JP3889769B2 - High-strength cold-rolled steel sheet and automotive steel parts with excellent coating film adhesion, workability, and hydrogen embrittlement resistance - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet and an automotive steel part excellent in coating film adhesion, workability and hydrogen embrittlement resistance, and particularly has excellent coating film adhesion and tensile strength. Is a cold-rolled steel sheet (residual austenite-containing steel sheet) optimal for the production of automotive steel parts that exhibit excellent workability and hydrogen embrittlement resistance at 780 MPa or higher, and high strength and hydrogen embrittlement resistance obtained using the steel sheet The present invention relates to a steel part for automobiles having excellent crystallization characteristics.

  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 secures chemical conversion treatment by regulating 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 more than the Ac ₁ transformation point. 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 coating crystal is generated to the same extent as Mn ₂ SiO ₄ , it is difficult to say that the chemical conversion treatment performance 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, a retained austenite steel sheet in which ductility is improved by generating residual austenite in the structure and inducing a transformation (strain-induced transformation: TRIP) of the retained austenite during work deformation. There are known general methods for stably presenting the retained austenite at room temperature, including a method of containing about 1-2% of Si and a method of containing about 1-2% of Al instead of Si. .

  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. Further, the hydrogen embrittlement resistance, which is generally regarded as a defect of the retained austenitic steel sheet, is not improved.
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 purpose is cold rolling that has excellent coating film adhesion and exhibits excellent workability and hydrogen embrittlement resistance when the tensile strength is 780 MPa or more. It is providing the steel plate for motor vehicles obtained using a steel plate and this steel plate.

The high-strength cold-rolled steel sheet according to the present invention is expressed by mass% (the same applies to chemical components below), C: 0.06-0.6%, Si: 0.1-2%, Al: 0.01-3. %, Si + Al: 1-4%, Mn: 1-6%, satisfying Si / Mn ≦ 0.40,
The metal structure is the space factor (the same applies to the metal structure below)
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% or more,
While the tensile strength is 780 MPa 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”).

  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 C: 0.06 to 0.6%, Si: 0.1 to 2%, Al: 0.01 to 3%, Si + Al: 1 to 4% , Mn: satisfying 1 to 6%,
The metal structure is
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% or more,
While the tensile strength is 780 MPa 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”).

Still another steel sheet of the present invention that can solve the above problems is C: 0.06 to 0.6%, Si: 0.1 to 2%, Al: 0.01 to 3%, Si + Al: 1 to 4 %, Mn: 1 to 6%, satisfying Si / Mn ≦ 0.40,
The metal structure is
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% or more,
In addition to having a tensile strength of 780 MPa or more, the tensile strength is characterized by satisfying the above requirements (I) and (II) (hereinafter sometimes referred to as “the present steel plate 3”).

  The present invention also includes steel parts for automobiles obtained using any of the above steel sheets.

  According to the present invention, a steel plate optimal for the production of automotive steel parts exhibiting excellent paint film adhesion and exhibiting excellent workability and hydrogen embrittlement resistance at a tensile strength of 780 MPa or more is provided with a cladding. It can be efficiently realized without constituting or adding an expensive element. Moreover, the steel part for motor vehicles obtained using this steel plate exhibits the excellent hydrogen embrittlement resistance in the high strength region of 780 MPa or more.

  As a result of various investigations to obtain the above-described steel sheet, it has been found that the following requirements (I) and / or (II) may be satisfied particularly in order to ensure excellent coating film adhesion. I came up with it. Furthermore, while satisfying these requirements, the component composition, metal structure, and manufacturing conditions for ensuring excellent workability and hydrogen embrittlement resistance at a tensile strength of 780 MPa or more were also examined.

(I) On the steel plate surface (when viewed in plan)
(I) 10/100 μm ² or more of Mn—Si composite oxide having a major axis of 0.01 μm or more and 5 μm or less with an atomic ratio (Mn / Si) of Mn to Si of 0.5 or more, and (ii) Si The steel sheet surface coverage of an oxide mainly composed of (an element other than oxygen constituting the oxide, in which Si occupies more than 67% by atomic ratio) is 10% or less.

