JP4698967B2 - High-strength cold-rolled steel sheet with excellent coating film adhesion and workability - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having excellent coating film adhesion and workability. In particular, the present invention has excellent coating film adhesion and excellent workability (especially deep) with a tensile strength of 340 MPa or more. The present invention relates to a cold-rolled steel sheet that is optimal as a steel sheet for automobile parts that exhibits (drawability).

  Steel materials are required to have high strength against the background of improving fuel economy and weight reduction, and high strength (high strength) is also progressing in the field of cold-rolled steel sheets. On the other hand, since cold-rolled steel sheets are subjected to press forming at the time of production of parts, it is assumed that sufficient workability such as deep drawability is ensured when increasing strength. Addition of an alloy element is effective for increasing the strength, but the ductility (workability) tends to decrease as the amount of the alloy element increases.

  However, among the above alloy elements, Si is an element having a relatively small ductility reduction, and is an element effective for achieving high strength while ensuring ductility. However, when the Si content is increased, the chemical conversion processability is deteriorated and the coating film adhesion after coating is lowered. For this reason, when the chemical conversion treatment is important, the Si content has to be reduced. Moreover, when Si content increased, it became easy to generate | occur | produce the crack resulting from the Si containing grain-boundary oxide produced | generated on the steel plate surface, and this became a factor which deteriorates coating-film adhesiveness.

  Until now, as a technology to achieve both mechanical properties and chemical conversion treatment, the steel sheet surface is coated with a clad material, and a low Si concentration layer is provided on the steel sheet surface to improve chemical conversion treatment. There is a technique for ensuring the target characteristics (for example, Patent Document 1). However, since the clad structure is required, there is a problem that the manufacturing process is complicated and the cost is increased. In addition, the clad material has been designed with priority given to ensuring chemical conversion treatment, and does not have improved mechanical properties, and it is difficult to say that the entire steel plate is sufficiently excellent in mechanical properties.

  There is also a conventional technique in which a special alloy element is added so that Si that inhibits chemical conversion treatment does not concentrate on the surface (for example, Patent Document 2 and Patent Document 3). In this method, by adding Ni or Cu, concentration of Si on the steel sheet surface layer is suppressed, and chemical conversion processability is ensured. However, this method has a problem that the cost is increased because expensive Ni or Cu is used.

  In Patent Document 4, a large amount of NbC is precipitated, and this is used as a nucleation site for zinc phosphate crystals to achieve both chemical conversion treatment and high strength. This technology, however, has a C concentration of 0.006% or higher, which is relatively high as an extremely low carbon steel sheet, and deep drawability is ensured by controlling the texture. However, the precipitation amount of NbC is very high. Therefore, there is a problem that YR is high and adversely affects the dimensional accuracy during molding.

Patent Document 5 proposes a retained austenite-containing steel sheet that ensures chemical conversion processability by defining the SiO ₂ / Mn ₂ SiO ₄ ratio of the surface layer. In this technique, in order to control the surface layer oxide or to control the element ratio of Si / Fe, the surface after continuous annealing is pickled or brushed to remove the Si oxide, or the Ac ₁ transformation point or higher. It is necessary to adjust the dew point to −30 ° C. or higher with temperature to suppress the amount of Si oxide produced.

However, when the pickling or brush treatment is performed, the manufacturing cost increases due to an increase in the number of steps. Although the dew point control is performed in a continuous annealing furnace, as long as the examples shown in the literature are seen, even if the dew point is controlled, the SiO ₂ / Mn ₂ SiO ₄ ratio in the outermost layer is about 1.0. Further, since SiO ₂ that inhibits the formation of the chemical conversion film is generated to the same extent as Mn ₂ SiO ₄ , it is difficult to say that the chemical conversion property is sufficiently improved.

  Patent Document 6 proposes a technique for observing the surface of a steel sheet by XPS and suppressing the ratio of Si to Mn (Si / Mn) constituting the oxide to 1 or less to improve chemical conversion property.

As steel having a Si / Mn ratio of 1 or less, it is generally known that, for example, mild steel with almost no Si content or steel sheet with an Si content of 0.1% or less has excellent chemical conversion properties. However, as described above, in order to increase both strength and ductility, it is necessary to contain Si to some extent, and there is a limit in reducing the Si amount and making the Si / Mn ratio 1 or less. Even when the Si / Mn ratio is controlled to 1 or less by controlling the amount of Mn while securing an appropriate amount of Si, a steel sheet exhibiting good chemical conversion properties is not always stably obtained.
Japanese Patent Laid-Open No. 5-78752 Japanese Patent No. 2951480 Japanese Patent No. 3266328 Japanese Patent No. 3049147 JP 2003-201538 A JP-A-4-276060

  The present invention has been made in view of the above circumstances, and its purpose is cold rolling that has excellent coating film adhesion and exhibits excellent workability (particularly deep drawability) at a tensile strength of 340 MPa or more. It is to provide a steel sheet.

