet is disclosed.
JP 58-055535 A Japanese Patent Laid-Open No. 56-116883 JP 58-066666 A JP 59-159987 A JP-A 61-149492 JP-A 61-023794 Japanese Patent Laid-Open No. 03-075382 Japanese Patent Laid-Open No. 03-086302 Japanese Patent Laid-Open No. 03-126879 Japanese Patent Application Laid-Open No. 07-278843 JP-A 61-136694 Japanese Patent Laid-Open No. 01-139728 Japanese Patent Laid-Open No. 02-270969

  However, the above method is intended to improve the phosphatability of general mild steel, and is not effective for improving the phosphatability of high strength steel sheets containing Si. This is because in Si-added steel, the Si-based oxide formed on the surface of the steel sheet during annealing hinders the phosphatability, so that the phosphatability is far inferior to that of mild steel.

  The present invention is intended to solve the above-mentioned problems, and an object thereof is to provide a Si-added high-strength steel sheet excellent in phosphate processability.

As a result of investigating the relationship between the Si amount on the steel sheet surface and the phosphate treatment property in the Si-added steel, the present inventors have found that in order to obtain a good phosphate treatment property, It has been found that the amount of Si-based oxide on the surface of a certain steel sheet may be set to a certain value or less. The Si-based oxide as used herein refers to an oxide containing Si produced by selective oxidation, and is a good phosphate even within the above condition range while controlling the amount of Si-based oxide. It has been found that when the processability cannot be obtained, the phosphate processability can be further improved by applying Ni or S to the steel sheet surface under certain conditions. The high-strength steel sheet steel obtained from the above knowledge and excellent in phosphatability that solves the above problems is as follows.
(1) The high-strength steel plate excellent in phosphate treatment according to the present invention is characterized in that the amount of Si-based oxide on the surface of the steel plate is 20 mg / m ² or less in terms of SiO ₂ .
(2) The high-strength steel plate with excellent phosphatability according to the present invention has a Si-based oxide content on the steel plate surface of 20 mg / m ² or less in terms of SiO ₂ and a Ni compound content on the steel plate surface in terms of Ni. It is characterized by being 1 mg / m ² or more and 200 mg / m ² or less, and the atomic ratio of Ni to Fe on the steel sheet surface is 0.08 or more and 2 or less.
(3) The high-strength steel sheet with excellent phosphatability according to the present invention has a Si-based oxide content on the steel sheet surface of 20 mg / m ² or less in terms of SiO ₂ and an S compound content on the steel sheet surface in terms of S. It is characterized by being 1 mg / m ² or more and 100 mg / m ² or less, and the atomic ratio of S to Fe on the steel sheet surface is 0.05 or more and 1.5 or less.

  The present invention defines the upper limit of the amount of Si-based oxides on the steel sheet surface that inhibits the phosphate treatment, and further provides a Ni compound or S compound on the steel sheet surface, thereby improving the phosphate treatment property of the Si-added high-strength steel sheet. It is possible to improve and obtain a Si-added high-strength steel sheet excellent in phosphate processability.

  The reason for limitation of the present invention will be described below.

(A) Si-based oxide amount: 20 mg / m ² or less in terms of SiO _{2 The} present inventors have developed a Si-based oxide formed on the steel sheet surface when annealing Si-added steel (the essential composition will be described later). Paying attention, the relationship between the amount and phosphate chemical conversion treatment was investigated.

  The amount of Si-based oxide on the surface of the steel sheet was adjusted by using steel sheets with different Si addition amounts or pickling the steel sheet for 3 to 60 seconds using a 10 mass% hydrochloric acid aqueous solution. Under the present circumstances, the cold-rolled steel plate which manufactured the steel of the component shown in Table 1 by the method shown in the Example mentioned later and the conditions shown in Table 2 was used as an original plate.

