TWI577808B - Steel plate - Google Patents

Description

Steel plate Technical field

The present invention relates to a steel sheet which can attain excellent chemical treatment.

Background technique

In recent years, in the automotive field, the demand for high-strength cold-rolled steel sheets for vehicles or parts has been reduced for the purpose of reducing the fuel consumption or the reduction of CO ₂ emissions. improve.

Like the soft steel sheet, the high-strength cold-rolled steel sheet can be formed in a large amount and inexpensively by press working, and supplied as various members. Therefore, high-strength cold-rolled steel sheets also require high ductility and good processability. In addition, in general, high-strength cold-rolled steel sheets are subjected to chemical treatment such as zinc phosphate treatment for the purpose of improving corrosion resistance or coating film adhesion. In the chemical treatment, for example, a zinc phosphate film of 2 g/m ² to 3 g/m ² is formed. A Zr-based film is sometimes formed by chemical treatment. Further, cationic electrodeposition coating is often performed on the coatings (chemical treatment layers). When cationic electrodeposition coating is applied, the surface of the chemical treatment layer is exposed to strong alkalinity. Therefore, the chemical treatment layer should have alkali resistance. The indicator indicating the alkali resistance is a parameter called a P ratio. The phosphate contained in the chemical treatment layer may, for example, be a zinc-zinc ore composed of Zn-PO; and a phosphorite composed of Zn-Fe-PO. Phosphorus is a reaction product of Fe and zinc phosphate in a steel sheet. The P ratio can be derived from the peak intensity of the X-ray diffraction device. The peak intensity of the zinc phosphate is manifested at a diffraction angle of 2 θ = 14.5 °, and the peak intensity of the phosphorite appears at a diffraction angle of 2 θ = 14.88 °. When the X-ray peak intensity in 14.55° is H and the X-ray peak intensity in 14.88° is P, the P ratio is represented by “P/(P+H)”. Phosphite exhibits excellent alkali resistance compared to phosphozinc. Therefore, the higher the P ratio, the higher the alkali resistance can be obtained.

In general, the higher the content of Si and Mn, the easier it is to obtain high ductility and good processability. However, Si and Mn contained in steel are easily oxidized. Therefore, if a steel containing a large amount of Si and Mn is to be used to produce a high-strength cold-rolled steel sheet, Si and Mn are oxidized during the annealing of the process, and an oxide is formed on the surface of the high-strength cold-rolled steel sheet. The oxide formed on the surface lowers chemical handleability and corrosion resistance.

Therefore, if high ductility and good workability are required to increase the content of Si and Mn, it is difficult to obtain good chemical treatment and corrosion resistance. For example, the zinc phosphate coating film is formed by crystallization of zinc phosphate. However, when the chemical treatment property is low, zinc phosphate is less likely to adhere to the surface of the steel sheet, and a portion where the chemical treatment layer is not formed may be generated. Further, by the oxide, the reaction between Fe and zinc phosphate in the steel sheet is hindered, and it is difficult to form the phosphorite, and sufficient alkali resistance may not be obtained. As a result of these, the cationic electrodeposition coating was not performed properly after the chemical treatment, and good corrosion resistance could not be obtained.

In the past, various proposals have been made for the purpose of improving chemical resistance, corrosion resistance, or both (Patent Documents 1 to 9). However, in the prior art, it is difficult to sufficiently improve the chemical treatment property, or even if the chemical treatment property is improved, the corrosion resistance is also lowered, or the tensile strength or the fatigue strength is lowered.

Advanced technical literature Patent literature

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2004-323969

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2009-221586

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2010-47808

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2010-53371

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2012-122086

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2008-121045

[Patent Document 7] Japanese Patent Laid-Open Publication No. 2005-307283

[Patent Document 8] Japanese Patent Laid-Open Publication No. 2010-90441

[Patent Document 9] Japanese Patent Laid-Open No. 4-247849

Summary of invention

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel sheet which can avoid a decrease in corrosion resistance and a decrease in strength, and which can attain excellent chemical treatment.

The inventors conducted a keen review in order to solve the above problems. As a result, the following matters were identified.

(a) The oxides present on the surface of a steel sheet containing a large amount of Si and Mn are cerium oxide and manganese ruthenate.

(b) Although manganese citrate can be easily removed by acid which does not cause pitting corrosion of the steel sheet, cerium oxide cannot be removed by acid which does not cause pitting corrosion of the steel sheet.

(c) The cerium oxide remaining after pickling can be roughly distinguished as a denser and a porous one.

(d) Dense cerium oxide has excellent chemical handling properties compared to manganese ruthenate and porous cerium oxide.

(e) Even if porous ceria is left, by performing electroplating of Ni, the porous ceria is covered with Ni to improve chemical treatment.

Based on this knowledge, the inventors further reviewed the results of the review and found various aspects of the invention shown below.