(II) When observing a cross section near the surface of the steel sheet at 2000 times using SEM,
In any 10 fields of view, cracks having a width of 3 μm or less and a depth of 5 μm or more should not be present.

  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 (Mn / Si) of Mn to 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 formation 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, ten 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.

In all of the Mn—Si composite oxides present, the electrochemical heterogeneous field is not necessarily formed effectively, and is preferably 50 or more, more preferably 100 or more, even more preferably 100 μm ^2. 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 steel is high, the Mn—Si composite oxide may take the form of a Mn—Si—Al composite oxide containing Al.

<Stainless steel sheet surface coverage of oxide mainly composed of Si: 10% or less>
Even if an appropriate amount of Mn-Si composite oxide effective for the formation of zinc phosphate crystals is present, if there are other substances that inhibit the chemical conversion treatment, excellent chemical conversion treatment performance will not be exhibited, resulting in coating adhesion. It becomes inferior.

  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 (FIG. 1) 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. Since it is difficult for zinc phosphate crystals to adhere to cracks with a thickness of 5 μm or more, cracks with 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%) ≦ 0.40>
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 and Mn content in steel (Si / Mn) to 0.40 or less, The chemical conversion processability was improved. Si / Mn is preferably 0.3 or less.

<C: 0.06 to 0.6%>
C is an element necessary for ensuring the strength, and is preferably contained in an amount of 0.06% or more (preferably 0.09% or more), but if it exists in excess, weldability is lowered. Therefore, C content is suppressed to 0.6% or less. Preferably it is 0.30% or less, More preferably, it is 0.20% 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, Si oxides are also generated at the grain boundaries and cracks are likely to occur after pickling, and the solid solution strengthening action becomes excessive and the rolling load increases, so that it is 2% or less. Keep it down. 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. Such oxide inclusions are present in a large amount in steel and may cause local workability deterioration. 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 0.02% or less of S as an element brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. (Sulfur), 0.01% or less of N (nitrogen), 0.01% or less of O (oxygen) and other inevitable impurities are allowed to be included, as well as adversely affecting the function of the present invention. It is also possible to positively contain Cr, Mo, Ti, Nb, V, P, and B as other elements within a range that does not give the above.

  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: 0.005% or more, P: 0.0005% or more, B: 0.0003% or more may be contained. Therefore, it is preferable to suppress Cr and Mo to 1% or less, Ti, Nb, and P to 0.1% or less, V to 0.3% or less, and B to 0.01% or less, respectively.

  In the present invention, the matrix structure of the steel sheet is bainitic ferrite and polygonal ferrite, and the retained austenite is present in the structure, and the retained austenite 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 bainitic ferrite and polygonal ferrite is 75% or more, preferably 80% or more, but the upper limit is controlled by the balance with the amount of retained austenite described later, and desired high workability is obtained. It is recommended to make appropriate adjustments. The bainitic ferrite in the present invention is different from the bainite structure in that it does not have carbides in the structure. It is also different from polygonal ferrite with extremely low dislocation density and quasi-polygonal ferrite structure with a substructure such as fine subgrains (see “Stein Bainite Photobook-1” published by the Japan Iron and Steel Institute Basic Research Group). . Of the above matrix structure, bainitic ferrite contributes to ensuring strength and improving hydrogen embrittlement resistance, and polygonal ferrite contributes to securing ductility, and must be controlled to an appropriate balance.

  Therefore, bainitic ferrite is 40% or more, and polygonal ferrite is 1 to 50%. More preferably, the bainitic ferrite is 50% or more, and the polygonal ferrite is 30% or less.