The high-strength cold-rolled steel sheet according to the present invention is suppressed by mass% (the same applies to chemical components below), C: 0.005% or less, N: 0.01% or less, and S: 0.01% or less. Mn: 0.1 to 4%, Si: 0.2 to 1%, P: 0.001 to 0.2%, Si / Mn ≦ 0.4 and Ti: 0.005 to 0.1% And / or Nb: 0.005 to 0.1%, and the metal structure is a steel sheet having a ferrite single phase structure,
The tensile strength is 340 MPa or more, the tensile strength (TS: unit MPa) and the elongation (El: unit%) satisfy the following formula (1), and the Rankford value (r value) is 1.2 or more.
(I) 10 Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less with an atomic ratio (Mn / Si) of Mn and Si of 0.5 or more on the surface of the steel sheet (when viewed in plan) / 100 μm ² or more, and the feature is that the steel sheet surface coverage of the oxide mainly composed of Si is 10% or less (hereinafter sometimes referred to as “the steel sheet 1 of the present invention”).
TS × El ≧ 13000 (1)
The oxide mainly composed of Si refers to an element other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio. Further, the oxide is considered to be amorphous as a result of analysis.

  As shown in the examples to be described later, the surface coverage of the oxide mainly composed of Si is obtained by observing a sample processed by the extraction replica method with a transmission electron microscope (TEM) and analyzing with an EDX (Energy Dispersive X-ray) analysis. Mapping and quantitative analysis of Si, O (oxygen), Mn, and Fe were performed, and the data was obtained by an image analysis method. If TEM observation of the extracted replica is complicated, surface mapping is performed on Si, O, Mn, and Fe at a magnification of 2000 to 5000 using AES (auger electron spectroscopy), and the data is subjected to image analysis. Good.

Another steel plate of the present invention that can solve the above problems is suppressed to C: 0.005% or less, N: 0.01% or less, S: 0.01% or less, Mn: 0.1 to 4% , Si: 0.2 to 1%, P: 0.001 to 0.2%, Si / Mn ≦ 0.4, and Ti: 0.005 to 0.1% and / or Nb: 0.005 Containing 0.1%, and the metal structure is a steel sheet of ferrite single phase structure,
The tensile strength is 340 MPa or more, the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1), and the Rankford value (r value) is 1.2 or more.
(II) When a cross section near the surface of the steel sheet is observed at a magnification of 2000 using a scanning electron microscope (SEM), it is characterized in that there are no cracks with a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view. (Hereinafter sometimes referred to as “the present steel plate 2”).

  The width and depth of the cracks refer to the portion shown in FIG. 1 (schematic cross section of steel plate) when the vicinity of the surface of the steel plate cross section is observed at 2000 times using SEM (S-4500 manufactured by Hitachi, Ltd.). Shall.

Still another steel sheet of the present invention that can solve the above problems is suppressed to C: 0.005% or less, N: 0.01% or less, S: 0.01% or less, and Mn: 0.1 to 4 %, Si: 0.2 to 1%, P: 0.001 to 0.2%, Si / Mn ≦ 0.4, and Ti: 0.005 to 0.1% and / or Nb: 0.00. 005 to 0.1%, and the metal structure is a steel sheet having a ferrite single phase structure,
The tensile strength is 340 MPa or more, the tensile strength (TS: unit MPa) and the elongation (El: unit%) satisfy the above formula (1), and the Rankford value (r value) is 1.2 or more. It has a feature that satisfies the requirements (I) and (II) (hereinafter sometimes referred to as “the steel plate 3 of the present invention”).

  The steel sheet of the present invention may further contain B: 0.005% or less (not including 0%) as another element.

  According to the present invention, a steel plate optimal for automobiles exhibiting excellent paint film adhesion and exhibiting excellent workability (particularly deep drawability) at a tensile strength of 340 MPa or more, constituting a cladding, It can be efficiently realized without adding expensive elements such as Ni and Cu, or after-treatment such as brush treatment.