(1) A steel sheet characterized by having a chemical composition represented by % by weight: C: 0.050% to 0.400%; Si: 0.10% to 2.50%; Mn: 1.20% to 3.50% ; P: 0.100% or less; Al: 1.200% or less; N: 0.0100% or less; Cr, Mo, Ni, and Cu: 0.00% to 1.20% in total; Nb, Ti, and V: 0.000% to 0.200% in total; : 0.0000%~0.0075%; Ca, Mg, Ce, Hf, La, Zr, Sb and REM: a total of 0.0000% to 0.1000%; and the remainder: Fe and impurities; further, the surface is made by using high-sensitivity reflection the Fourier transform infrared spectroscopy, within a number of 1200cm ^-1 ~ 1300cm ^-1 wave number range on the display reflectance of 50% or more, 85% or less of the absorption peak, and in the 1000cm ^-1 ~ 1100cm ^-1 wavenumber range absorption peak is not displayed, or display of the reflectance of 85% or more absorption peaks in the wave number range of 1000cm ^-1 ~ 1100cm ^-1, and adhered 3mg / m on the surface ² ~ 100mg / m Ni ² of.

(2) The steel sheet according to (1), wherein the surface is a reflectance of 60% or more in a wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by Fourier transform infrared spectroscopy using a high-sensitivity reflection method, Absorption peak below 85%.

According to the present invention, excellent chemical treatment properties can be obtained even if the treatment such as reduction in corrosion resistance and reduction in strength is not performed.

Fig. 1 is a graph showing a particularly good degree of adhesion of zinc phosphate crystals.

Fig. 2 is a graph showing a good degree of adhesion of zinc phosphate crystals.

Fig. 3 is a graph showing a poor adhesion of zinc phosphate crystals.

Form for implementing the invention

Hereinafter, embodiments of the present invention will be described.

First, the chemical composition of the steel sheet according to the embodiment of the present invention and the steel used for the production thereof will be described. As described later, the steel sheet according to the embodiment of the present invention is produced by hot rolling of steel, pickling after hot rolling, cold rolling, annealing, pickling after annealing, plating, and the like. Therefore, the chemical composition of steel sheets and steels will take into account these treatments and not only the characteristics of the steel sheets. In the following description, the content "%" of each element contained in the steel sheet means "% by mass" unless otherwise stated. The steel sheet according to the present embodiment has a chemical composition represented by the following: C: 0.050% to 0.400%; Si: 0.10%~2.50%; Mn: 1.20%~3.50%; P: 0.100% or less; Al: 1.200% or less; N: 0.0100% or less; Cr, Mo, Ni and Cu: 0.00% to 1.20% in total; Nb, Ti and V: 0.000% to 0.200% in total; B: 0.0000% to 0.0075%; Ca, Mg, Ce, Hf, La, Zr, Sb and rare earth metal (REM): a total of 0.0000%~ 0.1000%; the remainder: Fe and impurities. The impurities may be exemplified by those contained in raw materials such as ore or scrap, and included in the production steps.

(C: 0.050%~0.400%)

C is an element that forms the hard tissue of the Ma Tian bulk, the tempered Ma Tian bulk, the tough body and the residual Worth field and enhances the strength of the steel sheet. If the C content is less than 0.050%, the effect of utilizing this effect cannot be sufficiently obtained. Therefore, the C content is 0.050% or more. In order to obtain higher strength, the C content is preferably 0.075% or more. On the other hand, if the C content is more than 0.400%, sufficient weldability cannot be obtained. Therefore, the C content is 0.400% or less.

(Si: 0.10%~2.50%)

Si is an element that ensures good processability and enhances strength. If the Si content is less than 0.10%, the effect of utilizing this action cannot be sufficiently obtained. Therefore, the Si content is 0.10% or more. In order to secure good workability and obtain higher strength, the Si content is preferably 0.45% or more, and more preferably 0.86% or more. On the other hand, when the Si content is more than 2.50%, the toughness is lowered, and the workability is deteriorated. Therefore, the Si content is 2.50% or less.

(Mn: 1.20%~3.50%)

Like Si, Mn is an element that ensures good processability and enhances strength. If the Mn content is less than 1.20%, the effect of utilizing the effect cannot be sufficiently obtained. fruit. Therefore, the Mn content is 1.20% or more. In order to ensure good processability and obtain higher strength, the Mn content is preferably 1.50% or more. On the other hand, if the Mn content is more than 3.50%, sufficient weldability cannot be obtained. Therefore, the Mn content is 3.50% or less.

(P: 0.100% or less)

P is not an essential element and is contained as an impurity, for example, in steel. From the viewpoint of workability, weldability, and fatigue characteristics, the P content is preferably as low as possible. In particular, if the P content is more than 0.100%, the workability, weldability, and fatigue characteristics are significantly reduced. Therefore, the P content is made 0.100% or less.

(Al: 1.200% or less)

Al is not an essential element and is contained as an impurity, for example, in steel. From the viewpoint of workability, the lower the Al content, the better. In particular, if the Al content is more than 1.200%, the workability is markedly lowered. Therefore, the Al content is made 1.200% or less.