The amount of Si-based oxide on the surface of these cold-rolled steel sheets was determined as follows using a fluorescent X-ray analyzer (FX). First, using a standard sample the amount of deposition of SiO ₂ is known to prepare a calibration curve corresponding to the amount of deposition of X-ray intensity and in terms of SiO ₂ of Si. Next, after pickling the test sample and the test sample with a 1 mass% hydrofluoric acid aqueous solution (at 25 ° C. for 15 seconds), the X-ray intensity of Si in the sample was measured, and the difference between the X-rays of the Si-based oxide was measured. As the line intensity, the SiO ₂ equivalent amount of the Si-based oxide was determined from the calibration curve.

  After that, phosphate treatment was performed by a surf dyne SD2800HN system manufactured by Nippon Paint Co., Ltd. by a dipping method at a treatment solution temperature of 43 ° C. to compare phosphate treatment properties. The phosphate processability was evaluated by visual observation and observation with a scanning electron microscope (hereinafter, SEM). In the visual evaluation, a case with a uniform appearance was indicated by ◯, a case with a uniform but somewhat uneven appearance was indicated by Δ, and a case having a non-uniform appearance was indicated by ×. Evaluation by SEM shows that fine phosphate crystals cover 90% or more of the underlying steel sheet, and phosphate crystals cover 90% or more of the underlying steel sheet. The case where the salt crystals were present was indicated by Δ, and the case where the phosphate crystals were able to cover the base steel sheet with less than 90% was indicated as x. The coverage of the phosphate crystals was obtained from the SEM images of a 120 μm × 100 μm field of view and taken from their simple arithmetic average values.

Table 3 shows the relationship between the amount of Si-based oxide on the surface of the steel sheet and the phosphatability. In Table 3,
1) Visual evaluation: ○ and SEM evaluation: ○
2) Visual evaluation: ○ and SEM evaluation: △
3) Visual evaluation: △ and SEM evaluation: ○
4) Visual evaluation: Δ and SEM evaluation: Δ
Of these, any of the evaluation results is judged to have good phosphatability. Further, in the case of the above 1), it is judged to have particularly good phosphate treatment properties. From this result, it became clear that the phosphate treatment ability is improved by reducing the amount of Si-based oxide on the steel sheet surface. Moreover, the Si-based oxide amount on the border of 20 mg / m ² vicinity, phosphating property becomes good, it was found that the phosphate treatment is improved. Therefore, in the present invention, the amount of Si-based oxide is limited to 20 mg / m ² or less in terms of SiO ₂ . Furthermore, in order to obtain a particularly favorable phosphate treatment property, it is preferably 10 mg / m ² or less.

Here, in No. 11 where the amount of Si-based oxide is 20 mg / m ² or less, both visual evaluation and SEM evaluation are Δ. In addition, No. 14 has an evaluation of SEM observation of Δ despite the small amount of Si-based oxide, 12 mg / m ² . Thus, despite the fact that the amount of Si-based oxide is 20 mg / m ² or less, the detailed reason that the phosphate treatment ability may be somewhat degraded is unknown, but in steel other than Si on the steel sheet surface. The influence of components, heat treatment methods (atmosphere, temperature, etc.) can be considered.

  Next, the Ni compound and S compound present on the surface of the steel sheet satisfying the conditions shown in (a) above will be described.

(B) Ni compound amount: 1 mg / m ² or more and 200 mg / m ² or less in terms of Ni, atomic ratio of Ni to Fe: 0.08 or more, 2 or less Conventionally, metallic Ni and water have been used as means for improving phosphate treatment Although it is known that Ni oxide and Ni oxide are attached to the surface of the steel sheet, as described above, the phosphate treatment properties of the Si-added steel cannot be improved by these methods. However, as a result of investigations by the inventors, even in the case of Si-added steel, when the condition (a) is satisfied, Ni compounds (metal nickel, nickel hydroxide, nickel oxide, nickel chloride, nickel sulfate, etc.) Or two or more of these compounds) was found to be able to improve the phosphatability by adhering to the steel sheet surface. Furthermore, as a result of examining the optimum amount and form of Ni deposition, the Ni compound should be 1 mg / m ² or more and 200 mg / m ² or less in terms of Ni, and the atomic ratio of Ni to Fe on the steel sheet surface (Ni / Fe) It has been found that good phosphate treatment properties can be obtained when the ratio is 0.08 or more and 2 or less. Therefore, the amount of Ni compound is limited to 1 mg / m ² or more and 200 mg / m ² or less in terms of Ni, and the atomic ratio of Ni to Fe (Ni / Fe) on the steel sheet surface is limited to 0.08 or more and 2 or less.