  In order to obtain a steel sheet having a tensile strength of 340 MPa or more excellent in coating film adhesion and workability (particularly deep drawability), the following requirements (I) and The inventors have found that it is only necessary to satisfy // (II) and have arrived at the present invention. Furthermore, while satisfying these requirements, the component composition and manufacturing conditions for ensuring excellent workability (particularly deep drawability) at a tensile strength of 340 MPa or more were also examined.

(I) On the steel plate surface (when viewed in plan)
(i) Mn—Si composite oxide having a major axis of 0.01 μm or more and 5 μm or less having an atomic ratio (Mn / Si) of Mn to Si of 0.5 or more is present at 10 pieces / 100 μm ² or more, and
(ii) The steel sheet surface coverage of an oxide mainly composed of Si (an oxide other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio) is set to 10% or less.
(II) When observing a cross section in the vicinity of the steel sheet surface at a magnification of 2000 using an SEM, no cracks having a width of 3 μm or less and a depth of 5 μm or more are present in any 10 visual fields.

  The reason why the requirements (I) and (II) are specified will be described in detail below.

<Mn-Si composite oxide having a major axis of 0.01 to 5 μm with an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more on the steel sheet surface: 10/100 μm ² or more>
The present inventors have been researching for a long time to obtain a high-strength steel sheet excellent in coating film adhesion, and have already proposed a chemical conversion treatment improving technique for a steel sheet containing a relatively large amount of Si (Japanese Patent Application). 2003-106152). This technique aims to improve chemical conversion treatment by finely dispersing amorphous Si oxide that adversely affects chemical conversion treatment by controlling the annealing atmosphere. However, in the region where the Si concentration is relatively low, not the amorphous Si oxide but the Mn—Si composite oxide is generated as the main oxide. This composite oxide is also considered to reduce the adhesion of the coating film in the same manner as the amorphous Si oxide. Therefore, the Mn—Si composite oxide is considered to be actively used for improving chemical conversion treatment, and research has been advanced along that line.

  As a result, the Mn-Si composite oxide is finely dispersed in the iron-based oxide matrix formed on the surface layer portion of the steel sheet, and as described later, the “oxide interface that acts as a nucleation site for zinc phosphate crystals. The chemical conversion processability could be improved by forming an "electrochemical heterogeneous field". The reason why the Mn—Si composite oxide defined in the present invention is effective for the nuclei of zinc phosphate crystals is not clear, but is considered as follows.

  In the chemical conversion treatment process, it is known that zinc phosphate crystals are likely to be generated in the “electrochemical inhomogeneous field” formed around the grain boundaries and around the Ti colloid previously deposited on the steel plate surface during the surface conditioning treatment. It has been. Also in the present invention, an electrochemical heterogeneous field is formed around the Mn-Si composite oxide, so that zinc phosphate crystals are easily attached during chemical conversion treatment, and good chemical conversion treatment performance is exhibited. It is thought that.

It is said that the zinc phosphate crystal after the chemical conversion treatment is preferably several μm or less from the viewpoint of coating film adhesion. Therefore, it is considered that the above-mentioned electrochemical non-uniform field is desirably on the order of several μm or less. Therefore, 10 or more Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less having an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more are present in 100 μm ² (on average 10 μm ²⁾ . One or more particles were present), and the average particle spacing of the composite oxide particles was set to several μm so that an electrochemical heterogeneous field having the above size was easily formed.

Note that, in all the Mn—Si composite oxides present, the electrochemical heterogeneous field is not necessarily formed effectively, so preferably 50 or more per 100 μm ² , more preferably 100 or more, even more preferably. It is preferable that 150 or more of the above Mn—Si composite oxide exist. Examples of the Mn—Si composite oxide include Mn ₂ SiO ₄ . The size of the Mn—Si composite oxide that can be observed seems to be about 50 nm.

<Stainless steel sheet surface coverage of oxide mainly composed of Si: 10% or less>
Even if an appropriate amount of a Mn-Si composite oxide effective as a zinc phosphate crystal nucleus is present, if there is another substance that inhibits the chemical conversion treatment, the excellent chemical conversion treatment performance will not be exhibited. It becomes inferior to adhesiveness.

  As described above, when an oxide mainly composed of Si (an oxide other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio) is present on the surface of the steel sheet, the region contains phosphorus. Zinc acid crystals are not formed, and the chemical conversion treatment performance is significantly reduced. Therefore, the steel sheet surface coverage of the oxide mainly composed of Si is set to 10% or less.