(N: 0.0100% or less)

N is not an essential element and is contained as an impurity, for example, in steel. From the viewpoint of workability, the lower the N content, the better. In particular, if the N content is more than 0.0100%, the workability is markedly lowered. Therefore, the N content is made 0.0100% or less.

(Cr, Mo, Ni, and Cu: totaling 0.00% to 1.20%)

Cr, Mo, Ni and Cu contribute to the further improvement of the strength of the steel sheet. Therefore, it may contain Cr, Mo, Ni or Cu, or any combination of these. However, if the total content of Cr, Mo, Ni, and Cu is more than 1.20%, the effect is saturated and the cost is increased. Moreover, if the total content of Cr, Mo, Ni, and Cu is greater than 1.20%, When the casting occurs, the slab is broken, and sometimes it cannot be made into a steel sheet. Therefore, the total content of Cr, Mo, Ni, and Cu is 1.20% or less.

(Nb, Ti, and V: 0.000% to 0.200% in total)

Nb, Ti and V contribute to the further improvement of the strength of the steel sheet. Therefore, it may also contain Nb, Ti or V, or any combination of these. However, if the total content of Nb, Ti, and V is more than 0.200%, the effect is saturated and the cost is increased. Moreover, when the total content of Nb, Ti, and V is more than 0.200%, sufficient weldability may not be obtained. Therefore, the total content of Nb, Ti, and V is 0.200% or less.

(B: 0.0000%~0.0075%)

B contributes to the further improvement of the strength of the steel sheet. Therefore, it can also contain B. However, if the B content is more than 0.0075%, the effect is saturated and the cost is increased. Further, when the B content is more than 0.0075%, the cast piece may be broken during casting, and the steel sheet may not be produced. Therefore, the B content is 0.0075% or less.

(Ca, Mg, Ce, Hf, La, Zr, Sb, and REM: totaled from 0.0000% to 0.1000%)

Ca, Mg, Ce, Hf, La, Zr, Sb, and REM contribute to the improvement of the formability of the steel sheet. Therefore, it may contain Ca, Mg, Ce, Hf, La, Zr, Sb or REM, or any combination of these. However, if the total content of Ca, Mg, Ce, Hf, La, Zr, Sb, and REM is more than 0.1000%, the effect is saturated and the cost is increased. Further, when the total content of Ca, Mg, Ce, Hf, La, Zr, Sb, and REM is more than 0.1000%, the cast piece may be broken during casting, and the steel sheet may not be produced. Therefore, the total content of Ca, Mg, Ce, Hf, La, Zr, Sb, and REM is 0.1000% or less.

REM refers to a total of 17 elements of Sc, Y and lanthanides. The content of REM means the total content of the 17 elements. Lanthanides are industrially added, for example, as rare earth metal alloys.

Next, the surface of the steel sheet according to the embodiment of the present invention will be described. The surface of the steel sheet according to the present embodiment exhibits a reflectance of 50% or more and 85% or less in a wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by Fourier transform infrared spectroscopy using a high-sensitivity reflection method. It is desirable to have an absorption peak of 60% or more and 85% or less. Further, the surface morphology of the steel sheet relating to the present embodiment in the number of 1000cm ^-1 ~ 1100cm ^-1 absorption peak wave range is not displayed, or to display more than a reflectance of 85% within the wave number range of ^-1 ~ 1100cm ^-1 1000cm The absorption peak. Further, Ni is contained in the range of 3 mg/m ² to 100 mg/m ² on the surface of the steel sheet according to the present embodiment.

As described above, the steel sheet according to the present embodiment is produced by hot rolling of steel, pickling after hot rolling, cold rolling, annealing, pickling after annealing, plating of Ni, and the like. At the time of annealing, an oxide is formed on the surface of the cold-rolled steel sheet obtained by cold rolling, and an oxide is present on the surface of the annealed steel sheet obtained by annealing. This is because Si and Mn are substances which are easily oxidized, and therefore, Si and Mn are selectively oxidized in the vicinity of the surface of the cold-rolled steel sheet. The oxide is cerium oxide and manganese citrate. Since manganese ruthenate is easily dissolved in an acid, it can be easily removed by an acid which does not cause pitting corrosion, however, cerium oxide cannot be removed by acid which does not cause pitting corrosion of a cold-rolled steel sheet. Therefore, if the acid is washed by annealing using such an acid, part or all of the manganese citrate can be removed, and cerium oxide remains. The cerium oxide present after pickling after annealing can be roughly distinguished as a denser and a porous one. If Ni is attached to the annealed steel sheet by electroplating in the presence of dense ruthenium dioxide and porous ruthenium dioxide, Porous ceria will be covered by Ni. Ni is also attached to the portion of the annealed steel sheet where the cerium oxide is not present, that is, the surface of the base material. Therefore, cerium oxide is present on the surface of the steel sheet according to the present embodiment, and Ni adheres to the surface of the cerium oxide and the base material.

