JP4698968B2 - 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 excellent in coating film adhesion and workability, and in particular, has excellent coating film adhesion and exhibits excellent workability at a tensile strength of 550 MPa or more. The present invention relates to a cold-rolled steel sheet (residual austenite-containing steel sheet) that is optimal as a steel sheet for automobile parts.

  Higher strength of steel materials is demanded against the background of improving fuel economy and weight reduction of automobiles, and high tempering (hardening) is also progressing in the field of cold-rolled steel sheets. On the other hand, since cold-rolled steel sheets are press-formed at the time of component production, it is assumed that sufficient ductility such as elongation is secured. Addition of an alloy element is effective for increasing the strength, but the ductility 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, there is 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.

  Moreover, the steel materials used in these methods have a low C content of 0.005% or less, and the redrawing temperature is regulated to control the texture to improve the deep drawability. Although it relates to a so-called IF steel sheet, it is difficult to achieve a high strength as intended by the present invention with an IF steel sheet having a very small amount of C.

  In Patent Document 4, NbC is precipitated, and this is used as a nucleation site for zinc phosphate crystals to ensure chemical conversion treatment. However, this technique is also a technique that secures deep drawability by controlling the texture in a low C concentration range of 0.02% or less. Although the C concentration is slightly higher than the IF steel, the strength is insufficient. Absent.

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.

By the way, as a steel sheet capable of simultaneously improving both strength and ductility, the retained austenite (γ _R ) is generated in the structure, and γ _R undergoes an induced transformation (strain-induced transformation: TRIP) during work deformation, thereby improving the ductility. Residual austenitic steel sheet is known, and as a general method for stably presenting this retained austenite at room temperature, a method of containing about 1-2% of Si and about 1-2% of Al instead of Si There is a way to make it.

  In the method of positively containing Si, the strength and ductility can be increased at the same time, but since the Si-based oxide film is easily generated on the steel sheet surface, the chemical conversion treatment property is inferior. On the other hand, in the method of positively containing Al, a steel sheet having relatively good chemical conversion treatment properties can be obtained, but the strength and ductility are inferior to those of the Si-containing steel. Further, since Al is not an element having strengthening ability, it is necessary to add a large amount of strengthening elements such as C and Mn in order to increase the strength, which causes deterioration of weldability and the like.

Further, from the viewpoint of improving mechanical properties, a retained austenite-containing steel sheet in which both Si and Al are positively added has been proposed (for example, Patent Document 7). However, this steel plate is also considered to be inferior in chemical conversion treatment property because a Si-based oxide film is easily generated on the surface of the steel plate due to Si added in a large amount.
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 Japanese Patent Laid-Open No. 5-117761

  The present invention has been made in view of the above circumstances, and its object is to provide a cold-rolled steel sheet having excellent coating film adhesion and exhibiting excellent workability at a tensile strength of 550 MPa or more. is there.

The high-strength cold-rolled steel sheet according to the present invention is expressed by mass% (the same applies to chemical components below), C: 1% or less (not including 0%), Si: 0.1 to 2%, Al: 0.01 -3%, Si + Al: 1-4%, Mn: 1-6%, satisfying Si / Mn ≦ 0.4,
Composite in which the metal structure satisfies the space factor (the same applies to the metal structure hereinafter), the total amount of ferrite and bainite: 75% or more, ferrite: 5-80%, bainite: 5-80%, residual austenite: 5% or more A structured steel plate,
The tensile strength is 550 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1),
(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 ≧ 19000 (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 has solved the above problems is C: 1% or less (excluding 0%),
Si: 0.1 to 2%, Al: 0.01 to 3%, Si + Al: 1 to 4%, Mn: 1 to 6%,
A metallographic structure steel sheet satisfying a total amount of ferrite and bainite: 75% or more, ferrite: 5 to 80%, bainite: 5 to 80%, residual austenite: 5% or more,
The tensile strength is 550 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1),
(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 C: 1% or less (not including 0%), Si: 0.1-2%, Al: 0.01-3%, Si + Al: 1 -4%, Mn: 1-6%, satisfying Si / Mn ≦ 0.4,
A metallographic structure steel sheet satisfying a total amount of ferrite and bainite: 75% or more, ferrite: 5 to 80%, bainite: 5 to 80%, residual austenite: 5% or more,
It is characterized in that the tensile strength is 550 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1) and the above requirements (I) and (II). (Hereinafter sometimes referred to as “the present steel plate 3”).