  The inventors of the present invention have proposed a technique for finely dispersing an oxide mainly composed of Si as described above to improve the chemical conversion treatment property. However, the present invention utilizes the above-described action of the Mn—Si composite oxide. In the above, it was found that it is preferable that an oxide mainly composed of Si is not present as much as possible. Therefore, the steel sheet surface coverage of the oxide mainly composed of Si is more preferably suppressed to 5% or less, and most preferably 0%.

<When observing a cross section near the steel sheet surface at 2000 times using SEM, there should be no cracks with a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view>
If sharp cracks are present on the surface of the steel plate, it is considered that zinc phosphate crystals do not adhere to the site during the chemical conversion treatment, and as a result, corrosion of the site is likely to proceed, resulting in a decrease in coating film adhesion. That is, in order to improve the adhesion of the coating film, it is important to suppress as much as possible sharp cracks to which zinc phosphate crystals do not adhere.

  The present inventors have already proposed a technique for improving the adhesion of a coating film by setting the existing depth of a linear compound (width: 300 nm or less) containing Si and oxygen to 10 μm or less. In this technique, it is assumed that pickling is not performed after continuous annealing, but the steel sheet is more often subjected to pickling after continuous annealing, in which case, the linear oxide is removed and cracks occur. Occurs.

  Although the quantitative relationship between the crack depth and the linear oxide is not clear, it is considered that the linear oxide is dissolved in the acid as described above, or mechanically dropped to cause cracks, and the linear oxidation described above. It is considered that cracks formed after removal of the oxide are deeper than the existence depth of the linear oxide because dissolution of the crack portion proceeds by acid or the like even after the material is removed.

  Therefore, in the present invention, it is considered that controlling the cracks can more reliably improve the adhesion of the coating film than controlling the depth of existence of the linear oxide as in the proposed technique. When the shape of the power crack was examined, if the crack width was the same as or smaller than the zinc phosphate crystal grain size, it was difficult for the zinc phosphate crystal to adhere to the crack, and the depth was particularly 5 μm or more. Since zinc phosphate crystals hardly adhere to the cracks, cracks having a width of 3 μm or less and a depth of 5 μm or more were targeted for suppression.

  And when the said crack observed the cross section near the steel plate surface by 2000 times using SEM, it made it a requirement that it did not exist in arbitrary 10 visual fields.

  In the present invention, the chemical components are defined as follows in order to efficiently precipitate the Mn—Si composite oxide and to suppress the specified cracks and to provide the characteristics as a high-strength steel sheet.

<Si (mass%) / Mn (mass%) ≦ 0.4>
As described above, since an oxide mainly composed of Si adversely affects chemical conversion properties, it is preferable to suppress it as much as possible rather than finely dispersing the oxide. Therefore, the inventors suppress the oxide mainly composed of Si by setting the ratio of Si content (mass%) in steel to Mn content in steel (Si / Mn) to 0.4 or less, The chemical conversion processability was improved. From the viewpoint of suppressing the cracks, Si / Mn is preferably set to 0.4 or less. The Si / Mn is preferably 0.3 or less.

<C: 0.005% or less>
The steel sheet of the present invention achieves excellent deep drawability by reducing solute C as much as possible. If the amount of C exceeds 0.005%, it is necessary to add a large amount of Ti and Nb to reduce the solid solution C to the extent that no adverse effects due to aging appear, and the precipitates increase and the mechanical properties decrease. . Therefore, the C content is limited to 0.005% or less, preferably 0.003% or less. From the viewpoint of economy and productivity, the lower limit of the C amount is preferably 0.0003%.

<N: 0.01% or less>
The present invention is directed to a steel sheet that exhibits excellent deep drawability by reducing solid solution N together with solid solution C as much as possible. When N is excessively contained, In order to reduce N, it is necessary to add a large amount of nitride-forming elements (Ti, B, Al), which not only increases the cost, but also increases the precipitates, and the adverse effect on the mechanical properties becomes remarkable. Therefore, N must be suppressed to 0.01% or less, and is preferably 0.004% or less.

<S: 0.01% or less>
The steel sheet of the present invention also reduces solute S as much as possible as in C and N above. If S is excessively contained, the amount of sulfide-forming elements (Ti and Mn) added is increased to reduce the solid solution S as in the case of C and N, which not only increases the cost but also increases the amount of precipitates. Increases and the mechanical properties are significantly reduced. Therefore, S is suppressed to 0.01% or less, preferably 0.005% or less.