Manganese ruthenate hinders chemical handling and is easily dissolved in acidic ambient gases. Moreover, manganese citrate has a low barrier to corrosion factors. Therefore, if a large amount of manganese ruthenate is present on the surface of the steel sheet, good chemical treatment properties cannot be obtained, and since the chemical treatment layer cannot be formed properly, good corrosion resistance cannot be obtained. Cerium oxide can be roughly distinguished from those of dense and porous. The dense cerium oxide has good chemical treatment and excellent barrier to corrosion factors. The barrier property of the porous ceria to the corrosion factor is lower than that of the dense ceria to the corrosion factor. However, by attaching Ni to the porous ceria by electroplating, good chemical treatment can be obtained.

The absorption peak revealed by the Fourier transform-infrared spectroscopy (FT-IR) analysis using the reflection absorption spectrometry (RAS) method in the range of 1200 cm ^-1 to 1300 cm ^-1 is two The presence of cerium oxide. As described above, in the production of the steel sheet according to the present embodiment, cerium oxide and manganese ruthenate are formed during annealing, and some or all of manganese citrate is removed by pickling after annealing, however, in order to suppress the occurrence of pitting corrosion , there will be residual cerium oxide. Therefore, in the present embodiment, cerium oxide is present on the surface of the steel sheet, and the surface exhibits an absorption peak in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 . The reflectance in the wave number indicating the absorption peak indicates how much cerium oxide exists, and the lower the reflectance, the higher the absorption rate of infrared ray, and the presence of many cerium oxide. Further, if the reflectance is less than 50%, the cerium oxide is excessively present, and the porous cerium oxide is not sufficiently covered by Ni, and good chemical treatment property cannot be obtained. On the other hand, in order to make the reflectance greater than 85%, it is necessary to reduce the amount of cerium oxide generated during annealing or to increase the amount of cerium dioxide removed during pickling after annealing. In order to reduce the amount of cerium oxide generated during annealing, it is necessary to increase the dew point in the furnace during annealing, and it is possible to produce significant decarburization and reduce tensile strength and fatigue strength. In order to increase the amount of removal of cerium oxide, strong pickling must be performed, and significant pitting corrosion is caused to reduce bending workability. That is, if the reflectance is more than 85%, the desired mechanical properties cannot be obtained. Therefore, the surface of the steel sheet is formed by the FT-IR analysis by the RAS method, and the reflectance is 50% or more and 85% or less in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 , and more preferably 60% or more. , 85% or less of the absorption peak. Hereinafter, the "FT-IR analysis using the RAS method" may be simply referred to as "FT-IR analysis".

Analysis by FT-IR absorption peaks appearing in the wave 1000cm ^-1 ~ 1100cm ^-1 indicating the presence of a range of the number of manganese silicate. Since manganese ruthenate reduces chemical handleability, the less the better. Therefore, should the surface of the steel sheet by FT-IR analysis is not shown in the wave 1000cm ^-1 ~ 1100cm ^-1 the number range of the absorption peak. Shows the absorption peaks, as long as the reflectance is more than the absorption peak of the wave number in the range of 85% even though the number of display made on the wave 1000cm ^-1 ~ 1100cm ^-1, the silicate may be a small amount of manganese allowable amount. On the other hand, if the reflectance of the display wavenumber of the absorption peak appearing in the range of wave number of 1000cm ^-1 ~ 1100cm ^-1 of less than 85%, the manganese silicate is present excessively, and can not achieve good properties of chemical treatment, Further, since the chemical treatment layer cannot be formed properly, good corrosion resistance cannot be obtained. Therefore, it is the surface of a steel sheet made by FT-IR analysis in the number of 1000cm ^-1 ~ 1100cm ^-1 wave number range not shown absorption peaks, or reflectivity of the display 85 in the wavenumber range of ^-1 ~ 1100cm ^-1 1000cm Absorption peak above %.

Ni adhered to the surface of the steel sheet according to the present embodiment covers the porous cerium oxide to improve chemical treatment. If the adhesion amount of Ni is less than 3 mg/m ² , sufficient chemical handleability cannot be obtained. Therefore, the adhesion amount of Ni is 3 mg/m ² or more. In order to obtain more excellent chemical treatment properties, the adhesion amount of Ni is preferably 10 mg/m ² or more, and more preferably 40 mg/m ² or more. On the other hand, if the adhesion amount of Ni is more than 100 mg/m ² , the precious Ni is excessive and the sufficient corrosion resistance cannot be obtained as compared with Fe which is a main component of the steel sheet. Therefore, the adhesion amount of Ni is 100 mg/m ² or less. In order to obtain more excellent corrosion resistance, the adhesion amount of Ni is preferably 50 mg/m ² or less. Ni does not need to cover the entire porous cerium oxide, and does not need to cover the entire portion of the base material exposed from cerium oxide.

The amount of Ni attached can be measured using a fluorescent X-ray analyzer. For example, the X-ray intensity can be measured by using a sample having a known adhesion amount of Ni in advance, and a calibration curve showing the relationship between the adhesion amount of Ni and the X-ray intensity can be used, and the calibration curve can be used for self-determination. The X-ray intensity in the steel sheet of the object specifies the amount of Ni attached.