(C) S compound amount: 1 mg / m ² or more and 100 mg / m ² or less in terms of S, atomic ratio of S to Fe: 0.05 or more, 1.5 or less Further, the present inventors are substances that improve phosphate processability As a search for compounds other than Ni compounds. As a result, after satisfying the above condition (a), an S compound (thioglycolic acid, sulfate, FeS, or the like, or two or more of these compounds) is adhered to the surface of the steel sheet, thereby improving the phosphate processability. Succeeded to improve. As a result of examining the optimum S deposition amount and deposition form, the S compound is 1 mg / m ² or more and 100 mg / m ² or less in terms of S, and the atomic ratio of S to Fe on the steel sheet surface (S / Fe) is 0.05. As described above, it has been found that good phosphate treatment properties can be obtained when the ratio is 1.5 or less. Therefore, the amount of S compound is limited to 1 mg / m ² or more and 100 mg / m ² or less in terms of S, and the atomic ratio of S to Fe (S / Fe) on the steel sheet surface is limited to 0.05 or more and 1.5 or less.

  In addition, the steel plate surface in the above (a) to (c) indicates a range measured by a method generally recognized as a surface sensitive analysis method such as FX, XPS or EPMA.

  Moreover, as a composition of the high strength steel used as a raw material, the following conditions are desirable.

(D) Si: 0.5 mass% or more, 3 mass% or less
Si strengthens steel by solid solution strengthening, stabilizes austenite, and has an action of promoting the formation of residual austenite phase. Such an effect is recognized when the Si content is 0.5 mass% or more. On the other hand, when it contains exceeding 3 mass%, ductility will deteriorate. For this reason, Si is limited to the range of 0.5 mass% or more and 3 mass%. More preferably, it is 1 mass% or more and 2.5 mass% or less.

  Other components can appropriately contain the following components in accordance with required properties of steel.

(E) C: 0.05 mass% or more, 0.35 mass% or less
C is an indispensable element for increasing the strength of steel, and further has an effect on the formation of retained austenite and a low-temperature transformation phase, and is an indispensable element. However, if the C content is less than 0.05 mass%, the desired high strength cannot be obtained. On the other hand, if the C content exceeds 0.35 mass%, the weldability is deteriorated. For this reason, C is limited to the range of 0.05 mass% or more and 0.35 mass% or less. More preferably, it is 0.1 mass% or more and 0.2 mass% or less.

(F) Mn: 0.5 mass% or more, 3 mass% or less
Mn strengthens the steel by solid solution strengthening, improves the hardenability of the steel, and has an action of promoting the formation of retained austenite and a low-temperature transformation phase. Such an effect is observed when the Mn content is 0.5 mass% or more. On the other hand, even if the content exceeds 3 mass%, the effect is saturated, and an effect commensurate with the content cannot be expected, resulting in an increase in cost. For this reason, Mn is limited to the range of 0.5 mass% or more and 3.0 mass% or less. More preferably, it is 1 mass% or more and 2 mass% or less.

(G) P: 0.05 mass% or less
P is a solid solution strengthening element, which is usually an element effective for obtaining a high-strength steel sheet, but if it exceeds 0.05 mass%, spot weldability is reduced, so the upper limit is 0.05 mass% or less. To do. A more preferable range is 0.02 mass% or less.

(H) S: 0.005 mass% or less
S is an impurity element that forms MnS in the steel and lowers the stretch flangeability of the steel sheet. For this reason, the S content is limited to 0.005 mass% or less. A more preferable range is 0.003 mass% or less.