<Mn: 0.1 to 4%>
Mn is an element effective for fixing S (sulfur), which causes embrittlement of steel, as MnS. In order to exert such an effect, it is contained in an amount of 0.1% or more, preferably 0.2% or more. Let However, when the amount of Mn becomes excessive, both ductility and weldability deteriorate, so the content is suppressed to 4% or less, preferably 2% or less.

<Si: 0.2 to 1%>
Si is an element effective for ensuring an excellent balance between strength and ductility. In order to sufficiently exhibit such an effect, Si is contained in an amount of 0.2% or more, preferably 0.3% or more. On the other hand, if the Si content is excessive, the solid solution strengthening action becomes excessive and the rolling load increases, so the content is suppressed to 1% or less. Preferably it is 0.6% or less.

<P: 0.001 to 0.2%>
P is an element effective for ensuring the strength, and may be contained in an amount of 0.001% or more. However, if the P content is excessive, the steel tends to become brittle, so it is suppressed to 0.2% or less. Preferably it is 0.15% or less.

<Ti: 0.005-0.1% and / or Nb: 0.005-0.1%>
Ti and Nb are effective elements for fixing as precipitates in order to reduce the above-mentioned solid solution C, solid solution N, and solid solution S, and in order to exert such an effect, 0.005 Ti is used. % Or more (preferably 0.01% or more) and / or Nb is added 0.005% or more (preferably 0.01% or more). On the other hand, when these elements are added excessively, the recrystallization temperature tends to increase and the mechanical properties tend to be deteriorated. In addition, the effects are effective in the range of C amount, N amount and S amount specified in the present invention. Ti and Nb are each preferably added in the range of 0.1% or less (preferably 0.05% or less) because they are saturated and economically useless.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but as an element brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc., 0.01% or less Of course, inevitable impurities such as O (oxygen) and Al of 0.1% or less are allowed to be included, and B is added as another element within a range that does not adversely affect the operation of the present invention. It is also possible to contain it positively.

  B is an element effective for strengthening the grain boundary and suppressing the secondary work brittleness, and 0.0003% or more is preferably added to exert such an effect. However, since deep drawability deteriorates if contained excessively, it is preferable to add in the range of 0.005% or less.

  The present invention is directed to a steel sheet having a ferrite single phase structure. The “ferrite” in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density.

  The steel sheet of the present invention, in addition to the above ferrite, may inevitably remain in the manufacturing process, and may contain an extremely small amount of carbide, etc. as a structure included in a range not impairing the action of the present invention. The total content should be suppressed to 3% or less, and preferably 0%.

The steel plate of the present invention is a steel plate satisfying the basic components, has the metal structure, and as a characteristic,
-Tensile strength is 340 MPa or more (particularly 390 MPa or more),
-Tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1), and the right side of the following formula (1) is particularly 15000 or more, strength and workability This is preferable because of its excellent balance.
TS × El ≧ 13000 (1)
-Furthermore, it has excellent deep drawability with an r value of 1.2 or more.

  In order to enhance the chemical conversion treatment property, in order to control the form of the oxide deposited on the steel sheet surface as specified in the above requirement (I), in addition to satisfying the component composition, in the production process, after the hot rolling, the liquid temperature is It is effective to immerse in 1 to 18% by mass hydrochloric acid at 65 to 90 ° C. for 40 seconds or more (preferably 60 seconds or more) and to suppress the dew point during continuous annealing to −40 ° C. or less (preferably −45 ° C. or less). It is. In addition, as for the immersion time in hydrochloric acid, when a plurality of hydrochloric acid baths are installed and intermittent immersion is performed, the total immersion time may be 40 seconds or more.

  Further, as specified in the above requirement (II), in order not to generate cracks, in addition to satisfying the component composition, in the manufacturing process, the hot rolling coiling temperature is 500 ° C. or lower (preferably 480 ° C. or lower). And after hot rolling, the liquid temperature is 70 to 90 ° C. and immersed in 1 to 18% by mass of hydrochloric acid for 40 seconds or more (preferably 60 seconds or more), and the dew point during continuous annealing is −40 ° C. or less (preferably −45 ° C. or less) is effective. Furthermore, when the steel sheet is exposed to a water vapor atmosphere in the cooling process of continuous annealing, it is effective to cool the steel sheet to 550 ° C. or lower (preferably 400 to 450 ° C.) in advance.