Next, a method of manufacturing a steel sheet according to an embodiment of the present invention will be described. In this method, hot rolling of a steel having the above chemical composition, pickling after hot rolling, cold rolling, annealing, pickling after annealing, and plating of Ni are performed.

Hot rolling, pickling after hot rolling and cold rolling can be carried out by general conditions. get on.

Annealing after cold rolling is carried out by forming cerium oxide and manganese cerium on the surface of a cold-rolled steel sheet obtained by cold rolling, and internal oxidation is less likely to occur. Annealing is preferably carried out by continuous annealing. By adjusting the amount of cerium oxide generated during annealing, the surface of the steel sheet according to the present embodiment can be controlled to exhibit the wave number of the absorption peak appearing in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis. Reflectivity. The amount of cerium oxide formed during annealing can be controlled, for example, by adjusting the temperature of the annealing and the ambient gas. The higher the annealing temperature, the more cerium oxide is formed. The annealed ambient gas is preferably controlled by adjusting the oxygen potential in the N ₂ ambient gas containing oxygen atoms (O). The higher the oxygen potential, the more cerium oxide is formed, and the absorption rate of infrared rays is increased and the reflectance is lowered. There is no particular limitation on the method of adjusting the amount and reflectance of cerium oxide. When manufacturing a steel sheet, it is preferable to investigate in advance the conditions of the desired amount of cerium oxide formation, that is, the reflection in the wave number of the absorption peak which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis. The ratio is 50% or more and 85% or less, and more preferably 60% or more and 85% or less, and this condition is employed. For example, in an N ₂ ambient gas having an O ₂ concentration of 50 ppm or less, if the H ₂ concentration is 3% and the dew point is less than -35 ° C or more than -20 ° C, the reflectance is liable to lower.

If the oxygen potential is too high, the cerium oxide is not easily formed on the surface of the cold-rolled steel sheet, and internal oxidation is performed. Therefore, absorption which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis is exhibited. The reflectance in the wavenumber of the peak increases. When internal oxidation is performed, the decrease in tensile strength accompanying decarburization and the decrease in fatigue strength become remarkable. The degree of decarburization can be confirmed by the thickness of the decarburized layer. For example, when the area fraction of the hard tissue having a thickness of 1/4 of the steel sheet is S1 and the area fraction of the hard structure of the surface layer of the steel sheet is S2, the ratio S2/S1 can be 0.40. The maximum depth of the above part is regarded as the thickness of the decarburized layer. In order to avoid a decrease in tensile strength and a decrease in fatigue strength, the thickness of the decarburized layer is preferably 3 μm or less. The term "hard tissue" as used herein refers to a granulated loose body, a tempered granulated loose body, a toughened body or a residual Worth field body, or a structure composed of any combination of the above. For example, in an N ₂ ambient gas having an O ₂ concentration of 50 ppm or less, if the H ₂ concentration is 3% and the dew point is greater than -10 ° C, decarburization is remarkable, and the value of S 2 /S 1 is less than 0.40.

As by "H ₂ O The equilibrium formula of H ₂ +1/2(O ₂ )” also shows that the higher the O ₂ concentration in the annealing furnace, the higher the H ₂ O concentration or the lower the H ₂ concentration, the more the oxygen potential in the annealing furnace will be. high. The H ₂ O concentration is sometimes expressed by the water vapor concentration or dew point.