  According to the present invention, a steel plate optimal for automobiles exhibiting excellent coating film adhesion and exhibiting excellent workability at a tensile strength of 550 MPa or more is constituted by a cladding or adding an expensive element. Can be realized efficiently.

  In order to obtain a steel sheet having an excellent coating film adhesion and workability and a tensile strength of 550 MPa or more, the following requirements (I) and / or (II) are particularly required to ensure excellent coating film adhesion: The present inventors have found that it should be satisfied and have come up with the present invention. Furthermore, while satisfying these requirements, the component composition and manufacturing conditions for ensuring excellent workability at a tensile strength of 550 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 considered a thing.

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. When the} Al content in the steel is high, the Mn—Si composite oxide may take the form of a Mn—Si—Al composite oxide containing Al. 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 are difficult to 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. Si / Mn is preferably 0.3 or less.

<C: 1% or less (excluding 0%)>
C is an element necessary for ensuring the strength, and is preferably contained in an amount of 0.05% or more, but if it exists in excess, weldability is lowered. Therefore, C content is suppressed to 1% or less. Preferably it is 0.23% or less, More preferably, it is 0.18% or less.

<Si: 0.1 to 2%>
Si is an element effective to promote C concentration to austenite and to retain austenite at room temperature to ensure an excellent strength-ductility balance. In order to sufficiently exhibit such an effect, Si is contained in an amount of 0.1% or more, preferably 0.5% 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 2% or less. Preferably it is 1.5% or less.

<Al: 0.01 to 3%>
Al is an element having a deoxidizing action. When Al deoxidation is performed, if the Al content is less than 0.01%, sufficient deoxidation cannot be performed at the molten steel stage, and excess oxygen is added to MnO, SiO _2. As oxide inclusions such as these, they are present in large amounts in steel and may cause local workability degradation. In addition, Al is an element effective for promoting C concentration to austenite and retaining austenite at room temperature to ensure an excellent strength-ductility balance, similar to Si. Therefore, it is preferable to contain 0.01% or more of Al. Preferably it is 0.2% or more. On the other hand, when the Al content is excessive, not only the effect of securing retained austenite is saturated, but also the steel becomes brittle and the cost is increased, so it is suppressed to 3% or less (preferably 2% or less).

<Si + Al: 1-4%>
In order to sufficiently secure the retained austenite and exhibit excellent workability stably, it is preferable to contain Si and Al in total of 1% or more (preferably 1.2% or more in total). However, even if Si and Al are present in excess, the steel itself is likely to become brittle, so the total is suppressed to 4% or less (preferably 3% or less).

<Mn: 1 to 6%>
Mn is an element necessary for securing strength, and is also an element effective for securing retained austenite and improving workability. In order to exhibit such an effect, it is contained 1% or more, preferably 1.3% or more. However, if it is excessive, both ductility and weldability deteriorate, so it is limited to 6% or less, preferably 3% or less.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but N is 0.01% or less as an element to be brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. (Nitrogen), and inevitable impurities such as O (oxygen) of 0.01% or less are allowed to be included, as well as other elements as long as they do not adversely affect the operation of the present invention. It is also possible to positively contain Cr, Mo, Ti, Nb, V, P, and B.

  That is, Cr, Mo, Ti, Nb, V, P, and B may be added from the viewpoint of increasing the strength of the steel sheet, and Cr: 0.01% or more, Mo: 0.01% or more, Ti: 0.0. 005% or more, Nb: 0.005% or more, V: 0.0005% or more, P: 0.005% or more, B: 0.0003% or more. Therefore, Cr and Mo are preferably 1% or less, Ti, Nb, P and V are each 0.1% or less, and B is preferably 0.01% or less.

In the present invention, the parent phase structure of the steel sheet is ferrite and bainite, and retained austenite (γ _R ) exists in the structure, and the γ _R undergoes an induced transformation (strain-induced transformation: TRIP) during work deformation. Thus, it is intended for a so-called TRIP steel sheet exhibiting excellent ductility.

  The total amount of ferrite and bainite is 75% or more, preferably 80% or more, but the upper limit is controlled by a balance with the amount of retained austenite to be described later, so that the desired high workability can be obtained. It is recommended to adjust. The “ferrite” in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density. Of the above matrix structure, ferrite is a structure that contributes to securing ductility, and bainite is a structure that contributes to securing strength, and it is necessary to maintain these in an appropriate area ratio from the viewpoint of strength and ductility. Too much or too little is not preferable. Accordingly, ferrite and bainite are each in the range of 5 to 80%. Preferably, ferrite should be 30% or more, and the upper limit of bainite is preferably 50%.