  The present invention is not limited to other production conditions, and may be cast after melting and hot-rolled as usual. Moreover, in the Example mentioned later, although pickling is performed after continuous annealing, the presence or absence of this pickling is not ask | required.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  A steel material having the chemical composition shown in Table 1 was melted and cast using a slab obtained by casting, and then pickled. Table 2 shows the coiling temperature after hot rolling and the pickling time after hot rolling as the production conditions. In addition, pickling was performed using the hydrochloric acid aqueous solution whose temperature is 70-90 degreeC, and whose density | concentration is 5-15 mass%. Thereafter, cold rolling (cold rolling ratio: 70%) was performed to obtain a steel sheet having a thickness of 1.2 mm whose metal structure is a ferrite single phase structure. And the steel plate obtained by the method shown in FIG. 2 or FIG. 3 was subjected to continuous annealing. When cooling after soaking and slow cooling in continuous annealing is brackish water (mist) cooling, cooling by gas spraying (GJ), cooling by water cooling roll heat removal (RQ), the cooling is performed by the method of FIG. In the case of quenching (WQ), the method shown in FIG. 3 was used. After the cooling, tempering was performed as shown in FIG. In addition, when brackish water (mist) cooling and water quenching (WQ) are performed, after tempering, it is immersed (pickling) for 5 seconds in hydrochloric acid having a liquid temperature of 50 ° C. and a concentration of 5% by mass. The surface scale was removed. The heating temperature, annealing end point temperature, and tempering temperature in Table 2 indicate the temperatures at the locations shown in FIGS. The dew point is the atmospheric dew point of the continuous annealing furnace.

  Mechanical properties and coating film adhesion were evaluated using the obtained steel plates. Mechanical properties were measured by taking a JIS No. 5 test piece, obtaining tensile strength (TS), El (total elongation), yield point (YP), r value, and tensile strength (TS) of 340 MPa or higher. The case where the product of strength and elongation (TS × El) was 13000 or more and the r value was 1.2 or more was evaluated as excellent in mechanical properties.

  As the coating film adhesion, chemical conversion property and presence of cracks were examined. For the chemical conversion treatment, the state of the oxide on the surface of the steel sheet was examined as follows, and the chemical conversion treatment was performed under the following conditions, and the steel sheet surface after the chemical conversion treatment was observed by SEM at 1000 times, and 10 phosphate crystals of zinc phosphate were observed. The adhesion state of was investigated. The case where the zinc phosphate crystals were uniformly attached in all 10 fields of view was evaluated as “◯”, and the case where the portion where no zinc phosphate crystals were adhered was present as “x”. The results are shown in Table 3.

・ Chemical conversion treatment liquid: Palbond L 3020 manufactured by Nihon Parkerizing Co., Ltd.
・ Chemical conversion treatment process: Degreasing → Washing → Surface adjustment → Chemical conversion treatment The number of Mn-Si oxides was prepared by extracting an extracted replica film on the surface of the steel material, and TEM observation was performed at 15000 times (H-800, manufactured by Hitachi, Ltd.) The average number (per 100 μm ² ) of any 20 visual fields was examined.

As for the steel sheet surface coverage of the oxide mainly composed of Si, the sample treated by the extraction replica method was observed by TEM, and the coverage was determined by an image analysis method. The extraction replica method was performed according to the following procedures (a) to (d).
(A) Carbon is vapor-deposited on the surface of the steel material.
(B) A grid-like cut of 2 to 3 mm square is made on the sample plane.
(C) The carbon is levitated by being corroded with 10% acetylacetone-90% methanol etching solution.
(D) Store in alcohol and use for observation.

  Using the sample processed in this manner, a TEM image of 10 fields of view (13 cm × 11 cm) was taken at a magnification of 15000 times, and an oxide mainly composed of Si (among elements other than oxygen constituting the oxide) The area of Si (over 67% by atomic ratio) was measured, and the coverage of the oxide mainly composed of Si was determined.

  The presence or absence of cracks was examined using an SEM (S-4500, manufactured by Hitachi, Ltd.) at a magnification of 2000 and by observing any 10 visual fields (1 visual field: 13 cm × 11 cm) near the surface of the cross section of the steel sheet. These measurement results are shown in Table 3.

  From Tables 1 to 3, the following can be considered (the following No. indicates the experiment No.). That is, no. No. 30 satisfies the prescribed requirements as the steel sheet 1 of the present invention, and therefore has excellent chemical conversion treatment properties and excellent coating film adhesion. In this example, it can be seen that, in order to suppress cracks and ensure better coating film adhesion, it is particularly preferable to control the coiling temperature and the annealing end temperature as production conditions.