After annealing, some or all of the manganese citrate produced in the annealing is removed by pickling after annealing. After the remaining amount of manganese silicate by adjusting the annealing after pickling, the steel sheet may be related to the surface morphology of the control of the present embodiment displays wave by FT-IR analysis in the wave number range of 1000cm ^-1 ~ 1100cm ^-1 of the absorption peaks appearing The reflectivity in the number. The amount of residual manganese ruthenate can be controlled, for example, by adjusting the conditions of pickling after annealing. The higher the acid concentration, the higher the acid temperature, and the longer the time that the annealed steel sheet is in contact with the acid, the less manganese manganeseate will be. In the pickling after annealing, for example, the surface of the annealed steel sheet is maintained in a state of being wetted by hydrochloric acid at a concentration of 3.0% by mass to 6.0% by mass and a temperature of 50 ° C to 60 ° C for 3 seconds to 10 seconds. The annealed steel sheet can be obtained by immersing the annealed steel sheet in hydrochloric acid in a state of being wetted with hydrochloric acid, or by spraying hydrochloric acid on an annealed steel sheet. If the concentration of hydrochloric acid is less than 3.0% by mass, manganese ruthenate is not easily dissolved. Therefore, the concentration of hydrochloric acid is preferably 3.0% by mass or more. If the concentration of hydrochloric acid is more than 6.0% by mass, fine pitting will occur on the surface of the annealed steel sheet. Therefore, the concentration of hydrochloric acid is preferably 6.0% by mass or less. If the temperature of hydrochloric acid is less than 50 ° C, manganese citrate is not easily dissolved. Therefore, the temperature of hydrochloric acid is preferably 50 ° C or more. If the temperature of the hydrochloric acid is more than 60 ° C, fine pitting will occur on the surface of the annealed steel sheet. Therefore, the temperature of hydrochloric acid is preferably 60 ° C or less. If the time of wetting by hydrochloric acid is less than 3 seconds, manganese citrate is not easily dissolved. Therefore, the time should be more than 3 seconds. If the time is more than 10 seconds, fine pitting will occur on the surface of the annealed steel sheet. Therefore, the time is 10 seconds or less. The pickling after annealing is preferably carried out by removing the manganese hydride formed in the annealing and not easily annealing the steel sheet to cause pitting corrosion, and is not limited to the above examples. Even if pitting corrosion occurs, the number of pittings having a depth of 1 μm or more may be 5 or less in a field of view having an arbitrary cross-sectional width of 100 μm. If there are more than five pittings having a depth of 1 μm or more and a field of view having a cross-sectional width of 100 μm, sufficient corrosion resistance cannot be obtained or sufficient fatigue strength cannot be obtained. The acid used for pickling after annealing is not limited to hydrochloric acid. Further, the smaller the amount of manganese silicate, show the reflectance by wave number of FT-IR analysis in the wave number range of 1000cm ^-1 ~ 1100cm ^-1 of the absorption peaks appearing in the greater will be, when there is not the manganese silicate At this time, the absorption peak does not appear in this range. There is no particular limitation on the method of adjusting the amount of manganese citrate and the reflectance. In manufacturing the steel sheet, it also includes the kind of acid, the investigation should be less likely to occur in advance in annealed steel pitting and manganese silicate constituting the range of desired conditions, i.e., by FT-IR analysis at 1000cm ^-1 ~ 1100cm ^-1 The absorption peak does not appear in the wave number range, or even if it appears, the reflectance in the wave number of the absorption peak is set to be 85% or more, and this condition is employed.

After pickling after annealing, Ni is attached to the surface of the annealed steel sheet by electroplating. As a result, the porous cerium oxide is covered by Ni. As the treatment liquid used for the electroplating, for example, a general treatment liquid such as a nickel sulfate aqueous solution, a nickel chloride aqueous solution, or a nickel carbonate aqueous solution can be used. The amount of Ni attached can be adjusted, for example, by changing the concentration of the treatment liquid and the current density at the time of plating. As described above, Ni does not need to cover the entire porous ceria, and does not need to cover the entire portion of the base material exposed from the cerium oxide.

According to this, a steel sheet according to an embodiment of the present invention can be produced.

There is no particular limitation on the use of the steel sheet according to the embodiment of the present invention. For example, it is preferable to form it by press processing or the like, and then apply it by chemical treatment such as zinc phosphate treatment. More desirably, it is used by performing electrodeposition coating on the chemical treatment layer formed by chemical treatment.

The above-described embodiments are merely examples of the specific embodiments of the present invention, and the technical scope of the present invention is not limited by the terms. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features.

Example

Next, an embodiment of the present invention will be described. The conditions in the examples are a conditional example used to confirm the workability and effects of the present invention, and the present invention is not limited to the one condition example. As long as it does not deviate from the gist of the present invention To achieve the object of the present invention, various conditions can be employed in the present invention.

In this test, a cold rolled steel sheet having a thickness of 1.2 mm was obtained by hot rolling of steel having a chemical composition shown in Table 1, pickling after hot rolling, and cold rolling. The blank column in Table 1 indicates that the content of the element is less than the detection limit, and the remainder is Fe and impurities.

Next, the cold-rolled steel sheet was annealed at a maximum temperature of 820 ° C by a continuous annealing apparatus to obtain an annealed steel sheet. The gas atmosphere in the annealing furnace is made into an N ₂ ambient gas containing H ₂ and water vapor (H ₂ O). Table 2 shows the H ₂ concentration at the time of annealing. The amount of water vapor is managed by the dew point in the furnace shown in Table 2.

Next, the annealed steel sheet is annealed and then pickled. In the pickling after annealing, the three conditions shown in Table 2 were employed. In one condition (weak pickling), hydrochloric acid having a concentration of 5% by mass and a temperature of 60 ° C was sprayed on the annealed steel sheet for 6 seconds, and then washed with water. Under the other conditions (first strong pickling), hydrochloric acid having a concentration of 10% by mass and a temperature of 90 ° C was sprayed on the annealed steel sheet for 20 seconds, and then washed with water. In another condition (second strong pickling), the annealed steel sheet was immersed in hydrochloric acid having a concentration of 2% by mass and a temperature of 70 ° C for 2 seconds, and then washed with water.

Next, Ni is attached to the surface of the annealed steel sheet by electroplating. The plating bath was a nickel sulfate aqueous solution adjusted to have a Ni concentration of 2 g/L. The bath temperature was made at 40 °C. The amount of adhesion of Ni is adjusted by changing the voltage. The amount of Ni attached was measured using a fluorescent X-ray analyzer. Table 2 shows the amount of Ni attached.