  Further, as described above, the steel sheet of the present invention contains 3% or more, preferably 5% or more of retained austenite in order to exhibit excellent ductility. On the other hand, if the retained austenite is excessive, stretch flangeability deteriorates, so the upper limit is preferably set to 25%. The retained austenite is preferably present in lath form in bainitic ferrite from the viewpoint of improving hydrogen embrittlement resistance. Here, the term “lass” means that the average axial ratio (major axis / minor axis) is 2 or more (preferably 4 or more, and a preferred upper limit is 30 or less).

  The space factor of bainitic ferrite in the present invention is determined by subtracting the space factor occupied by polygonal ferrite and retained austenite from the entire structure (100%), as shown in the examples described later. The space factor of bainitic ferrite obtained in this way may include bainite and martensite that can be inevitably formed in the production process of the present invention within a range that does not impair the function of the present invention.

  The production method for obtaining the steel sheet of the present invention is not particularly limited, but in order to improve the chemical conversion processability, the component composition is satisfied in order to control the form of oxide deposited on the steel sheet surface as defined as the above requirement (I). In addition, in the manufacturing process, after hot rolling, the liquid temperature is 70 to 90 ° C. and immersed in 5 to 16% by mass of hydrochloric acid for 40 seconds or more (preferably 60 seconds or more), and the dew point during continuous annealing is −40 It is effective to suppress the temperature to below ℃ (preferably below -45 ℃). 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 at the time of continuous annealing, cooling by gas blowing without using water (GJ) or cooling by heat removal from a water-cooled roll (RQ) 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).

In addition, as a matrix structure, in order to secure a mixed structure of bainitic ferrite with a space factor of 40% or more and polygonal ferrite, heat treatment is performed as follows while controlling the dew point during continuous annealing to the above-mentioned conditions. It is recommended to do it under conditions. That is,
(A) heating and holding at a temperature of 850 ° C. or higher for 10 to 200 seconds;
(B) cooling to a bainite transformation temperature range (about 500 to 350 ° C.) while avoiding pearlite transformation at an average cooling rate of 3 ° C./s or more, and (C) holding for 10 seconds or more in the temperature range, Is recommended.

  First, as described in (A) above, soaking at a temperature of 850 ° C. or more is effective for completely dissolving the carbide to form the desired retained austenite, and in the cooling step after soaking. It is also effective in obtaining bainite having a high dislocation density. The holding time at the above temperature is preferably 10 to 200 seconds. This is because if it is too short, the above-mentioned effect due to heating cannot be fully enjoyed. On the other hand, if the holding time is too long, the crystal grains become coarse. More preferably, it is 20 to 150 seconds.

  Next, as in (B) above, the average cooling rate is 3 ° C./s or higher, preferably 5 ° C./s or higher, and cooling to the bainite transformation temperature range (about 500 to 350 ° C.) is avoided while avoiding pearlite transformation. . By controlling the average cooling rate, a large amount of dislocations can be introduced into the bainitic ferrite, and a desired strength can be ensured. From the viewpoint of increasing the strength, the upper limit of the average cooling rate is not particularly specified and should be as large as possible. However, it is recommended to appropriately control in consideration of actual operation.

  The cooling rate is preferably controlled up to the bainite transformation temperature range. If the control is terminated earlier in the higher temperature range than the temperature range, and then cooled, for example, at a moderate rate, dislocation cannot be sufficiently introduced, and residual austenite is difficult to generate, This is because excellent processability cannot be ensured. On the other hand, even when cooled to the lower temperature range at the above cooling rate, retained austenite is hardly generated, and excellent workability cannot be secured, which is not preferable.

  After cooling, the temperature is preferably maintained for 10 seconds or more in the temperature range as described in (C) above. Thereby, C concentration to the retained austenite can be efficiently advanced in a short time, and a large amount of stable retained austenite can be obtained. As a result, the TRIP effect by the retained austenite is sufficiently exhibited. On the other hand, if the holding time is too long, dislocation recovery occurs, dislocations formed by the cooling decrease, and the strength cannot be ensured.