  No. Since No. 31 satisfies the requirements defined as the steel sheet 2 of the present invention, cracks are not generated, and a steel sheet having excellent coating film adhesion is obtained. In this example, in order to ensure the chemical conversion treatment and further improve the adhesion of the coating film, it is preferable to control the component composition so that the form of the oxide deposited on the steel sheet surface is as specified.

  No. 1 to 13 and 24 are the requirements regarding the surface oxide and crack defined by the steel plate 3 of the present invention (that is, the requirements regarding the surface oxide defined by the steel plate 1 of the present invention and the requirements regarding the crack defined by the steel plate 2 of the present invention). Since it is satisfied, excellent chemical conversion processability can be secured, and the occurrence of cracks is suppressed and excellent coating film adhesion is exhibited.

  In contrast, no. Nos. 14 to 23 and 25 to 29 do not satisfy any of the requirements of the steel plates 1 to 3 of the present invention, and are not excellent in coating film adhesion, or are not excellent in the strength-ductility balance, and are high in strength and excellent. No product that exhibits excellent workability has been obtained.

  No. 14-20 and 27-29 did not satisfy the component composition prescribed | regulated by this invention, Therefore It became a result inferior to a mechanical characteristic or inferior to coating-film adhesiveness. That is, no. No. 14 has a small amount of Si. No. 17 has too much Mn. No. 19 has a C content exceeding the upper limit, carbides are excessively precipitated, and the elongation is low. No. 20 was inferior in the strength-ductility balance because the N amount was excessive. No. No. 17 has a small r value and inferior deep drawability.

  No. No. 15 has an excessive amount of Si, and the Si / Mn ratio exceeds the upper limit, so that it does not become the steel sheet surface to be defined, resulting in poor coating film adhesion. No. In No. 16, the Si amount and the Mn amount are within the specified ranges, respectively, but the Si / Mn ratio exceeds the upper limit, so that the coating film adhesion is inferior. No. No.18, the amount of Si is within the specified range, but the amount of Mn is too small and the Si / Mn ratio exceeds the upper limit. Therefore, the specified Mn-Si composite oxide is small, and a large amount of Si-based oxide is generated. As a result, the chemical conversion processability was inferior. Cracks were also observed at the surface grain boundary.

  No. No. 27 was inferior in the strength-ductility balance because the amount of S was excessive. No. No. 28 has an excessive amount of Ti. No. 29 has an unfavorable balance between strength and ductility because the amount of Nb is excessive.

  No. 21 to 23, 25, and 26 are not manufactured under the recommended conditions, and are not in the form of oxides defined in the present invention, so that they are inferior in chemical conversion treatment, and cracks are also generated, resulting in poor coating film adhesion. .

  That is, no. No. 21 has a high coiling temperature after hot rolling, which facilitates Si surface concentration in hot rolling. No. 22 has a short pickling time, so that removal of the concentrated Si layer is insufficient. Since No. 23 has a high dew point, the surface concentration of Si is promoted at the annealing stage, and in both cases, a large amount of oxide mainly composed of Si is inferior in chemical conversion treatment property. Produced and cracks occurred after pickling, resulting in poor coating adhesion.

  No. In Nos. 25 and 26, the conditions until the cooling after annealing were controlled to a more suitable range, In No. 25, mist cooling was performed after annealing. In No. 26, water cooling was performed after annealing to expose it to a high temperature and moisture-rich atmosphere, so a large amount of Si-based oxides were formed on both the surface and grain boundaries, resulting in poor chemical conversion properties. Moreover, the said oxide melt | dissolved in the subsequent pickling process and the crack also generate | occur | produced.

  For reference, a micrograph obtained by TEM observation of an extracted replica of the steel sheet obtained in this example and a SEM observation photograph of the steel sheet surface after chemical conversion treatment are shown. FIG. FIG. 4 shows that the surface area of the steel sheet is covered with an oxide layer mainly composed of Si.

  FIG. 5 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment observed with an SEM. From FIG. 15 shows that the zinc phosphate crystal is large and the gap is large.

  On the other hand, FIG. 6 is a TEM observation photograph in the cross section of the steel plate of No. 6, but in the steel plate surface layer region, the above-mentioned No. 15 is not formed, and instead, Mn-based Mn—Si composite oxide is finely dispersed. That is, no. It can be confirmed that the surface layer region of the steel plate 6 has almost no Si-based oxides that lower the chemical conversion property, and there are many Mn-Si composite oxides effective for improving the chemical conversion property.

  FIG. 7 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment, which was observed with an SEM. 6 shows that the zinc phosphate crystals are small and have no gaps.