  The balance is Fe and inevitable impurities. Inevitable impurities include elements such as Ca, Zr, and Mg that are expected to be mixed before the steelmaking process, or alloy elements that do not adversely affect the phosphate treatment, and do not cause problems in toughness and phosphate treatment. Allowed in range.

  Next, an example of the manufacturing method of the high strength steel plate according to the present invention will be described.

  First, a steel having a component appropriately selected from the range (d) to (h) above is made into a slab by continuous casting, and then the slab is heated to 1170 ° C or lower, and finished at Ar3 point or higher and Ar3 point + 100 ° C or lower. Rolling was performed, and cooling was performed at an average cooling rate of 20 ° C./s or more to a temperature of 400 to 650 ° C., and a rolled hot-rolled steel sheet was obtained. And after subjecting the obtained hot-rolled steel sheet to cold rolling at a reduction rate of 30% or more and 60% or less, after further heating to 700 ° C or more and holding for 30 seconds or more, to a temperature of 300 ° C or more and 480 ° C or less Cool at a cooling rate of 10 ° C / s or more, hold at that temperature range for 60 seconds or more and 600 seconds or less, and cool to 50 ° C or less at a rate of 30 ° C / s or more to obtain a cold-rolled steel sheet.

  Thereafter, the amount of Si-based oxide on the surface of the cold-rolled steel sheet was changed by pickling the steel sheet for 3 to 60 seconds using a 10 mass% hydrochloric acid aqueous solution. In addition to the adjustment of the amount of Si-based oxide on the surface of the steel plate, mechanical grinding and removal of the surface of the steel plate, such as pickling in general and shot blasting, can be mentioned.

Similarly to the above (a), the value of the Si-based oxide amount on the steel sheet surface was obtained in terms of SiO ₂ using a fluorescent X-ray analyzer (hereinafter referred to as FX).

  After confirming whether the condition (a) is satisfied, a Ni compound is adhered to the steel sheet surface. As a method for attaching the Ni compound, within the range studied by the inventors, Ni electroplating is applied to the steel sheet, and an aqueous solution of Ni compound (nickel chloride, nickel sulfate, etc.) is applied and dried. It was. In these methods, as long as the Ni adhesion amount was the same, the same effect could be obtained regardless of the adhesion method.

  Finally, the amount of Ni compound on the steel sheet surface is confirmed. For the determination of the amount of Ni compound, FX was used as in the case of the Si-based oxide. Specifically, the X-ray intensity of Ni before and after pickling was measured, and the adhesion amount of Ni compound was determined from the difference. A 12 mass% nitric acid-2 mass% hydrochloric acid aqueous solution was used for the pickling solution. The atomic ratio of Ni to Fe (Ni / Fe) on the steel sheet surface was determined using an X-ray photoelectron spectrometer (hereinafter referred to as XPS). AXIS-HS from KRATOS is used for XPS, Al Kα radiation monochromatized with a monochromator is used for the X-ray source, and the steel sheet outermost surface cleaned with acetone without pretreatment such as sputtering is used. Measured as is. From the measurement results, the bottom of the peak was connected by a straight line, the background was removed, the integrated intensity of the Ni and Fe peaks was corrected with the relative sensitivity coefficient, and the atomic ratio of Ni to Fe was obtained.

  Or after satisfy | filling the conditions of said (a), S compound is made to adhere to the steel plate surface. As a method of attaching the S compound, within the range examined by the inventors, a method such as immersing or applying an aqueous solution to an aqueous solution of thioglycolic acid or thiourea was possible. In these methods, as long as the S deposition amount was the same, the same effect could be obtained regardless of the deposition method.

  Finally, the amount of S compound on the steel sheet surface is confirmed. For the determination of the amount of S compound, FX was used as in the case of the Si-based oxide. Specifically, the X-ray intensity of S before and after pickling was measured, and the adhesion amount of the S compound was determined from the difference. A 10 mass% hydrochloric acid aqueous solution was used for the pickling solution. Further, the atomic ratio of S to Fe (S / Fe) on the steel sheet surface was calculated according to the calculation method of the Ni / Fe atomic ratio.