  Other production conditions are not particularly limited, and as usual, after smelting, continuous casting or mold casting is performed to obtain a slab, and then hot rolling (hot rolling) and then cold rolling (cold rolling). It may be done. In the hot rolling step, normal conditions other than the coiling temperature may be adopted. After the hot rolling is finished at 850 ° C. or higher, the cooling is performed at an average cooling rate of about 30 ° C./s, and the temperature is about 400 to 500 ° C. It is sufficient to employ conditions such as winding with In the cold rolling process, it is recommended to perform cold rolling at a cold rolling rate of about 30 to 70%. Of course, this is not intended to be limited to this. In the examples described later, pickling is performed after continuous annealing, but the presence or absence of the pickling is not questioned. 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 1.6 mm. Cooling after soaking in continuous annealing was performed by any one or a combination of mist cooling, GJ, and RQ, and holding was performed under the conditions (temperature and time) shown in Table 2 after the cooling. In the case of mist cooling, after holding, 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 dew point is the atmospheric dew point of the continuous annealing furnace excluding the mist cooling section.

  The metal structure of the obtained steel sheet was examined as follows. That is, after the steel plate was repeller-corroded and the structure was identified by observation with an SEM and an optical microscope (magnification 1000 times), the area ratio of polygonal ferrite was calculated. The area ratio of retained austenite was determined by XRD (X-ray diffraction analyzer). The area ratio of bainitic ferrite is obtained by subtracting the area ratio occupied by the polygonal ferrite and retained austenite from the total structure (100%), and inevitably formed martensite and other structures. Including.

  Moreover, the mechanical characteristics and coating-film adhesiveness were evaluated using the obtained steel plate. The mechanical properties were measured by taking a JIS No. 5 test piece, obtaining the tensile strength (TS), total elongation (El), and yield point (YP), the tensile strength (TS) being 780 MPa or more, and the tensile strength and When the product of elongation (TS × El) was 19000 (17000 when the strength was 1180 MPa or more, 15000 when the strength was 1370 MPa or more), it was evaluated that the workability was excellent.

  The resistance to hydrogen embrittlement was obtained by using each obtained steel sheet as a test piece of 15 mm × 65 mm size and applying a stress of 780 MPa by 4-point bending (0.5 mol sulfuric acid + 0.01 mol-KSCN) in a potentiostat in the solution. This was evaluated by measuring the time (cracking life) until cracking occurred when a potential of −80 mV, which is lower than the natural potential, was applied using the. In this example, the crack generation life exceeding 1000 seconds was evaluated as “excellent in hydrogen embrittlement resistance”.

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 → water washing → surface adjustment → chemical conversion treatment

As for the number of Mn-Si oxides, an extraction replica film on the surface of a steel material was prepared, and this was observed by TEM at 15000 times (H-800, manufactured by Hitachi, Ltd.), and the average number of 20 arbitrary fields (per 100 μm ² ) was examined. It was.

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) 10% acetylacetone-90% methanol etchant
Corrosion to raise the carbon.
(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. These results are shown in Table 3.

  It can be considered as follows from Tables 1 to 3 (note that the following No. indicates the experiment No.). That is, no. Nos. 24 and 27 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 set the recommended conditions for the cooling at the coiling temperature and continuous annealing.

  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, and 26 satisfy the requirements specified by the steel plate 3 of the present invention (that is, the requirements specified by the steel plate 1 of the present invention and the steel plate 2 of the present invention), thus ensuring excellent chemical conversion treatment properties. And the occurrence of cracks is suppressed and excellent coating film adhesion is exhibited.

  In contrast, no. Nos. 17 to 20, 28, and 29 do not satisfy any of the requirements of the steel sheets 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. 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. . Furthermore, since the amount of bainitic ferrite is too small, the hydrogen embrittlement resistance is inferior.

  No. Nos. 28 and 29 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. No. 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. 2 shows that the surface area of the steel sheet is covered with an oxide layer (white part) mainly composed of Si.

  FIG. 3 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment observed with an SEM. From FIG. 18 shows that the zinc phosphate crystal is small but the gap is large.