  Further, as described above, the steel sheet of the present invention contains 5% or more, preferably 7% or more of retained austenite in order to exhibit excellent ductility. On the other hand, when the residual austenite is excessively present, the sensitivity to hydrogen embrittlement represented by delayed fracture increases, so it is preferable to keep it to 25% or less.

  In addition to the above-described structure (that is, ferrite, bainite, and retained austenite), martensite that may inevitably remain in the production process of the present invention may be included within a range that does not impair the function of the present invention. However, the martensite is preferably suppressed to 5% or less in terms of space factor.

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 550 MPa or more (particularly 590 MPa or more), and
-Tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1). Those in which the right side of the following formula (1) is 21,000 or more are particularly preferable because of excellent balance between strength and workability.
TS × El ≧ 19000 (1)
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 5 to 16 mass% hydrochloric acid at 70 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 70-90 ° C. and immersed in 5-16 mass% 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 Is -45 ° C. or lower), and further, as a cooling method during continuous annealing, GJ (cooling by gas blowing) or RQ (cooling by water cooling roll heat removal) not using water is adopted, or in the case of mist cooling, It is effective to perform the mist cooling from a state where the steel plate temperature is 550 ° C. or lower (preferably 450 ° C. or lower).

  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. Further, if a small amount of Ni flash plating is performed on the steel material after annealing or the steel material pickled after annealing, there is an effect of making the chemical conversion treatment film fine and effective.

  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. The manufacturing conditions are shown in Table 2. The pickling was performed using a hydrochloric acid aqueous solution having a temperature of 70 to 90 ° C. and a concentration of 10 to 16% by mass. Thereafter, cold rolling was performed to obtain a steel plate having a thickness of 1.4 mm. And the continuous annealing was given to the steel plate obtained by the method shown in FIG. Cooling after soaking and gradual cooling in continuous annealing is performed by any one of brackish water (mist) cooling, gas spray cooling (GJ), and water cooling roll heat removal (RQ). Tempering was performed as shown in FIG. In the case of brackish water (mist) cooling, after tempering, it was immersed (pickled) for 5 seconds in hydrochloric acid having a liquid temperature of 50 ° C. and a concentration of 5% by mass. The heating temperature, annealing end point temperature, and tempering temperature in Table 2 indicate the temperatures at the locations shown in FIG. The dew point is the atmospheric dew point of the continuous annealing furnace.

  The metal structure of the obtained steel sheet was examined as follows. That is, the steel sheet was repeller-corroded and the structure was identified by observation with an optical microscope (magnification 1000 times), and then the ferrite area ratio in the optical micrograph (magnification 1000 times) was calculated. The area ratio of retained austenite was determined by XRD (X-ray diffraction analyzer). The area ratio of bainite was determined as the balance of ferrite and retained austenite (including martensite and other structures).

  Moreover, the mechanical characteristics and coating-film adhesiveness were evaluated using the obtained steel plate. The mechanical properties are measured by taking a JIS No. 5 test piece, obtaining the tensile strength (TS), El (total elongation) and yield point (YP), the tensile strength (TS) is 550 MPa or more, and the tensile strength and A case where the product of elongation (TS × El) was 19000 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 exceeding 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.

  From Tables 1 to 3, the following can be considered (the following No. indicates the experiment No.). That is, no. Nos. 31 and 33 satisfy the prescribed requirements as the steel sheet 1 of the present invention, and therefore have excellent chemical conversion 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 Nos. 21 and 22 satisfy the requirements defined as the steel plate 2 of the present invention, cracks are not generated, and a steel plate 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 16, 23, 25, 26, and 32 satisfy the requirements defined by the steel plate 3 of the present invention (that is, the requirements defined by the steel plate 1 and the steel plate 2 of the present invention), and therefore excellent chemical conversion treatment properties. And the occurrence of cracks is suppressed, and excellent coating film adhesion is exhibited.

  In contrast, no. 17-20, 24, 27-30 do not satisfy any of the requirements of the steel sheets 1-3 of the present invention, are not excellent in coating film adhesion, or are not excellent in the strength-ductility balance, and have high strength. In addition, a material that exhibits excellent workability has not been obtained.