According to this, 56 kinds of steel sheets were produced. Further, FT-IR analysis of the surface of the steel sheets was carried out. The FT-IR analysis was performed using a FT-IR6200 type Fourier transform type infrared spectroscopic analyzer manufactured by JASCO Corporation. In the FT-IR analysis, the particular wave number of infrared absorption spectrum of the absorption peak at 1200cm ^-1 of the range of ^-1 ~ ^1300cm, and the absorption peak at 1000cm ^-1 of the range of ^-1 ~ 1100cm, and obtains the display The reflectance in the wavenumber of the absorption peak. Table 2 shows the results. As described above, the reflectance of the display in an absorption peak within a wavenumber of 1300 cm ^-1 ~ Number of wave ^-1 1200cm range is reflected in the amount of silicon dioxide, shown in a number of absorption peaks within 1000cm ^-1 ~ 1100cm ^-1 wave range The reflectance in the wavenumber reflects the amount of manganese citrate. The bottom line in Table 2 indicates that the value is outside the scope of the present invention.

Investigate the pitting corrosion of each steel plate. In the investigation, the vicinity of the surface layer of any cross section of the steel sheet was observed by a scanning electron microscope, and the number of pittings having a depth of 1 μm or more in a field of view having an arbitrary cross-sectional width of 100 μm was examined. Table 3 shows the results.

The thickness of the decarburized layer of each steel sheet was investigated. In this investigation, the area fraction S1 of the hard tissue having a thickness of 1/4 of the thickness of the steel sheet and the area fraction S2 of the hard structure of the surface layer portion were measured, and the ratio S2/S1 was made the thickness of the decarburized layer. In the measurement of the area fraction S1 and the area fraction S2, a plate thickness section parallel to the rolling direction of the steel sheet is formed as an observation surface, and the observation surface is polished and oxidized by the etchant, and the electric field radiation type scanning type is used. Electron microscopy (FE-SEM) was observed at a magnification of 500 to 3000 times. At this time, a line parallel to the plate surface of the steel sheet is drawn, and the total length L of the line and the hard structure is determined, and the ratio L/L0 to the length L0 of the line is made to be the area fraction of the hard tissue at the depth position. . Table 3 shows the results.

The tensile strength, chemical treatment, and corrosion resistance after coating of each steel sheet were also evaluated.

In the evaluation of the tensile strength, the JIS No. 5 test piece was cut out from the steel sheet at a right angle to the rolling direction, and subjected to a tensile test at normal temperature. Moreover, when the tensile strength was 780 MPa or more, it was evaluated as ○, and when it was less than 780 MPa, it was evaluated as ×. Table 3 shows the results.

In the evaluation of the chemical treatment property, first, a test piece of 70 mm × 150 mm was cut out from the steel sheet, and degreasing and chemical treatment of the test piece were performed. In the degreasing, an aqueous solution of a degreasing agent having a concentration of 18 g/L was sprayed on the sample at 40 ° C for 120 seconds, and washed with water. Degreaser is made using Japanese Pakajing (PARKERIZING) company FINE CLEANER E2083. In the chemical treatment, the test piece was immersed in an aqueous solution of a surface treatment agent having a concentration of 0.5 g/L at room temperature for 60 seconds, and immersed in a zinc phosphate treatment agent for 120 seconds, and washed with water and dried. Forming a chemical treatment film. The surface treatment agent was PREPARENE XG manufactured by Paccarat Co., Ltd., and the zinc phosphate treatment agent was PALBOND L3065 manufactured by Paccarat Co., Ltd., Japan.

In addition, the appearance of the chemically-treated film was evaluated by scanning electron microscopy (SEM), and the upper part, the center part, and the lower part of the test piece were observed at a magnification of 1000 times, and the degree of crystallinity of zinc phosphate was observed. In addition, the ratio of the area where the zinc phosphate film was not formed was less than 5 area% was evaluated as ○, and the area of 5 area% or more and less than 20 area% was evaluated as Δ, and the area of 20 area% or more was evaluated as ×. Table 3 shows the results. Fig. 1 shows an SEM photograph of a sample prepared as ○ evaluation, Fig. 2 shows an SEM photograph of a sample subjected to Δ evaluation, and Fig. 3 shows a SEM photograph of a sample prepared as × evaluation.

The amount of adhesion of the chemical treatment coating using fluorescent X-rays was also measured. In this measurement, in terms of the P intensity of the fluorescent X-rays, a calibration curve prepared by using a steel sheet in which the amount of adhesion of the zinc phosphate chemical treatment film is known is used. The lower the adhesion amount of the chemical treatment film, the lower the chemical treatment property, and the chemical treatment property is good if the adhesion amount is 2 g/m ² or more. In this evaluation, the amount of adhesion is 2 g/m ² or more, ○, 1.5 g/m ² or more, less than 2 g/m ² is made into Δ, and less than 1.5 g/m ² is made into ×. Table 3 shows the results.