  On the other hand, FIG. 7 is a TEM observation photograph on the steel plate surface. The layer like 18 is not formed, and the granular material is finely dispersed instead. That is, from FIG. It can be confirmed that in the steel plate surface region of No. 7, there is almost no Si-based oxide that lowers the chemical conversion processability, and there are many Mn—Si composite oxides that are effective in improving the chemical conversion processability.

  FIG. 5 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment, which was observed with an SEM. 7 shows that the zinc phosphate crystals are small and have no gaps.

  Experiment No. 1 in Example 1 above. No. 7 steel plate (1.6 mm thick) was pressed and shaped into a hat-channel shaped test body imitating the center pillar reinforcement, which is a vehicle body component. Moreover, a 1.8 mm thick iron ream (Japan Iron and Steel Federation) standard JSC590Y was used as a comparative material, and a specimen having the same shape was molded.

  When both ends of the test body were fixed and a three-point bending test was performed in which a load was applied to the central portion with an Amsler type tester, both showed almost the same load-displacement behavior. From this result, it can be seen that if the steel plate of the present invention is used for the production of automobile body parts, it is possible to reduce the thickness compared to the case where a conventional steel plate is used, and it is effective for reducing the weight of the vehicle.

It is the figure which showed typically the crack in a steel plate cross section. No. in the examples. 18 is a TEM observation photograph (extracted replica, magnification: 15000 times) of 18 (Comparative Example). No. in the examples. It is a SEM observation photograph of the steel plate surface (after chemical conversion treatment) of 18 (comparative example). No. in the examples. 7 is a TEM observation photograph (extraction replica, magnification: 15000 times) of No. 7 (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 7 (invention example).

Claims (8)

% By mass (the same applies to chemical components)
C: 0.06 to 0.6%,
Si: 0.1 to 2%,
Al: 0.01 to 3%,
Si + Al: 1-4%
Mn: 1 to 6%
Si / Mn ≦ 0.40
The filling,
The metal structure is the space factor (the same applies to the metal structure below)
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% 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-rolled steel sheet excellent in coating film adhesion, workability, and hydrogen embrittlement resistance, characterized by having a tensile strength of 780 MPa or more. C: 0.06 to 0.6%,
Si: 0.1 to 2%,
Al: 0.01 to 3%,
Si + Al: 1-4%
Mn: 1 to 6%
Ti: 0.005 to 0.1%,
Balance: Fe and inevitable impurities are satisfied, and the metal structure is
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% or more,
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,
A high-strength cold-rolled steel sheet excellent in coating film adhesion, workability, and hydrogen embrittlement resistance, characterized by having a tensile strength of 780 MPa or more. C: 0.06 to 0.6%,
Si: 0.1 to 2%,
Al: 0.01 to 3%,
Si + Al: 1-4%
Mn: 1 to 6%
Si / Mn ≦ 0.40
Meet the metal structure,
Total amount of bainitic ferrite and polygonal ferrite: 75% or more,
Bainitic ferrite: 40% or more,
Polygonal ferrite: 1-50%
Residual austenite: a steel sheet containing 3% 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-rolled steel sheet excellent in coating film adhesion, workability, and hydrogen embrittlement resistance, characterized by having a tensile strength of 780 MPa or more. The high strength cold-rolled steel sheet according to any one of claims 1 to 3, which is Si + Al: 1.2 to 4%. The high-strength cold rolling according to any one of claims 1 to 4, further comprising 1% or less (not including 0%) of Cr and / or 1% or less (not including 0%) of Mo. steel sheet. Further, Ti is 0.1% or less (not including 0%), Nb is 0.1% or less (not including 0%), and V is 0.3% or less (not including 0%). The high-strength cold-rolled steel sheet according to any one of claims 1 and 3 to 5, which contains at least one selected from. The high-strength cold-rolled steel sheet according to any one of claims 1 to 6, further comprising 0.01% or less (excluding 0%) of B. An automotive steel part obtained by using the steel plate according to any one of claims 1 to 7 .