It is the figure which showed typically the crack in a steel plate cross section. It is a figure which shows the manufacturing process (part) in an Example. It is a figure which shows another manufacturing process (part) in an Example. No. in the examples. 15 is a TEM observation photograph (extracted replica, magnification: 15000 times) of 15 (comparative example). No. in the examples. It is a SEM observation photograph of the steel plate surface (after chemical conversion treatment) of 15 (comparative example). No. in the examples. 6 is a TEM observation photograph (extraction replica, magnification: 15000 times) of No. 6 (example of the present invention). No. in the examples. It is a SEM observation photograph of the steel plate surface (after chemical conversion treatment) of 6 (invention example).

Claims (4)

% By mass (the same applies to chemical components)
C: 0.005% or less,
N: 0.01% or less,
S: suppressed to 0.01% or less,
Mn: 0.1-4%
Si: 0.2-1%,
P: 0.001 to 0.2%,
While satisfying Si / Mn ≦ 0.4,
Ti: 0.005 to 0.1% and / or Nb: 0.005 to 0.1%, the balance is iron and inevitable impurities,
The metal structure is a steel sheet having a ferrite single phase structure,
In the steel sheet surface, with the atomic ratio of Mn and Si (Mn / Si) is 5μm or less of Mn-Si composite oxide major diameter 0.01μm or more and 0.5 or more is present 10/100 [mu] m ² or more, mainly of Si The steel sheet surface coverage of the oxide is 10% or less,
When tensile strength is 340 MPa or more, tensile strength (TS: unit MPa) and elongation (El: unit%)
Satisfies the following formula (1) and has a Rankford value (r value) of 1.2 or more, a high-strength cold-rolled steel sheet excellent in coating film adhesion and workability.
TS × El ≧ 13000 (1) C: 0.005% or less,
N: 0.01% or less,
S: suppressed to 0.01% or less,
Mn: 0.1-4%
Si: 0.2-1%,
P: 0.001 to 0.2%,
While satisfying Si / Mn ≦ 0.4,
Ti: 0.005 to 0.1% and / or Nb: 0.005 to 0.1%, the balance is iron and inevitable impurities,
The metal structure is a steel sheet having a ferrite single phase structure,
When observing a cross section in the vicinity of the steel sheet surface at 2000 times using SEM, there are no cracks having a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view,
When tensile strength is 340 MPa or more, tensile strength (TS: unit MPa) and elongation (El: unit%)
Is the following formula (1)
TS × El ≧ 13000 (1)
The meet, and Lankford value (r value) Ri der 1.2 or more,
A high-strength cold-rolled steel sheet excellent in coating film adhesion and workability , wherein the cracks are not generated by performing the following steps (a) to (d) .
(A) The hot rolling coiling temperature is 500 ° C. or less,
(B) After hot rolling, the liquid temperature is 70-90 ° C. and immersed in 1-18 mass% hydrochloric acid for 40 seconds or more,
(C) The dew point during continuous annealing is −40 ° C. or lower,
(D) When the steel sheet is exposed to a water vapor atmosphere in the cooling step during continuous annealing, the steel sheet temperature is gradually cooled to 550 ° C. or lower in advance. C: 0.005% or less,
N: 0.01% or less,
S: suppressed to 0.01% or less,
Mn: 0.1-4%
Si: 0.2-1%,
P: 0.001 to 0.2%,
While satisfying Si / Mn ≦ 0.4,
Ti: 0.005 to 0.1% and / or Nb: 0.005 to 0.1%, the balance is iron and inevitable impurities,
The metal structure is a steel sheet having a ferrite single phase structure,
In (I) the steel sheet surface, with the atomic ratio of Mn and Si (Mn / Si) is 5μm or less of Mn-Si composite oxide major diameter 0.01μm or more and 0.5 or more is present 10/100 [mu] m ² or more, The steel plate surface coverage of the oxide mainly composed of Si is 10% or less, and (II) When the cross section near the steel plate surface is observed at 2000 times using SEM, the width is 3 μm or less in any 10 fields of view. There are no cracks with a depth of 5 μm or more,
When tensile strength is 340 MPa or more, tensile strength (TS: unit MPa) and elongation (El: unit%)
Satisfies the following formula (1) and has a Rankford value (r value) of 1.2 or more, a high-strength cold-rolled steel sheet excellent in coating film adhesion and workability.
TS × El ≧ 13000 (1) The high-strength cold-rolled steel sheet according to any one of claims 1 to 3 , further comprising B: 0.005% or less (not including 0%) as another element.

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