In the evaluation of the corrosion resistance after coating, first, the chemical treatment film is formed on the steel sheet and coated on the same as the evaluation of the chemical treatment property. Electrodeposited coatings. The electrodeposition coating was performed using POWERNICS manufactured by Japan Paint Co., Ltd. (PAINT). In this coating, a voltage was applied while the test piece was immersed in an electrodeposition paint having a temperature of 30 ° C, and the thickness of the coating film was set to 20 μm in accordance with the dry film thickness by a voltage of 150 V to adjust the energization time. The power-on time is about 3 minutes. The film thickness was measured using an electromagnetic film thickness meter.

Further, in the center of the test piece, a cutter-shaped material (steel plate) is applied from the coating film to the test piece to form a X-shaped cut, and the end surface (side surface) of the side is sealed by a tape, thereby producing corrosion resistance. Test sample. This was subjected to a salt spray test by the method disclosed in JIS Z 2371. When the test time was 1000 hours, the maximum expansion width from the cut was 2 mm or less on one side, and it was evaluated as ○, and if it was more than 2 mm and 3 mm, it was evaluated as Δ, and when it was more than 3 mm, it was evaluated as ×. Table 3 shows the results. The bottom line in Table 3 indicates that the value is out of the desired range.

In Test Nos. 1, 3, 6~8, 10~14, 16~18, 21, 23, 27~29, 32, 34, 38~40, 43~45 and 49~51, due to the scope of the present invention Therefore, excellent chemical treatment properties and corrosion resistance after coating can be obtained. The test number 1, 6 to 8, 11 to which the reflectance in the wave number of the absorption peak which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by the FT-IR analysis is 60% or more and 85% or less is shown. Among the 14, 16 to 18, 21, 27 to 29, 32, 38 to 40, 43 to 45, and 49 to 51, in particular, excellent chemical treatment properties and corrosion resistance after coating can be obtained.

In Test Nos. 2,9,22 and 33, since the display by the reflection FT-IR analysis of the wave number appearing within the range of the wave number of 1000cm ^-1 ~ 1100cm ^-1 of the absorption peak of less than 85%, therefore, the chemical The handleability is low, and along with this, the corrosion resistance after application is also low. It is generally believed that this is because a large amount of manganese citrate remains.

In Test Nos. 15, 26, 37, and 48, since the adhesion amount of Ni was less than 3 mg/m ² , the chemical treatment property was low, and accordingly, the corrosion resistance after application was also low. In Test Nos. 19, 30, 41 and 52, since the adhesion amount of Ni was more than 100 g/m ² , although good chemical treatment properties were obtained, the corrosion resistance after coating was low.

In Test Nos. 4, 5, 24, 25, 35, 36, 46, and 47, annealing was performed by conditions such as decarburization, that is, annealing was performed under an ambient gas having a high dew point and a high oxygen potential. Therefore, a thick decarburized layer is formed. Therefore, the fatigue strength is lowered. Further, it is shown that the reflectance in the wave number of the absorption peak which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis is more than 85%.

In Test Nos. 20, 31, 42 and 53, since the pickling was followed by pickling by conditions which are likely to cause pitting corrosion, that is, the first strong pickling was performed, a large number of pitting corrosion occurred. Therefore, the bending workability is lowered. Further, it is shown that the reflectance in the wave number of the absorption peak which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis is more than 85%.

In Test Nos. 54 to 56, since the composition of steel deviated from the range of the present invention, the tensile strength was low.

In Test Nos. 57 to 60, since the second pickling was carried out by annealing after the annealing, that is, the second strong pickling was carried out, the pitting was caused. Therefore, the bending workability is lowered. Further, it is shown that the reflectance in the wave number of the absorption peak which appears in the wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by FT-IR analysis is more than 85%.

Industrial availability

The present invention can be utilized in industries such as those associated with steel sheets suitable for automobile bodies or parts.

Claims (2)

A steel sheet characterized by having a chemical composition represented by mass % by C: 0.050% to 0.400%; Si: 0.10% to 2.50%; Mn: 1.20% to 3.50%; P: 0.100% or less; Al: 1.200% or less; N: 0.0100% or less; Cr, Mo, Ni, and Cu: 0.00% to 1.20% in total; Nb, Ti, and V: 0.000% to 0.200% in total; B: 0.0000% ~0.0075%; Ca, Mg, Ce, Hf, La, Zr, Sb and REM: a total of 0.0000% to 0.1000%; and the remainder: Fe and impurities; in addition, the surface is converted by Fourier transform using high-sensitivity reflection infrared spectroscopy, within a number of 1200cm ^-1 ~ 1300cm ^-1 wave number range on the display reflectance of 50% or more, 85% or less of the absorption peak, and in the number of 1000cm ^-1 ~ 1100cm ^-1 absorption wave range does not appear peak, or the display of the reflectance of 85% or more absorption peaks in the wave number range of 1000cm ^-1 ~ 1100cm ^-1, and adhered 3mg / m on the surface ² ~ 100mg / m Ni ² of. The steel sheet according to claim 1, wherein the surface is a reflectance of 60% or more and 85% or less in a wavenumber range of 1200 cm ^-1 to 1300 cm ^-1 by Fourier transform infrared spectroscopy using a high-sensitivity reflection method. The absorption peak.