  No. Since 17-20 did not satisfy the component composition prescribed | regulated by this invention, the result was inferior to mechanical characteristics or inferior to coating-film adhesiveness. That is, no. No. 17 has a small amount of Si. No. 20 had a small total amount of Si and Al, so that all of them could not secure sufficient retained austenite, and the strength-ductility balance was inferior. No. In No. 18, since the Si amount was excessive and the Si / Mn ratio exceeded the upper limit, the surface of the steel sheet was not defined, and the coating film adhesion was inferior.

  No. No. 19 has a small amount of Mn, so it cannot secure sufficient retained austenite, is inferior in the strength-ductility balance, cannot secure the specified Mn-Si composite oxide, and is inferior in chemical conversion treatment. .

  No. Nos. 24 and 27 to 30 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. In No. 24, mist cooling is performed from a state in which the end point temperature of the annealing is relatively high, and a large amount of oxide mainly composed of Si is formed on both the surface and the grain boundary by being exposed to the water vapor atmosphere at such a high temperature. The result was inferior to chemical conversion. Moreover, since the said oxide melt | dissolved in the subsequent pickling process and the crack generate | occur | produced, it became inferior to coating-film adhesiveness significantly.

  No. Nos. 27 and 30 have a relatively high coiling temperature, so that Si surface concentration during hot rolling is promoted. No. 28 has a short pickling time, so that removal of the concentrated Si layer is insufficient. No. 29 has a high dew point, so that the surface concentration of Si is promoted in the annealing stage, and in each case, a large amount of Si-based oxides exist, and Si oxides are formed at the grain boundaries and cracks occur after pickling. As a result, the coating film adhesion was inferior.

  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. 3 shows that the surface region of the steel sheet is covered with an oxide layer mainly composed of Si.

  FIG. 4 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment observed with an SEM. From FIG. 22 shows that the zinc phosphate crystal is small but the gap is large.

  On the other hand, FIG. 2 is a TEM observation photograph on the steel plate surface. The layer like 22 is not formed, and the granular material is finely dispersed instead. That is, from FIG. It can be confirmed that the surface area of the steel plate 2 has almost no Si-based oxides that lower the chemical conversion processability, and there are many Mn-Si composite oxides effective for improving the chemical conversion processability.

  FIG. 6 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment, which was observed with an SEM. 2 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. No. in the examples. 22 is a TEM observation photograph (extraction replica, magnification: 15000 times) of No. 22 (comparative example). No. in the examples. It is a SEM observation photograph of the steel plate surface (after chemical conversion treatment) of 22 (comparative example). No. in the examples. 2 is a TEM observation photograph (extraction replica, magnification: 15000 times) of No. 2 (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 2 (invention example).

Claims (3)

% By mass (the same applies to chemical components)
C: 1% or less (excluding 0%),
Si: 0.1 to 2%,
Al: 0.01 to 3%,
Si + Al: 1-4%
Mn: 1 to 6%
Si / Mn ≦ 0.4
The balance is iron and inevitable impurities,
The metal structure is the space factor (the same applies to the metal structure below)
Total amount of ferrite and bainite: 75% or more,
Ferrite: 5-80%,
Bainite: 5-80%
Residual austenite: a composite structure steel plate satisfying 5% or more,
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,
A high-strength cold excellent in coating film adhesion and workability, characterized in that the tensile strength is 550 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1): Rolled steel sheet.
TS × El ≧ 19000 (1) C: 1% or less (excluding 0%),
Si: 0.1 to 2%,
Al: 0.01 to 3%,
Si + Al: 1-4%
Mn: 1 to 6%
Si / Mn ≦ 0.4
The balance is iron and inevitable impurities,
The metal structure is
Total amount of ferrite and bainite: 75% or more,
Ferrite: 5-80%,
Bainite: 5-80%
Residual austenite: a composite structure steel plate satisfying 5% or more,
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,
A high-strength cold excellent in coating film adhesion and workability, characterized in that the tensile strength is 550 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1): Rolled steel sheet.
TS × El ≧ 19000 (1) As other elements,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%,
V: 0.0005 to 0.1%,
P: 0.005-0.1% and B: 0.0003-0.01%
The high-strength cold-rolled steel sheet according to claim 1 or 2 , containing one or more selected from the group consisting of:

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