  In the description of the present embodiment, the amount of Si-based oxide, the amount of Ni compound, and the amount of Si compound were determined as FX, and the Ni / Fe atomic ratio and Si / Fe atomic ratio were determined as XPS, respectively. It is not limited to. It may be determined appropriately in consideration of the accuracy required for each value, the measurement limit of the analysis method, the time required for analysis, or the cost of the analyzer.

  The case where a Ni compound or an S compound is adhered to the surface of the cold rolled steel sheets A to C in Table 1 will be described as an example.

  A slab having the components shown in Table 1 manufactured by the converter melting and continuous casting processes was used as a cold-rolled steel sheet according to the manufacturing conditions shown in Table 2. Further, the amount of Si-based oxide on the surface of the cold-rolled steel sheet was changed by pickling the cold-rolled steel sheet for 3 to 60 seconds using a 10 mass% hydrochloric acid aqueous solution. Whether or not the amount of the Si-based oxide on the surface of the cold-rolled steel sheet after pickling is within the range of the invention was determined using a fluorescent X-ray analyzer (FX) as in the embodiment.

  After this judgment, if the amount of Si-based oxide on the surface was within the range of the invention, Ni electroplating or thioglycolic acid aqueous solution was applied to the cold-rolled steel sheet and dried. Ni electroplating was performed under the conditions of a treatment liquid temperature of 50 ° C. using a mixed aqueous solution of sulfuric acid Ni (240 g / L) and boric acid (30 g / L) (pH adjusted to 3.6 with sulfuric acid) as a treatment liquid. The amount of adhesion was controlled by the electrolysis time, and the Ni / Fe ratio was controlled by the current density. Moreover, the thioglycolic acid aqueous solution treatment was performed by spraying the thioglycolic acid aqueous solution (1.0 to 5.0 g / L) in the form of a mist on the surface of the cold-rolled steel sheet and then drying it. The adhesion amount was controlled by the coating amount of the treatment liquid, and the S / Fe ratio was controlled by changing the particle size of the mist. The adhesion amount of Ni and S, the Ni / Fe ratio, and the S / Fe ratio were measured by the methods described above.

  After attaching the prescribed Ni compound or S compound to the surface, degrease the cold-rolled steel sheet with SP-250 made by Nippon Paint Co., Ltd. Phosphate treatment was carried out with SD-2800 manufactured.

  The samples prepared as described above were evaluated in the same manner as the evaluation of phosphate treatment in the Si-based oxide shown in the embodiment. Tables 4 and 5 show the conditions and evaluation results for each sample. Table 4 shows the results when Ni compounds were deposited, and Table 5 shows the results when S compounds were deposited. From these results, it became clear that the high-strength steel plates within the scope of the present invention examples show good phosphatability even when Si is added.

Claims (3)

A high-strength steel sheet excellent in phosphatability, characterized in that the amount of Si-based oxide on the steel sheet surface is 20 mg / m ² or less in terms of SiO ₂ . The amount of Si-based oxide on the steel sheet surface is 20 mg / m ² or less in terms of SiO ₂ , and the amount of Ni compound on the steel sheet surface is 1 mg / m ² or more and 200 mg / m ² or less in terms of Ni, and Ni against Fe on the steel sheet surface A high-strength steel sheet excellent in phosphate treatment, characterized by having an atomic ratio of 0.08 or more and 2 or less. The amount of Si-based oxide on the steel sheet surface is 20 mg / m ² or less in terms of SiO ₂ , and the amount of S compound on the steel sheet surface is 1 mg / m ² or more and 100 mg / m ² or less in terms of S, and the amount of S relative to Fe on the steel sheet surface A high-strength steel sheet excellent in phosphate processability characterized by an atomic ratio of 0.05 or more and 1.5 or less.